The Tibetan Plateau (TP) is the largest alpine plateau on
Earth and plays an important role in global climate dynamics. On the TP,
climate change is happening particularly fast, with an increase in air
temperature twice the global average. The particular sensitivity of this
high mountain environment allows observation and tracking of abiotic and
biotic feedback mechanisms. Closed lake systems, such as Nam Co on the
central TP, represent important natural laboratories for tracking past and
recent climatic changes, as well as geobiological processes and interactions
within their respective catchments. This review gives an interdisciplinary
overview of past and modern environmental changes using Nam Co as a case
study. In the catchment area, ongoing rise in air temperature forces
glaciers to melt, contributing to a rise in lake level and changes in water
chemistry. Some studies base their conclusions on inconsistent glacier
inventories, but an ever-increasing deglaciation and thus higher water
availability have persisted over the last few decades. Increasing water
availability causes translocation of sediments, nutrients and dissolved
organic matter to the lake, as well as higher carbon emissions to the
atmosphere. The intensity of grazing has an additional and significant
effect on CO2 fluxes, with moderate grazing enhancing belowground
allocation of carbon while adversely affecting the C sink potential through
reduction of above-surface and subsurface biomass at higher grazing intensities.
Furthermore, increasing pressure from human activities and livestock grazing
are enhancing grassland degradation processes, thus shaping biodiversity
patterns in the lake and catchment. The environmental signal provided by
taxon-specific analysis (e.g., diatoms and ostracods) in Nam Co revealed
profound climatic fluctuations between warmer–cooler and wetter–drier
periods since the late Pleistocene and an increasing input of freshwater and
nutrients from the catchment in recent years. Based on the reviewed
literature, we outline perspectives to further understand the effects of
global warming on geodiversity and biodiversity and their interplay at Nam Co,
which acts as a case study for potentially TP-level or even worldwide processes
that are currently shaping high mountain areas.
Introduction
The Tibetan Plateau (TP), often referred to as “The Third Pole” and “The
Water Tower of East Asia”, is the highest and largest alpine plateau on
Earth (Qiu, 2008). With an area of about 2.5 million km2 at
an average altitude of > 4000 m above sea level (a.s.l.), it
includes the entire southwestern Chinese provinces of Tibet and Qinghai;
parts of Gansu, Yunnan, and Sichuan; and neighboring countries (Fig. 1). The
southern and eastern plateau and the adjacent Himalayas regions form
the headwaters of several major rivers (i.e., Brahmaputra, Ganges, Hexi,
Indus, Mekong, Salween, Yangtze, and Yellow rivers), providing freshwater
for ∼1.65 billion people and many ecosystems in greater
Asia (Cuo and Zhang, 2017). Large proportions of the inner TP are endorheic
and therefore do not drain into the large river systems. On the TP, the
effects of climate change are expressed more strongly than the global average,
showing a steep rise in air temperature of about 0.3 ∘C per decade
since 1960 (Yao et al., 2007) and a moderate rise in precipitation during
the last few decades (Dong et al., 2018). The warming rate increases with
altitude (Pepin et al., 2015), which is why the air temperature on the TP is
soaring roughly twice the global average, thus substantially affecting the
geodiversity and biodiversity. Glaciers and lakes are the dominant components for
the Tibetan water sources, and their actual status and future development
are strongly impacted by global warming. Since the 1990s, nearly all
glaciers on the TP have exhibited retreat, causing a 5.5 % increase in
river runoff from the Tibetan Plateau (Yao et al., 2007). The consequences of
deglaciation and permafrost degradation (Wu et al., 2010) are observable in
higher water and sediment fluxes, relief changes and arising natural hazards
(floods, rockfalls, landslides, desertification, ecosystem degradation).
Consequently, landscapes are continuously being rearranged which alters the
spatial distribution and composition of the inhabiting species, many of
which are endemic to the TP (Walther et al., 2002). Even conservative
estimates predict substantial species extinction and considerable changes to
the ecosystems (Chen et al., 2011; Bellard et al., 2012). The future
trajectory of such complex processes is difficult to map accurately, thus it
is important to monitor the current state as well as the evolution of this
highly sensitive region. The large number of water bodies on the TP and its
geological diversity, climatic setting and sensitivity to climate
change make it a unique natural laboratory, which could be used as an early
warning system for other alpine environments. Many lakes on the TP are
superficially closed systems, which is why they are particularly suitable as
“thermometers” and “rain gauges” to measure the climatic, hydrological,
geomorphological, pedological and ecological changes in their respective
catchments. With an area of 2018 km2, Nam Co is the second
largest lake on the central TP. Currently, Nam Co represents an
endorheic system, acting as a sink for water, sediment and carbon fluxes.
The existence of a former drainage (“Old Qiangtang Lake”) towards
northwestern Siling Co and further east down from the TP is still under
discussion (Li et al., 1981; Kong et al., 2011) (see Sect. 3.1). With good
accessibility and infrastructure such as the Nam Co Monitoring and Research
Station for Multisphere Interactions (NAMORS), the Nam Co catchment has
become a frequent study location for monitoring and tracking of
environmental changes over various timescales.
Here we present an interdisciplinary overview of how Earth surface fluxes
have developed with changing environmental conditions and which consequences
are to be expected for biodiversity, as well as for water, sediment and
carbon fluxes within the study area of the Nam Co catchment on the central
TP. In particular, this review considers past and modern
geobiodiversity changes with a focus on glacier retreat in relation to
hydrological patterns and changes in lake water chemistry. The corresponding
changes in terrestrial ecosystems concerning carbon cycle, greenhouse gas
releases and pasture degradation are discussed. We provide an
overview of how the paleoenvironment on the Tibetan Plateau with respect to
landscape evolution around Nam Co was shaped by geodiversity, lake level
changes and Holocene vegetation cover. Lastly, based on the available
studies, this review identifies the major research gaps that are awaiting
further exploration and comparison with other high-altitude environments.
Major atmospheric systems governing the climate in China (a). The Nam
Co study site (b). Characteristics of Nam Co's catchment (c). (a) Continuous arrows indicate systems active in summer. These are the Indian
Summer Monsoon (ISM) in red, the East Asian Summer Monsoon (EASM) in orange
and the westerlies in blue. Dashed arrows represent systems active in
winter. These are the Asian Winter Monsoon (AWM) in green and southern parts
of the westerlies in blue. The dotted black lines denote the Summer Monsoon
Transition Zone (SMTZ) (following Wünnemann et al., 2018). Background
elevation data according to SRTM digital elevation model v4 (Jarvis et al., 2008). (b) Nam Co
catchment, including the current lake extent (based on Copernicus Sentinel
data 2018, processed by ESA), its bathymetric depth in 2007 (Wang et al.,
2009a), the outline of the catchment (following Keil et al., 2010), glaciers of
the Nyainqêntanglha Range (GLIMS and NSDIC 2005, updated 2018) and
rivers discharging into Nam Co (SRTM digital elevation model v4; Jarvis et al., 2008). The dotted red
line indicates the profile position of Fig. 2. (c) Characteristics
of Nam Co: lake elevation (Jiang et al., 2017), lake surface area (Zhang et
al., 2017), catchment area, lake pH and salinity (Keil et al., 2010).
Environmental changes in Nam Co and its catchmentClimatic characteristics of the Nam Co basin
The prevailing climate at Nam Co is characterized by strong seasonality,
with long, cold winters and short but moist summers. During winter, the
westerlies control the general circulation and lead to cold and dry weather,
with daily temperature minima below -20 ∘C. In springtime, the TP
heats up and allows the meltwater to percolate to deeper soil layers. The
drought situation increases gradually until the monsoon rains arrive,
typically between May and June. During autumn, weather shifts again to
clear, cold and dry conditions (Yao et al., 2013). The mean annual
temperature measured at the NAMORS research station (Fig. 1) between 2006
and 2017 was -0.6 ∘C, and the annual precipitation was between
291–568 mm (mean = 406 mm), with the majority occurring during the
monsoon season from May to October (Table 1). The onset and strength of
monsoonal precipitation varies substantially between individual years and
can be delayed by up to 6 weeks, depending on the altitude and latitude on
the TP (Miehe et al., 2019). Precipitation rates are subject to spatial
variations due to the > 7000 m high Nyainqêntanglha range,
which represents the southern border of the lake catchment. This leads to
considerably larger glacial areas in the southwestern area (∼700 km2) than in the northeastern area of the mountain range
(∼100 km2 ) (Bolch et al., 2010).
Average daily air temperature (maximum, mean and minimum in
∘C) and average daily precipitation (sum in mm) from NAMORS from
2006 to 2017. Calculations were performed using the tidyverse package family
in R on RStudio environment (Wickham, 2017; RStudio Team, 2018; R Core Team,
2019). Data provided by ITP Beijing; for details about sensor equipment, see
Ma et al. (2009).
JanFebMarAprMayJunJulAugSepOctNovDec∅/∑Tmax-0.7-1.51.54.711.613.312.612.211.38.72.51.26.4Tmean-10.8-9.7-5.7-1.43.17.99.18.36.50.3-6.5-8.4-0.6Tmin-21.5-20.5-14.3-7.3-4.51.15.23.2-1.3-14.7-15.3-19.1-9.1Precip.41313234185117813451406Glacier retreat and hydrological patterns of Nam Co
The rise of satellites such as Envisat, CryoSat, and ICESat and the
increasingly widespread availability of their data has enabled the
accurate study of lake and glacier parameters as far back as the early
1970s (Wu and Zhu, 2008; Zhu et al., 2010b; Liao et al., 2013). The size of
Nam Co and the extent and distribution of glaciers in the
Nyainqêntanglha range have been the subject of many publications over
the recent years (Yao et al., 2007; Frauenfelder and Kääb, 2009;
Bolch et al., 2010; Wang et al., 2013; Fig. 2; Table 1). Due to
different data sources with varying resolutions as well as different mapping
procedures, the estimated glacier area varies between different studies
(Fig. 2; Table 1), as the delineation of debris- and snow-covered glaciers
is rather subjective (Wu et al., 2016). This is especially true for the
first glacier inventory (Li et al., 2003 in Bolch et al., 2010), which has been discussed in
various studies due to inaccuracies and the quality of its base data
(Frauenfelder and Kääb, 2009; Bolch et al., 2010). Nevertheless,
recent studies show glacier shrinkage in the Nyainqêntanglha range at a
rate of 0.3 %–0.5 % yr-1, as measured since 1970 when the first
satellite images were acquired (Fig. 2; Table 1). As a result of this
glacier melting, the lake surface area has expanded from ca. 1930 km2 to ca. 2018 km2 at a rate of 2.1 km2 yr-1 (Fig. 3a), and the lake level rose at a
rate of 0.3 m yr-1 until approximately 2009 and at lower rates since
then (Fig. 3b). The initial rising trends of both lake level and surface
area are mirrored by most lakes in the southern part of the TP, but the
slowdown of this trend observed at Nam Co around 2009 seems unique
(Jiang et al., 2017). This suggests that the lakes on the TP react to
changing environmental parameters in a variety of different ways, and that
geographical proximity among lakes does not necessarily produce similar
reactions to change. The effects on freshwater input to the lake are
discussed in Sect. 2.3. Although changes in monsoonal
precipitation and wind direction may influence glacial retreat rates (Wang
et al., 2013), rising temperatures remain their primary cause (Ji et al.,
2018). The total contribution of glacial meltwater as surface runoff to
this lake level increase has been estimated as ranging from 10 % to 53 %
(Zhu et al., 2010b; Lei et al., 2013; Wu et al., 2014; Li and Lin, 2017),
with recent studies being at the lower end of this spectrum. Increased
precipitation is estimated to be responsible for 50 %–70 % of lake growth
(Zhu et al., 2010b; Lei et al., 2013). Whether there is a change in
evaporation remains unclear, as studies for approximately the same time
period have suggested both a slightly increasing and a slightly decreasing
evaporation rate since the late 1970s (Lazhu et al., 2016;
Ma et al., 2016).
Overview of glacier area changes (%) in the western
Nyainqêntanglha range (changed after Wu et al., 2016).
PeriodRegion of the NyainqêntanglhaGlacierReferencerangeshrinkage (%)1970–2000Southeastern slope-5.2Shangguan et al. (2008)1970–2000Northwestern slope-6.9Shangguan et al. (2008)1970–2000Western-5.7Shangguan et al. (2008)1977–2010Western-22.4±2.9Wang et al. (2013)1970–2009Western-21.7±3.4Wu et al. (2016)1970/80–2000Southwestern-19.8Frauenfelder and Kääb (2009)1970–2000Nam Co basin-15.4Wu and Zhu (2008)1976–2001Nam Co basin-6.8±3.1Bolch et al. (2010)1976–2001Southeastern slope-5.8±2.6Bolch et al. (2010)1976–2009Detailed glaciers: Zhadang, Tangse No. 2, Lalong, Xibu, Panu-9.9±3.1Bolch et al. (2010)
The rises in temperature and precipitation are also affecting permafrost
soils that extend over an area of ca. 1.4 million km2 (Yang et
al., 2004) on the TP. The permafrost layers can be described as relatively
warm and thin, with temperatures mostly > -1.5 ∘C and
< 100 m thickness (Wu et al., 2010). The mean annual soil
temperature of permafrost in particular areas of the TP has increased by
0.1–0.3 ∘C between 1970 and 1990 (Cheng and Wu, 2007). Simulation
studies have shown that due to climate warming the permafrost extent
may decrease by 9 %–19 % by 2049 and by 13 %–58 % by 2099 (Li and Cheng,
1999; Nan, 2005). Although there is no clear estimate of permafrost extent
in the Nam Co basin, Tian et al. (2009) reports a lower limit of permafrost
at an elevation around 5300 m a.s.l. along the northern slopes of Mt.
Nyainqêntanglha (7162 m). A frost lens was also encountered 9 m below
the surface (4738 m a.s.l.) while sampling an outcrop along the right bank of
the Gangyasang Qu close to the northwestern lake shore in 2005 (Schütt
et al., 2010). Thus, due to increasing temperatures, permafrost degradation
may serve as an additional recharge factor to groundwater, resulting in
increased subsurface inflow into the lakes.
Focusing on Nam Co, the hydraulic interaction between lake and
groundwater is still uncertain, as previous studies either neglected or
ignored the influence of groundwater due to a lack of reliable data (Zhang
et al., 2011). However, recent studies revealed a water imbalance, which was
explained by lake water seepage with an estimated outflow of 1.9×109 and 1.5×109 m3 during 1980–1984 and
1995–2009, respectively (Zhou et al., 2013; Du et al., 2018).
Glacier area reduction in the southwestern Nyainqêntanglha range
since 1970 as evaluated in various studies.
Enhanced water availability controls changes in lake water chemistry
The maximum recorded depth of Nam Co is 122 m (Li et al., 2008a), with
brackish water characterized by an alkaline pH of 7.8–9.5 and a
conductivity of 1920 µS mm-1 (Keil et al., 2010). The chemical
composition of a lake is essentially a function of its climate (which
affects its hydrology) and the basin geology. Increased freshwater input
from precipitation, melting glaciers and thawing permafrost alters the
chemical composition of the lake water and enhances surface runoff,
infiltration rates and subsurface flow. Together with the input of
freshwater, streams transport dissolved organic matter (DOM), which is
composed of a wide range of dissolved components and particles (≤0.45µm), thus affecting the water chemistry in the lake (Spencer et al.,
2014). Excessive landscape disturbance through removing vegetative cover
causes higher rates of DOM leaching, more erosion and increasing water
runoff velocity, resulting in additional input of minerals and nutrients
into the lake. Since the process of DOM leaching and translocation itself is
largely dependent on water and sediment cycles (Kaiser and Kalbitz, 2012),
it represents both the seasonal and interannual variation in an ecosystem
as well as its long-term trend. As the glaciers on the TP retreat, highly
bioavailable DOM may provide additional nutrients to downstream environments
and amplify the trend of eutrophication of lotic and lacustrine ecosystems.
Furthermore, the rivers on the TP have been shown to transport dissolved
organic carbon from thawing permafrost areas (Qu et al., 2017), which is
likely rapidly degraded via microbial activity, resulting in CO2
emissions, thus potentially producing a positive feedback on global warming.
However, the research into DOM as an important allochthonous source of
nutrients and as a capture of biodiversity and geodiversity of its respective
catchment area is largely lacking for High Asia. The concentration and
ratios of different ions in the water have a regulatory impact on the
structure of biotic communities (microbes, invertebrates and fish) that can
best tolerate abiotic conditions (Wrozyna et al., 2012). In Nam Co, water
conductivity has been regarded as the most important environmental factor
for shaping communities such as archaea, bacteria, phytoplankton, and
micro-invertebrates (Hu et al., 2010; Wang et al., 2011). Studies
demonstrated that ammonia-oxidizing archaea (autotrophic microorganisms) are
key contributors to ammonia oxidation in deep and oligotrophic lakes
(Callieri et al., 2016). This has implications for CO2 fixation in the
hypolimnion or the benthic zone, where there is insufficient irradiance to
support photosynthesis, implying that archaea would perform the final step
in the decay of organic matter via methanogenesis, resulting in carbon
dioxide accumulation (e.g., when they decrease during winter). Although
nitrification does not directly change the inventory of inorganic nitrogen in
freshwater ecosystems, it constitutes the only known biological source of
nitrate and as such represents a critical link between mineralization of
organic N and its eventual loss as N2 by denitrification or anaerobic
ammonia oxidation to the atmosphere (Herber et al., 2020). Ultimately, the
changes in the communities of primary producers could alter the lake's
trophic structure, which also affects the top predators of the ecosystem.
The primary productivity, as an indicator of nutrient supply and a longer
growing period associated with a shorter ice cover duration, has increased
markedly at Nam Co within the last 100 years (Lami et al., 2010). Wang et
al. (2011) reported the increasing abundance of the diatom species
Stephanodiscusminutulus during the last few decades (ca. 1970–2001). This species is generally viewed
as an indicator of water phosphorus enrichment, suggesting increasing inputs
from the lake's catchment and stronger mixing in spring season. To predict
future consequences of ongoing climate change, it is essential to understand
the responses of biotic communities to hydrological variations. Thus,
long-term monitoring is needed to adequately address the feedbacks of recent
environmental changes, while climatic conditions of the past can be
reconstructed through the study of organisms such as diatoms and ostracods
that are sensitive to hydrologic and chemical variations (see Sect. 3.2).
(a) Lake level changes of Nam Co since 2000 (there is notable shift in the
water balance in 2009), and (b) changes of the lake surface area since 1970,
as evaluated in previous studies. The overall increase rate of lake area is 2.1 km2 yr-1. Red lines denote LOESS curves, with the 95 %
confidence interval shown in gray.
Vegetation, soils and pasture degradation in the catchment
Nam Co is located in the transitional zone between the central Tibetan
Kobresia pygmaea pastures and the northwestern alpine steppe ecosystem (Miehe et al., 2019)
(Fig. 4). Situated on the northern slope of the Nyainqêntanglha range,
the vegetation pattern changes according to elevation, moisture availability
and temperature. Grazing intensity and abundance of small rodents, such as
the plateau pika (Ochotona curzoniae), may contribute to the shaping of the vegetation cover
(Dorji et al., 2014; Miehe et al., 2014). The area close to the lake
(< 4800 m) is covered mainly with alpine steppe vegetation
consisting of Artemisia, Stipa, Poa, Festuca and Carex (Li, 2018; Nölling, 2006). Soils developed in the
drier steppe areas consequentially tend to show lower organic carbon
contents, naturally lowering their total C sink or source potential, as
indicated by a study from Ohtsuka et al. (2008). Only one evaluable soil
investigation exists from the area of Nam Co. Wang et al. (2009b)
investigated two lake terrace sites, situated in the alpine steppe biome.
According to their findings, the soils reflect the cold semiarid climate of
the area by showing low biologic activity, while the influence of physical
weathering is dominant. The soils showed several decimeter-thick layers of
loess in which mainly the A horizons were developed. Although only very
sparse to moderate vegetation cover occurs, an almost 30 cm thick organic-rich topsoil with granular structure was developed there (Wang et al.,
2009b). Further organic-rich buried horizons were found and dated in both
profiles, showing phases of climatic conditions enabling the buildup of
organic material related to warm wet periods in the past (before 2.4 and 1.6 ka cal BP) and interchanging with phases of erosion, leading to, e.g., sheet
erosion and the formation of gullies and alluvial fans supposedly during
colder periods. These results fit well to climate reconstructions presented
in Sect. 3.3 of our review. In accordance with the World Reference Base for
Soil Resources (WRB) classification, we propose that the soils described by
Wang et al. (2009b) can be classified as Calcisols, as there is evidence of
carbonate translocation.
Higher up the slope (4800–5200 m), the alpine steppe is replaced by
Kobresia pygmea pasture. Wang et al. (2007) and Kaiser et al. (2008) investigated the
relationship between plant communities and development of soil types on the
High Asian Plateau and for pasture soils in the wider area. Vegetation
strongly controls the input of organic material into the soil but beyond
that also stabilizes fine materials (< 0.1 mm) and governs the
degree of chemical weathering. The authors found soils with stronger signs
of biologic activity and chemical weathering (e.g., Cambisols) associated
with alpine pasture sites. Kobresia root mats are usually developed in up to 40 cm
thick loess layers and form a distinctive felty horizon that protects
against erosion. The genesis of this felty root mat is attributed to
Kobresia pygmaea, since this shallow-rooted, small plant allocates most of its biomass
belowground and is able to reproduce vegetatively, making it well adapted to
the high grazing pressure (Miehe et al., 2008). The curious dominance of K. pygmaea is
often linked to grazing: (i) K. pygmaea replaces taller plants at sites where grazing
pressure is increased experimentally. (ii) Several enclosures show that
other grasses and shrubs increase in dominance after grazing competition ceased
(Miehe et al., 2008). Hence, the felty root mat can be seen as an effect of
an anthropozoogenic plagioclimax. At higher elevation (5200–5900 m), only
sparse alpine vegetation associated with initial soil processes occurs
(Ohtsuka et al., 2008).
Where water availability is abundant, alpine swamps with Carex sagensis and Kobresia schoenoides are formed,
especially at source areas, along riverbanks and in waterlogged
depressions, some of which can cover large areas (Li et al., 2011).
Concerning soil development in alpine wetlands, the database is sparse
compared to the alpine pasture and steppe biomes. It was pointed out for
alpine pastures that a strong relationship exists between plant communities
and (top)soil genesis. This relationship probably also holds true for alpine
wetlands, with the exception that the influences of waterlogging and
seasonal fluctuations and frost–melt cycles in the water table are likely to
have an effect on soils. This can be expressed in terms of formation of
gleyic features, frost turbations, heaves or other azonal features related
to the soil-forming effects of water (Chesworth et al., 2008). It still
needs to be clarified how these waterlogged areas affect the cycling and
processing of organic matter and nutrients. There is no evidence of tree
species, only the evergreen shrubs of Juniperus pingii var. wilsonii, which are mainly found on the
south-facing slopes of the northern Nam Co catchment, and shrubs of Salix spp. in
the Niyaqu Valley in the eastern lake catchment (Li, 2018). Alpine steppe
is comprised of more plant species compared to pasture and marsh ecosystems, which
are predominantly covered with Carex spp. and Kobresia spp. (Miehe et al., 2011b). Alpine
pastures are often described as “golf-course-like” (Miehe et al., 2014)
with the intention of illustrating their unique plane surface. However,
small-scale structures such as thufa or hummocks are also present. The
origin of these structures around Nam Co remains unclear; however, frost
heave and permafrost degradation processes are seen to play a major
role (Adamczyk, 2010). The landscape, generally dominated by endemic
Kobresiapygmea sedges, harbors only a few other species (Miehe et al., 2019), but the
additional microhabitats provided by thufa and hummocks enable rarer and
less competitive species to settle in niches in these heterogeneous
structures (Vivian-Smith, 1997). Compared to the surroundings, the
microtopography of thufa possesses different degrees of wetness, exposition
and insulation; depth of soil material; and type of topsoil. Local studies of
the Nam Co area state that slightly degraded bare soil patches and gullies
are often areas where plants have the chance to evade the suppression of the
closed Kobresiapygmea root mat (Schlütz et al., 2007; Dorji et al., 2014). Thus, the
genesis of thufa and mild, limited degradation processes are likely to
increase species richness and diversity by cracking open the closed root mat
of alpine pastures. These structures can also be formed by grass species
that grown in tussocks (i.e., clumps, bunches or tufts), such as the endemic
species Stipa purpurea (Liu et al., 2009) or Kobresia tibetica (Yu et al., 2010) and Kobresia schoenoides (Nölling, 2006).
The often-cited degradation of alpine pastures is likely initiated by
natural polygonal cracking (Miehe et al., 2019), which can occur through
drying (Velde, 1999) and then tends to be amplified by livestock trampling
and plateau pikas using the cracks as highways (Liu et al., 2017b; Hopping
et al., 2016). Overgrazing in alpine pastures is one of the most frequently
mentioned causes of pasture degradation (Unteregelsbacher et al., 2012;
Harris, 2010; Miehe et al., 2008), as excessive trampling by livestock might
aggravate the initial conditions of polygonal cracking (Miehe et al., 2019).
This effect, however, seems to be limited to the direct vicinity of
herder's settlements and camps (piosphere centers), and many
factors that are usually attributed to degradation are instead proof of
environmental control, especially in drier areas (Wang et al., 2018b).
Some researchers argue that climate change is the dominant or even sole
driver of degradation (Wang et al., 2007), although the effects of rising
temperatures and increasing precipitation appear to be an intensifier rather
than the cause of degradation (Zhou et al., 2005; Harris, 2010). In turn,
both Wang et al. (2018b) and Cao et al. (2019) point out that a multitude of
effects might be in play, with a locally differing magnitude or even
reversion, while usually moderate grazing was not found to cause
degradation. Certainly there are more factors than just grazing pressure,
and there might be site-specific effects leading to nonequilibrium behavior
of the study object, be it pasture or steppe (Wang and Wesche, 2016).
Plot-level experiments from the Nam Co area found warming to have
significant effects on the shallow-rooted Kobresia pygmaea by reducing the number of flowers
and delaying its reproductive phenology. These changes were provoked by
simulating increasing precipitation by means of snow addition (Dorji et al.,
2013) and also by maintaining a moderate level of grazing combined with snow
addition (Dorji et al., 2018). This underlines the importance of climate
forcing on the terrestrial systems in the Nam Co catchment. Grazing should
not be seen as a disturbance but as an integral part of a non-steady-state
but plagioclimax environment. Currently there are no estimates of the extent
of degraded land at Nam Co, but the degradation of wide areas of alpine
pastures is not without consequences for pastoralist communities. The
severe degradation and sloughing off of the whole topsoil removes the basis
for business and might lead to unknown consequences for the lake ecosystem
by means of enhancing or terminating nutrient exchanges. The economic
rationale of herders might be to increase the numbers of livestock, as this
represents a form of social security (Simpson et al., 1994). The bottom line
is that conflicts arise as less land is available for grazing (Hopping et
al., 2016).
The Chinese government has favored policies such as sedentariness and
fostered the construction of stationary settlements, which have, in turn,
created hotspots of overgrazing (Miehe et al., 2008). In these hotspots,
large portions of the topsoil are lost by erosion and denudation, leaving
only an area of humic material or subsoil, thus being called “black beach”
(Miehe et al., 2008) or “black-soil patch” (Liu et al., 2017a). The
remaining landscapes are usually dry, poor in plant cover and prone to
further degradation. Increasing areas of bare soil patches enhance
evapotranspiration, causing earlier cloud cover formation, especially before
noon. This may, in turn, lead to reduced radiation and temperature at the
surface, thus hampering photosynthesis and consequently overall carbon
sequestration (Babel et al., 2014). However, the evolution of grasslands on
the TP has been accompanied by herbivore communities; thus, the plants have
developed coping mechanisms to persist under continuous grazing pressure
(Miehe et al., 2011a). According to the intermediate disturbance hypothesis,
species diversity is higher under moderate disturbances, which suggests the
positive effect of intermediate level of grazing pressure. Indeed, a plant
clipping experiment to simulate grazing demonstrated that under the effect
of climate warming, the grazing activities mitigated the negative effects of
rising temperature by maintaining a higher number of plants (Klein et al.,
2008). Many studies hold the traditional nomadic practice to be a
sustainable one (Miehe et al., 2008; Babel et al., 2014; Hafner et al.,
2012), but the current policy of removing pastoralist lifestyles from
certain regions could potentially reduce overall species richness.
(I) Cross section from Damxung valley to the Nam Co study area
(A'–A), as shown in Fig. 1b. Schematic depiction of altitude-dependent
biomes and azonal landforms; changes in chroma denote height-dependent biome
shifts. Approximate biome heights were gained from satellite imagery
(Sentinel-2B) and herewith-derived vegetation indices, field excursions, and
the literature review (Ohtsuka et al., 2008; Wang and Yi, 2011). (II) Frequency,
direction and velocity of mean daily wind measurements at the NAMORS
(30∘46′22′′ N, 90∘57′47′′ E) between 2005 and 2015.
Effects on carbon cycling in alpine ecosystems
Changes in temperature and moisture have a significant effect on the biotic
community structure with feedbacks on ecosystem productivity. Alpine meadows
respond with increased plant productivity to warming, while productivity may
be hampered in alpine steppe ecosystems (Ganjurjav et al., 2016). As soil
moisture governs the community response to warming, negative effects of
warming on plant productivity likely occur due to limited water
availability (Ganjurjav et al., 2016). Warming was also reported to have a
negative effect on plant species diversity in both alpine meadow and steppe
ecosystems (Klein et al., 2008; Ganjurjav et al., 2016). Possible
explanations for a decline in plant species diversity include changes in
small mammal activity, storage of belowground nutrient resources and
water stress and microclimate in general (soil temperature and moisture)
(Ganjurjav et al., 2016; Klein et al., 2008, 2004). Thus, climate change may
reduce the habitat quality for the local populations of grazers and reduce
the well-being of the pastoralists by diminishing the abundance of palatable and
medicinal plant groups. The changes in the plant productivity levels, as well
as community changes, affect the local carbon cycle. Alpine grassland root
mats on the TP are estimated to store up to 10 kg of carbon (C) per
square meter (Li et al., 2008b), summing up to roughly 2.5 % of the
global terrestrial carbon stocks (Wang et al., 2002). At Nam Co, the topsoils contain an almost 30 cm thick organic-rich layer (Wang et al., 2009b),
thus representing considerable soil organic carbon (SOC) stocks. Due to
higher plant productivity, alpine meadows in general represent a CO2
sink; however, the interannual and seasonal uptake is highly variable (Kato
et al., 2004, 2006; Gu et al., 2003). Like plant productivity,
the CO2 uptake depends on water availability and temperature, which
exhibit diurnal, seasonal and annual fluctuations. The overall great
importance of water availability and temperature on ecosystem–atmosphere
CO2 exchange in the central Tibetan alpine Kobresia meadows was demonstrated in
several studies through eddy covariance measurements (Zhang et al., 2018),
chamber measurements (Zhang et al., 2018; Zhao et al., 2017), decomposition
of cellulose cotton strips (Ohtsuka et al., 2008) and altitudinal
transplantation experiments (Zhao et al., 2018). Similarly, carbon fluxes in
alpine steppe biomes are driven by precipitation and temperature on a daily to
seasonal and annual timescale. The interannual flux variability follows
the varying monsoonal precipitation, showing a stronger tendency towards functioning
as a C sink in wetter years and as a C source in drier years (Wang et al.,
2018a; Zhu et al., 2015b). Soils that develop in the drier steppe areas tend
to show lower organic carbon contents, therefore lowering the total C sink
and source potential (Ohtsuka et al., 2008). Although the production of
plant biomass may be hampered in steppes, the ecosystem may still act as a
carbon sink through microbial CO2 fixing activities, as shown by a
recent study on the TP that reported relatively high CO2 fixation
capacity (29 mg kg-1 soil d-1, Zhao et al., 2018). Interestingly,
this study also found that alpine steppe soils demonstrated significantly
higher microbial CO2 fixation capacity compared to meadow soils (29
vs. 18 mg kg-1 soil d-1, respectively).
As a result of increasing precipitation and glacier runoff, wetlands in the
Nam Co area are expanding, thus increasing emissions of CH4, which is
28 times more climate active than CO2 (IPCC, 2013). A study conducted in the alpine wetlands around Nam
Co reported that CH4 emissions have increased exponentially with
increasing precipitation, especially when soil moisture exceeded 80 %
(Wei et al., 2015). However, there was a large difference between swamp
meadows and swamps (67 and 1444 µg CH4 m-2 h-1,
respectively). Swamps are permanently inundated, while swamp meadows are
usually seasonally inundated. Furthermore, SOC stocks are higher in swamps
compared to swamp meadows (Wei et al., 2015). Large amounts of SOC in
combination with anoxic conditions are the main precursors for methanogens
activity, which results in increasing CH4 emissions to the atmosphere
(Kato et al., 2013). Thus, the saturated soils with high SOC content produce
higher CH4 emissions (Deng et al., 2013). Observations from 2008 to
2013 at Nam Co have shown that alpine steppe and alpine meadows show annual
uptake rates of 72 and 59 µg CH4 m-2 h-1, respectively
(Wei et al., 2015); however, the corresponding emission rates are much
higher. Generally, it is expected that the alpine wetland acts as a CH4
source, while the aerated soils of alpine steppe and alpine meadow act mainly
as a CH4 sink.
As the grasslands on the TP are widely used for yak and sheep grazing,
carbon cycling is influenced particularly through human activities and the
degree of degradation. The intensity of grazing has a significant effect on
CO2 fluxes, with moderate grazing enhancing belowground allocation of
carbon (Hafner et al., 2012), while adversely affecting the C-sink potential
through reduction of aboveground and belowground biomass at higher grazing
intensities (Babel et al., 2014). Overgrazing, along with the increase in
burrowing pikas in the Tibetan grasslands, may increase nitrous oxide
(N2O) emissions (Zhou et al., 2018), an important greenhouse gas with
297 times larger warming potential compared to CO2 (IPCC, 2013).
Despite several studies focusing on greenhouse gas emissions on the TP, the
magnitude of the N2O emissions in different ecosystems has not yet been
estimated. Experimental studies on the eastern TP demonstrated that the rate
of N2O emission may increase with increasing soil temperature and soil
moisture under a future climate change scenario (Yan et al., 2018; Yingfang
et al., 2018). Expanding wetland areas provide anoxic conditions for the
release of methane and, due to the greater temperature sensitivity of
permafrost areas, subsurface SOC is at high risk of loss, which may decrease
the carbon sequestration potential in the region (Li et al., 2018). Besides
carbon cycling through decomposition processes, responses to changing
temperature and precipitation depend on the composition of decomposer
communities (Glassman et al., 2018). Thus, the conclusive effects and
feedback mechanisms (i.e., positive vs. negative loop) on warming are complex
and not always clear.
Paleoenvironments on the Tibetan Plateau and landscape evolution at Nam CoGeodiversity and evolution of biodiversity
Topography, geological context, climate and their complex interplay are key
determinants for the distribution of organisms. In general, the ecoregion
can serve as a proxy for community- and species-level biodiversity, which
best describe communities of mammals, birds and plants (Smith et al., 2018).
The TP forms a distinctive zoographical region, an “ecological island”
(Deng et al., 2019), characterized by fauna that are adapted to high
altitudes, drought, low temperatures and low oxygen levels (He et al.,
2016). The TP forms unique high-altitude biogeographical biota by
also harboring many unique lineages of other organisms, with a higher endemism
of low dispersal species (Yang et al., 2009; Clewing et al., 2016). As
mountain building has been directly associated with the development of
biodiversity (Hoorn et al., 2013; Antonelli et al., 2018), the biodiversity
hotspots are located in the south and southeast of the TP especially. There
is also a pattern of increasing biodiversity from west to east, which
correlates positively with increasing precipitation. In contrast, the harsh
central areas of the TP show much lower richness but nevertheless harbor
various endemics (Päckert et al., 2015). Throughout the geological
formation of the TP, the mountainous southeastern parts have been
hypothesized to serve as center of species diversification (Mosbrugger et
al., 2018), although the core TP region is also suggested to represent a
center of origin (Deng et al., 2011). The TP has been a source area for
several mammalian lineages (“Out of Tibet hypothesis”; Deng et al., 2011),
including the snow leopard and the arctic fox (Wang et al., 2015), as well
as birds, such as redstarts (Voelker et al., 2015), and plants, such as
Gentiana (Favre et al., 2015). These mountainous areas may also have acted as
refugia, which preserved unique lineages over long periods (López-Pujol
et al., 2011; Lei et al., 2014). Whether some endemic taxa represent relics
of a formerly more diverse clade or have never extensively diversified
remains unclear (Päckert et al., 2015). Besides being a center of
origin, the TP may represent a center of accumulation as proposed by the
examples of Saxifraga (Ebersbach et al., 2017), warblers (Johansson et al., 2007) and
hynobiid salamander (Zhang et al., 2006). Overall, the regional biota of the
TP is comprised mainly of Palearctic and Oriental species, Nearctic species
from the Bering land bridge, as well as species from speciation in situ and
postglacial recolonization from adjacent areas. The evolution of
biodiversity on the TP has been affected by the combination of geological
and climatic changes over the time of the uplift phases (Mosbrugger et al.,
2018). Although many studies have associated recent in situ radiations to
different uplift phases of the TP, Renner (2016) pointed out that the
evidence for recent rapid uplift (9–8 or 3.6–2.6 Ma) remains doubtful and
controversial. As proposed by the “mountain geobiodiversity” hypothesis,
the evolution of biodiversity on the TP is a result of an increasing local
geodiversity in combination with rapid climatic oscillations and steep
ecological gradients (Mosbrugger et al., 2018).
The combination of geological, climatic and ecological changes has left its
footprint in the history of Nam Co. There are at least seven different
levels of continuous terraces around Nam Co, with the highest being over 30
meters above the current lake level, corresponding well with the elevation of
the natural spillway in the northeast of Nam Co. Several authors claim the
existence of a much larger fluvial lake system called Old Qiangtang
Lake, which covered an area of around 30 000–50 000 km2 or more (Li
et al., 1981; Zhu et al., 2002). The connections provided by a large lake
allowed gene flow between drainages, which is reflected, for example, by
the closely related clades of schizothoracine fish (Cyprinidae,
Osteichthyes) from Nam Co and the surrounding lakes, compared with more
distant parts of the TP (He et al., 2016). In contrast, due to a
vector-mediated passive dispersal across large areas, other aquatic taxa,
such as freshwater snails, seem to have been less influenced by drainage
histories (Oheimb et al., 2011). Higher lake terraces are older, suggesting
a long-term reduction in lake level (Zhu et al., 2002). This may be
associated with an evolution from wet to dry phase, which Li et al. (1981)
connects to the gradual uplift of the plateau from early Pleistocene to the
Holocene. However, there is an alternative suggestion to this interpretation
of a rather modern uplift proposed by Renner (2016) who states that large
parts of the TP had already reached average heights of 4000 m and more
during the mid-Eocene (∼40 Ma ago). Recent findings of palm
leave fossils on the central part of the TP, dated to ca. 25.5±0.5 million years, do not suggest a presence of such a high plateau before the
Neogene (Su et al., 2019). Thus, although it is suggested that the final
large lake phase took place ca. 40–25 ka cal BP (Lehmkuhl et al.,
2002; Zhu et al., 2002), the complex relationship between evolution of the
TP and the development and the temporal existence of Old Qiangtang Lake
are not completely resolved.
Holocene lake level changes and climate reconstruction based on aquatic
bioindicators
Lake sediments contain important indicators or proxies that can be used to
reconstruct limnological and (hydro-) climatic conditions over long time
periods (Zhu et al., 2010a; Wrozyna et al., 2010). Widely used environmental
indicators include communities of diatoms (Bacillariophyceae) and ostracods
(Crustacea) as they are abundant and usually preserve well in
sediments (Kasper et al., 2013). For example, the investigations of
Quaternary ostracods, modern assemblages, and stable isotopes from Nam Co
and nearby water bodies represent the most detailed application of ostracod
analysis in the south–central region of the TP (Mischke, 2012). Different
approaches (stratigraphy, paleoecology, etc.) detected several climatic
fluctuations between warmer–cooler and wetter–drier periods (Fig. 5). In
general, higher lake levels based on aquatic fauna suggest a more humid
environment during the early and middle Holocene, which displayed a shift
pattern compared to the northern TP (Wünnemann et al., 2018). Together
with the indicator species approach and the application of transfer
functions for Nam Co sediments, different stages can be recognized. In Stage I
(8.4–6.8 ka cal BP), the climate changed from warm humid to cold arid with
water depth being much lower than today (Zhu et al., 2010a). In Stage II
(6.8–2.9 ka cal BP), environmental conditions returned to warm and humid
(Zhu et al., 2010a). During 4–2 ka cal BP, lake water depth initially
remained much shallower than today but then gradually increased due to high
rates of precipitation (Frenzel et al., 2010). The presence of the diatom
taxa Stephanodiscus in this stage indicates stronger monsoon activity and higher
availability of nutrients (Kasper et al., 2013). Finally, in stage III (2.9 ka cal BP to present), the climate again became warm humid, with a cold dry
event between 1.7 and 1.5 ka cal BP (Zhu et al., 2010a). Between 2 and 1.2 ka cal BP, benthic diatoms inferred a lower water level and drier climate
(Kasper et al., 2013). Subsequently, wetter conditions and an increase in
lake level was detected (1.2 ka cal BP until 250 cal BP), possibly
corresponding to the Medieval Warm Period (MWP), with high planktonic diatom
species and high ostracod diversity (Kasper et al., 2013). During the late
Holocene, the minimum water level occurred throughout the Little Ice Age
(LIA) (∼1490 and 1760 CE) (Frenzel et al., 2010). However,
the lake level increased towards the present, which is plausibly linked to
the melting of the glacier due to the current warming.
Although a large number of studies describe profound hydrological changes
and general climate fluctuations, there are several uncertainties regarding
taxonomy, resolution and proxy sensitivities. For example, modern ostracod
data detects several morphological variations, characterized by different
nodding or shell sizes, which could lead to an erroneous ecological
interpretation and later, vague paleoenvironmental conclusions in relation
to salinity changes (Fürstenberg et al., 2015). In paleo-studies,
different sedimentation rates and uncertainties in the core chronologies
also cause a lack of correspondence between signals detected by different
proxies (Wang et al., 2012). For this reason, it is surrogate to understand
the precise causal relationships between a complex environmental gradient
(e.g., water depth, water chemistry, temperature, etc.) and the response of
bioindicators. Although ecological information is still poorly known for
many species, ostracod and diatom assemblages represent reliable proxies to
trace the climatic history of Nam Co.
Further emphasis should be placed on combining morphology and DNA analysis
to corroborate the classification of the species already described.
Furthermore, experiments with living individuals should be performed under
controlled environmental variables to allow the setup of a transfer function
that could be used to evaluate quantitative data for paleo-reconstructions.
Comparisons of the reconstructed climate conditions based on
fossils of pollen (Li et al., 2011; Adamczyk, 2010; Herrmann et al., 2010),
ostracods (Zhu et al., 2010a), Ostracod δ18O (Wrozyna et al., 2010, 2012) and diatoms (Kasper et al., 2013) from sediment cores
in, and at the shoreline of, Nam Co. Ostracod-based water depth transfer
function (Zhu et al., 2010a) (blue line) was used to indicate long-term
hydrological changes and all reconstructed water depth values were adjusted
to the maximum water level of the lake according to the 45 m difference
between this study site (60 m) and the deepest site (105 m) at Nam Co. The main
species are also shown in different periods.
Holocene vegetation cover and climate reconstruction based on pollen
records
The comparison of modern pollen assemblages with those from sediment cores
allows the reconstruction of floristic diversity and distribution across
various timescales. Vegetation patterns contribute to the reconstruction of past
climate and the assessment of the degree of local human influence. Modern
vegetation belts around Nam Co reveal that alpine steppes contain
mostly species of Artemisia (Asteraceae) and Poaceae, while alpine meadows and swamps
are dominated by Cyperaceae (Li et al., 2011). The sedimentary pollen ratio
of Artemisia to Cyperaceae (A/Cy) can, within certain limitations, be used to
reconstruct past climates (Li et al., 2011; Li, 2018; Zhu et al., 2015a)
provided that vegetation belts move with altitude during climate change. For
example, when the climate is warmer and drier, alpine steppe reaches higher
up the mountain, displacing alpine meadow into areas further away from the
lake, leading to a higher input of Artemisia pollen into the nearby lake and
consequently a higher A/Cy pollen ratio in the sediments. However, the A/Cy
pollen ratio and abundance of tree pollen originating from a short distance
can be altered by human-driven change of plant composition, hence the
beginning of pastoral economy might limit the explanatory power of pollen
records (Adamczyk, 2010; Miehe et al., 2014). Pollen composition inferred
from sediment cores reveals a downward shift of the altitudinal vegetation
belts since 8.4 ka BP (Li et al., 2011). A major extension of alpine pasture
and alpine sparse vegetation closer to the lake shore during the late
Holocene is corroborated by a pollen-based climate reconstruction from a
peat core near Nam Co (Herrmann et al., 2010) and two other pollen records
from the eastern lake shore (Adamczyk, 2010). They found a trend of
increasing temperatures from the late glacial until the early Holocene,
accompanied by an extension of alpine steppe, tree and shrub vegetation.
Already in this period, synanthrope taxa pollen are increasing in the data
used by Adamczyk (2010) with the only small occurrence of, for example, Plantago lanceolata in the whole
profile. This very early signal shows that a lot of room still exists for studies
of pollen archives around Nam Co, with much doubt still persisting at present.
Climate fluctuated between dry and humid from 8.5 to 4.8 ka BP, with an
intense cold regression between 8.1 to 7.8 ka BP. The onset of human activity
at Nam Co is dated to 5.6 ka BP according to synanthrope taxa proxies
(Li et al., 2011; Herrmann et al., 2010). Between 4.8 and 0.7 ka BP, a
relatively stable climate with predominantly humid conditions developed
(Fig. 5), and the vegetation pattern already showed trends of a human-made
steppe biome, potentially a plagioclimax (Adamczyk, 2010). Since 0.7 ka BP,
drier conditions prevailed.
Whether and to what extent the central Tibetan Plateau was forested and
what caused the forest decline are the subject of ongoing discussion (Miehe
et al., 2006, 2019). This matter is closely related to the
prior discussed onset of more intense human activity in the area, since
parts of the discussion involve a human-made forest clearing in combination
with a natural forest decline. As stated, there are only occurrences of
shrubs (Juniperus pingii var. wilsonii and Salix, Nölling, 2006) in the Nam Co area. No remains and no
reliable evidence of a once tree-rich vegetation have yet been found in the Nam Co
catchment. According to locals, there exist several caves with potentially
(pre)historic tree depictions of unknown age. Unfortunately, there is no
verification of their existence nor any dating approach. Since the area of
Damxung still does feature larger occurrences of Juniperus pingii var. wilsonii and, around 4250 m a.s.l.,
tree stands of Juniperus tibetica in enclosed areas, there is the potential to discuss
that these species have been more numerous in this area in the past (i.e.,
last tooth theory). Miehe et al. (2019) show locations of forest relicts and
give a drought line of 200–250 mm precipitation and elevations between 3600
and 4000 m a.s.l. as the upper tree line. Questions arise as to whether there has
been an expansion of J. tibetica into the Nam Co catchment in earlier times, which
would be feasible within certain limitations according to the presented
thresholds. Charred micro remains as a potential sign of fire-driven forest
decline are missing in one of the profiles of Adamczyk (2010) but can be
found throughout the Holocene until 1 ka cal BP (Herrmann et al., 2010). The
authors attribute the size and shape of the charcoal remains to local,
small-scale burning of wood and leaves, not showing signs of larger forest
clearings. In addition to the burning of Juniperus trees for religious reasons (Miehe
et al., 2006), trees and shrubs may have been burned for heating or clearing
of pastures by nomads. Following the presumptuous argumentation of some
authors, the trees were previously able to spread again due to sufficient
precipitation provided by the summer monsoon. Furthermore, the occurrence of
synanthropic taxa has been observed in the nearby Damxung valley since 8.5 ka cal BP, corroborating the strong anthropogenic influence on the formation
and restructuring of the vegetation patterns in the area (Schlütz et
al., 2007). The decrease in summer precipitation and temperature, in
conjunction with ongoing human activity, ultimately led to the total
disappearance of trees and the formation of the alpine grasslands and steppe
as we know them today (see Sect. 2.4). Furthermore, the occurrence of
synanthropic taxa has been observed in the nearby Damxung valley since 8.5 ka cal BP (Schlütz et al., 2007). This corroborates the strong
anthropogenic influence on the formation and restructuring of vegetation
patterns in the area but leaves a time gap of almost 3 ka between the
evidence from Damxung valley and Nam Co. Hence, further research is needed
to address the question of onset of human activity and degree of landscape
modification.
Conclusions and perspectives
This literature review summarizes the manifold environmental changes
affecting abiotic and biotic processes in the area caused by past and
ongoing climate change. Ecosystems on the Tibetan Plateau experience an
increase in air temperature roughly twice the global average. This has
accelerated deglaciation of the Nyainqêntanglha range during the last
few decades, leading to substantial inflow of freshwater and various solutes
resulting from weathering to the lake. The combined effects of overgrazing
by livestock and warming-accelerated degradation processes of the alpine
grasslands further increase surface runoff in the catchment. Moreover,
warmer and wetter climate, as well as pasture degradation, may turn alpine
wetlands and steppe pasture ecosystems into an overall source of methane and
carbon dioxide, respectively. Based on the reviewed literature focusing on
the catchment of Nam Co, we outline perspectives to improve the
understanding of the close connections between geodiversity and biodiversity. (1) Permafrost areas act as buffers of the water budget and influence the
behavior of geomorphological processes and periglacial landforms. Although a
significant warming and consequent decay of permafrost have been reported
throughout the TP in recent decades, studies on permafrost in the Nam Co
catchment and in the immediate Nyainqêntanglha range are missing. (2) The rising lake level trend, starting in late 1970, had a point of
reflection around 2009, which indicates changes of variable precipitation
and evaporation trends, reduced water inflow from already melted glaciers,
and additional ground water seepage out of the lake. Therefore, long-term
monitoring is necessary to calibrate and validate models properly and to
achieve a more accurate climate prognosis. (3) To improve climate
modeling approaches, the dynamics of DOM, CO2 and CH4 fluxes need
further clarification by in-depth analysis of the different biomes and
in situ observations. (4) The development of molecular methods for
biomonitoring and water quality assessment has advanced greatly during the last
decade with the aim of providing clear monitoring standards. These offer time-
and cost-effective approaches for complementary studies to tackle community
shifts of various water quality indicator organisms. (5) Alongside the
“traditional” paleo-bioindicator analysis, DNA-based taxa identification
methods hold also a great potential for application in paleoecological
studies to provide improved taxa differentiating accuracy. Various
biological and geochemical proxies in Nam Co sediments have enabled the
tracking of historical events and the reconstruction of past environments,
which provide information about the magnitudes and directions of past
climate change and thus a key to assess future changes. Both the formation
of high-elevation environments and pronounced past climate oscillations have
contributed to the development of biota on the TP. Interdisciplinary
research of the Nam Co catchment has provided vast insights into how warming
trends may affect ecosystems from microbes to the top of the food chain.
Recognizing the impacts of a warming climate is the base for establishing
effective climate change adaptation strategies and actions in the TP region
and in alpine regions in general.
Data availability
As this paper is reviewing existing literature findings, there were no
data analyzed that are not already published in the studies we cite. The
dataset from which Table 1 was generated was provided by the Institute of
Tibetan Plateau Research and is publicly available under
https://data.tpdc.ac.cn/en/data/4deeb2b4-4fc1-4c7c-b0c6-6263a547d53f/, last access: 4 March 2020 (Wang
and Wu, 2018) and
https://data.tpdc.ac.cn/en/data/3767cacc-96e3-48b2-b66c-dac92800ca69/, last access: 4 March 2020 (Wang,
2019).
Author contributions
SA, MAR, JB, PEG, JK, WK, LK, PM, FN, ER, HT, TVT and YW contributed equally to
the content of the paper. AS conceived the idea and was responsible for
funding acquisition. SA, JB, PM and FN structured the main text body. The
following authors were in charge of the corresponding sections: JB, ER, TVT –
“Glacier retreat and hydrological patterns of Nam Co”; PEG, LK – “Enhanced water availability controls changes in lake water chemistry”; PM, FN – “Vegetation, soils and pasture degradation in the catchment”, “Effects on carbon cycling in alpine ecosystems”, and “Holocene vegetation cover and climate reconstruction based on pollen records”; SA, PEG, WK – “Geodiversity and evolution of biodiversity” and “Paleo-lake level changes and climate reconstruction”. The tables and figures
were prepared as follows: ER – Fig. 1; JB and FN – Tables 1 and 2 and Figs. 2 and
3; PM – Fig. 4; WK – Fig. 5.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
We thank all colleagues who contributed to this review. We especially thank
Miguel Vences and Bernd Wünnemann for their enriching comments that
greatly improved the manuscript. We also thank Binbin Wang for providing
precipitation data. The authors are very grateful to Georg Miehe and one
other anonymous reviewer for their helpful comments that greatly helped to
improve the review. This research is a contribution to the International
Research Training Group (GRK 2309/1) “Geo-ecosystems in transition on the
Tibetan Plateau (TransTiP)” funded by Deutsche Forschungsgemeinschaft
(DFG).
Financial support
This research is a contribution to the International Research Training Group (GRK 2309/1) “Geoecosystems in transition on the Tibetan Plateau (TransTiP)” funded by Deutsche Forschungsgemeinschaft (DFG).
Review statement
This paper was edited by Kirsten Thonicke and reviewed by Georg Miehe and one anonymous referee.
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