Review EGUsphere 2023-789

This paper describes a research plan to investigate a comparatively little-studied ecosystem, the coastal temperate evergreen rainforest of southern Chile, using a critical zone observatory framework.  The stated aim of the research effort described in the paper is to gain quantitative understanding of carbon sinks, “biota-driven landscape evolution,” water, biogeological and energy fluxes, and disturbance regimes. Their work focuses on roles of trees and forests in carbon cycling. 

The study is entitled “Pumalin CZO” in reference to the Pumalin National Park, a 400,000-hectare area which was created and endowed by Doug Tompkins, founder of The North Face and Esprit outdoor clothing companies, and his wife, Kris Tompkins, former CEO of Patagonia, Inc.  The Pumalin Park, which has been donated the government of Chile, encompasses land from the coastal fjords to the border with Argentina in the Andes. The geology and geomorphology of the park is dominated by past glaciation and volcanism. Glaciation created extremely steep, hard rock slopes and broad flat valleys buried in sediment from glacial retreat. Volcanism has overprinted some of the glacial sculpting, including the massive Michinmahuida volcano and Chaitén volcano, which erupted in 2008 with major effects on the landscape of most of the Pumalin Park. 

The authors emphasize the need for work on understudied coastal temperate rainforsts of the global south. They present results of pilot studies in a small (16 km²) study watershed in which they (1) used lidar to census trees and estimate biomass, (2) used seismic sensors to detect winds, high rainfall intensity, and tree movement that may be related to landslide initiation, and (3) used seismic sensors to detect streamflow events and the possible signature of large wood moving in the stream.  The reliance upon remote sensing and sensor arrays both builds on contemporary technological advances and also is necessary in this landscape, which is extremely difficult to access.  The authors should be commended for their work in such a challenging setting.

Major comments:

• The authors could improve their explanation of the physical setting of the study basin and how it relates to the larger landscape. The study area for “Pumalin CZO” described in this paper represents a very small and particular fraction of the landscape. It is less than 0.5% of Pumalin Park, a 1630-ha catchment draining to the fjord just west of the ferry port of Caleta Gonzalo. Small, steep catchments draining directly to fjords represent a rather small portion of the landscape of Pumalin Park and this portion of southern Chile, which is dominated by much larger river basins (e.g. Amarillo, Rio Blanco Chaitén, Rio Rayas, Rio Michinmahuida) where steep glaciated slopes mantled with volcanic deposits drain to broad, flat valleys with meandering rivers and wetlands. It is unclear from the paper how the studies from this small catchment can be generalized to the larger landscape.
• The paper could provide more clarity on how the narrowly focused work is related to the ecological and geomorphic processes that govern carbon cycling. The actual work described in this paper is focused on three principal efforts: (1) lidar based estimates of forest structure and topography, (2) seismic sensors to measure rainfall intensity and tree movement, to address the role of wind in toppling trees and their effects on landslide generation, and (3) water stage information and characteristic frequency band from seismic sensors to identify large wood in motion in the stream channel, in order to determine rates of large wood movement in this “undisturbed” catchment. It is unclear how the results of these focused studies will contribute to the understanding of broader ecological and geomorphic processes.
• The work lacks context within a broader conceptual framework for carbon cycling and disturbances.
  1. The authors could do a better job of placing their work within the context of how cascading disturbance effects influence carbon cycling. The authors state, “The local disturbance regime is comparably simple.” But this does not seem accurate.  The study area received substantial tephra deposits from the 2008 eruption of Chaitén volcano.  Many studies have been conducted of the effects of this volcanic eruption on forests, landslides, and rivers, including several by the lead author of this paper. Published work has revealed how tephra deposition and other processes during the Chaitén eruption killed large areas of forest on hillslopes and in river valleys (Swanson et al., 2013), how this tree mortality combined with tephra deposition accelerated landslides, and how subsequent extraordinary elevated sediment export (Major et al. 2016) transported large wood in large rivers.  This is not a simple disturbance regime.  The map of landslides in Figure 4 does not refer to the role of tephra deposits from the Chaitén eruption, although one of the authors of this paper has published work on post-eruption landslides in the area.  Nor does the paper explain how tephra deposition from the Chaitén eruptions (or from other volcanic eruptions) may have contributed to the finding that “the highest denudation rates along the entire Chilean Andes [were found] under dense Patagonian CTRs [coastal temperate rainforests].” This paper does not set the work on wind-triggered landslides in this broader context. The concept of disturbance cascades in space and time could be useful for their framing.
  2. The authors could better explain the expected importance of the carbon transfer processes they are studying, which appear to have the effect of storing, rather than transporting carbon in the landscape. The authors note that extremely low concentrations of suspended sediment (<0.001 g/l) immediately after a rainfall-runoff event in March 2022 suggest “a high recycling rate of hillslope debris within the catchment and not necessarily high sediment and organic carbon export into the fjords.” Does this mean that the authors believe that chronic processes such as small landslides produce low export of carbon, and that instead carbon export from this landscape is dominated by less frequent disturbance-mediated events, such as the combined effects of volcanic eruption, elevated landsliding, and sediment and wood-laden floods? This paper does not make clear how the research they are conducting will be combined with prior work to understand the relative roles of chronic vs. disturbance-mediated carbon transport processes.
  3. The authors could improve their explanations of the spatial and temporal scales of their studies. The authors state, “disturbances are predicted to change in this biome not only quantitatively but also qualitatively: The disturbance regime has likely already started to change.” The landscape has been recently (in the past two decades) changed by a volcanic eruption.  What are the time scales and spatial scales of changes in disturbance that motivate the research described in this paper?
• The paper is presented as an introduction to a CZO, but the work is not evaluated in the context of how CZOs are framed. There is no mention of soils, for example, which are a key part of many CZOs.
• The paper is presented as an introduction to a place, rather than a set of findings about geomorphic and ecological processes.
• There are a lot speculative statements scattered through the paper, which are not phrased as hypotheses, nor are they tested.
• The paper seems to describe a research plan, rather than research results. It seems more focused on the measurement technology than on findings. It does not provide clear hypotheses or interpretations about what the results reveal about geomorphic and ecological processes.

Major, J.J., Bertin, D., Pierson, T.C., Amigo, Á., Iroumé, A., Ulloa, H. and Castro, J., 2016. Extraordinary sediment delivery and rapid geomorphic response following the 2008–2009 eruption of Chaitén Volcano, Chile. Water Resources Research, 52(7), pp.5075-5094.

Swanson, F.J., Jones, J.A., Crisafulli, C.M. and Lara, A., 2013. Effects of volcanic and hydrologic processes on forest vegetation: Chaitén Volcano, Chile. Andean Geology, 40(2), pp.359-391.

This is a beautiful description of a new CZO site and its scientific goals. The site is of great interest from an ecological point of view and from the point of view of interactions between geomorphology and trees. The authors present the site, describe their measurements and installations, and discuss the topics they will be studying. The paper is written clearly and coherently and was a pleasure to read. There are a few typos or mis-prints, but there are fewer mistakes than most published papers! My one small quibble is that the paper is quite long. I wonder if the authors might be able to condense the paper and put some of the information into tables and/or figures to lower the word count. Several of the figures are multi-panel and they already perform the function of summarizing the story, but perhaps a few tables might be used instead of lists of instrumentation or sites. Anything the authors can do to tighten up the paper would be good (if they can do it). 

The authors are to be applauded at putting together this study on such an interesting site in a location where we need a CZO.