Bottomland hardwood forest growth and stress response to 1 hydroclimatic variation: Evidence from dendrochronology and 2 tree-ring δ 13 C values

. Wetland forests around the world have been reduced to a small proportion of their 9 original expanse due to changing climatic conditions and intensification of human land use 10 activities. As a case in point, the Columbia bottomland hardwood forests along the Brazos- 11 Colorado Coastal Basin on the Gulf coast of Texas are currently threatened by an increasingly 12 erratic hydroclimate in the form of both extreme floods as well as droughts, and by urban 13 expansion. In this study, we use dendrochronology and tree-ring carbon isotopes to understand the 14 effect of changing hydroclimatic conditions on the functional attributes of these forests. We 15 examined tree-rings of Quercus nigra at four sites within the Columbia bottomlands, of which one 16 site experiences frequent and prolonged flooding, while the other three are less flood-prone. The 17 objectives of this study were to: (i) understand the impact of hydroclimatic variation on growth 18 rates using tree-ring width analysis, (ii) assess the magnitude of physiological stress inflicted by 19 extreme hydroclimatic conditions using tree-ring δ 13 C measurements, and (iii) evaluate the 20 relationship between physiological stress and growth inhibition. Growth rates across the landscape 21 were influenced most strongly by mid-growing season climate, while early-growing season 22 climate inflicted the greatest physiological stress. Neither growth inhibition nor changes in δ 13 C 23 values were observed in trees at the wetter site under extreme hydrologic conditions such as 24 droughts or floods. In addition, trees at the wet site were less sensitive to precipitation and showed 25 no response to higher temperatures. In contrast, trees of the three drier sites experienced growth 26 inhibition and had higher tree-ring δ 13 C values during dry periods. Our results indicate higher 27 physiological resilience in trees growing under wetter conditions. Management and conservation 28 strategies dependent on site-specific conditions are critical for the health of these wetland forests 29 under a rapidly changing hydroclimate. This study provides the first dendrochronological baseline 30 for this region and thresholds of optimum conditions for the growth and health of these forests 31 which can assist management decisions such as streamflow regulation and conservation plans.

no response to higher temperatures. In contrast, trees of the three drier sites experienced growth 26 inhibition and had higher tree-ring δ 13 C values during dry periods. Our results indicate higher 27 physiological resilience in trees growing under wetter conditions. Management and conservation 28 strategies dependent on site-specific conditions are critical for the health of these wetland forests 29 under a rapidly changing hydroclimate. This study provides the first dendrochronological baseline 30 for this region and thresholds of optimum conditions for the growth and health of these forests 31 which can assist management decisions such as streamflow regulation and conservation plans. Bernard River and Colorado River combine to form the Columbia bottomland hardwood forests, 40 an area of high biodiversity with a critical role in regional hydrology. Large portions of the 41 Columbia basin forest have been cleared and land cover is now a mix of isolated forest patches, 42 cropland, and pasture (Griffith, 2004), with only a few larger forest patches remaining (Fig. 1A). 43 The pre-settlement distribution of these forests was >283,000 ha along a 150 km long corridor 44 inland from the coast, but has since been reduced to about 72,000 ha (USFWS, 1997; Barrow and altering hydrologic conditions over short temporal scales. Annual precipitation amounts have been highly variable with up to 61% more rainfall than average during some years, while up to 53% 50 deficit during others, in addition to at least five major tropical storms and hurricanes. Rapid  Leavitt et al., 2002). As tree rings are distinguished by their high temporal (annual or sub-annual) 62 and spatial resolution, regional tree-ring chronologies and δ 13 C values have the potential to identify 63 a wide range of growth and stress response of vegetation to hydroclimatic variability. However, 64 tree-ring δ 13 C is also influenced by the changing δ 13 C value of atmospheric CO2. The increase in 65 atmospheric CO2 concentration mainly due to fossil fuel combustion has led to a significant 66 decrease in δ 13 C of atmospheric CO2 over the last century (Graven et al., 2017). Although this 67 change is relatively small over short temporal scales, correction methods are available to remove 68 this signal from tree-ring records when using tree-ring δ 13 C to understand plant physiological   These include disruption of water and nutrient uptake due to anoxic conditions in the root zone 93 (Jackson and Drew, 1984), lowered root hydraulic conductivity (Davies and Flore, 1986), 94 https://doi.org/10.5194/bg-2020-131 Preprint. Discussion started: 28 April 2020 c Author(s) 2020. CC BY 4.0 License. increased abscisic acid concentrations (Kozlowski and Pallardy, 1984) and accumulation of 95 metabolic toxins from flooding (Jackson and Drew, 1984). 96 In this study, we investigated how bottomland hardwood wetland forests of eastern Texas, 97 USA respond to hydroclimatic variation and extremes under different edaphic conditions. The 98 study was conducted at four sites, of which one was a frequently flooded wet site, while at the 99 other three sites waterlogging and surface flooding were much less frequent and more ephemeral.

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Our first objective was to understand how growth rates are affected by hydroclimatic variation 101 using tree-ring width analysis in water oak (Quercus nigra L.), a dominant species in the Columbia 102 bottomland hardwood forest. We hypothesized that at relatively drier sites, trees have lower growth 103 rates on average over long time scales compared to wetter sites. Periods of higher rainfall will be 104 associated with increases in growth. However, in extremely wet conditions, at frequently 105 waterlogged sites, trees will show a decline in growth caused by flooding and hypoxic conditions.

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Our second objective was to assess the magnitude of physiological stress inflicted by hydroclimatic 107 conditions on these forests. We hypothesized that tree-ring δ 13 C in trees growing under relatively 108 drier soil conditions will decrease during periods of higher rainfall. In contrast, the opposite trend 109 is expected at wetter sites where increasing moisture would induce flooding stress. In addition, we 110 hypothesized that trees growing where waterlogging is common are less stressed during dry 111 periods than those at the drier sites because of slower depletion of soil water reserves. Our third 112 objective was to evaluate the relationship between physiological stress and growth inhibition.

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Although a negative relationship between stress (tree-ring δ 13 C) and growth (tree-ring width) is 114 expected in this study, the strength of this relationship could vary with site conditions. Given that 115 water oaks are moderately tolerant of flooding, and dry conditions are also common in this 116 ecosystem, we hypothesized that drought stress had a stronger effect on growth than flooding stress. Thus, we expected a stronger negative relationship between growth and physiological stress 118 at drier sites.

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The study was conducted at four different sites located within the Brazos-Colorado Coastal  Table 1). Site DB was observed to be flooded most frequently after significant rain events from sites. Therefore, we refer to this site as the "wet site". The sites are located in Ecoregion III Western   Smiley, 1968). Three cores were extracted at breast height from every tree spaced equally around 159 the circumference. Two cores were processed and used for ring-width measurements and the third 160 core was used for δ 13 C analysis after cellulose extraction.  Tree cores not utilized for ring-width analyses were hand-sanded using a sandpaper (220 187 grit) to enhance ring-visibility. Tree-rings were selected from years with a wide range of 188 precipitation to cover the maximum breadth of the dry-wet hydroclimatic spectrum (235-1120 189 mm/year). Selected tree-rings were precisely excised using an X-Acto knife. For δ 13 C analysis, α-190 cellulose was extracted from the tree-rings using a slightly modified version of the Jayme-Wise

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Method (Green, 1963), in which a Soxhlet extraction assembly is used (Leavitt and Danzer, 1993;192 Cullen and Macfarlane, 2005). δ 13 C in tree-ring α-cellulose was analyzed using a Costech ECS    years (1991,1992,1997,2001,2003 and 2007) (Fig. 2). The divergent responses to these wet 232 years apparently reduced the pooled series intercorrelation (Table 3). Hence, this multi-site 233 chronology was not used for further analyses. High series intercorrelation for the drier sites indicates lower tree-to-tree differences at these sites, as compared to our wettest site, DB, which 235 had a slightly lower series intercorrelation (Table 3). All site-level chronologies were found to be 236 sensitive enough for dendroclimatological analysis as mean sensitivity, which is a measurement  (Table 3).  value as compared to two drier sites, BP (p=0.03) and OT (p=0.02) (Fig. 3).

Dendroclimatology analysis 261
Comparisons between ring-width index and climate data reveal that growth rates are most 262 strongly influenced by mid-growing season climate (May-July precipitation and maximum 263 temperatures; July PDSI) (Table S1). Since a larger proportion of annual growth occurs during the 264 mid-growing season, higher rainfall and lower maximum temperatures during this period strongly 265 drive annual growth rates. Similar comparisons between tree-ring δ 13 C measurements and climate 266 a ab b a data indicate that climatic conditions early in the growing season (April) are critical for causing 267 physiological stress in these forests (Table S2).

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As hypothesized, we observed a strong increase in tree-ring widths with mid-growing 269 season precipitation. Although this positive relationship was expected for trees growing in drier 270 conditions, we observed a similar but weaker positive relationship between growth and 271 precipitation even at the wet site (Table 4). We had hypothesized that at extremely wet conditions, 272 growth rates at the wet site would decline due to flood stress, however, no such decline was 273 observed even during extremely wet phases (Fig. 4a). Drought conditions and maximum 274 temperatures during the mid-growing season result in decreasing growth at the drier sites, but not 275 at the wet site, as expected (Table 4; Fig. 4b, c).

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In line with our second hypothesis, we observed a depletion in tree-ring δ 13 C values with 285 increase in early-growing season precipitation at the drier sites (Table 5, Fig. 5a). We had 286 hypothesized that high precipitation at the wet site will enrich tree-ring δ 13 C values as a result of 287 physiological stress caused by possible flooding stress. However, we found no relationship 288 between tree-ring δ 13 C and precipitation at the wet site (

Relationship between growth and tree-ring δ 13 C 302
The comparison between tree-ring δ 13 C values and tree-ring width from corresponding years 303 supports our third hypothesis only at the drier sites. Ring-width index was correlated with δ 13 C 304 values only at sites OT (p<0.05; R 2 =0.53) and BC (p<0.05; R 2 =0.58). Tree-ring δ 13 C values were 305 not correlated with annual growth at the wet site DB, which indicates that trees at this site were 306 able to sustain their growth rates even during stressful conditions (Fig. 6).     We found that mid-growing season precipitation (from May to July) is most critical for 371 growth in this landscape. Similarly, high temperatures during the same period were associated with  biodiversity that is dependent on this ecosystem. We provide evidence that wetter portions of this 419 landscape are more resilient to hydroclimatic changes than drier areas, as well as better adapted to 420 periods of flooding and waterlogging. Trees in drier areas grew more slowly during dry and warm 421 periods and were more sensitive to seasonal physiological stress. We observed variation in growth