Graduate Program in Environmental Science, College of Environmental Science and Forestry, State University of New York, Syracuse, New York, USA.
Marine Science Institute, University of California, Santa Barbara, California, USA.
Glob Chang Biol. 2022 Nov;28(22):6771-6788. doi: 10.1111/gcb.16376. Epub 2022 Aug 31.
Dryland riparian woodlands are considered to be locally buffered from droughts by shallow and stable groundwater levels. However, climate change is causing more frequent and severe drought events, accompanied by warmer temperatures, collectively threatening the persistence of these groundwater dependent ecosystems through a combination of increasing evaporative demand and decreasing groundwater supply. We conducted a dendro-isotopic analysis of radial growth and seasonal (semi-annual) carbon isotope discrimination (Δ C) to investigate the response of riparian cottonwood stands to the unprecedented California-wide drought from 2012 to 2019, along the largest remaining free-flowing river in Southern California. Our goals were to identify principal drivers and indicators of drought stress for dryland riparian woodlands, determine their thresholds of tolerance to hydroclimatic stressors, and ultimately assess their vulnerability to climate change. Riparian trees were highly responsive to drought conditions along the river, exhibiting suppressed growth and strong stomatal closure (inferred from reduced Δ C) during peak drought years. However, patterns of radial growth and Δ C were quite variable among sites that differed in climatic conditions and rate of groundwater decline. We show that the rate of groundwater decline, as opposed to climate factors, was the primary driver of site differences in drought stress, and trees showed greater sensitivity to temperature at sites subjected to faster groundwater decline. Across sites, higher correlation between radial growth and Δ C for individual trees, and higher inter-correlation of Δ C among trees were indicative of greater drought stress. Trees showed a threshold of tolerance to groundwater decline at 0.5 m year beyond which drought stress became increasingly evident and severe. For sites that exceeded this threshold, peak physiological stress occurred when total groundwater recession exceeded ~3 m. These findings indicate that drought-induced groundwater decline associated with more extreme droughts is a primary threat to dryland riparian woodlands and increases their susceptibility to projected warmer temperatures.
旱地河岸林地被认为由于浅层且稳定的地下水位而局部免受干旱影响。然而，气候变化导致更频繁和更严重的干旱事件，伴随着更温暖的温度，共同通过增加蒸发需求和减少地下水供应来威胁这些依赖地下水的生态系统的持续存在。我们进行了树木年代学分析和季节性（半年度）碳同位素分馏（ΔC），以调查加利福尼亚州范围内空前的干旱（2012 年至 2019 年）对河岸棉白杨林的影响，研究地点沿着南加州最大的剩余自由流动的河流。我们的目标是确定旱地河岸林地干旱胁迫的主要驱动因素和指标，确定其对水文气候胁迫的耐受阈值，并最终评估其对气候变化的脆弱性。河岸树木对河流沿岸的干旱条件高度敏感，在干旱高峰期表现出生长受抑制和强烈的气孔关闭（从减少的ΔC 推断）。然而，在气候条件和地下水下降率不同的地点，径向生长和ΔC 的模式变化很大。我们表明，地下水下降率而不是气候因素是导致干旱胁迫的站点差异的主要驱动因素，并且在地下水下降较快的地点，树木对温度的敏感性更高。在各个地点，树木个体之间的径向生长和ΔC 的相关性更高，以及树木之间的ΔC 的相关性更高，表明干旱胁迫更大。树木对地下水下降的耐受阈值为 0.5m/年，超过该阈值，干旱胁迫变得越来越明显和严重。对于超过该阈值的地点，当总地下水退水超过~3m 时，出现最大的生理胁迫。这些发现表明，与更极端干旱相关的干旱引起的地下水下降是旱地河岸林地的主要威胁，并增加了它们对预计变暖温度的敏感性。
