Abstract：
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 (Δ13 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 Δ13 C) during peak drought years. However, patterns of radial growth and Δ13 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 Δ13 C for individual trees, and higher inter-correlation of Δ13 C among trees were indicative of greater drought stress. Trees showed a threshold of tolerance to groundwater decline at 0.5 m year-1 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.
摘要：
旱地河岸林地被认为是由浅层和稳定的地下水位在干旱中局部缓冲的。然而,气候变化导致更频繁和严重的干旱事件，伴随着温暖的温度，通过增加蒸发需求和减少地下水供应共同威胁这些依赖地下水的生态系统的持久性。我们对径向生长和季节性（半年度）碳同位素识别（Δ13C）进行了树枝状同位素分析，以调查河岸棉木林分对2012年至2019年加利福尼亚州空前的干旱的响应。南加州剩余的自由流动河流。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，确定他们对气候压力源的耐受阈值，并最终评估他们对气候变化的脆弱性。河岸树对沿河的干旱状况高度敏感，在干旱高峰期表现出抑制的生长和强烈的气孔关闭（从Δ13C降低推断）。然而,在气候条件和地下水下降速率不同的地点之间，径向生长和Δ13C的模式变化很大。我们证明了地下水下降的速度，与气候因素相反，是干旱胁迫部位差异的主要驱动因素，在地下水下降速度较快的地点，树木对温度表现出更大的敏感性。跨站点,单株树木的径向生长和Δ13C之间的相关性更高，树木之间Δ13C的相互关系较高，表明干旱胁迫更大。树木在0.5m年1时显示出对地下水下降的耐受性阈值，超过该阈值，干旱胁迫变得越来越明显和严重。对于超过此阈值的站点，当地下水总衰退超过约3m时，生理压力达到峰值。这些发现表明，干旱引起的地下水减少与更极端的干旱有关，是对旱地河岸林地的主要威胁，并增加了它们对预计温度升高的敏感性。
