NOAA/WDS Paleoclimatology - Southern Sierra Nevada 60 Year Tree Ring Carbon Isotope and Resin Duct Data

Tree mortality associated with drought and concurrent bark beetle outbreaks is expected to increase with further climate change. When these two types of disturbance occur in concert it complicates our ability to accurately predict future forest mortality. The recent extreme California USA drought and bark beetle outbreaks resulted in extensive tree mortality and provides a unique opportunity to examine questions of why some trees die while others survive these co-occurring disturbances. We use plot-level data combined with a three-proxy tree-level approach using radial growth, carbon isotopes, and resin duct metrics to evaluate 1) whether variability in stand structure, tree growth or size, carbon isotope discrimination, or defenses precede mortality, 2) how relationships between these proxies differ for surviving and now-dead trees, and 3) whether generalizable risk factors for tree mortality exist across pinyon pine (Pinus monophylla), ponderosa pine (P. ponderosa), white fir (Abies concolor), and incense cedar (Calocedrus decurrens) affected by the combination of drought and beetle outbreaks. We find that risk factors associated with mortality differ between species, and that few generalizable patterns exist when bark beetle outbreaks occur in concert with a particularly long, hot drought. We see evidence that both long-term differences in physiology and shorter-term beetle-related selection and variability in defenses influence mortality susceptibility for ponderosa pine, whereas beetle dynamics may play a more prominent role in mortality patterns for white fir and pinyon pine. In contrast, incense cedar mortality appears to be attributable to long-term effects of growth suppression. Risk factors that predispose some trees to drought and beetle-related mortality likely reflect species-specific strategies for dealing with these particular disturbance types. The combined influence of beetles and drought necessitates the consideration of multiple, species-specific risk factors to more accurately model forest mortality in the face of similar extreme events more likely under future climates.

随着气候变化进一步加剧，与干旱及同期树皮甲虫（bark beetle）暴发相关的树木死亡事件预计将愈发频发。当这两类干扰协同发生时，我们准确预测未来森林死亡动态的能力将变得更为复杂。美国加利福尼亚州近期的极端干旱与树皮甲虫暴发事件造成了大规模树木死亡，为我们探究为何部分树木在这类协同干扰中死亡、而部分得以存活的问题提供了独特契机。本研究采用样地水平数据结合单木水平的三类替代指标方法，包括径向生长（radial growth）、碳同位素（carbon isotopes）与树脂道指标（resin duct metrics），来评估三个核心问题：1）林分结构（stand structure）、树木生长或大小、碳同位素判别（carbon isotope discrimination）以及防御能力的变异是否先于树木死亡出现；2）存活树木与已死亡树木之间的这些替代指标关联存在何种差异；3）在受干旱与甲虫暴发协同影响的矮松（Pinus monophylla）、西黄松（Pinus ponderosa）、白冷杉（Abies concolor）以及香雪松（Calocedrus decurrens）中，是否存在通用的树木死亡风险因子。本研究发现，与树木死亡相关的风险因子因物种而异；当树皮甲虫暴发与持续时间长、温度偏高的极端干旱协同发生时，通用的风险模式极为罕见。研究证据表明，生理特性的长期差异、短期甲虫介导的选择作用以及防御能力的变异，均会影响西黄松的死亡易感性；而对于白冷杉与矮松而言，甲虫种群动态可能在其死亡模式中占据更为主导的地位。与之相对，香雪松的死亡似乎可归因于生长受抑的长期效应。使部分树木更易遭受干旱与甲虫相关死亡的风险因子，大概率反映了各物种应对这类特定干扰类型的物种特异性策略。甲虫与干旱的协同影响要求我们纳入多种物种专属风险因子，才能在未来气候背景下更易出现的类似极端事件中，更精准地模拟森林死亡动态。