NOAA/WDS Paleoclimatology - Peltier - Southwest Ecological Memory Project Site A10 - PPTR - ITRDB UT557

Trees are long-lived organisms that integrate climate conditions across years or decades to produce secondary growth. This integration process is sometimes referred to as 'climatic memory.' While widely perceived, the physiological processes underlying this temporal integration, such as the storage and remobilization of non-structural carbohydrates (NSC), are rarely explicitly studied. This is perhaps most apparent when considering drought legacies (perturbed post-drought growth responses to climate), and the physiological mechanisms underlying these lagged responses to climatic extremes. Yet, drought legacies are likely to become more common if warming climate brings more frequent drought. To quantify the linkages between drought legacies, climate memory and NSC, we measured tree growth (via tree ring widths) and NSC concentrations in three dominant species across the southwestern USA. We analyzed these data with a hierarchical mixed effects model to evaluate the time-scales of influence of past climate (memory) on tree growth. We then evaluated the role of climate memory and the degree to which variation in NSC concentrations were related to forward-predicted growth during the hot 2011-2012 drought and subsequent 4-year recovery period. Populus tremuloides exhibited longer climatic memory compared to either Pinus edulis or Juniperus osteosperma, but following the 2011-2012 drought, P. tremuloides trees with relatively longer memory of temperature conditions showed larger (more negative) drought legacies. Conversely, Pinus edulis trees with longer temperature memory had smaller (less negative) drought legacies. For both species, higher NSC concentrations followed more negative (larger) drought legacies, though the relevant NSC fraction differed between P. tremuloides and P. edulis. Our results suggest that differences in tree NSC are also imprinted upon tree growth responses to climate across long time scales, which also underlie tree resilience to increasingly frequent drought events under climate change.

树木作为长寿生物，可整合数年至数十年的气候条件以产生次生生长。这一整合过程有时被称为“气候记忆”。尽管该现象已被广泛认知，但支撑这种时间整合的生理过程——例如非结构性碳水化合物（non-structural carbohydrates, NSC）的储存与再动员——却鲜有得到明确研究。这一点在探讨干旱遗留效应（即干旱后受扰动的生长对气候的响应）以及这类极端气候滞后响应的生理机制时体现得尤为突出。

然而，若气候变暖导致干旱发生愈发频繁，干旱遗留效应势必会愈发普遍。为量化干旱遗留效应、气候记忆与非结构性碳水化合物之间的关联，我们在美国西南部的三个优势树种中测定了树木生长（通过树木年轮宽度）以及非结构性碳水化合物浓度。我们采用分层混合效应模型对这些数据进行分析，以评估过往气候（即气候记忆）对树木生长的影响时长；随后进一步评估气候记忆的作用，以及非结构性碳水化合物浓度的变异与2011-2012年高温干旱及后续4年恢复期内的预测生长之间的关联程度。

颤杨（Populus tremuloides）相较于多节松（Pinus edulis）和圆柏（Juniperus osteosperma）表现出更长的气候记忆，但在2011-2012年干旱过后，对温度条件记忆更长的颤杨个体呈现出更强（更负向）的干旱遗留效应。与之相反，温度记忆更长的多节松个体的干旱遗留效应则更弱（负向程度更低）。对于两个树种而言，更高的非结构性碳水化合物浓度均对应更强（负值更大）的干旱遗留效应，不过颤杨与多节松相关的非结构性碳水化合物组分存在差异。我们的研究结果表明，树木非结构性碳水化合物的差异同样印记在长期尺度下树木对气候的生长响应之中，这也构成了气候变化背景下树木应对愈发频繁干旱事件的恢复力基础。