NOAA/WDS Paleoclimatology - Peltier - Southwest Ecological Memory Project Site J2-P6 - JUOS - ITRDB NM622

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.

树木是长寿生物，可整合数年乃至数十年的气候信息以形成次生生长。这一整合过程有时被称为‘气候记忆(climatic memory)’。尽管该现象已被广泛认知，但其背后支撑这一时间整合过程的生理机制——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研究中体现得尤为显著。然而，若气候变暖导致干旱发生愈发频繁，干旱遗留效应或许会愈发普遍。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，我们对美国西南部三个优势树种的树木生长（通过树轮宽度测定）与非结构性碳水化合物浓度开展了测定。我们采用分层混合效应模型对上述数据进行分析，以评估过往气候（记忆）对树木生长的影响时间尺度。随后我们评估了气候记忆的作用，以及非结构性碳水化合物浓度的变异与2011-2012年高温干旱期及后续4年恢复期内的生长预测量之间的关联程度。颤杨（Populus tremuloides）的气候记忆时长相较于多节松（Pinus edulis）与北美圆柏（Juniperus osteosperma）更长，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（负向程度更高）的干旱遗留效应。与之相反，温度记忆时长更长的多节松个体，其干旱遗留效应则更弱（负向程度更低）。对于两个物种而言，更高的非结构性碳水化合物浓度往往伴随更强（负向程度更高）的干旱遗留效应，不过涉及的非结构性碳水化合物组分在颤杨与多节松中存在差异。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会烙印在长期尺度上树木生长对气候的响应模式中，这也是气候变化背景下树木应对愈发频繁的干旱事件的韧性基础。