Environmental Studies Program, Santa Barbara, California, USA.
Bren School of Environmental Science & Management, UC Santa Barbara, Santa Barbara, California, USA.
Glob Chang Biol. 2023 Aug;29(15):4368-4382. doi: 10.1111/gcb.16740. Epub 2023 May 16.
Predicted increases in extreme droughts will likely cause major shifts in carbon sequestration and forest composition. Although growth declines during drought are widely documented, an increasing number of studies have reported both positive and negative responses to the same drought. These divergent growth patterns may reflect thresholds (i.e., nonlinear responses) promoted by changes in the dominant climatic constraints on tree growth. Here we tested whether stemwood growth exhibited linear or nonlinear responses to temperature and precipitation and whether stemwood growth thresholds co-occurred with multiple thresholds in source and sink processes that limit tree growth. We extracted 772 tree cores, 1398 needle length records, and 1075 stable isotope samples from 27 sites across whitebark pine's (Pinus albicaulis Engelm.) climatic niche in the Sierra Nevada. Our results indicated that a temperature threshold in stemwood growth occurred at 8.4°C (7.12-9.51°C; estimated using fall-spring maximum temperature). This threshold was significantly correlated with thresholds in foliar growth, as well as carbon (δ C) and nitrogen (δ N) stable isotope ratios, that emerged during drought. These co-occurring thresholds reflected the transition between energy- and water-limited tree growth (i.e., the E-W limitation threshold). This transition likely mediated carbon and nutrient cycling, as well as important differences in growth-defense trade-offs and drought adaptations. Furthermore, whitebark pine growing in energy-limited regions may continue to experience elevated growth in response to climate change. The positive effect of warming, however, may be offset by growth declines in water-limited regions, threatening the long-term sustainability of the recently listed whitebark pine species in the Sierra Nevada.
预计极端干旱的增加将可能导致碳固存和森林组成的重大转变。尽管干旱期间的生长下降已被广泛记录，但越来越多的研究报告了对同一干旱的积极和消极反应。这些不同的生长模式可能反映了由主导树木生长的气候限制因素变化所推动的阈值（即非线性响应）。在这里，我们测试了树干生长是否对温度和降水表现出线性或非线性响应，以及树干生长阈值是否与限制树木生长的源和汇过程中的多个阈值同时出现。我们从内华达山脉白皮松（Pinus albicaulis Engelm.）的气候范围内的 27 个地点中提取了 772 个树干芯、1398 个针叶长度记录和 1075 个稳定同位素样本。我们的结果表明，树干生长中的温度阈值出现在 8.4°C（7.12-9.51°C；使用秋冬最大温度估计）。该阈值与叶片生长以及干旱期间出现的碳（δC）和氮（δN）稳定同位素比的阈值显著相关。这些同时出现的阈值反映了能量和水分限制树木生长之间的过渡（即 E-W 限制阈值）。这种过渡可能介导了碳和养分循环，以及生长-防御权衡和干旱适应的重要差异。此外，在内华达山脉能源受限地区生长的白皮松可能会继续因气候变化而经历更高的生长。然而，变暖的积极影响可能会被水分受限地区的生长下降所抵消，这威胁到内华达山脉最近被列为白皮松物种的长期可持续性。
