Abstract

Increased drought frequency and severity are a pervasive global threat, yet the capacity of mesic temperate forests to maintain resilience in response to drought remains poorly understood. We deployed a throughfall removal experiment to simulate a once in a century drought in New Hampshire, USA, which coupled with the region-wide 2016 drought, intensified moisture stress beyond that experienced in the lifetimes of our study trees. To assess the sensitivity and threshold dynamics of two dominant northeastern tree genera (Quercus and Pinus), we monitored sap flux density (J_s), leaf water potential and gas exchange, growth and intrinsic water-use efficiency (iWUE) for one pretreatment year (2015) and two treatment years (2016–17). Results showed that J_s in pine (Pinus strobus L.) declined abruptly at a soil moisture threshold of 0.15 m³ m⁻³, whereas oak’s (Quercus rubra L. and Quercus velutina Lam.) threshold was 0.11 m³ m⁻³—a finding consistent with pine’s more isohydric strategy. Nevertheless, once oaks’ moisture threshold was surpassed, J_s declined abruptly, suggesting that while oaks are well adapted to moderate drought, they are highly susceptible to extreme drought. The radial growth reduction in response to the 2016 drought was more than twice as great for pine as for oaks (50 vs 18%, respectively). Despite relatively high precipitation in 2017, the oaks’ growth continued to decline (low recovery), whereas pine showed neutral (treatment) or improved (control) growth. The iWUE increased in 2016 for both treatment and control pines, but only in treatment oaks. Notably, pines exhibited a significant linear relationship between iWUE and precipitation across years, whereas the oaks only showed a response during the driest conditions, further underscoring the different sensitivity thresholds for these species. Our results provide new insights into how interactions between temperate forest tree species’ contrasting physiologies and soil moisture thresholds influence their responses and resilience to extreme drought.

中文翻译：

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Pinus strobus 和 Quercus spp 的灵敏度和阈值动态。应对实验性和自然发生的严重干旱

摘要

干旱频率和严重程度的增加是普遍存在的全球威胁，但中温带森林在应对干旱时保持恢复力的能力仍然知之甚少。我们部署了一项贯穿式清除实验，以模拟美国新罕布什尔州百年一遇的干旱，再加上 2016 年全地区的干旱，加剧了水分胁迫，超出了我们研究树木的生命周期。为了评估东北两个主要树种（栎属和松属）的敏感性和阈值动态，我们监测了树液通量密度（J _s)、叶水势和气体交换、生长和内在水分利用效率 (iWUE) 一个预处理年 (2015) 和两个处理年 (2016-17)。结果表明，松树 ( Pinus strobus L.) 的J _s在土壤水分阈值为 0.15 m ³  m ^{-3 时}突然下降，而橡树 ( Quercus rubra L. 和Quercus velutina Lam.) 的阈值为 0.11 m ³  m ^-3 —这一发现与松树的等氢策略一致。然而，一旦超过橡树的水分阈值，J _s突然下降，这表明虽然橡树很好地适应了中度干旱，但它们极易受到极端干旱的影响。由于 2016 年干旱，松树的径向生长减少是橡树的两倍多（分别为 50% 和 18%）。尽管 2017 年降水量相对较高，但橡树的生长继续下降（低恢复），而松树则表现出中性（处理）或改善（控制）的生长。2016 年，处理和对照松树的 iWUE 均有所增加，但仅限于处理橡树。值得注意的是，松树在不同年份的 iWUE 和降水之间表现出显着的线性关系，而橡树仅在最干燥的条件下显示出响应，进一步强调了这些物种的不同敏感性阈值。