Department of Natural Resources and the Environment, University of New Hampshire, 56 College Rd, Durham, NH 03824, USA.
Earth Systems Research Center, University of New Hampshire, 8 College Rd, Durham, NH 03824, USA.
Tree Physiol. 2021 Oct 4;41(10):1819-1835. doi: 10.1093/treephys/tpab056.
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 (Js), 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 Js 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, Js 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.
干旱频率和严重程度的增加是一种普遍存在的全球威胁，但中湿性温带森林在应对干旱时保持恢复力的能力仍知之甚少。我们在美国新罕布什尔州部署了一个穿透雨去除实验，以模拟百年一遇的干旱，再加上该地区 2016 年的大旱，使水分胁迫程度加剧，超出了我们研究树木的寿命。为了评估两个主要的东北树种（栎属和松属）的敏感性和阈值动态，我们监测了 sap flux density（Js）、叶片水势和气体交换、生长和内在水分利用效率（iWUE）在预处理一年（2015 年）和两年（2016-17 年）。结果表明，松树（Pinus strobus L.）的 Js 在土壤湿度阈值为 0.15 m3 m-3 时突然下降，而栎属（Quercus rubra L. 和 Quercus velutina Lam.）的阈值为 0.11 m3 m-3，这一发现与松树更等水的策略一致。然而，一旦橡木的水分阈值超过，Js 就会突然下降，表明虽然橡木适应中度干旱，但它们极易受到极端干旱的影响。与橡木相比，2016 年干旱对松树的径向生长减少了两倍以上（分别为 50%和 18%）。尽管 2017 年降雨量相对较高，但橡木的生长仍在继续下降（恢复不佳），而松树的生长则保持中性（处理）或改善（对照）。2016 年，无论是处理组还是对照组的松树，iWUE 都有所增加，但只有处理组的橡木才会增加。值得注意的是，松树的 iWUE 与多年来的降水呈显著线性关系，而橡木只在最干旱的条件下才会有反应，这进一步强调了这两个物种的不同敏感性阈值。我们的研究结果提供了新的见解，说明温带森林树种不同生理特性和土壤水分阈值之间的相互作用如何影响它们对极端干旱的反应和恢复力。
