Forest Ecology, Department of Environmental Systems Science, Institute of Terrestrial Ecosystems, ETH Zürich, Universitätstrasse 22, 8092, Zürich, Switzerland.
Institute of Systematic Botany and Ecology, Ulm University, Albert-Einstein-Allee 11, 89081, Ulm, Germany.
Glob Chang Biol. 2017 Apr;23(4):1675-1690. doi: 10.1111/gcb.13535. Epub 2016 Nov 12.
Tree mortality is a key factor influencing forest functions and dynamics, but our understanding of the mechanisms leading to mortality and the associated changes in tree growth rates are still limited. We compiled a new pan-continental tree-ring width database from sites where both dead and living trees were sampled (2970 dead and 4224 living trees from 190 sites, including 36 species), and compared early and recent growth rates between trees that died and those that survived a given mortality event. We observed a decrease in radial growth before death in ca. 84% of the mortality events. The extent and duration of these reductions were highly variable (1-100 years in 96% of events) due to the complex interactions among study species and the source(s) of mortality. Strong and long-lasting declines were found for gymnosperms, shade- and drought-tolerant species, and trees that died from competition. Angiosperms and trees that died due to biotic attacks (especially bark-beetles) typically showed relatively small and short-term growth reductions. Our analysis did not highlight any universal trade-off between early growth and tree longevity within a species, although this result may also reflect high variability in sampling design among sites. The intersite and interspecific variability in growth patterns before mortality provides valuable information on the nature of the mortality process, which is consistent with our understanding of the physiological mechanisms leading to mortality. Abrupt changes in growth immediately before death can be associated with generalized hydraulic failure and/or bark-beetle attack, while long-term decrease in growth may be associated with a gradual decline in hydraulic performance coupled with depletion in carbon reserves. Our results imply that growth-based mortality algorithms may be a powerful tool for predicting gymnosperm mortality induced by chronic stress, but not necessarily so for angiosperms and in case of intense drought or bark-beetle outbreaks.
树木死亡是影响森林功能和动态的关键因素,但我们对导致树木死亡的机制以及与树木生长速率相关的变化的理解仍然有限。我们从同时采集了死亡和存活树木的地点(190 个地点的 2970 棵死亡树木和 4224 棵存活树木,包括 36 个物种)编制了一个新的泛大陆树木年轮宽度数据库,并比较了经历特定死亡事件的死亡树木和存活树木的早期和近期生长速率。我们观察到,约 84%的死亡事件中,树木在死亡前的径向生长会减少。由于研究物种之间以及死亡源之间的复杂相互作用,这些减少的程度和持续时间变化很大(96%的事件中为 1-100 年)。我们发现,裸子植物、耐荫和耐旱物种以及因竞争而死亡的树木的生长衰退幅度大且持续时间长。被子植物和因生物攻击(特别是树皮甲虫)而死亡的树木通常表现出相对较小且短期的生长减少。我们的分析没有突出表明在一个物种内早期生长和树木寿命之间存在任何普遍的权衡关系,尽管这一结果也可能反映出各地点采样设计的高度变异性。死亡前的生长模式的站点间和种间变异性为死亡率过程的性质提供了有价值的信息,这与我们对导致死亡率的生理机制的理解是一致的。在死亡前生长的突然变化可能与普遍的水力失效和/或树皮甲虫的攻击有关,而长期的生长减少可能与液压性能的逐渐下降以及碳储量的枯竭有关。我们的结果表明,基于生长的死亡率算法可能是预测慢性胁迫引起的裸子植物死亡率的有力工具,但对于被子植物则不一定如此,而且在严重干旱或树皮甲虫爆发的情况下也不一定如此。