Competition amplifies drought stress in forests across broad climatic and compositional gradients

Abstract

Forests around the world are experiencing increasingly severe droughts and elevated competitive intensity due to increased tree density. However, the influence of interactions between drought and competition on forest growth remains poorly understood. Using a unique dataset of stand‐scale dendrochronology sampled from 6405 trees, we quantified how annual growth of entire tree populations responds to drought and competition in eight, long‐term (multi‐decadal), experiments with replicated levels of density (e.g., competitive intensity) arrayed across a broad climatic and compositional gradient. Forest growth (cumulative individual tree growth within a stand) declined during drought, especially during more severe drought in drier climates. Forest growth declines were exacerbated by high density at all sites but one, particularly during periods of more severe drought. Surprisingly, the influence of forest density was persistent overall, but these density impacts were greater in the humid sites than in more arid sites. Significant density impacts occurred during periods of more extreme drought, and during warmer temperatures in the semi‐arid sites but during periods of cooler temperatures in the humid sites. Because competition has a consistent influence over growth response to drought, maintaining forests at lower density may enhance resilience to drought in all climates.

Lagged mortality among tree species four years after an exceptional drought in east Texas

“…One caveat to this is that mean predicted loblolly weather mortality was consistently underestimated each year (i.e., observations were 29-66% greater than predictions across all years; Appendix S1: Fig. Recent tree-ring studies of pine species across the U.S. highlight the effectiveness of thinning and reduced stand densities at mitigating water stress (D'Amato et al 2013, Bottero et al 2017, Gleason et al 2017) lending support to our observations in east Texas. Despite this, observed mean loblolly pine weather mortality and standard errors were consistently lower than other species.…”

Taxonomy, together with ontogeny and growing conditions, drives needleleaf species' sensitivity to climate in boreal North America

“…These relationships could have originated from lower carbon assimilation and carbohydrate reserve formation originating from reduced water availability (Gleason et al, 2017). The growth of trees in densely vegetated (high BA) stands that were composed of taller individuals (high CI) was more negatively correlated with excess heat during previous-year summers and less positively correlated with current-year wet summers than in a less competitive environment.…”

“…By modifying resource availability, interindividual competition can exacerbate tree sensitivity to harsh climatic conditions (Buechling, Martin, & Canham, 2017;Ford et al, 2016;Gleason et al, 2017;Jiang et al, 2018;Nicklen et al, 2018) or buffer growth gains from favorable periods (Cortini, Comeau, & Bokalo, 2012). Ultimately, the capacity of a tree to efficiently use resources will also dictate its response to climate (Carrer & Urbinati, 2004).…”

Moisture-Limited Tree Growth for a Subtropical Himalayan Conifer Forest in Western Nepal

“…Decreasing trends in winter and pre-monsoon precipitation were observed, with increasing drought and rising temperatures in recent decades across western and northwestern Nepal [24,46]. Under severe drought conditions, high competition for moisture between neighboring trees will further exacerbate drought stress for tree growth [47][48][49][50]. As shown in a recent study, low winter and spring precipitation can cause a delay of the initiation of xylogenesis and contribute to the occurrence of the locally missing rings in years with extremely dry springs [51].…”