Competition‐induced tree mortality across Europe is driven by shade tolerance, proportion of conspecifics and drought

Abstract

Forest stand densities are increasing in the boreal and temperate biomes, suggesting that tree‐tree competition is intensifying. Anticipating the consequences of this intensified competition is difficult because competition‐induced mortality may depend not only on the occurrence of extreme climatic events such as drought, but also on stand composition, since tree species differ in their ability to compete and tolerate competition. A better understanding of the effects of stand composition and drought on competition‐induced mortality would help to anticipate future changes in forest ecosystems. We studied the tree‐level probability of competition‐induced mortality using National Forest Inventory data from three European countries (Finland, France and Germany), covering a latitudinal gradient from the Mediterranean to the Arctic. We investigated how (i) the proportion of conspecifics, (ii) the shade tolerance (ST) of the focal tree and its competitors and (iii) drought events modify the effect of competition on tree mortality. We used a generalized linear mixed model on a dataset of 461,109 trees representing 39 species on 48,088 individual plots. Competition, measured as the basal area of larger trees, was a stronger driver of background mortality (BM) than tree size and climate. A higher proportion of conspecifics increased the competition effect on mortality, showing that conspecific individuals had a higher competitive effect compared to heterospecific individuals. The competition effect on mortality also increased as a function of the ST of neighbouring trees, suggesting an increased shading effect. A higher ST of a focal tree decreased the competition effect on mortality. Drought anomalies increased the competition effect, resulting in a higher mortality probability for the most suppressed trees. Synthesis. Competition was the main driver of background mortality. Increasing stand density increased competition‐induced tree mortality in both monospecific and mixed stands, but to different extents depending on the proportion of conspecifics and tree species shade tolerance (ST). Drought periods increase mortality, especially among the most suppressed trees, suggesting an interaction between competitive status and drought. Incorporating more detailed information on stand composition and tree species ST into tree mortality models will improve our understanding of forest dynamics in a changing climate.

The competitive effect of neighbouring plants on the growth of two eucalypts is stronger in dry conditions

“…This finding contradicts predictions of the SGH (Bertness & Callaway, 1994) and contributes to an increasing body of evidence that in severely water-limited environments, plant-plant interactions often become increasingly competitive, rather than facilitative (Barron-Gafford et al, 2017;Maestre & Cortina, 2004;O'Brien et al, 2017;Tielbörger & Kadmon, 2000). Further, our finding that stronger competition was associated with reduced precipitation is supported by previous similar studies that assess how temporal variation in rainfall determines the nature of plant-plant interactions (Diez et al, 2021;Tielbörger & Kadmon, 2000) and by a recent study that found drought tended to increase competition-induced tree mortality across parts of Europe (Kulha et al, 2023). Stronger competition in dry conditions has important implications for tree F I G U R E 1 Partial residual plots depicting the relationship between (a) Eucalyptus delegatensis and (b) Eucalyptus regnans annual absolute basal area increment (BAI) (cm 2 ) and neighbourhood metrics (NMs) (community-and functional group-level neighbourhood basal area; cm 2 ) in three sampling periods with different climatic conditions.…”

Current and future drought vulnerability for three dominant boreal tree species

“…In addition, uneven‐aged forests may improve resilience to drought (Piedallu et al., 2022; Pretzsch et al., 2022). Management could also be adapted to reduce competition within forest stands (Kulha et al., 2023). For example, Sohn et al.…”

“…Management could also be adapted to reduce competition within forest stands (Kulha et al, 2023). For example, Sohn et al (2016) observed that heavy thinning improved growth responses following drought in both conifers and broadleaves, although it could be sometimes offset by increased soil water evaporation (Pretzsch et al, 2018).…”

“…Accordingly, maps of forests' drought tolerance have proven to be a valuable tool for managers to mitigate the effects of climate change (Piedallu et al., 2022). National forest inventories (NFI) that perform systematic forest surveys across space and time along large environmental gradients are powerful tools to understand how climate controls forest growth at wide geographical scales, and may at the same time account for the effects of local abiotic and biotic interactions (Kulha et al., 2023). The Swedish NFI provides a dense network of plots with tree‐ring data and information on forest characteristics (Fridman et al., 2014), useful for meaningful reconstructions and interpretations of regional‐scale patterns of tree drought responses.…”