Low forest productivity associated with increasing drought‐tolerant species is compensated by an increase in drought‐tolerance richness

Abstract

Many temperate forests are changing in composition due to a combination of changes in land‐use, management and climate‐related disturbances. Previous research has shown that in some regions these changes frequently favour drought‐tolerant tree species. However, the effects of these changes in composition on forest functioning (e.g. productivity) are unclear. We studied 25 years of change in individual tree biomass growth, ingrowth and mortality, and community composition and total plot biomass across 2663 permanent forest plots in Catalonia (NE Spain) comprising 85,220 trees of 59 species. We focused on the relationship between community‐level forest productivity and drought tolerance (DT), which was estimated using hydraulic traits as well as biogeographic indicators. We found that there was a small increase (1.6%–3.2% on average) in community‐mean DT (DTcwm) during the study period, concurrent with a strong increase (12.4%–19.4% on average) in DT richness (DTric; i.e. trait range). Most importantly, we found that the mean DT was negatively related to forest productivity, which was explained because drought‐tolerant tree species have lower tree‐level growth. In contrast, DT richness was strongly and positively related to forest productivity, probably because it allowed for a more stable production along wet and dry periods. These results suggest a negative impact of ongoing climate change on forest productivity mediated by functional composition shifts (i.e. selection of drought‐tolerant species), and a positive effect of increased DT richness as a consequence of land‐use legacies. Such a trend towards functional diversification, although temporary, would increase forests’ capacity to resist drought and place them in a better position to face the expected change in climate.

Wood density and hydraulic traits influence species’ growth response to drought across biomes

“…Hydraulic Safety Margins have been proposed as a key trait to determine species vulnerability to drought (Choat et al, 2012; Oliveira et al, 2021), including species drought‐induced mortality (Anderegg et al, 2016) and effects on ecosystem fluxes (Anderegg et al, 2018; García‐Valdés et al, 2021). In our case, HSM was positively related to growth resistance and negatively to recovery, which indicates that species operating with narrower safety margins are more sensitive to extreme drought events.…”

“…Hydraulic traits summarise cross‐species differences in water transport, with important implications for species physiology and ecology (Oliveira et al, 2021; Venturas et al, 2017). Hydraulic traits such as the water potential at 50% loss of conductivity (P50), the minimum leaf and xylem water potentials (Ψmin), and the hydraulic safety margin (HSM = Ψmin − P50) correlate with cross‐species vulnerability to drought and drought‐induced mortality (Anderegg et al, 2016; Choat et al, 2012), as well as ecosystem fluxes in response to drought (Anderegg et al, 2018; García‐Valdés et al, 2021).…”

Responses in Growth and Anatomical Traits of Two Subtropical Tree Species to Nitrogen Addition, Drought, and Their Interactions

“…Atmospheric nitrogen (N) deposition and drought are two major stressors that influence the structures, functionalities, and services of forest ecosystems ( Wamelink et al, 2009 ; Ma et al, 2012 ; Garcia-Valdes et al, 2021 ). The frequency and intensity of seasonal and periodic droughts are escalating globally, causing an increased decline of forests in many regions ( Allen et al, 2010 ; Senf et al, 2020 ).…”

Functional traits and propagule pressure explain changes in the distribution and demography of non‐native trees in Spain

“…Xylem hydraulics represents a direct limit to the drought tolerance of vascular plants (Brodribb & Cochard, 2009) and hydraulic failure is a major global cause of productivity loss and plant mortality during droughts (Choat et al, 2012; Greenwood et al, 2017). P50 is considered a key trait in predicting hydraulic efficiency during drought stress (Choat et al, 2012; Greenwood et al, 2017) and it has been successfully related to changes in composition in European forests (Ruiz‐Benito et al, 2017; García‐Valdés et al, 2021) and responses to climate change (García‐Valdés et al, 2021).…”