How neighbourhood interactions control the temporal stability and resilience to drought of trees in mountain forests

Abstract

Over the coming decades, the predicted increase in frequency and intensity of extreme events such as droughts is likely to have a strong effect on forest functioning. Recent studies have shown that species mixing may buffer the temporal variability of productivity. However, most studies have focused on temporal stability of productivity, while species mixing may also affect forest resilience to extreme events. Our understanding of mechanisms underlying species mixing effects on forest stability and resilience remains limited because we ignore how changes from intraspecific to interspecific interactions in the neighbourhood of a given tree might affect its stability and resilience to extreme drought (i.e. response during and after this drought). This is crucial to better understand forests’ response to climate change and how diversity may help maintain forest functioning. Here we analysed how local intra‐ or interspecific interactions may affect the temporal stability and resilience to drought of individual trees in French mountain forests, using basal area increment data over the previous 20 years for Fagus sylvatica, Abies alba and Quercus pubescens. We analysed the effect of interspecific competition on (a) the temporal stability and (b) the resilience to drought (resistance and recovery) of individual tree radial growth. We found no significant interspecific competition effect on temporal stability, but species‐specific effects on tree growth resilience to drought. There was a positive effect of heterospecific proportion on the drought resilience of Q. pubescens, a negative effect for A. alba and no effect for F. sylvatica. These differences may be related to interspecific differences in water use or rooting depth. Synthesis: In this study, we showed that stand composition influences individual tree growth resilience to drought, but this effect varied depending on the species and its physiological responses. Our study also highlighted that a lack of biodiversity effect on long‐term stability might hide important effects on short‐term resilience to extreme climatic events. This may have important implications in the face of climate change.

Neighbourhood diversity mitigates drought impacts on tree growth

“…Our results are in contrast to findings from temperate forests, where Jucker, Bouriaud, Avacaritei, Dǎnilǎ, et al () demonstrated the strongest decline in biodiversity effects (relative to community tree species richness) during dry years for species associated with the lowest drought tolerance. Similarly, the proportion of heterospecific neighbours was shown to positively affect drought resilience of drought‐tolerant species ( Quercus pubescens ), while a neutral ( Fagus sylvatica ) and negative ( Abies alba ) effect was evident for less drought‐tolerant species (Jourdan et al, ). These differences might be largely attributed to differences in biomes (level of tree species diversity, climate and soil conditions).…”

“…Consequently, BPRs at the local neighbourhood scale should become stronger during periods of water deficits, meaning that the relative importance of biodiversity effects increases during drought (Figure a,b). The few evidence on climate–growth relationships in response to local neighbourhood conditions comes from observational studies performed in less diverse temperate forests with a limited taxonomic tree diversity (Jourdan, Kunstler, & Morin, ), and most of these studies accounted for neighbourhood diversity using a contrast of neighbourhood composition (conspecific vs. heterospecific neighbours; Mölder & Leuschner, ; Vitali, Forrester, & Bauhus, ) or neighbourhood competition (intraspecific vs. interspecific competition; Aussenac, Bergeron, Gravel, & Drobyshev, ). Similarly, one recent experimental study explored drought resistance of tropical tree seedlings in response to neighbourhood composition (conspecific vs. heterospecific neighbours; O’Brien, Reynolds, Ong, & Hector, ).…”

European beech stem diameter grows better in mixed than in mono-specific stands at the edge of its distribution in mountain forests

“…However, the large variability in site conditions covered by both types of stands (Supplement Table 1) and the large number of trees sampled made the comparison reliable and representative of European beech's growth trends in mountain regions. Nevertheless, the identified mixing effect on growth trends could be partly masked by the different identities of the admixed species, as species interactions vary among different beech mixtures (del Río et al 2014;Pretzsch and Forrester 2017;Jourdan et al 2020).…”

Constraining Amazonian land surface temperature sensitivity to precipitation and the probability of forest dieback

“…Thus, in this range, the relation between sensitivity and forest cover before the tipping point is weak and the forestcover fraction remains almost stable. Possible mechanisms are that forests can resist drought through, for instance, investing more carbohydrates into root growth for higher soil water availability 32 , increase stomatal closure during the middle of the day for less transpiration, thus increasing water-use efficiency (WUE) 33 and leading to a more stable forest cover even during drought events 34 . The assumption of stable vegetation cover under drought is also supported by satellite-observed dry-season green up in Amazonian forest.…”