Tree growth is more limited by drought in rear-edge forests most of the times

J Julio Camarero,Raquel Alfaro-Sánchez,Antonio Gazol,Gabriel Sangüesa-Barreda,Sergio M Vicente-Serrano,Marta Vergarechea,Nicolás Cattaneo

doi:10.1186/s40663-021-00303-1

Abstract

BackgroundEquatorward, rear-edge tree populations are natural monitors to estimate species vulnerability to climate change. According to biogeographical theory, exposition to drought events increases with increasing aridity towards the equator and the growth of southern tree populations will be more vulnerable to drought than in central populations. However, the ecological and biogeographical margins can mismatch due to the impact of ecological factors (topography, soils) or tree-species acclimation that can blur large-scale geographical imprints in trees responses to drought making northern populations more drought limited.MethodsWe tested these ideas in six tree species, three angiosperms (Fagus sylvatica, Quercus robur, Quercus petraea) and three gymnosperms (Abies alba, Pinus sylvestris and Pinus uncinata) by comparing rear-edge tree populations subjected to different degrees of aridity. We used dendrochronology to compare the radial-growth patterns of these species in northern, intermediate, and southern tree populations at the continental rear edge.Results and conclusionsWe found marked variations in growth variability between species with coherent patterns of stronger drought signals in the tree-ring series of the southern populations of F. sylvatica, P. sylvestris, and A. alba. This was also observed in species from cool-wet sites (P. uncinata and Q. robur), despite their limited responsiveness to drought. However, in the case of Q. petraea the intermediate population showed the strongest relationship to drought. For drought-sensitive species as F. sylvatica and P. sylvestris, southern populations presented more variable growth which was enhanced by cool-wet conditions from late spring to summer. We found a trend of enhanced vulnerability to drought in these two species. The response of tree growth to drought has a marked biogeographical component characterized by increased drought sensitivity in southern populations even within the species distribution rear edge. Nevertheless, the relationship between tree growth and drought varied between species suggesting that biogeographical and ecological limits do not always overlap as in the case of Q. petraea. In widespread species showing enhanced vulnerability to drought, as F. sylvatica and P. sylvestris, increased vulnerability to climate warming in their rear edges is forecasted. Therefore, we encourage the monitoring and conservation of such marginal tree populations.

Highlights

Equatorward, rear-edge tree populations are natural monitors to estimate species vulnerability to climate change
Lower growth rates in the southern sites were more evident for drought-sensitive species such as F. sylvatica and P. sylvestris (Table 1; Fig. S3)
There is a notable exception in the case of sessile oak, a drought-sensitive species (Aranda et al 2000), whose growth is more impacted by drought in intermediate than in southern populations, pointing to the importance of other ecological or local factors modulating tree growth sensitivity to drought (Cavin and Jump 2017)

Summary

Introduction

Equatorward, rear-edge tree populations are natural monitors to estimate species vulnerability to climate change. The ecological and biogeographical margins can mismatch due to the impact of ecological factors (topography, soils) or tree-species acclimation that can blur large-scale geographical imprints in trees responses to drought making northern populations more drought limited. The role of drought in determining tree growth is spatially variable (Anderegg et al 2015), with tree populations located near the equatorward distribution limit being more vulnerable to drought (e.g., SánchezSalguero et al 2017; but see Cavin and Jump 2017). Recent studies have demonstrated that populations on the equatorward limit display adaptations to drought (Dorado-Liñán et al 2019; Muffler et al 2020) and that ecological factors can blur or override geographical patterns of greater vulnerability to drought as latitude decreases (Vilà-Cabrera and Jump 2019). Marginality has different components (i.e., geographical, ecological, and genetic) and geographical range margins do not always dictate population performance (Vilà-Cabrera et al 2019)

Methods

Results

Discussion

Conclusion