Abstract
Despite growing evidence that diverse forests play an important role in ecosystem functioning, ensuring the provision of different ecosystem services, whether such diversity improves their response to drought events remains unclear. In this study, we use a large tree-ring database from thirty case studies across nine European countries and eleven species, covering from Mediterranean to hemiboreal forests, to test if the growth response to site specific drought events that occurred between 1975 and 2015 varied between mixed and monospecific stands. In particular, we quantify how stands resist those specific drought events and recover after them, thus analyzing their resilience. For each drought event and forest stand we calculated resistance, recovery, resilience and relative resilience and we related the variation in these indices between monospecific and mixed stands with type of admixture, tree species identity, site aridity gradient, stand basal area and stand age. We found a large variability among case studies, even for those that share similar species composition and have similar climates. On average, mixed stands showed higher resistance, resilience and relative resilience to drought events than monospecific stands. However, the beneficial effect of mixtures could not be generalized, being greatly modulated by the type of admixture and tree species identity, and depending on site water supply and stand characteristics, such as basal area and age. The increase in resilience in mixtures compared with monocultures was greater on the conifer-broadleaved admixtures, and to a lesser extent in the broadleaved-broadleaved combinations. The observed response patterns to drought largely varied among the eleven studied species, thus revealing the importance of functional traits for understanding a species’ response to drought across its distribution range. Along the site aridity gradient, resilience and relative resilience to drought increased in drier sites for both monospecific and mixed stands, with an observed trend towards higher resilience in mixed stands in the drier and hotter sites. Our results confirm the complexity of the relationships found of resistance, recovery, resilience and relative resilience with drought when comparing pure vs mixed stands.
Highlights
Mixed forests, characterized by the coexistence of at least two tree species, represent more than two thirds of the total forested areas on Earth (FAO, 2016)
When considering the whole database with 30 case studies (Hy pothesis I) we observed that overall mixed stands showed greater resistance (P-value = 0.0564), resilience (P-value < 0.0001) and relative resilience (P-value = 0.0283) compared to monospecific stands, while value < 0.0001) and mixed stands (P-value < 0.0001), resilience differed between species only in monospecific stands (P-value = 0.0053)
The greatest recovery was shown in Pinus pinea for mono specific and mixed stands, while the lowest values were observed in Abies alba for monospecific stands and for Quercus pubescens in mixed stands
Summary
Mixed forests, characterized by the coexistence of at least two tree species, represent more than two thirds of the total forested areas on Earth (FAO, 2016). Mixed forests can be more stable to different disturbances, both biotic (Jactel et al 2017) and abiotic (Anderegg et al, 2018; Pretzsch et al 2019), while they can increase the temporal stability of community productivity (Jucker et al 2014; del Rio et al, 2017) or even increase such productivity (Jucker et al 2014; Zhang et al 2012; Forrester 2014; Pretzsch 2017; Jactel et al 2018). This will result in higher amounts of carbon stored above- and belowground in mixed forests (Forrester et al 2006; Epron et al 2013)
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