Tree Species Are Differently Impacted by Cumulative Drought Stress and Present Higher Growth Synchrony in Dry Places

Antonio Gazol,Lluís Coll,Raúl Sánchez-Salguero,Gabriel Sangüesa-Barreda,J Julio Camarero,Pere Casals,Xavier Serra-Maluquer

doi:10.3389/ffgc.2020.573346

Abstract

The increase in frequency and intensity of droughts due to climate change might threaten forests under stress levels causing dieback and mortality episodes. Thus, deciphering how tree species from within a region respond to drought along environmental gradients should help us to understand forest vulnerability to climate change. To enlighten contrasting drought responses of dominant tree species, we reconstructed vegetation activity using Normalized Difference Vegetation Index (NDVI) and radial growth using tree-ring width series. We studied six tree species, three angiosperms (Fagus sylvatica,Quercus humilis, andQuercus ilex) and three gymnosperms (Pinus sylvestris,Pinus nigra, andPinus halepensis), inhabiting a Mediterranean region in north-eastern Spain. We investigated if reduced growth resilience and increased growth synchrony after successive droughts (1986, 1989, 2005, and 2012): (i) were related to cumulative drought stress and (ii) preceded forest dieback in dry sites as compared to wet sites. In 2016, dieback affectedQ. ilexandP. sylvestrisstands in dry sites showing lower growth rates and NDVI. No dieback symptoms were observed in other species from dry (P. nigra,P. halepensis) or wet (F. sylvatica,Q. humilis,P. sylvestris) sites. Hot and dry summer conditions constrained growth and reduced NDVI. During 2005, a severe drought affected all species, but growth drops were more marked in dry places. All species were able to recover after extreme droughts, albeit angiosperms displayed lower than expected values of growth after the 2012 drought. Growth synchrony was higher in dry sites than in wet sites, and the differences were higher after the 2005 drought. This study reveals that the sensitivity of tree species to drought in species inhabiting the same region is species dependent, and it is contingent on local conditions with higher effects in dry sites than in wet sites. We describe how a cumulative impact of successive droughts increases growth synchrony and triggers the occurrence of dieback events in Mediterranean forests.

Highlights

The occurrence of extreme climate events such as droughts and heat waves among others has been identified as a key factor reducing tree growth and leading to forest dieback (Allen et al, 2015; Senf et al, 2018; Resende et al, 2020)
The results of the Linear mixed-effect models (LME) confirmed that the relationship of tree growth with the 12-month August Standardized Precipitation-Evapotranspiration index (SPEI) was stronger in dry than in wet sites (Supplementary Tables S1, S2 and Figure 2)
We combined tree-ring measures and satellite-derived vegetation indexes to show that droughts severely constrain growth and that this has an impact on forest productivity

Summary

Introduction

The occurrence of extreme climate events such as droughts and heat waves among others has been identified as a key factor reducing tree growth and leading to forest dieback (Allen et al, 2015; Senf et al, 2018; Resende et al, 2020). Leaf shedding and canopy dieback often happen during and after drought occurrence (Camarero et al, 2015), and methods allowing the retrospective quantification of droughts impact on long-term growth and vigor are required In this sense, tree rings are natural archives on how trees responded to past droughts (Fritts, 1976; Ogle et al, 2000). Combining tree ring growth with estimates of primary productivity and vegetation activity (e.g., Correa-Díaz et al, 2019; Vicente-Serrano et al, 2019) enhances our ability to upscale growth patterns from individuals or stands to regional scales (Babst et al, 2018; Gazol et al, 2018; Kannenberg et al, 2019) This information, combined with high-resolution climate data, improves our understanding on how forests respond to drought at different spatial scales and allow us to understand drought-driven forest mortality (Hartmann et al, 2018)

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Discussion

Conclusion