Timing of Drought Triggers Distinct Growth Responses in Holm Oak: Implications to Predict Warming-Induced Forest Defoliation and Growth Decline

J Camarero,Gabriel Sangüesa-Barreda,Magí Franquesa

doi:10.3390/f6051576

Abstract

Droughts negatively impact forests by reducing growth and increasing defoliation leading to forest dieback as the climate becomes warmer and drier. However, the timing and severity of droughts determine how differently or intensively water shortage affects primary (shoot and leaf formation) and secondary growth (stem radial growth based on tree-ring widths). We compare the impact of two severe droughts (2005, 2012), showing different climatic characteristics on the growth responses of three Mediterranean holm oak stands in northeastern Spain. We also quantify climate trends and drought severity. Then, we use remote sensing data to infer how those droughts impacted forest productivity. Both droughts were characterized by warm and dry spring conditions leading to reduced budburst, low shoot production, asynchrony in primary growth and decreased productivity and scarce radial growth, particularly in 2005. However, defoliation peaked in 2012 when radial growth showed minimum values and early spring and late summer temperatures reached maximum values. We discuss how uncoupled and resilient are the responses of primary and secondary growth to drought. Finally, these findings are used to gain insight into the drought-related drivers of defoliation in Spanish holm oak forests.

Highlights

Abrupt climatic events such as droughts affect forests and other terrestrial ecosystems [1]
Both drought during the growing season and masting can lead to a loss in forest productivity by inducing a massive loss of old leaves and shoot abscission which can be regarded as self-pruning processes
We report diverse growth responses to two different droughts (2005, 2012) in three holm oak stands

Summary

Introduction

Abrupt climatic events such as droughts affect forests and other terrestrial ecosystems [1]. Forest responses to droughts are manifold since severe water shortage diminishes leaf carbon fixation through inducing stomatal closure [4,5], triggers growth decline, and forest dieback [6,7,8]. Droughts are complex climatic phenomena [9] They involve the stress due to water scarcity caused by low precipitation, and entail high atmospheric vapor pressure deficits, low soil water-holding capacity, and sometimes heat stress causing the degradation of plant membranes and radiation stress leading to photoinhibition [10]. Drought stress is expected to be more intense when soil water reserves are minimum, evaporative demand is maximum, and both transpiration rates and leaf area peak, i.e., during summer in most Northern Hemisphere forests

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