Patterns and Drivers of Tree Mortality in Iberian Forests: Climatic Effects Are Modified by Competition

Paloma Ruiz-Benito,Miguel A Zavala,David A Coomes,Lorena Gómez-Aparicio,Emily R Lines,Andrew Hector

doi:10.1371/journal.pone.0056843

Abstract

Tree mortality is a key process underlying forest dynamics and community assembly. Understanding how tree mortality is driven by simultaneous drivers is needed to evaluate potential effects of climate change on forest composition. Using repeat-measure information from c. 400,000 trees from the Spanish Forest Inventory, we quantified the relative importance of tree size, competition, climate and edaphic conditions on tree mortality of 11 species, and explored the combined effect of climate and competition. Tree mortality was affected by all of these multiple drivers, especially tree size and asymmetric competition, and strong interactions between climate and competition were found. All species showed L-shaped mortality patterns (i.e. showed decreasing mortality with tree size), but pines were more sensitive to asymmetric competition than broadleaved species. Among climatic variables, the negative effect of temperature on tree mortality was much larger than the effect of precipitation. Moreover, the effect of climate (mean annual temperature and annual precipitation) on tree mortality was aggravated at high competition levels for all species, but especially for broadleaved species. The significant interaction between climate and competition on tree mortality indicated that global change in Mediterranean regions, causing hotter and drier conditions and denser stands, could lead to profound effects on forest structure and composition. Therefore, to evaluate the potential effects of climatic change on tree mortality, forest structure must be considered, since two systems of similar composition but different structure could radically differ in their response to climatic conditions.

Highlights

Tree mortality driven by an inability to tolerate harsh environmental conditions is critical for structuring forest communities [1,2] acting alongside disturbance events such as fires, logging or extreme climatic events to drive regional patterns of species abundance and forest size structure [3,4]
The two exceptions to this rule were P. uncinata, for which the mortality rate was virtually invariant of tree size, and P. nigra, for which mortality continued to decrease with size without leveling off (Fig. 1a)
Tree mortality generally increased with the basal area of larger trees, with this increase being strong for pine species and Q. pyrenaica (Fig. 1b and Table S5)

Summary

Introduction

Tree mortality driven by an inability to tolerate harsh environmental conditions is critical for structuring forest communities [1,2] acting alongside disturbance events such as fires, logging or extreme climatic events to drive regional patterns of species abundance and forest size structure [3,4]. The ability of a tree to withstand harsh conditions may be strongly modulated by competitive interactions, because when light, water and soil nutrients are preempted by neighbouring trees, it limits the amounts available for effective metabolism [5,6,7]. A better understanding of the interaction between climatic-driven factors and competition on tree mortality patterns is critically needed to understand likely future successional trajectories [19], in Mediterranean water-limited forests where climatic change effects could be severe [20,21]

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