Climate Differently Impacts the Growth of Coexisting Trees and Shrubs under Semi-Arid Mediterranean Conditions

Jesús Julio Camarero,Antonio Gazol,Michele Colangelo,Cristina Valeriano,Raúl Sánchez-Salguero

doi:10.3390/f12030381

Jesús Julio Camarero, Antonio Gazol + Show 3 more

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https://doi.org/10.3390/f12030381

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Abstract

Background and Objectives—Coexisting tree and shrub species will have to withstand more arid conditions as temperatures keep rising in the Mediterranean Basin. However, we still lack reliable assessments on how climate and drought affect the radial growth of tree and shrub species at intra- and interannual time scales under semi-arid Mediterranean conditions. Materials and Methods—We investigated the growth responses to climate of four co-occurring gymnosperms inhabiting semi-arid Mediterranean sites in northeastern Spain: two tree species (Aleppo pine, Pinus halepensis Mill.; Spanish juniper, Juniperus thurifera L.) and two shrubs (Phoenicean juniper, Juniperus phoenicea L.; Ephedra nebrodensis Tineo ex Guss.). First, we quantified the intra-annual radial-growth rates of the four species by periodically sampling wood samples during one growing season. Second, we quantified the climate–growth relationships at an interannual scale at two sites with different soil water availability by using dendrochronology. Third, we simulated growth responses to temperature and soil moisture using the forward, process-based Vaganov‒Shashkin (VS-Lite) growth model to disentangle the main climatic drivers of growth. Results—The growth of all species peaked in spring to early summer (May–June). The pine and junipers grew after the dry summer, i.e., they showed a bimodal growth pattern. Prior wet winter conditions leading to high soil moisture before cambium reactivation in spring enhanced the growth of P. halepensis at dry sites, whereas the growth of both junipers and Ephedra depended more on high spring–summer soil moisture. The VS-Lite model identified these different influences of soil moisture on growth in tree and shrub species. Conclusions—Our approach (i) revealed contrasting growth dynamics of co-existing tree and shrub species under semi-arid Mediterranean conditions and (ii) provided novel insights on different responses as a function of growth habits in similar drought-prone regions.

Highlights

The Mediterranean Basin is a climate warming and biodiversity hotspot, where aridification trends have been observed, which are expected to be magnified by warmer conditions during the late 21st century, negatively impacting its diverse woody flora [1].In this region, droughts during the late 20th century and early 21st century have been among the most intense of the past millennium [2]
Maximum radial growth rates were observed in May and early June (0.01–0.02 mm day−1 ; 0.008 mm day−1 in Ephedra), followed by drops in growth rates from July to September, and a second peak in September and October (0.003–0.007 mm day−1 ), which was more noticeable for the P. halepensis and the two juniper species but not for the Ephedra (Figure 2)
The analyses showed a similar impact of Standardized Precipitation Evapotranspiration Index (SPEI) on the growth rates (RWI, ring width indices) in both cases (t = 5.31, p < 0.01), indicating that the growth responses to drought were significant but did not vary significantly between shrubs and trees or between dry and very dry sites

Summary

Introduction

The Mediterranean Basin is a climate warming and biodiversity hotspot, where aridification trends have been observed, which are expected to be magnified by warmer conditions during the late 21st century, negatively impacting its diverse woody flora [1].In this region, droughts during the late 20th century and early 21st century have been among the most intense of the past millennium [2]. The long-term effects of climate and drought on the growth of these gymnosperm tree species have been identified through dendroecological and ecophysiological analyses [5,6,7,8], but we lack comparative analyses on coexisting gymnosperm trees and shrubs (see [9]) This gap of research is very relevant because in the driest regions of the Mediterranean Basin and other semi-arid regions, treeless steppe-like landscapes are dominated by shrub species, often forming shallow roots with poor access to deep soil water, which may be sensitive to a severe winter-to-spring water deficit if they show an early growth onset [8]. The VS-Lite model identified these different influences of soil moisture on growth in tree and shrub species

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