Environmental Controls on the Seasonal Variation in Gas Exchange and Water Balance in a Near-Coastal Mediterranean Pinus halepensis Forest

Andreas Matzarakis,Tala Awada,Mariangela N Fotelli,Spyridon A Paparrizos,Kalliopi Radoglou,Evangelia Korakaki

doi:10.3390/f10040313

Abstract

Aleppo pine (Pinus halepensis Mill.) is widespread in most countries of the Mediterranean area. In Greece, Aleppo pine forms natural stands of high economic and ecological importance. Understanding the species’ ecophysiological traits is important in our efforts to predict its responses to ongoing climate variability and change. Therefore, the aim of this study was to assess the seasonal dynamic in Aleppo pine gas exchange and water balance on the leaf and canopy levels in response to the intra-annual variability in the abiotic environment. Specifically, we assessed needle gas exchange, water potential and δ13C ratio, as well as tree sap flow and canopy conductance in adult trees of a mature near-coastal semi-arid Aleppo pine ecosystem, over two consecutive years differing in climatic conditions, the latter being less xerothermic. Maximum photosynthesis (Amax), stomatal conductance (gs), sap flow per unit leaf area (Ql), and canopy conductance (Gs) peaked in early spring, before the start of the summer season. During summer drought, the investigated parameters were negatively affected by the increasing potential evapotranspiration (PET) rate and vapor pressure deficit (VPD). Aleppo pine displayed a water-saving, drought avoidance (isohydric) strategy via stomatal control in response to drought. The species benefited from periods of high available soil water, during the autumn and winter months, when other environmental factors were not limiting. Then, on the leaf level, air temperature had a significant effect on Amax, while on the canopy level, VPD and net radiation affected Ql. Our study demonstrates the plasticity of adult Aleppo pine in this forest ecosystem in response to the concurrent environmental conditions. These findings are important in our efforts to predict and forecast responses of the species to projected climate variability and change in the region.

Highlights

Aleppo pine (Pinus halepensis Mill.) is widespread in most countries of the Mediterranean area [1,2].In Greece, Aleppo pine forms forests of economic importance [1], comprising 26% of the coniferous forests in the country.Aleppo pine can reach heights of 10–20 m, depending on the precipitation regime in the area, and has relatively shallow roots, usually not exceeding depths of 5 m [3]
The site is in a natural Aleppo pine (Pinus halepensis) stand with a mean tree height of 16 m, a mean diameter at breast height of 45 cm, a mean tree basal area of
The results of the present study demonstrate the potential of this dominant Mediterranean forest tree species, to overcome the adverse conditions during summer droughts and to take advantage of more favorable water regimes occurring in early spring and occasionally during autumn and winter, if other environmental parameters are not limiting, in a semi-arid ecosystem in Chalkidiki, Greece

Summary

Introduction

Aleppo pine (Pinus halepensis Mill.) is widespread in most countries of the Mediterranean area [1,2].In Greece, Aleppo pine forms forests of economic importance (e.g., wood, resin, medicinal, and honey products) [1], comprising 26% of the coniferous forests in the country.Aleppo pine can reach heights of 10–20 m, depending on the precipitation regime in the area, and has relatively shallow roots, usually not exceeding depths of 5 m [3]. The species is adapted to the xerothermic conditions (high temperatures and droughts) of the Mediterranean, due to its drought avoidance strategy of reducing stomatal conductance under water shortage [4]. This isohydric response allows Aleppo pine to limit the reduction of needle water potential and xylem cavitation, to which it is quite vulnerable [5,6]. Aleppo pine has adapted its physiological activity to the seasonally changing climatic regime in the region It actively grows during two periods of the growing season (spring and autumn) when temperatures are favorable and water is available. Extreme winter and summer temperatures and intensive summer droughts may cause growth activity to cease [8] and lead to extensive dieback and growth declines in Aleppo pine forests [9]

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