Sap flux and stem radius variations in mature Cedrus libani trees during the growing season

Aylin Güney,Reiner Zimmermann,Klaus Haas

doi:10.3906/tar-1802-115

Abstract

Analysis of continuous sap flux and stem growth measurements can help to elucidate how environmental conditions influence water status and stem growth in trees. Stem radius variations and sap flux densities of the drought-tolerant conifer Cedrus libani were measured using automatic point dendrometers and heat dissipation sensors, respectively. The study was conducted at the Elmali Cedar Research Forest at 1665 m a.s.l. From April to September 2009, stem radius variation was measured hourly for nine C. libani trees. Concurrently, environmental variables (e.g., air temperature, precipitation, relative humidity, soil water content) were monitored within the study site. From the dendrometer records, maximum daily shrinkage (MDS) of the stem radius was calculated. From June to August, a time period of 2 months, four of the nine study trees were selected for hourly measurements of sap flux density (Js). Daily courses of stem radius variation and Js were very uniform and similar for all studied trees. A daily hysteresis effect was observed, reflecting the pattern of water storage and retrieval. Means of MDS and Js varied between 0.09 and 0.15 mm and between 4.3 and 10.6 g m-2 s-1, respectively. Daily maximum Js varied between 8.0 and 33 g m-2 s-1. Correlation and regression analyses between MDS and Js and environmental variables revealed that MDS and Js were most closely related to vapor pressure deficit and photosynthetic active radiation, respectively. We conclude that C. libani is well adapted to the current environmental conditions at the study site (natural habitat) and that atmospheric conditions play a primary role in transpiration and tree growth.

Highlights

Mediterranean ecosystems are expected to face an increase in temperature, with higher frequencies of drought and a decrease in spring precipitation (IPCC, 2013), which will likely affect tree growth (Ciais et al, 2005; Boisvenue and Running, 2006). Sarris et al (2007) observed that growth of Pinus brutia Ten. individuals on Samos already showed reduced annual increments caused by a decline in precipitation
We investigated radial stem growth dynamics and sap flux of this tree species in order to improve our understanding of its ecophysiological characteristics and responses to environmental variables
Radial stem variations and sap flux densities of C. libani during the 2009 growing season Dendrometer measurements revealed a uniform seasonal growth pattern without the summer dormancy reported for Cupressus sempervirens (L.) (Senitza, 1989; Güney et al, 2017), a species that can be found in mixed stands with C

Summary

Introduction

Mediterranean ecosystems are expected to face an increase in temperature, with higher frequencies of drought and a decrease in spring precipitation (IPCC, 2013), which will likely affect tree growth (Ciais et al, 2005; Boisvenue and Running, 2006). Sarris et al (2007) observed that growth of Pinus brutia Ten. individuals on Samos already showed reduced annual increments caused by a decline in precipitation. Mediterranean ecosystems are expected to face an increase in temperature, with higher frequencies of drought and a decrease in spring precipitation (IPCC, 2013), which will likely affect tree growth (Ciais et al, 2005; Boisvenue and Running, 2006). Studies using high-resolution observations of growth dynamics and sap flux in combination with environmental parameters help to decipher trees’ responses to short-term and long-term changes in environmental conditions and how these might affect tree vitality and forest dynamics (Rossi et al, 2006; Horna et al, 2011; Michelot et al, 2012). Xylem sap flux can be measured using heat dissipation sensors (constant heating method) after Granier (1985).

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