Soil water availability and evaporative demand aff

Authors: Walter Oberhuber

Publish Date: 2016/09/29

Volume: 31, Issue: 2, Pages: 467-478

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Abstract

Highresolution time series of stem radius variations SRVs record fluctuations in tree water status and temporal dynamics of radial growth The focus of this study was to evaluate the influence of tree size ie saplings vs mature trees and soil water availability on SRVs Dendrometers were installed on Pinus sylvestris at an open xeric site and on Picea abies at a drymesic site and the SRVs of cooccurring saplings and mature trees were analyzed during two consecutive years The results revealed that irrespective of tree size radial growth in P sylvestris occurred in April–May whereas the main growing period of P abies was April–June saplings and May–June mature trees Linear relationships between growthdetrended SRVs SSRVs of mature trees vs saplings and climateSSRV relationships revealed greater use of water reserves by mature P abies compared with saplings This suggests that the strikingly depressed growth of saplings compared with mature P abies was caused by source limitation ie restricted photosynthesis beneath the dense canopy In contrast a tree size effect on the annual increment SSRV and climate–SSRV relationships was less obvious in P sylvestris indicating comparable water status in mature trees and saplings under an open canopy The results of this study provided evidence that water availability and a canopy atmosphere can explain differences in temporal dynamics of radial growth and use of stem water reserves among mature trees and saplingsThe anticipated changes in climate include significant warming in future decades changes in seasonal precipitation patterns and an increase in both the frequency and intensity of severe droughts especially during spring and summer in many midlatitude regions IPCC 2013 Hence we should be aware that droughtexposed forests face an uncertain future which underline the need to gain more knowledge on tree susceptibility to drought Allen et al 2010 Williams et al 2013 McDowell and Allen 2015 Several authors also reported sizemediated climate sensitivity of forest trees Mencuccini et al 2005 Mérian and Lebourgeois 2011 Schuster and Oberhuber 2013a Bennett et al 2015 which is expected to affect forest structure composition productivity and climate feedbacks Bonan 2008 Gaining more knowledge on the influence of tree size on susceptibility to drought will enable us to more accurately model the dynamics of forest ecosystems under climate changeDrought affects tree water status and impairs radial stem growth because cell division and cell enlargement require adequate cell turgor Zweifel et al 2006 Eilmann et al 2011 Rossi et al 2013 Deslauriers et al 2016 The close coupling of radial stem growth to atmospheric conditions in conifers and broadleaved tree species Deslauriers et al 2003 Oberhuber and Gruber 2010 Köcher et al 2012 points to the importance of tree water status for intraannual growth of droughtprone conifers Zweifel et al 2005 Steppe et al 2005 Tissue water storage in trees was found to be an important factor in transiently regulating tree water relations Meinzer et al 2004 Čermak et al 2007 and Swidrak et al 2011 2014 reported that conifers exposed to drought at the start of the growing season exploit water reserves in the stem to allow temperatureinduced growth resumption to occurTime series of stem radius variations SRVs recorded by automated dendrometers are composed of two components i reversible stem shrinking and swelling caused by dynamics in water storage in elastic tissues outside the cambium and ii irreversible radial stem growth eg Deslauriers et al 2007 Drew and Downes 2009 Steppe et al 2015 The reversible component of changes in stem size ie SRV detrended for growth is the result of changing water potential gradients within the stem and is a measure of water translocation from the elastic tissues outside the cambium to the xylem and vice versa Zweifel et al 2001 Sevanto et al 2011 De Swaef et al 2015 Tracheid enlargement is regarded as the major driving force of radial stem growth ie the irreversible component in dendrometer traces Deslauriers et al 2003 Although water status may strongly affect SRV especially in slowgrowing trees Zweifel and Häsler 2001 Linares et al 2009 and Oberhuber and Gruber 2010 reported a close agreement in intraannual radial growth determined using dendrometers and xylem cell development Intraannual radial growth was also reported to peak synchronously in cooccurring early and late successional conifers Cuny et al 2012 Oberhuber et al 2014 Both components ie reversible tree water status and irreversible radial growth were found to be controlled by environmental factors on a daily timescale eg Deslauriers et al 2003 Köcher et al 2012A multitude of studies have employed dendrometers to study water relations and temporal growth dynamics for a review see De Swaef et al 2015 To date however little is known about stem radial variations and intraannual growth dynamics of saplings in comparison to mature trees of the same species cooccurring within the same stand although there is evidence that functional processes change with tree size Mencuccini et al 2005 affecting the timing duration and rate of intraannual radial growth Rossi et al 2008 Rathgeber et al 2011 Small trees also have minor ability to buffer their interactions with the environment due to lower biomass for storage Lachenbruch et al 2011 Scholz et al 2011

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