Diurnal and seasonal changes in stem radius increment and sap flow density indicate different responses of two co-existing oak species to environmental stress

摘要

Using continuous monitoring of stem radius combinedudwith sap flow measurements weudassessed the effects of environmental conditions onudtree radial growth and water status of two co-udexisting oak species (udQuercus petraeaudandudQuercus cerrisud) at high resolution time in growingudseasons of 2008 and 2009. The forest (95–100 yr) isudsituated in a xeric site in the transition zoneudbetween forested and forest-steppe regions in northud-eastern Hungary, Bükk mountains (47udoud90’N,ud20udoud46’E, elevation 320–340 m a.s.l.). Weather conditioudns in the growing season of 2008 (totaludrainfall 354 mm, mean daily temperature 17.0udoudC) was less extreme than in 2009 (total rainfallud299 mm, temperature 17.9udoudC). Rainfall strongly determined the course of radiudal growth incrementudin trees. Radial growth of trees was limited in 200ud9 due to the drought in spring. The maximumudradial increment of both species was achieved threeudweeks earlier (4udthudweek of June) than in 2008ud(4udthudweek of July). We used dendrometer monitoring dataudfor estimation of stem (tree) wateruddeficit (ud∆udW) by measuring water-related changes in stem radiuuds (Zweifel et al. 2005). Theudmagnitude of tree water deficit variation (ud∆udW) was always smaller inudQ. cerrisudthan inudQ. petraea.udIn contrast,udQuercus cerrisudalways exhibited larger daytime averages and maximaudof sap flowuddensity. In August of 2009 when drought became seveudre there were larger increases in tree wateruddeficit (ud∆udW) (50–55 %) in both species compared to July as itudcould be expected from the extentudof decreases in sap flow density (24–28%). Our dataudsuggested that due to the low SWC theudtranspiration was supported mainly from the inner wudater storage of trees during prolonged droughtudwhich resulted in high stem water deficit (ud∆udW).