Diurnal and Seasonal Changes in Stem Radius Increment and Sap Flow Density Indicate Different Responses of Two Co-existing Oak Species to Drought Stress

Using continuous monitoring of stem radius combined with sap flow measurements weassessed the effects of environmental conditions on tree radial growth and water status of two coexistingoak species (Quercus petraea and Quercus cerris) at high resolution time in growingseasons of 2008 and 2009. The forest (95–100 yr) is situated in a xeric site in the transition zonebetween forested and forest-steppe regions in north-eastern Hungary, Bükk mountains (47o90’N,20o46’E, elevation 320–340 m a.s.l.). Weather conditions in the growing season of 2008 (totalrainfall 354 mm, mean daily temperature 17.0 oC) was less extreme than in 2009 (total rainfall299 mm, temperature 17.9 oC). Rainfall strongly determined the course of radial growth incrementin trees. Radial growth of trees was limited in 2009 due to the drought in spring. The maximumradial increment of both species was achieved three weeks earlier (4th week of June) than in 2008(4th week of July). We used dendrometer monitoring data for estimation of stem (tree) waterdeficit (W) by measuring water-related changes in stem radius (Zweifel et al. 2005). Themagnitude of tree water deficit variation (W) was always smaller in Q. cerris than in Q. petraea.In contrast, Quercus cerris always exhibited larger daytime averages and maxima of sap flowdensity. In August of 2009 when drought became severe there were larger increases in tree waterdeficit (W) (50–55 %) in both species compared to July as it could be expected from the extentof decreases in sap flow density (24–28%). Our data suggested that due to the low SWC thetranspiration was supported mainly from the inner water storage of trees during prolonged droughtwhich resulted in high stem water deficit (W).