Climatic influences on intra-annual stem radial increment of Pinus sylvestris (L.) exposed to drought.

Within a dry inner Alpine valley in the Eastern Central Alps (750 m a.s.l., Tyrol, Austria) the influence of climate variables (precipitation, air humidity, temperature) and soil water content on intra-annual dynamics of tree-ring development was determined in Scots pine (Pinus sylvestris L.) at two sites differing in soil water availability (xeric and dry-mesic site). Radial stem development was continuously followed during 2007 and 2008 by band dendrometers and repeated micro-sampling of the developing tree rings of mature trees. Daily and seasonal fluctuations of the stem radius, which reached almost half of total annual increment, primarily reflected changes in tree water status and masked radial stem growth especially during drought periods in spring. However, temporal dynamics of intra-annual radial growth determined by both methods were found to be quite similar, when onset of radial growth in dendrometer traces was defined by the occurrence of first enlarging xylem cells. Radial increments during the growing period, which lasted from early April through early August showed statistically significant relationships with precipitation (Kendall τ = 0.234, p < 0.01, and τ = 0.184, p < 0.05, at the xeric and dry-mesic site, respectively) and relative air humidity (Pearson r = 0.290, p < 0.05, and r = 0.306, p < 0.05 at the xeric and dry-mesic site, respectively). Soil water content and air temperature had no influence on radial stem increment. Culmination of radial stem growth was detected at both study plots around mid-May, prior to occurrence of more favourable climatic conditions, i.e. an increase in precipitation during summer. We suggest that the early decrease in radial growth rate is due to a high belowground demand for carbohydrates to ensure adequate resource acquisition on the drought prone substrate.