Soil water availability and evaporative demand affect seasonal growth dynamics and use of stored water in co-occurring saplings and mature conifers under drought

High-resolution 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 (i.e., 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 dry-mesic site, and the SRVs of co-occurring 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 growth-detrended SRVs (SSRVs) of mature trees vs. saplings and climate-SSRV 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, i.e., 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 saplings.