The study investigates the influence of climate conditions on radial increment of oak, with special concern to the situations when analysed trees formed conspicuously wider or narrower tree-rings. The research material was collected in four locations in central Poland within natural range of pedunculate and sessile oaks. The elaborated residual chronologies were correlated with CRUTS 2.1 climate data. The analyses included thermal and pluvial conditions spanning from April of the year prior to ring formation to September of the current growth year. Special interest was paid to simple water ability index that combined both temperature and precipitation during the vegetation season. Additionally, pointer year analysis was carried out to determine situations when conspicuously smaller or larger increment was formed. Investigated chronologies cover the period of 1927–1992 (Łochów), 1845–1992 (Płońsk), 1868–1992 (Pułtusk), and 1796–1992 (Sokołów). The analysed oaks from sites in central Poland exhibit growth patterns comparable with those known form previous studies concerning that species, where influence of precipitation (higher and positive) and temperature (negative) have been observed. Extreme growth reactions expressed by negative and positive pointer years turned to present high dependence of analysed oak’ growth on water availability during vegetation season.

Since the 1990s, a decline of riparian black alder (Alnus glutinosa Gaertn.) has been observed over Europe. The fungus-like eukaryotic pathogen Phytophthora alni subsp. alni is thought to be a causal agent of this process; however, abiotic factors may also be involved. Previous studies suggest that climate conditions and, especially, depletion of groundwater level may be among the most important factors that trigger this phenomenon. We investigated the radial growth and wood vessel diameter of black alder trees of various vigour classes as well as their response to groundwater level changes to search for the link between soil water resources availability and mortality related to alder dieback. Samples were collected in the natural stand located near Sieraków village in the Kampinoski National Park, central Poland, in the area where alder dieback has been recently observed. Based on the crown defoliation level, three vigour classes (healthy, weakened, and dead trees) were distinguished. Cross sections were prepared with a sliding microtome, and Cell P image analysis software was used for the measurements. Tree-ring width (TRW) and vessel diameter (VD) were determined and correlated with the monthly values of precipitation and groundwater level. Alders of the analysed vigour classes exhibited similar patterns of TRW and VD changes over the analysis time. The narrowest tree rings were observed in weakened alders, while the largest vessels were noted in healthy trees. In the case of TRW and VD chronologies, the weakest, and hence insignificant, resemblance was found for healthy and dead trees. TRW and VD of the analysed alders were not correlated with the monthly sum of precipitation, but a negative influence of rainfall in April was observed. In turn, groundwater level had an impact on the radial growth and wood anatomical features of the analysed trees. A negative effect of the highest water table level was found for TRW of weakened and dead trees as well as for VD of healthy and weakened alders. The lowest groundwater level and the amplitude of the water table positively affected VD of the dead trees. Alder decline has a polyetiological nature, and groundwater level fluctuations are one of many factors contributing to disease development.