Long−term tree mortality patterns in the natural forest stands of Białowieża National Park in northeast Poland
Główne wzorce i prawidłowości wieloletniego procesu zamierania drzew w naturalnych drzewostanach Białowieskiego Parku Narodowego w płn.−wsch. Polsce
Abstract: Natural tree mortality is an important element of the long−term dynamics of strictly protected forest stands. This study aimed to analyse tree mortality patterns in Białowieża National Park over a 80 year period (1936−2012) using an extensive data set from five permanent sample plots (total area=15.44 ha, a sample of ca. 10,000 trees with DBH≥5 cm × 6 census intervals). Mean annual mortality rates for particular tree species for the whole study period ranged between 1−3% · y
–1. The smallest (ca. 1% · y
–1) values of mortality rates occurred for hornbeam and pine, and the largest (ca. 3%· y
–1) for aspen, birch, ash and spruce. The mortality rates varied significantly between census intervals, suggesting the occurrence of two basic types of mortality,
i.e. regular (baseline) and catastrophic mortality. While the regular mortality was clearly the most prevalent, episodes of catastrophic mortality occurred as well (for ash, elm, spruce, aspen and birch trees). For the smallest trees (5−15 cm DBH) a strong, negative correlation between mortality rates and the degree of shade tolerance was observed (the lower the tolerance, the higher the mortality rate). In general, trees representing intermediate diameter classes were distinguished by lower values of mortality rates than the smallest and largest trees. The average residence time, calculated from mortality rates, varied strongly among individual tree species (from
ca. 30 years for aspen, birch, ash and spruce to ca. 90−100 years for pine and hornbeam, for trees with DBH≥5 cm). Similarly, there were large differences between tree species in respect to the estimated length of time period needed to allow a recent population density to decline below an arbitrary threshold value of 1 ind.· ha
–1. The value of this parameter was extremely low for aspen (only 3 years). For the remaining species it varied from
ca. 60 years for birch to
ca. 600 years for hornbeam. In future studies, an attempt should be made to elucidate the impact of the mortality type (regular vs. catastrophic, or dispersed
vs. clumped) on the basic parameters of the recruitment process (composition, intensity). In addition, extending the current methodology of field work by including the most probable causes of tree mortality would help to understand better the underlying mechanisms of the long−term dynamics of natural forests.
Key words: catastrophic mortality, life history strategy, long−term study, mortality rate, permanent plot, tree population dynamics, regular mortality