Declines in populations of Salix caprea L.during forest regeneration after strong herbivore pressure

Abstract

Communities of broadleaved forest subject to strong pressure from large herbivores underwent degeneration. The relief of this pressure led to regeneration of the community, in which an important role was played by the sallow <em>Salix caprea</em> and other light-seeded pioneer species of tree (<em>Populus tremula</em>, <em>Betula pendula</em> and <em>B. pubescens</em>). Regeneration involving <em>Salix caprea</em> proceeded following the conservatorial protection of the degenerate stands in a reserve and later in Białowieża National Park. The emergence and development of the population of <em>Salix caprea</em> proceeded following the invasion of spruce, which coincided with the period of enhanced animal pressure on broadleaved forest. <em>Salix caprea</em> filled all the gaps in the tree stand arising as a result of the destruction of trees and undergrowth by herbivores (in the years 1892-1915). It also appeared en masse on old, at that time unforested, clearings and felled areas. In these places, <em>Salix caprea</em> created very abundant populations, with particular trees being in good condition, with a habit typical of forest trees and attaining considerable heights. The majority of trees were 50-60 years old at the time of death, although individuals reached 74 years of age. The process of extinction of the sallow population - observed over 19 years on permanent plots and fixed trees - proceeded very quickly, especially in the first decade of observation. It led to the almost complete disappearance of sallow for the forest communities of Białowieża National Park. The death of individual trees is preceded by impairment of their health and reduced annual increments in the 4-9 last years of life. The extinction of the population is associated with the loss of its primary phenological differentiation and with a change in the sex structure of the population from a prevalence of female trees to a near even distribution of the two sexes. The development of the populations of permanent constituents of the forest (<em>Carpinus betulus</em>, <em>Tilia cordata</em>, <em>Acer platanoides</em> and <em>Ulmus glabra</em>) under the canopy of light-seeded trees, and the non-creation of a new generation of pioneer species points to the imminent end of the process of regeneration in the forest communities of Białowieża National Park.