Aggregations of dead wood after wildfire act as browsing refugia for seedlings of Populus tremula and Salix caprea.

摘要

In the European boreal forest, early successional tree species in the genera Salix and Populus are among the most favoured by browsers, often causing poor regeneration. It is frequently assumed that the fast height growth rate of these species on favourable sites can increase their chances to outgrow browsers. Spatial or temporal variation in browsing pressure could be important mechanisms for escape too, but there are few examples of this. In 1999 a large area of old-growth mixed Pinus sylvestris and Picea abies forest burned in Tyresta National Park in central Sweden. In the following year, an abundance of Populus tremula and Salix caprea seedlings regenerated naturally. Four years later we analysed seedling height and browsing history for the tallest seedlings inside and outside natural aggregations of dead wood formed by windthrow of fire-killed trees. All seedlings outside the aggregations had been browsed (on average three times) and average height was 60+or-9 cm for Populus tremula and 54+or-12 cm for S. caprea. Inside aggregations, only 33% of the tallest seedlings had any evidence of browsing, and in most cases only from one episode. Average height for Populus tremula was 153+or-41 cm and for S. caprea 167+or-27 cm. Stem base diameter was also wider for seedlings growing in dead wood aggregations than in open areas. Droppings of moose, roe deer, and hare were abundant in the area, but judging from bite marks, most browsing damage was due to the ungulates. For seedlings inside dead wood aggregations, backward regression analysis showed that both the structure of the aggregations and the spatial position of seedlings influenced seedling height: for Populus tremula seedlings, there was a significant association with increasing height of the dead wood aggregation, increasing distance to the edge of the aggregation, and decreasing distance to the nearest dead wood stem. For S. caprea seedlings there was a significant association only with the height of the aggregation. The results show that the legacy of the pre-fire tree stand can allow seedlings of palatable early successional trees to escape browsers. The effectiveness of this mechanism depends on both the quality of the pre-fire stand and on the fire behaviour generating the dead wood. A fire regime of long fire-intervals (allowing for wide-crowned trees to develop) followed by a stand-replacing fire (allowing for large aggregates of dead wood) would be optimal.