Tree recruitment pulses and long-term species coexistence in mixed forests of western Québec

Abstract

:In sub-boreal forests of northeastern North America, late-successional stands often contain a fine-scale mixture of several coniferous and hardwood tree species, including an important proportion of relatively shade-intolerant species such as white birch (Betula papyrifera) and yellow birch (B. alleghaniensis). In order to better understand the recruitment of these species, we determined the age at 1 m height of each tree in sample plots located in three stand types representative of different site conditions in a small forest mosaic undisturbed by humans. Three recruitment pulses occurred synchronously in all stand types during the periods 1835-1860, 1910-1930, and 1970-1990, the last two being related to known spruce budworm (Choristoneura fumiferana) outbreaks. These pulsed recruitment episodes were identified for birch species, but also for more shade-tolerant species such as balsam fir (Abies balsamifera) and to a lesser degree white cedar (Thuja occidentalis). White cedar was the only species for which there was some recruitment during the 1940-1960 period, but this was restricted to only one stand type located on mesic sites. These results indicate that when examined over relatively long periods of time, resource availability after canopy disturbances is an important factor for the recruitment above 1 m height for most tree species. In sub-boreal forests, periodic and synchronous mortality of balsam fir can favour the emergence of multi-species, multi-cohort stands, especially where soil conditions are adequate for the recruitment of other tree species.