Compounded disturbances in sub-alpine forests in western Colorado favour future dominance by quaking aspen (Populus tremuloides)

Question As the extent, magnitude and/or frequency of various forest disturbances are increasing due to climate change, it is becoming increasingly likely that forests may be affected by more than one type of disturbance in short succession. We studied how compounded disturbances and pre-fire composition influence post-fire tree regeneration. Specifically, do compounded disturbances reduce overall regeneration and favour initial dominance of species that regenerate vegetatively? Location Sub-alpine forests in northwestern Colorado. Methods The study region was affected by a severe outbreak of Dendroctonus rufipennis in the 1940s, a severe wind storm in 1997 and severe fires in 2002. Permanent plots to monitor regeneration were established in 2003 and were re-measured in 2004, 2005 and 2010. Plots were located in stands that varied in long-term disturbance history (stands that originated following fires in 1879 or 1880 vs older stands), recent disturbance history (fire only; outbreak then fire; blowdown then fire) and pre-fire forest dominance (Populus tremuloides, Pinus contorta, or Picea engelmanniiAbies lasiocarpa). Results Combined density of regeneration of all tree species was highest in stands dominated by P. tremuloides prior to the 2002 fires. In P. contorta stands that were affected only by the 2002 fires, regeneration density was higher in stands that were younger prior to the fire (those that originated in the 1880s), in which cone serotiny is more prevalent, than in older stands (those that originated >200 yr ago). However, the advantage of relatively young P. contorta stands to regenerate following fire was inhibited by compounded disturbances of wind and then fire. Similarly, following compounded disturbances the combined density of conifer seedlings of all species was lower than following only fire. In contrast, the density of P. tremuloides was not lower following compounded disturbances than following fire only, and was higher than that of other species. Conclusions Pre-fire forest composition and disturbance history influence the abundance and composition of post-fire regeneration. Compounded disturbances generally reduce the regeneration of conifers, which regenerate exclusively from seed, and appear to favour initial stand dominance by P. tremuloides, which regenerates both sexually and asexually. Such differential effects may thereby alter trajectories of post-fire regeneration. As P. tremuloides are less susceptible than conifers to fires, bark beetle outbreaks and wind disturbances, increased dominance by P. tremuloides may contribute to a negative feedback that may diminish the probability and/or severity of future disturbances and thus increase overall forest ecosystem resiliency.