Warming across the western US is expected to cause dramatic shifts in tree species distributions, with changes in composition most pronounced at ecotone boundaries in the mixed conifer forest where the current range limits of the dominant species overlap. Yet microsite conditions and precipitation can strongly limit tree recruitment, and thus responses of tree species to warming. We expect that the timing of precipitation and variation in microsite characteristics (primarily light and microclimate) -- rather than seed production and dispersal -- drive colonization and regeneration patterns and that these effects will differ by seedling life stage and species. To address these questions we established a long-term study in a mixed conifer forest on the eastern slope of the Colorado Rockies, where floristic elements of lower, mid and high elevation forest types co-occur. The site is generally free of recent disturbance (e.g. fire, beetle kill) and has sufficient populations of Psuedotusga menziesii, Pinus contorta, Pinus ponderosa, Abies lasiocarpa, Picea engelmannii, and Populus tremuloides for study. Over two growing seasons we collected seed production and dispersal, seedling establishment and growth, microclimate and substrate conditions, and herbaceous ground cover data to create species-specific and spatially-explicit models of regeneration by climate and microsite.
Results/Conclusions
Germination, survival, and mortality rates of new recruits differed among species and was related to light availability and precipitation, depending on the life stage. For example, the majority of P. engelmannii germination followed large precipitation events. Mortality patterns were highly variable between species and unexplained by microsite and climate, except for P. contorta which had the majority of its mortality after a dry period. High propagule pressure did not ensure high seedling abundance -- e.g. P. contorta had the highest number of germinants, but a net survivorship of only 2.3% -- indicating recruitment limitation was the main determinant of regeneration. Microsite effects (e.g. seedbed substrates or understory vegetation) had no significant effects on regeneration. Variation in understory light and a species’ shade tolerance played a key role in recruitment patterns, as a number of species successfully colonized a wide range of microsites but failed to survive into year 2 or 3 in low light. A. lasiocarpa is the most shade tolerant species, and 87.9% of older juveniles were A. lasiocarpa, compared to only 69.5% of first-year seedlings. These results suggest that the availability of suitable microsites and water availability will be an important factor in the future distribution of this forest.