Seedling establishment pulses in California forests: Migration versus potential for local persistence

Background/Question/Methods

Many forest species will shift their distributions as the climate continues to change. Shifts in the distribution of sessile long-lived organisms, such as trees, will likely be dependent on the ability of their offspring to establish in new locations as well as to persist in current locations. Seedlings, however, experience climate change at higher temporal and spatial resolutions than adult trees. Consequently, climate variability and temporal pulses of seedling survival – years highly suitable for regeneration in a landscape – may be key to understanding potential migration and persistence of forests. Are microclimate pulses of seedling survival consistent with an upward and poleward shift of forest species? Is the frequency of these pulses changing over time?

We undertook a three-year, large scale common garden experiment along an altitudinal and latitudinal gradient in California (USA) for several pine and oak species. We measured climate at garden locations and assessed their first-year survival. Subsequently, we modeled the spatial and temporal distribution of microclimate in two different landscapes and projected the relationship between seedling survival and microclimate for several scenarios of climate change. 

Results/Conclusions

Results from model projections estimate that future seedling survival is displaced from where conspecific adults are currently located, suggesting future range displacement for the majority of species.  In general, we observe an upward shift in seedling survival to higher elevations. Changes in the frequency and variability of projected seedling survival through time is species- and location-specific. Our results show decreasing frequency of pulses of seedling survival for species found at lower elevations; this trend is less prominent for species found at higher elevations. Our projections suggest that for the majority of species variability in pulses of regeneration will decrease through time.

Our study projects an overall regime shift in pulses of seedling survival of some dominant California forest species. Consequently, this could have a cascading effect on the ability of trees to migrate to new locations and lead to a compositional turnover in California forests.