Weather patterns with no regional analog and altered habitats along human modified landscapes are likely to create abiotic environments that many species have not experienced in their evolutionary history. In addition, efforts predicting forest dynamics has been slow because we still lack critical information on tree species recruitment. Given these limitations, multisite demographic studies that account for the short-term responses to environmental variability and that also evaluate the variability of those responses among populations could be our best bet at generating more realistic predictions of forest dynamics.
We analyzed eight years of recruitment data, collected for two tree species at two latitudes, to quantify responses to a wide range of environmental conditions during the recruitment process (i.e., seed production, seedling establishment and seedling survival). We also ran simulations to assess recruitment trends under current climatic conditions as well as under two forecasted climatic scenarios.
Our analyses show that differences exist between recruitment stages and between populations in how trees respond to environmental cues, thus responses from one stage or site cannot be extrapolated to others. As expected, annual demographic transitions were affected by the particular conditions taking place during their onset, but the effects of similar environmental shifts differed among the life stages involved in recruitment, beneficial to some and detrimental to others. These effects also varied between populations, showing that similar changes in environmental conditions might have very different outcomes among populations. Our simulations illustrate that if we had only analyzed seed production we would have forecasted an increase in number of seeds and assumed an increase in recruitment. Focusing only on establishment or survival would have resulted on a more doomed outcome. The combined analysis of seed production and establishment would assert that recruitment rates are unlikely to change. It is only when we considered seed production, seedling establishment and seedling survival that we were able to assess a likely decrease in recruitment under the predicted conditions.