Seedling establishment is essential for population persistence and spread. The relative performance of different species and genotypes can strongly influence future community composition and eco-evolutionary interactions. In the forests of the Sierra Nevada, the ability of seedlings to establish following severe fire that removes nearby seed sources has long been an issue of concern. A dense canopy resulting from lack of fire could also reduce growth and survival, but moderately shady microclimates could also help seedlings to survive by reducing the rate of water loss. From 1999 to 2009, USGS researchers monitored seedlings in more than a dozen long-term plots in Sequoia-King’s Canyon and Yosemite National Parks, and some of these plots experienced low- to moderate-intensity fire during this period. We employed a Bayesian framework to model seedling and sapling survival as a function of species, size, and various environmental variables. The environmental predictors tested include temperature, precipitation, and aridity, and deviations of these measures from long-term plot averages. Predictive loss was used to compare fit across models.
Results/Conclusions
As expected, smaller seedlings experienced higher mortality, fire in the current or preceding year increased seedling mortality, and these factors interacted, with larger seedlings being more negatively impacted by fire. Of the climate variables tested, deviation from long term average annual temperature and precipitation best predicted survival. Most species, except for ponderosa pine and black oak, the two lowest-elevation species, had higher survival in warm years. Many also had lower survival in years with high precipitation in the spring of the census year, but higher survival when precipitation was high in the preceding year. This pattern could reflect the negative effects of fungal pathogens versus the positive effects of soil moisture. However, in both years black oak exhibited positive effects of precipitation, red fir exhibited to show negative effects, sequoia showed almost no effect, which may reflect the normally dry, snowy, and wet habitats of these species, respectively. Adult basal area within 10 m had a small but positive to neutral impact on survival in most species (except for red fir, incense cedar, and sugar pine) indicating a facilitative effect. We will discuss implications of these findings for forest dynamics and management under future climates.