Reforestation is one of our most promising natural climate solutions, and one that addresses the looming biodiversity crisis. Natural regeneration is often an optimal strategy to promote the reestablishment of native species. Yet, in many degraded landscapes, tree planting can catalyze forest community reassembly where natural regeneration is slow. However, tree survival rates vary remarkably across projects. Building a trait-based framework for tree survival could streamline species selection in a way that generalizes across ecosystems, thereby increasing the effectiveness of the global restoration movement.
We investigated how traits mediated seedling survival in a tropical dry forest restoration, and how traits were coordinated across plant structures. We examined growth and survival of 14 tree species for two years and measured six belowground traits and 22 aboveground traits, including a suite of aboveground ecophysiological characteristics. We tested the relative importance of above- vs. belowground traits and examined how interactions between trait combinations mediated seedling survival rates. Ultimately, by taking a broad scope, we aimed to identify a clear hierarchy of traits that should be considered to promote the survival of planted seedlings within tropical dry forest restoration.
Species-level survival ranged widely from 7.8-90.1%. Relative growth rate was the strongest single predictor of tree species survival rate, indicating that species with faster initial growth rates typically had higher survival. However, the principal component axes which captured the greatest amount of variation in either above- or belowground traits were poor predictors of survival on their own. The model that best explained variation in survival including growth rate, belowground traits, and their interaction (R2adj = 0.73). A strong interaction between belowground traits and growth rate indicated that selecting species with fast growth rates can promote establishment, but this effect was most apparent for species that invest in thicker fine roots and deep root structures. In general, belowground root traits were most predictive of seedling survival. However, one ecophysiological trait emerged as an important predictor of restoration outcomes, as seedlings with higher water use efficiency (measured using foliar δ13C) were more likely to survive.
Overall, results emphasize the prominent role of belowground traits in determining early restoration outcomes, and highlight minimal above- and belowground trait coordination, providing a path forward for tropical dry forest restoration efforts. As such, our analysis highlights the importance of considering belowground root characteristics, relative to aboveground stem and leaf traits, when predicting early restoration outcomes within these seasonally dry tropical ecosystems.
Restoration, tropical dry forest, functional traits, ecophysiology, growth rates