Applied nucleation as a forest restoration strategy

Background/Question/Methods

Ecologists have long recognized that forest succession in former agricultural lands proceeds by a process of “nucleation”, whereby pioneer shrubs and trees establish patchily and spread through growth and facilitation of the dispersal and establishment of other species.  Nonetheless, most forest restoration efforts plant large areas with trees in a homogeneous manner.  We established a large-scale study between 2004 and 2006 with 13, ~1-ha experimental sites spread across 100 km² in southern Costa Rica to test the applied nucleation approach to facilitating succession. The three restoration treatments at each site include planting four species of trees throughout (plantation), planting the same tree species in different-sized nuclei (4 × 4, 8 × 8, 12 × 12 m; nucleation treatment), and no tree planting (control).  Sites are surrounded by 9-89% forest cover within a 500-m radius, in order to be able to test interactions between the effect of restoration strategy and the landscape context. Over the past eight years we have monitored numerous changes in these experimental plots, including planted tree growth, seed rain and tree recruitment, overall vegetation composition, small-scale heterogeneity in soil nutrients and light availability, bird and bat communities and behavior, and a number of other variables.

Results/Conclusions

Our results provide several important insights into the applied nucleation restoration strategy.  First, both the diversity and abundance of seed rain and tree recruits in the first few years of succession are similar in plantation and nucleation treatments, and are much lower in controls.  Second, canopy openness, the primary variable that is correlated with tree seedling recruitment, is more heterogeneous in the nucleation than the plantation treatment, whereas heterogeneity of most soil nutrients was similar across treatments. Third, there seems to be a critical tree nucleus size (~100 m² in our system) below which tree nuclei do not serve to enhance seed rain or seedling establishment.  Fourth, planted tree nuclei are expanding rapidly, primarily due to growth of planted trees, but also due to recruitment around the edge of nuclei.  Fifth, bird communities are most similar to reference forests in plantation plots surrounded by high forest cover, demonstrating an interaction between restoration strategy and landscape context. Results on the early stages of succession suggest that applied nucleation is a promising and cost-effective restoration strategy that should be used more widely, but longer-term data are needed to compare the rate and direction of the successional trajectories of the different restoration strategies.