Does restoring tropical forest using an applied nucleation approach enhance habitat heterogeneity and seedling recruitment?

Background/Question/Methods

Planting trees islands (applied nucleation) is a strategy for restoring degraded habitat that simulates the natural forest recovery process and requires less resources than traditional tree plantations.  We tested the hypotheses that (1) applied nucleation would result in greater heterogeneity in light and nutrient conditions than planting trees across large areas or passive restoration (no tree planting), and (2) enhanced habitat heterogeneity would result in greater abundance and richness of naturally recruiting forest tree seedlings.  We compared canopy cover, understory vegetation cover, soil nutrients, and tree seedling (>20 cm and <1 m height) and sapling (≥1 m) recruitment in three restoration treatments at six, 5-7 yr forest restoration sites in southern Costa Rica.  Restoration treatments (50 × 50 m plots) included planting four species of trees throughout (plantation), planting the same tree species in different-sized nuclei (4 × 4, 8 × 8, 12 × 12 m; islands), and no tree planting (control).  We compared means and coefficients of variation of vegetation structure and soil variables within treatment plots using a randomized complete block ANOVA.  We compared means of seedling and sapling abundance using a similar design and calculated species rarefaction curves. 

Results/Conclusions

Canopy cover was greatest in plantations, intermediate and significantly more heterogeneous in islands, and lowest in the control.  Bare ground was highest in plantations, intermediate in islands, and lowest and most variable in controls.  Soil pH and percentage of base cations (K, Ca, and Mg) were lower in plantations than control or island treatments; in most cases the variation in soil nutrients was similar within all three treatments.  Seedling species richness and abundance were significantly lower in controls and similar in plantation and island treatments.  Saplings showed similar trends, although the differences were not significant.  Our results suggest that canopy cover, but not soil nutrient availability, are more heterogeneous in the island treatment than in the plantation.  Increased canopy heterogeneity and associated light availability did not translate into a greater number of tree species establishing, which is likely due to strong dispersal limitation in this system.  Nonetheless, the abundance and richness of tree seedlings establishing in the island treatment was similar to the plantation, despite the substantially smaller area planted.  Our results suggest that applied nucleation is a promising strategy to facilitate tropical forest recovery in degraded agricultural lands.