Diversity and structure of regenerating tropical dry forests in Costa Rica: Geographic patterns and environmental drivers

Much of the dry tropical forest biome has been converted to agricultural land uses over the past several centuries. However, in conservation areas such as those in the Guanacaste and Tempisque regions of Costa Rica, tropical dry forests are regenerating due to management practices including fire suppression. To better understand the patterns of secondary succession occurring in Costa Rican tropical dry forest, we established 60 20 × 50 m plots in mature and regenerating forests in the Sector Santa Rosa (formerly known as Parque Nacional Santa Rosa) and Palo Verde National Park. Plots were stratified into three plant communities: tropical dry oak forest (Quercus oleoides) (SROAK), Santa Rosa tropical dry forest (SRTDF), and Palo Verde tropical dry forest (PVTDF). In these plots we measured and identified and all individuals >10 cm DBH, measured but did not identify stems <10 cm but taller than 1.3 m, counted woody seedlings (<1.3 m height) and analyzed soil chemical and physical properties. Soil properties clearly differentiated vegetation communities and defined a gradient from rocky, siltier soils with low nutrient availability (SROAK soils) to clayey, nutrient-rich soils (PVTDF). Soils in the Santa Rosa dry forest had intermediate soil properties compared to the other two plant communities and had the highest tree species richness. Successional dynamics as assessed from plots of different age showed that the patterns of change in indices of stand structure, species richness and tree community composition varied with forest type (and hence soil properties). Forest structure (densities of stems in different size classes) recovered to levels found in mature forest within 4-5 decades in SRTDF and PVTDF, but increased with stand age in the oak forest. In all plots, we identified 135 species from 45 families. Simple and partial Mantel tests showed that across all plots, both stand age and soil properties explain variation in species composition, but that there is also unexplained spatial variation in tree community composition after accounting for spatial co-variation with soils. Additional analyses suggested that this is due to β-diversity, i.e. changes in the regional species pool from the northern (Santa Rosa) to more southern area (Palo Verde). Species composition in young stands was dominated by wind-dispersed species in SRTDF and PVTDF, and by animal-dispersed species in the oak forest. We conclude that the management strategy of fire control promotes passive regeneration of secondary dry forest in Costa Rica. However, if a specific forest composition is desired, more active restoration strategies may be necessary.