Tropical forest restoration: Survivorship, growth, resilience, and ecological services
Since 1999, over 160,000 trees of native species have been planted in a regional reforestation effort on the Pacific slope of Monteverde, Costa Rica where deforestation and fragmentation have been extensive. The Fundación Conservacionista Costarricense and the Monteverde Institute have collaborated to raise tree seedlings for landowners to restore forests on their properties, as well as investigate the success of the reforestation effort. Here, we evaluate the success of this effort and present data on transplant success, survival, and growth rates of tree seedlings planted on five sites, a subset of the broader reforestation project. Three distinct life zones span the region, requiring a large set of trees (90 species from 34 families produced to date) to restore habitat with native diversity. The production of at least 8000 trees per year is necessary to meet the voluntary demand by landowners, and the reasons for this willingness to reforest do not seem to be solely related to economic benefits. Reforestation costs have been low, funded principally by donations and small grants.
We have documented survival and growth rates on site where individual seedlings have been followed for a minimum of four years. Most illuminating is the variability among species in overall survival rates that range from 0 to 89.3% after 5 years. The timing of mortality varies among species, some species do not survive transplanting while others struggle to survive >500 days after planting (Kaplan-Meier survivorship models). Growth for most species exceeds expectations with some rates >0.5m per year. Removing competition from surrounding vegetation has a significant effect on survival (p<0.01) but may not be necessary for some species to become established (survivorship >40%). Adding a one-time fertilizer application at the time of planting had a modest effect on growth, but is not necessary when the cost vs. benefit is considered. Herbivory from leaf cutter ants can be locally substantial, but plants show resilience to the initial defoliation. We expect that climate change will impact reforestation efforts in two ways. First, we are investigating carbon sequestration rates (calculations for tropical allometry pending), which may lead to funding opportunities. Second, we are investigating variability of congeneric species survival and growth in life zones at higher elevations than their historical ranges. Overall, high survivorship and growth rates for most species, even with minimal maintenance, shows promise for tropical forest restoration in regions with adequate edaphic qualities.