Aboveground net primary productivity in Costa Rican secondary tropical dry forest over 3 years

Background/Question/Methods

Tropical forests are a significant component of the global carbon cycle but aboveground net primary productivity (ANPP), a major carbon flux, is poorly understood in these ecosystems. Quantifying productivity in secondary tropical forest is particularly important as they are increasing in area and have a growing role in taking up and storing atmospheric carbon dioxide, thus potentially mitigating climate change. We measured litterfall for 4 years and biomass increment for 3 years in secondary tropical dry forest to estimate ANPP. We used 18 20 × 50m plots that range in stand age from 5 to 60 years and varied in soil fertility. Litterfall was measured with 4 0.5 × 0.5m litter traps along a 50m transect in each plot. Litter was collected monthly and separated into leaves, flowers, fruit, twigs, and frass. We measured biomass increment with band dendrometers on all trees above 10cm. Dendrometers were read at the start and end of each wet season. We used an allometric equation that incorporated both tree diameter and wood density to estimate tree biomass. Biomass increment and litterfall were summed to estimate total ANPP.

Results/Conclusions

Mean ANPP across sites of all ages for 2009, 2010, and 2011 was 9.2, 10.5, and 10.8 Mg/ha respectively. Inter-annual differences in litterfall were small (1.5-8.4%) compared to biomass increment (10-35%) making changes in biomass increment responsible for most of the variation in total ANPP. These annual differences are likely explained by variation in precipitation. 2009 was a dry year at 1242mm and both 2010 and 2011 were very wet years, 2819 and 2551mm respectively, a meter above the 30-year average (1575mm). Among sites ANPP ranged from 4.2 to 19.8 Mg/ha with the highest values from plots of intermediate age (30-40 years). Grouped by age and averaged across years ANPP is 9.0, 10.2, and 12.1 Mg/ha for sites with stand ages less than 15 years, 15 – 30 years, and greater than 30 years respectively. ANPP values are similar to those of other studies in tropical dry forests, but many dry forest productivity estimates are based on litterfall alone. Our finding that annual variation in ANPP is being driven by biomass increment and not litterfall underscores the need for all components of ANPP to be measured.