Landscape and forest structural controls on wood density and aboveground biomass along a tropical elevation gradient in Costa Rica

Background/Question/Methods

This research seeks to understand how alpha diversity, wood density, and aboveground biomass (AGB) of trees varies along an elevation gradient in Braulio Carrillo National Park in Costa Rica. Utilizing forest inventory and botanical data from 20 1-ha plots ranging from <100 m to 2800 m above sea level and remote sensing data from an airborne Lidar sensor (LVIS), we quantified variations in topography and forest structure and related it to ground data. Montane forests act as perfect natural gradients on which to test various ecological theories. Regional AGB estimates are becoming increasingly important in current times of carbon trading and monetizing carbon stocks of countries. In addition, terrestrial carbon dynamics introduce most of the uncertainty with carbon uptake/ release models. Wood density is an important parameter of AGB, and it is not well understood how it is affected along elevation gradients. Database values tend to be collected from lowland areas, and might not be representative of how the same species is storing carbon at different elevations. Tree cores were collected from all survey plots ≥100m elevation, and wood density was measured at the species level. From wood density, AGB was calculated and compared to derived lidar topographical and forest structural parameters.

Results/Conclusions

In this study we report on the effectiveness of relating patterns of tree taxonomic diversity, wood density, and aboveground biomass (AGB) to topographical information and three-dimensional structure of a tropical montane forest using remote sensing observations. Topographic information includes elevation, slope, and aspect, while three-dimensional structure was assessed using LVIS waveform quartiles as derived height metrics. Preliminary results suggest that wood density has been overestimated across these sites, particularly at mid-elevations. This inaccuracy results in subsequently inflated AGB estimates within plots, and across the gradient. Through the comparison to active remote sensing imagery, our results show that there is a strong relationship between estimated AGB and topography and forest 3D-structure, likely controlled by variations in abiotic factors along the altitudinal gradient. Using spatial analysis with the aid of remote sensing data, we found distinct patterns along the environmental gradients defining species turnover and changes in carbon storage. The study provides novel approaches to use detailed spatial information from remote sensing data to study relations between diversity, AGB, topography, and forest structure.