Environmental drivers of forest structure and stem turnover across Venezuelan tropical forests.

Emilio Vilanova,Armando Torres-Lezama,Gregory J Ettl,Rafael Herrera,Cristabel Durán,Geertje Van Der Heijden,Gerardo Aymard,Oliver L Phillips,Lionel Hernández,Luis Gámez,Hirma Ramírez-Angulo

doi:10.1371/journal.pone.0198489

Abstract

Using data from 50 long-term permanent plots from across Venezuelan forests in northern South America, we explored large-scale patterns of stem turnover, aboveground biomass (AGB) and woody productivity (AGWP), and the relationships between them and with potential climatic drivers. We used principal component analysis coupled with generalized least squares models to analyze the relationship between climate, forest structure and stem dynamics. Two major axes associated with orthogonal temperature and moisture gradients effectively described more than 90% of the environmental variability in the dataset. Average turnover was 1.91 ± 0.10% year-1 with mortality and recruitment being almost identical, and close to average rates for other mature tropical forests. Turnover rates were significantly different among regions (p < 0.001), with the lowland forests in Western alluvial plains being the most dynamic, and Guiana Shield forests showing the lowest turnover rates. We found a weak positive relationship between AGB and AGWP, with Guiana Shield forests having the highest values for both variables (204.8 ± 14.3 Mg C ha-1 and 3.27 ± 0.27 Mg C ha-1 year-1 respectively), but AGB was much more strongly and negatively related to stem turnover. Our data suggest that moisture is a key driver of turnover, with longer dry seasons favoring greater rates of tree turnover and thus lower biomass, having important implications in the context of climate change, given the increases in drought frequency in many tropical forests. Regional variation in AGWP among Venezuelan forests strongly reflects the effects of climate, with greatest woody productivity where both precipitation and temperatures are high. Overall, forests in wet, low elevation sites and with slow turnover stored the greatest amounts of biomass. Although faster stand dynamics are closely associated with lower carbon storage, stem-level turnover rates and woody productivity did not show any correlation, indicating that stem dynamics and carbon dynamics are largely decoupled from one another.

Highlights

Tropical forests serve as habitats for more than 45,000 tree species [1], and store up to 262 Pg C or 66% of world’s terrestrial biomass [2]
Some of the highest values were found for actual evapotranspiration (AET) and annual precipitation (MAP) (Kendal’s tau τ = 0.80), and elevation and temperature (τ = - 0.78) (S2 Table and S2 Fig)
Using climate variables as predictors, we found that the best model describing above-ground wood productivity (AGWP) included AET interacting with region, followed by length of dry season, and Climatic Water Deficit (CWD)

Summary

Introduction

Tropical forests serve as habitats for more than 45,000 tree species [1], and store up to 262 Pg C or 66% of world’s terrestrial biomass [2]. Degradation and deforestation of tropical forests between 2005 and 2010 released between 0.56 and 1.69 Gt C year−1 respectively [6], a number that may account for 10–20% of global carbon emissions [7,8]. Forests in the tropics have helped mitigate climate change by sequestering large amounts of carbon. Between 1980 and 2010, net carbon sequestration in mature forests across countries in the Amazon region was estimated to be greater than carbon emissions from land-use change, and except for Venezuela, those from fossil fuels as well [9]

Methods

Results

Discussion

Conclusion