Abstract
Spatial variation of tropical forest tree height is a key indicator of ecological processes associated with forest growth and carbon dynamics. Here we examine the macroscale variations of tree height of humid tropical forests across three continents and quantify the climate and edaphic controls on these variations. Forest tree heights are systematically sampled across global humid tropical forests with more than 2.5 million measurements from Geoscience Laser Altimeter System (GLAS) satellite observations (2004–2008). We used top canopy height (TCH) of GLAS footprints to grid the statistical mean and variance and the 90 percentile height of samples at 0.5 degrees to capture the regional variability of average and large trees globally. We used the spatial regression method (spatial eigenvector mapping-SEVM) to evaluate the contributions of climate, soil and topography in explaining and predicting the regional variations of forest height. Statistical models suggest that climate, soil, topography, and spatial contextual information together can explain more than 60% of the observed forest height variation, while climate and soil jointly explain 30% of the height variations. Soil basics, including physical compositions such as clay and sand contents, chemical properties such as PH values and cation-exchange capacity, as well as biological variables such as the depth of organic matter, all present independent but statistically significant relationships to forest height across three continents. We found significant relations between the precipitation and tree height with shorter trees on the average in areas of higher annual water stress, and large trees occurring in areas with low stress and higher annual precipitation but with significant differences across the continents. Our results confirm other landscape and regional studies by showing that soil fertility, topography and climate may jointly control a significant variation of forest height and influencing patterns of aboveground biomass stocks and dynamics. Other factors such as biotic and disturbance regimes, not included in this study, may have less influence on regional variations but strongly mediate landscape and small-scale forest structure and dynamics.
Highlights
Humid tropical forests play an important role in the global carbon cycle by covering only 7%–10%of the Earth land surface, yet they contain about 40%–50% of the terrestrial carbon stock [1], and are Remote Sens. 2016, 8, 494; doi:10.3390/rs8060494 www.mdpi.com/journal/remotesensingRemote Sens. 2016, 8, 494 responsible for about 70% of terrestrial carbon sink [2]
The macroscale patterns of TCHm shown in the 0.5-deg gridded map (Figure 1) capture the known large-scale variations in forest structure along soil, elevation and climate gradients across three continents [1]
For other soil biological properties, we find that the soil organic carbon (OC) content has a strong negative correlation on top canopy height (TCH) metrics in America, while it shows positively correlated with TCHm in Africa, and less clear in Asia
Summary
Humid tropical forests play an important role in the global carbon cycle by covering only 7%–10%of the Earth land surface, yet they contain about 40%–50% of the terrestrial carbon stock [1], and are Remote Sens. 2016, 8, 494; doi:10.3390/rs8060494 www.mdpi.com/journal/remotesensingRemote Sens. 2016, 8, 494 responsible for about 70% of terrestrial carbon sink [2]. Of the Earth land surface, yet they contain about 40%–50% of the terrestrial carbon stock [1], and are Remote Sens. With increasing amount of high-resolution remote sensing data and availability of networks of ground plots, understanding the fine scale variations of forest structure and dynamics and their biotic and abiotic controls have improved significantly over the past decade [3,4,5,6,7,8]. Understanding macroscale variations of forest structure and their ecological and environmental controls lags behind This is because existing ground plots are sparsely distributed and not suitable for macroscale studies [9,10], and investments in design and implementation of regional scale ecological studies are inadequate. Recent or past human induced land use activities such as small and large-scale forest clearing or tree extractions may have influenced the variations of forest structure
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