Atypical soil carbon distribution across a tropical steepland forest catena
Highlights
► Some tropical steeplands do not follow typical models of soil carbon distribution. ► Vegetation/topographic interactions cause higher soil carbon on ridges. ► Biogeochemical and litter quality effects are of secondary importance. ► Alterations in vegetation and topography affect soil carbon distributions and pools.
Introduction
The spatial distribution of soil organic carbon (SOC) at landscape scales is controlled by interactions of edaphic, topographic, and biological factors through time, and understanding these interactions is essential to quantifying the role of SOC in the global carbon cycle (Amundson, 2001, Janzen, 2004, Post et al., 2001, Smith and Fang, 2010). Soil catena models have been widely applied to relate soil forming processes to the spatial distribution of soil properties (Jimenez and Lal, 2006, Scatena and Lugo, 1995, Silver et al., 1994), which can then be used to map soil properties, including SOC, at larger scales. In many dry, temperate, and humid landscapes, the largest SOC pools tend to occur in topographically low areas (i.e. valleys). This pattern of accumulation has been attributed to various factors, including the chemical stabilization and burial, decreased decomposition because of low redox conditions, and higher litter inputs from vegetation and upslope contributions (Berhe et al., 2007, Gregorich et al., 1998, Jenny, 1941). However, in the tabonuco (Dacroydes excelsa) forests that are the focus of this study, SOC pools are smallest in valley positions and largest on ridges (Scatena and Lugo, 1995, Silver et al., 1994, Silver et al., 1999, Soil Survey Staff, 1995).
Previous studies have identified factors that control the spatial patterns in soil development and trees species composition in the tabonuco forest and have suggested that the primary control on soil development (and SOC contents) is landscape stability. The tabonuco tree, which typically grows on ridges, forms root grafting networks that reduce damage from hurricanes and other disturbances (Basnet et al., 1993, Lugo and Scatena, 1995, Scatena and Lugo, 1995). Additionally, “low quality” litter inputs (e.g. higher C:N ratios, higher lignin contents) from trees such as the tabonuco, may decompose more slowly than other species and thus increase SOC (Fonte and Schowalter, 2004, Myster and Schaefer, 2003, Ostertag et al., 2003, Zalamea et al., 2007). Other factors that may influence SOC spatial distribution, and that have not been explicitly explored before, include soil Fe, Al, and texture and litter quantity via changes in tree mortality. It is well known that Fe and Al are not only important components of tropical soils, but are also often correlated with SOC and provide a mechanism of protection from microbial decomposition (e.g. Kleber et al., 2005, Powers and Schlesinger, 2002). Additionally, higher tree mortality may cause less C to accumulate in woody pools, and therefore reduce the amount of recalcitrant woody inputs that would otherwise be stabilized in soils over long periods of time (e.g. 300+ years; Johnson et al., 2010).
This study investigates the relative influence of vegetation and topographic factors on SOC accumulations in a humid tropical steepland environment. Although some of these topographic patterns have been previously documented for the tabonuco forest, their relative influence on the distribution of SOC has not been determined. To better understand the controls on SOC spatial distributions, our objectives were to: 1) determine the role of topography and soil physical characteristics with SOC content, and 2) investigate secondary controls, in particular litter quality, litter quantity, and Fe, Al, and soil texture. We used the Century model to address our second objective, and discuss the caveats and advantages to this approach in future analyses.
Section snippets
Study Area
The focus of this study is the tabonuco forest, one of four major forest types found in the Luquillo Experimental Forest, Puerto Rico, and which typically occurs between 200 and 600 masl. The underlying parent material is mostly volcaniclastic that has weathered to a saprolite as thick as 20 m in some areas (Schellekens et al., 2004). Four general categories describe the typical geomorphic settings (topographic positions) found — ridge, slope, upland valley and riparian valley (Scatena, 1989) (
Spatial distribution of combinations of landscape attributes
Before relating SOC to landscape attributes, we first quantified which attribute combinations were most common in the tabonuco watersheds. The prevailing pattern was shown by histograms of 0–10 cm samplings (one for each of the 84 plots) among topographic, vegetation, and stand age classes (c.f. Scatena and Lugo, 1995) (Fig. 2). Ridges were dominated by older tabonuco stands, valleys were dominated by younger non-tabonuco species, and slopes had a variety stand ages and species. In addition to
Discussion
Geomorphically stable areas in the tabonuco forest provide the best opportunities for SOC accumulation. The level of stability is related to topographic positions, resulting in an atypical pattern of SOC distribution across soil catenas. Stand age is an important indicator of soil stability and there was a clear association in this study between older tabonuco stands on ridge soils and high SOC. This is due to the resistance of tabonuco uprooting after disturbance and resultant biophysical
Conclusions
In these watersheds, landscape stability interplays with biotic controls so that most of the carbon is held in stable upland locations where tabonuco trees dominate. Terrestrial carbon modeling of topographically complex areas such as tropical steepland watersheds will have to grapple with SOC distributions that do not follow typical catena models and the processes responsible for these patterns. Predicting future changes in SOC in the tabonuco forest should consider the interactions of
Acknowledgments
We wish to thank A. Johnson, A. Plante, Y. Pan, B. Helliker, and an anonymous reviewer for their comments that helped improve the paper. We also thank T. Niemen for assistance in gathering field data. Funding for this research was provided by the University of Pennsylvania, the NSF supported Luquillo LTER program and the NSF supported Luquillo Critical Zone Observatory. Additional logistical and infrastructural support was provided by the USDA-FS International Institute of Tropical Forestry.
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