Landscape scale variability in vegetation and soil organic carbon dynamics in the Western Ghats

Abstract

Soil organic carbon (SOC) is the largest terrestrial carbon reservoir (1550 GT), holding more carbon than that by biomass and atmosphere. Despite being a key component of the global carbon cycle, large uncertainty remains in quantifying its response to climate change and in estimating how much more carbon it can sequester through active measures. One of the major sources of the uncertainty stems from the inaccurate estimation of SOC stocks and their stability. Limited understanding of the landscape-scale SOC dynamics resulting from the interaction of vegetation-climate-topography leads to uncertainties in assessing the response of the SOC reservoir to climate change. The type of vegetation that exists at a given location is an integrated response to climatic and topographical factors. Since vegetation type governs the quantity and quality of organic matter input to the soil, it plays a crucial role in controlling SOC dynamics. Limited studies address landscape-scale heterogeneity in SOC dynamics from the tropical region. To evaluate the landscape-scale heterogeneity in SOC dynamics, this study investigates (i) landscape-scale variability of vegetation distribution in the tropics and (ii) the influence of vegetation on mean residence time(MRT) as well as physical (soil aggregation) and chemical (SOC %, 13C of SOC, C/N ratio, humic and fulvic acid content) characteristics SOC. The former was done through analysing satellite-derived vegetation data while the latter involved field sampling and laboratory analysis. The slope-aspect, through its influence on soil properties, solar radiation received and microclimate, puts additional spatially heterogeneous constraints on tree growth at the landscape-scale. However, its influence on vegetation is mainly shown at mid-latitudes. This work through analysis of tree cover (TC), canopy height (CH) and vegetation type in 25 protected areas from the Western Ghats (WG), and of tree cover in 361 protected areas in tropics clearly demonstrated existence and prevalence of influence of slope-aspect on vegetation. Landscape-scale vegetation pattern was influenced by the combined effect to two modes: the increasing trend in TC from Equator- to Pole-facing slopes and from East- to West-facing slopes. The combined effect creates higher TC on slopes facing West, Pole-west, and Pole, and lower TC on slopes facing Equator, Equator-east and East. More evergreen forest was observed on the slopes facing West, North-west and North in the WG. The work further showed that the earlier reported N-S and W-E asymmetry in the vegetation is created by selectively lowering TC on the South- and East-facing slopes. Previously unrecognized role of the seasonal movement of Inter Tropical Convergence Zone (ITCZ) in creating N-S asymmetry in the vegetation was also revealed. This work proposed a novel and easy way of classifying vegetation types based on seasonal variation in NDVI (Normalized Difference Vegetation Index) values which reflects leaf phenology of different vegetation types. The NDVI-space was divided into various parts based on the NDVI values immediately after rainy season and the dry season. The 13C values of the surface soil samples collected from thus defined vegetation classes validated the vegetation classification scheme. To establish the influence of vegetation on stability and physical and chemical characteristics of SOC, several locations under different vegetation types were sampled. The analysis revealed a strong influence of vegetation type on SOC characterization. Various SOC parameters showed a systematic trend under vegetation from evergreen to grassland: (i) soil depth decreased especially toward grasslands, (ii) SOC stocks declined (128 ± 16 to 10 ± 5 t ha⁻¹), (iii) 13C increased (-26.5 ± 0.8 ‰ to -18.9 ± 1.5 ‰), (iv) mean weighted diameter of soil aggregates decreased (2.1 ± 0.07 mm to 1.3 ± 0.09 mm), (v) C/N ratio decreased (14.7 ± 2.4 to 9.2 ± 0.6 in macroaggregates), (vi) fulvic acid content decreased (0.31 % to 0.09 % in macroaggregates) (vii) ratio of humic to fulvic acid increased (1 to 4 in macroaggregates), and (viii) MRT increased (~120 to ~685 years). On a regional scale, rainfall controlled relative proportion of different vegetation types. Using these proportions and vegetation-type-dependent SOC stocks, regional SOC stocks were estimated. Regional SOC stocks increased with rainfall: High Rainfall Region (~3100 mm) had 89 ± 8 t ha⁻¹; Mid Rainfall Region (~1800 mm) had 70 ± 8 t ha⁻¹; and Low rainfall region (~600 mm) had 19 ± 7 t ha⁻¹). The SOC stocks on North- and West-facing watersheds were higher due to higher proportion of evergreen vegetation on them. The outcome of this work has implication on land carbon management. Tropical afforestation and reforestation programs, especially in hilly areas with 10 to 80% tree cover, should consider the influence of slope direction on TC establishment and adopt appropriate plantation strategies. Evergreen forest stores more carbon but it can be lost relatively quickly due to its shorter residence time. This, in the context of managing atmospheric carbon dioxide concentrations, underscored the importance of preserving the existing evergreen forest and increasing its extent wherever possible.