Abstract
Tree/shrub encroachment into drylands is a geographically widespread vegetation change that often modifies soil organic carbon (SOC) storage and dynamics, and represents an important yet uncertain aspect of the global carbon (C) cycle. We quantified spatial patterns of soil δ13C to 1.2 m depth in a subtropical savanna to evaluate the magnitude and timing of woody encroachment, and its impacts on SOC dynamics. Woody encroachment dramatically altered soil δ13C spatial patterns throughout the profile; values were lowest in the interiors of woody patches, increased towards the peripheries of those patches, and reached highest values in the surrounding grasslands. Soil δ13C and 14C revealed this landscape was once dominated by C4 grasses. However, a rapid vegetation change occurred during the past 100–200 years, characterized by (1) the formation and expansion of woody patches across this landscape, and (2) increased C3 forb abundance within remnant grasslands. Tree/shrub encroachment has substantially increased SOC and the proportion of new SOC derived from C3 plants in the SOC pool. These findings support the emerging perspective that vegetation in many dryland ecosystems is undergoing dramatic and rapid increases in SOC storage, with implications for the C cycle at regional and global scales.
Highlights
Arid and semiarid regions cover approximately 40% of the Earth’s land surface [1], and support approximately 20% of the human population [2]
Considerable effort has been invested to understand the ecological causes and consequences of globally widespread woody plant encroachment into grass-dominated ecosystems, there is growing recognition that we need to broaden our perspectives and approach woody encroachment as a social-ecological phenomenon in order to enhance our ability to adapt to, prevent, reverse, or otherwise manage this ongoing land cover change [92,93]. Vegetation dynamics across this landscape, especially the encroachment of C3 woody plants into the remnant grassland matrix, have created a heterogeneous landscape structure that is reflected in the spatial variation of soil δ13 C throughout the soil profile
Results from this subtropical savanna, which may be analogous to other dryland ecosystems in southwestern U.S, Africa, South America, and Australia, showed that vegetation across this landscape is experiencing dramatic changes characterized by a significant increase in abundance of both C3 woody plants and forbs in a system that was once dominated primarily by C4 grasses
Summary
Arid and semiarid regions (drylands) cover approximately 40% of the Earth’s land surface [1], and support approximately 20% of the human population [2]. The increase in woody plant abundance in deserts, grasslands, savannas, and other dryland ecosystems around the world is among the most significant ecological changes occurring globally [6,7] This vegetation change is likely a response to multiple local and global forcing factors, including the intensification of livestock grazing, reduced fire frequency, elevated atmospheric CO2 concentration, and changes in the climate system [7,8,9,10,11,12,13,14,15], and has dramatically altered the dryland C cycle across multiple spatial scales [16,17,18,19]. Up to 330 million hectares in dryland ecosystems are currently undergoing woody encroachment in the USA alone [17,20], and this conversion appears
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