Abstract
The Yarlung Zangbo River basin is an important alley to transport moisture from the Indian Ocean to the inner Tibetan Plateau. With a wide range of elevations from 147 m to over 7000 m above sea level (a.s.l.), ecosystems respond differently to climate change at various elevations. However, the pattern of elevation-dependent vegetation change and how it responds to recent warming have been rarely reported. Here, we investigated the pattern of vegetation greening at different elevations in this river basin using SPOT normalized difference vegetation index (NDVI) data during 1999–2013, and examined its relationship with elevation-dependent changes in temperature and precipitation. The annual NDVI has increased by 8.83% from 1999 to 2013. In particular, the NDVI increased more apparently at lower elevations, but remained relatively stable or even decreased at high elevations. It seems that rising temperature has driven the basin-wide vegetation greening, but the greening rate is in contrast to the pattern of elevation-dependent warming (EDW) with more significant temperature increase at higher elevations. It appears that decreasing precipitation does not reverse the overall increasing trend in NDVI, but relatively limited precipitation (<500 mm) may constrain the NDVI increases, causing apparently stable or even decreased NDVI at higher elevations (>4000 m).
Highlights
Climate change is one of the main, but uncertain, factors that have significant impact on terrestrial ecosystems, inducing vegetation and biodiversity changes, especially in fragile environments [1,2,3,4,5]
We investigate the pattern of elevation-dependent vegetation greening (EDVG) in the Yarlung Zangbo River (YZR) basin, an important alley to transport moisture from the Indian Ocean to the inner Tibetan Plateau [32], during the past 15 years (1999–2013) and examine how elevation-dependent warming (EDW) and precipitation affect this pattern
Using normalized difference vegetation index (NDVI) as an indicator of vegetation greenness [33,34,35], we aim to address three research questions: (1) does NDVI’s temporal increasing/decreasing trend depend on elevation? (2) do elevation-dependent temperature and precipitation changes exist in the YZR basin? and (3) would EDVG be explained by EDW effectively? If not, what are the roles of precipitation or other factors on vegetation greening? This study provides useful information to assess the impact of climate change on high elevations and promote a better understanding of ecological protection and restoration on the Tibetan Plateau
Summary
Climate change is one of the main, but uncertain, factors that have significant impact on terrestrial ecosystems, inducing vegetation and biodiversity changes, especially in fragile environments [1,2,3,4,5]. Since 1901, global mean surface temperature has increased by 0.89 ̋C according to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) [6]. This warming trend is complex with large spatial heterogeneity, varying from significant temperature increase at some regions to no change or even cooling at certain regions [7,8,9]. If rising temperature acts as a major driver of vegetation greening in a mountainous environment, a pattern of elevation-dependent vegetation greening (EDVG) would be expected in response to the EDW. Due to the variability of temperature, precipitation, and other potential factors, such as permafrost and snow cover, the mechanism is still elusive on how the pattern of EDVG is associated with changes in climatic factors at various elevations [11,22]
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