Environmental Humidity Regulates Effects of Experimental Warming on Vegetation Index and Biomass Production in an Alpine Meadow of the Northern Tibet

Abstract

Uncertainty about responses of vegetation index, aboveground biomass (AGB) and gross primary production (GPP) limits our ability to predict how climatic warming will influence plant growth in alpine regions. A field warming experiment was conducted in an alpine meadow at a low (4313 m), mid- (4513 m) and high elevation (4693 m) in the Northern Tibet since May 2010. Growing season vapor pressure deficit (VPD), soil temperature (Ts) and air temperature (Ta) decreased with increasing elevation, while growing season precipitation, soil moisture (SM), normalized difference vegetation index (NDVI), soil adjusted vegetation index (SAVI), AGB and GPP increased with increasing elevation. The growing season Ta, Ts and VPD in 2015 was greater than that in 2014, while the growing season precipitation, SM, NDVI, SAVI, AGB and GPP in 2015 was lower than that in 2014, respectively. Compared to the mean air temperature and precipitation during the growing season in 1963–2015, it was a warmer and wetter year in 2014 and a warmer and drier year in 2015. Experimental warming increased growing season Ts, Ta,VPD, but decreased growing season SM in 2014–2015 at all the three elevations. Experimental warming only reduced growing season NDVI, SAVI, AGB and GPP at the low elevation in 2015. Growing season NDVI, SAVI, AGB and GPP increased with increasing SM and precipitation, but decreased with increasing VPD, indicating vegetation index and biomass production increased with environmental humidity. The VPD explained more variation of growing season NDVI, SAVI, AGB and GPP compared to Ts, Ta and SM at all the three elevations. Therefore, environmental humidity regulated the effect of experimental warming on vegetation index and biomass production in alpine meadows on the Tibetan Plateau.

Effect of Long-Term Experimental Warming on the Nutritional Quality of Alpine Meadows in the Northern Tibet

“…Warming had significant effects on herbaceous biomass only when the mean annual temperature was no more than -2 ℃ (Lin et al, 2010). The temperature decreased from site A to C (Fu and Shen, 2016), and the mean annual temperature in this study was greater than those in the three previous studies (Li and Liu, 2017;Li et al, 2018;Xu et al, 2018). Fifth, the nutrient content may change under experimental warming conditions when precipitation is near a certain threshold but not above/below a certain threshold (Augustine et al, 2018).…”

“…Fourth, warming may have contrasting effects on plant production and nutritional quality (Li et al, 2018). For example, warming could decrease plant production only when it was a dry growing season at site A, but not at B or C (Fu and Shen, 2016). Warming had significant effects on herbaceous biomass only when the mean annual temperature was no more than -2 ℃ (Lin et al, 2010).…”

Growing Season Precipitation Rather than Growing Season Length Predominates Maximum Normalized Difference Vegetation Index in Alpine Grasslands on the Tibetan Plateau

“…First, longer GSL caused by advanced SGS may result in earlier snowmelt [13] and potential detrimental effects of early frosts [42], which in turn can alter vegetation productivity. Second, longer GSL can increase GSP, which in turn can increase soil moisture and decrease vapor pressure deficit [44]. Vegetation productivity increases with increasing soil moisture and decreasing vapor pressure deficit [15,44].…”

“…Second, longer GSL can increase GSP, which in turn can increase soil moisture and decrease vapor pressure deficit [44]. Vegetation productivity increases with increasing soil moisture and decreasing vapor pressure deficit [15,44].…”

Greening Implication Inferred from Vegetation Dynamics Interacted with Climate Change and Human Activities over the Southeast Qinghai–Tibet Plateau

“…Under the background of global warming, the energy balance has been altered greatly, which is more evident in the YZR basin distributed with extensive glaciers and snow cover [89]. Moreover, the snow melting process in this region will produce more shallow moisture and further promote vegetation greening [90][91][92]. In addition, the rising temperature has extended the length of the growing season, promoting vegetation photosynthesis [93], which is also an important reason for the vegetation greening in the YZR.…”