Causes for the unimodal pattern of biomass and productivity in alpine grasslands along a large altitudinal gradient in semi‐arid regions

Abstract

Questions How can we understand the limitations to plant growth at high altitudes? Our aim was to test the hypotheses that for alpine grasslands along a large altitudinal gradient in semi‐arid regions, plant growth is mainly limited by drought at low altitudes but by low temperature at high altitudes, resulting in a unimodal pattern of biomass and productivity associated with an optimal combination of temperature and precipitation. Such knowledge is important to understanding the response of alpine ecosystems to climate change. Location We conducted a 5‐yr livestock exclosure experiment along the south‐facing slope of the Nyaiqentanglha Mountains, central Tibetan Plateau. Methods We measured above‐ and below‐ground biomass, species richness, leaf δ13C and water potential, and related climate and soil variables across 42 fenced and unfenced quadrats near seven HOBO weather stations along the slope. The vegetation changed from alpine steppe‐meadow at 4390–4500 m to alpine meadow at 4600–5210 m. Results Total above‐ and below‐ground biomass across fenced and unfenced quadrats increased with increasing altitude up to 4950–5100 m, and then decreased above 5100 m. Altitudinal trends in leaf δ13C and water potential of dominant species also showed a unimodal pattern corresponding to that of vegetation biomass. Total above‐ and below‐ground biomass as well as sedge above‐ground biomass all showed a quadratic relationship with mean temperatures and the ratio of growing season precipitation (GSP) to ≥5 °C accumulated temperature (AccT; R2 = 0.83−0.88, P < 0.001). In general, above‐ and below‐ground biomass increased with increasing water availability when the GSP/AccT ratio was lower than the threshold level of 0.80–0.84, but decreased when the GSP/AccT ratio was higher than this threshold level. No significant relationship was found between residuals of above‐ground biomass and species richness after removing the effects of climate factors on both stand variables. Conclusions The results support our hypotheses, further suggesting a threshold of water limitation that is consistent with the model prediction over the Tibetan Plateau. Species richness per se appears to weakly affect community‐level productivity. The response of alpine grasslands to climate warming may vary with altitude because of altitudinal shifts in factors limiting plant growth.

Diversity and distribution of autotrophic microbial community along environmental gradients in grassland soils on the Tibetan Plateau

“…Elevation usually encompasses primary change in climatic conditions (temperature and precipitation) and consequently results in a corresponding change in vegetation and soil physicochemical factors (Guo et al 2013b;Nogues-Bravo et al 2008;Wang et al 2013). At this study site of the Tibetan Plateau, higher elevation exhibited higher precipitation and lower temperature, which consequently induced higher plant growth (Wang et al 2013), soil water content and nutrients (TOC and NH 4 + ), but lower soil pH (Table S1).…”

“…At this study site of the Tibetan Plateau, higher elevation exhibited higher precipitation and lower temperature, which consequently induced higher plant growth (Wang et al 2013), soil water content and nutrients (TOC and NH 4 + ), but lower soil pH (Table S1). It is reasonable to assume that the higher nutrients at higher elevations result from enhanced plant biomass (above and below ground) (Wang et al 2013). This gradient of plant and soil factors along the elevation provides an opportunity to explore the interaction of soil microbial communities and their ecological functions with environmental factors.…”

Spatial and climatic patterns of the relative abundance of poisonous vs. non-poisonous plants across the Northern Tibetan Plateau

“…The three climatic variables were GSP (GSP), ≥5°C accumulated temperatures (AccT), and habitat moisture index (HMI). We defined HMI as the ratio of GSP/AccT, which was accepted as a substitute for the habitat moisture condition in Wang et al (2013) and Wu et al (2014a). To calculate these climatic variables, daily meteorological records of temperature and precipitation, annually from May to September 2009-2012, were downloaded for the 39 stations in the Tibetan Autonomous Region from the National Meteorological Centre (NMIC) of China Meteorological Administration (CMA).…”

Clipping alters the response of biomass production to experimental warming: A case study in an alpine meadow on the Tibetan Plateau, China

“…Understanding the responses of this system to warming and clipping is crucial for predicting changes in C cycling with global climate change. However, the responses of GPP, AGB and ANPP to warming (De Boeck et al 2007;Wu et al 2011;Jiang et al 2012;Wang et al 2012Wang et al , 2013 and clipping (Wan and Luo 2003;Klein et al 2007) have been inconsistent with respect to vegetation types and initial climatic conditions. Although several warming experiments have been performed in alpine meadows on the Tibetan Plateau, the responses of plant biomass/ productivity to warming have been inconsistent (Klein et al 2007;Wang et al 2012;Fu et al 2013b).…”