Regime shift on the roof of the world: Alpine meadows converting to shrublands in the southern Himalayas
Highlights
► We reconstructed alpine change from 1950 to 2009 in northwest Yunnan, China. ► From 1990 to 2009, 39% of the alpine meadows converted to woody shrubs. ► Patterns of change suggest that a catastrophic regime shift is occurring. ► Shrub encroachment was driven by climate change, fire restrictions, and grazing. ► This regime shift threatens both endemic meadow biodiversity and local livelihoods.
Introduction
Worldwide, shrubs are encroaching in grasslands, affecting biodiversity, ecosystem function, and carbon storage (Knapp et al., 2008). Climate change, overgrazing, and land use all contribute to shrub encroachment (Walker and Wahren, 2006). When multiple interacting factors reach a critical threshold, a relatively small perturbation can trigger a regime shift, i.e., a sudden, and potentially irreversible, transition to an alternate state (Scheffer, 2009). To conserve grassland biodiversity, the drivers of shrub encroachment need to be identified and their critical thresholds determined. Untangling these factors, however, is challenging in coupled natural and human systems (Brock and Carpenter, 2010).
Shrub encroachment into alpine meadows represents a particular grave conservation threat (Montane et al., 2007, Xu and Wilkes, 2004). Alpine meadows often harbor many endemic plants adapted to harsh conditions, and orogenies have resulted in high levels of both paleo- and neo-endemism (Wu et al., 2007). With the largest alpine ecosystems in the world, alpine meadows in the Greater Himalayan region are of particular concern. Northwest Yunnan Province in southwest China represents a global biodiversity hotspot (Myers et al., 2000), and meadows here have higher species richness, beta diversity, and endemism than elsewhere in the Himalayas (Salick et al., 2009, Xu et al., 2009).
Historically, Himalayan meadows have been used by indigenous agro-pastoralists whose rangeland management practices sustained both local livelihoods and biodiversity (Klein et al., 2011). The highly nutritious meadows are critical to Tibetan transhumance herding, and the replacement of meadows by unpalatable shrubs threatens their livelihoods (Yi et al., 2007). Alpine meadow species can persist in the low-intensity land use systems typical for montane regions, but even subtle environmental changes may threaten them (Galop et al., 2011, Thuiller et al., 2005). For example, when climate change shifts woody vegetation to higher elevations, species of alpine mountaintop meadows may not be able to migrate to other suitable environments (Parmesan, 2006). Likewise, the abandonment of traditional land use practices can result in shrub encroachment (Baur et al., 2006, Laiolo et al., 2004). Indeed, climate change and land use change interact in alpine ecosystems making it difficult to identify the main driver(s) of shrub expansion.
Since the 1950s, numerous environmental changes affected Himalayan ecosystems. Climate warming rates here are higher than the global average, altering ecosystem productivity (Peng et al., 2010), phenology (Yu et al., 2010), and permafrost (Xu et al., 2009). Changing political and social structures have led to economic development and abandonment of traditional land use practices (Klein et al., 2011, Yeh and Gaerrang, 2010). However, despite grassland degradation throughout the Himalayas (Harris, 2010), rates of shrub encroachment have not been quantified, and its drivers are not well understood.
Our goals were (a) to measure rates of shrub expansion into alpine meadows in Northwest Yunnan and (b) to identify the likely mechanisms driving these changes. We used Landsat TM/ETM + satellite image analysis and dendrochronology to map land-cover distribution and change in the forest/shrub/meadow ecotone of the alpine zone (3800–4500 m) in 1974, 1990, and 2009. In addition, we reconstructed changes since the 1950s using interviews, livestock records, and climate data. We then tested the following hypotheses: H1 Climate change: We hypothesized that temporal patterns of shrub establishment and expansion were caused by changes in temperature and precipitation. Warming alters phenology (Sturm et al., 2005, Yu et al., 2010) and caused shrub encroachment in other alpine (Harte and Shaw, 1995, Klein et al., 2007) and arctic (Elmendorf et al., 2012a, Elmendorf et al., 2012b, Molau, 2010) environments. Snowpack variability can alter productivity of high-elevation systems (Peng et al., 2010) and cause shrub encroachment (Bjork and Molau, 2007, Wipf and Rixen, 2010). In the Himalayas, temperature, precipitation, snowpack, and phenology changed rapidly since the 1950s (Gao et al., 2012, Xu et al., 2009), and NW Yunnan represents a hotspot of change even within the Greater Himalayas (Haynes, 2011), resulting in receding glaciers and advancing tree lines (Baker and Moseley, 2007, Wong et al., 2010). H2 Burning cessation: We hypothesized that the 1988 region-wide burning ban triggered shrub establishment, and that areas where burning continued irrespectively suffered less shrub encroachment. Burning cessation has triggered shrub encroachment world-wide (Roques et al., 2001). In NW Yunnan, herders historically burned shrubs to improve meadows, but the 1988 burning ban may have fostered shrub encroachment (Sherman et al., 2008). H3 Shrub autocatalysis: We hypothesized that shrub autocatalysis, where existing shrubs facilitate further shrub expansion due to feedbacks (D’Odorico et al., 2010, Ratajczak et al., 2011, Sturm et al., 2005), was an important driver of shrub expansion. Feedbacks are critical components of regime shifts, because they push the original ecosystem towards an alternate stable state from which reversion to the original state is difficult or impossible (Scheffer, 2009). H4 Overgrazing: We hypothesized that areas of high grazing intensity had more shrub encroachment, because shrubs establish more easily when other vegetation is removed (Eldridge et al., 2011, Schlesinger et al., 1990). H5 Abandonment: We hypothesized that territories where alpine yak herding was abandoned have particularly high shrub encroachment rates because activities associated with humans (e.g., clearing of woody vegetation) may facilitate encroachment. Abandonment was the major driver of 20th century shrub encroachment in the European Alps (Galop et al., 2011, Montane et al., 2007). In our study area, economic development integrated rural areas into the regional economy of western China since the 1980s (Yeh and Gaerrang, 2010), leading to abandonment of yak herding in some villages (Yi et al., 2007).
Section snippets
Study area
Northwest Yunnan Province is at the southeast edge of the Qinghai–Tibetan Plateau (QTP), bordering Tibet and Sichuan Province (Fig. 1a), and an IUCN biodiversity hotspot (Myers et al., 2000). Three major rivers (the Yangtze, Mekong, and Salween) create steep gorges, with elevations ranging from 1800 to 6740 m. The alpine zone occurs between 4000 and 5000 m. Climate in the alpine zone is cold with near-constant rain during the monsoon season (June–September) and sporadic snowfall from October
Changes in alpine land cover
Land use and land cover in the alpine zone (Fig. 1b) were mapped with an overall accuracy of 94% and 93% in the recent and historic time period, respectively (Table 2). The overall accuracy of the shrub encroachment class was 91%, and the minimum accuracy of any class was 80%.
Changes in the proportions of grass and shrub between 1974 and 1990 were negligible, and forest cover remained largely unchanged from 1974 to 2009 (Fig. 1c). However, from 1990 to 2009, meadows shrank from 20% to 7%,
Implications of shrub encroachment for biodiversity
Alpine meadows remained unchanged from 1974 to1990 despite changes in climate and land use, but an abrupt shift occurred from 1990 to 2009 when at least 39% of all meadows were encroached by shrubs. The annual encroachment rate of 2.1% is among the highest reported in the literature (0.1–2.3% per year) for other areas of broad-scale shrub expansion (Briggs et al., 2005, Coop and Givnish, 2007, Sankey and Germino, 2008, Thompson, 2007).
Given the exceptionally rich flora with many endemics, this
Acknowledgements
This work was supported by the National Science Foundation under IGERT Grant No. DGE-0549369 and by a NASA Earth and Space Science Fellowship. We gratefully thank Fang Zhendong, J. Posner and T. Allendorf for logistical support, and two anonymous reviewers for suggestions that greatly improved the manuscript.
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Present address: U.S. Fish & Wildlife Service, National Conservation Training Center, 698 Conservation Way, Shepherdstown, WV 25443, United States.