Abstract
The Tibetan Plateau hosts one of the world's highest undisturbed alpine juniper shrublines. However, little is known about the dynamics of these shrublines in response to climate warming and shrub-to-shrub interactions. Since growth of shrubline junipers is limited more by moisture availability than by low temperatures, we tested if upslope advancement of alpine juniper shrublines was constrained by warmer temperatures and related recent droughts. We also evaluated whether facilitation among neighboring shrubs, as inferred from spatial analyses, influenced shrubline dynamics. Three rectangular plots crossing the Juniperus pingii var. wilsonii shrubline were sampled at elevations from 4810 to 4917ma.s.l. near the Nam Co Lake, central Tibetan Plateau. Location of each stem and its diameter at the root collar and age were measured. We reconstructed the spatial and temporal shrubline dynamics during the past 350years using standard dendrochronological methods. Independent, long-term summer temperature reconstructions also were associated with shrub recruitment. Point-pattern analyses were used to characterize spatial patterns of different size classes of shrubs. The three shrublines showed little long-term changes despite ongoing warming; no upward shift has occurred in the past 100years. Recruitment was negatively associated with summer temperatures and drought occurrence since the 1920s. Spatial patterns were characterized by clustering at local scales and attraction between the different size classes, suggesting facilitation. We conclude that moisture availability limits the recruitment and elevational advance of junipers in this area of the Tibetan Plateau. Dynamics of alpine shrublines are more contingent on positive interactions and local environmental factors than on regional climatic variability.
Highlights
A growing body of evidence shows that global climatic warming has been altering the composition, structure, and distribution of ecosystems worldwide (IPCC, 2014).Among these structural changes, the northern or upward expansion of shrublines—the highest latitude or uppermost altitude at which shrubs occur—into arctic tundra or alpine grasslands is often regarded as fingerprint of global climate warming (Sturm et al, 2001; Post et al, 2009)
Little is known about the range shift of alpine shrublines in mid-latitude mountains subjected to predictable seasonal changes in precipitation and moisture availability, even though alpine shrublands are a major community in these treeless regions (Körner, 2003)
Where plant growth at alpine treeline or shrubline is limited by moisture availability during the early growing season (Liang et al, 2012, 2014) or the previous winter (Pellizzari et al, 2014), the treelines or shrublines could be at risk of retreat if lack of water leads to growth decline, constrains recruitment, or increases mortality rates
Summary
A growing body of evidence shows that global climatic warming has been altering the composition, structure, and distribution of ecosystems worldwide (IPCC, 2014). Among these structural changes, the northern or upward expansion of shrublines—the highest latitude or uppermost altitude at which shrubs occur—into arctic tundra or alpine grasslands is often regarded as fingerprint of global climate warming (Sturm et al, 2001; Post et al, 2009). Little is known about the range shift of alpine shrublines in mid-latitude mountains subjected to predictable seasonal changes in precipitation and moisture availability, even though alpine shrublands are a major community in these treeless regions (Körner, 2003). Where plant growth at alpine treeline or shrubline is limited by moisture availability during the early growing season (Liang et al, 2012, 2014) or the previous winter (Pellizzari et al, 2014), the treelines or shrublines could be at risk of retreat if lack of water leads to growth decline, constrains recruitment, or increases mortality rates
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