The ecological respsonses and adaptability of alpine shrubs to global warming

Shrubland is defined as a vegetation type dominated by shrubs with heights of less than 5 m. Given the adaptation of alpine shrubs to the cold habitats characterizing high altitudes or latitudes, these plants are particularly sensitive to climate warming. In recent decades, many studies have identified a declining trend in tree growth sensitivity to summer temperatures in circumarctic regions. However, it remains unclear as to how alpine shrubs respond and adapt to climate warming, and whether there would be a reduction in the temperature sensitivity of shrub growth. In this study, we mainly review findings on the responses of alpine shrubs to climate warming at different spatiotemporal scales based on different research methods, and describe the associated research trends and current problems. Long-term sequence data, based on long-term positional observations, tree-ring data, remote sensing, and aerial and recurrent photography, have revealed the changing trends and possible factors influencing shrub expansion and growth. Transect surveys mainly show variations along altitudinal gradients, and studies have focused on changes in associated ecological indices, whereas manipulation experiments have identified changes in plant phenology and leaf traits. In general, warming has been found to promote the growth of alpine shrubs, which is reflected in increases in vegetation coverage, biomass, and related indices. With respect to leaf functional traits, those such as specific leaf area have been observed to undergo convergent adaptation in stressed habitats. However, given the limitations of scale, research methodology is constrained by boundedness. In the future, we should develop more comprehensive multi-scale global network observation systems. In terms of scientific issues, comparative studies between different functional types should be further conducted to examine the universality of convergence adaptation and the environmental thresholds of alpine shrubs. Moreover, we need to focus to greater extent on transect studies along water gradients, simulation studies examining changes in snow cover, and the responses of belowground ecological processes, such as freeze–thaw cycles and microbial activities, to climate change.