Research advances in disturbance and ecological processes of the treeline ecotone

Abstract

The treeline ecotone refers to the transition from the timberline to treeless alpine vegetation. As either the upper-elevation or northern-most limit of erect trees, treeline will be sensitive to environmental change. An increasing number studies have focused on influence of climatic change on treeline dynamics. Treeline shift rate is also considered to be a fingerprint of influence of global change on terrestrial ecosystems. However, less is known about how disturbance, as an important non-climatic factor, mediates the response of treelines to climate change. Such studies can provide useful information to guide implementation of key ecological protection and rehabilitation measures in these comparatively cold biomes. Herein, we review research progress relating to impacts of different disturbances on ecological processes on the treeline ecotone. Based on the Web of Science and CNKI database, only a modest 10.8% of 1002 treeline-related publications from 1950 to 2018 focused on disturbance. Disturbances include natural and human-caused categories according to their causes. Natural disturbances mainly include volcanic eruptions, glacier fluctuations, wildfire, snow avalanche, biological activities, extreme low temperature, and strong wind. Human-caused disturbances mainly include deforestation, grazing, human-caused fire, mining, farming, medicinal plant collecting and tourism. In most cases, due to differences in intensity, frequency and time duration, different disturbances could have different effects on the ecological processes of the treeline ecotone. Under some circumstances, the impacts of natural and human-caused disturbances on treelines may not differ significantly. For example, natural wildfires and human-caused fires, as well as wildlife activities and livestock grazing, may contribute similar forcing in driving treeline dynamics. In disturbed areas, based on soil habitat regimes, treeline changes would follow two main stages, i.e., primary succession or secondary succession processes driven by multiple environmental factors and climate-driven stand changes. Observational data for the common disturbance influences (e.g. fire and grazing) were rare in recent studies, failing to distinguish relative effects of disturbances and climate change on ecological processes in the treeline ecotone. Additionally, attention should be paid to both the negative and positive effects of disturbances at the treeline ecotone. In particular, more studies are necessary to better understand positive effects of disturbances driving treeline shifts and ecological strategies of trees at disturbed treelines. To adapt to frequent disturbances, treeline trees might counteract disturbance through bark and cone characteristics, by clumped distributions, or by clonal reproduction (e.g. sprout-regeneration). Ecological strategies of trees under different disturbance regimes provide a new perspective for explaining non-linear responses of treeline shift to climate change. Humans cannot control most natural disturbances (e.g. volcanic eruption, glacier fluctuations, wildfire and snow avalanche), whereas we can control human-caused disturbances (e.g. grazing, deforestation and tourism). In this context, human-caused disturbance is a key issue for forest management policies. Further work should consider incorporating disturbance factors into treeline modelling. In most forest regions, monitoring data for common disturbance factors are very scarce. In order to have a better understanding of treeline ecological processes, it is necessary to model changes of disturbed treelines in response to disturbance regimes. Based on multiple approaches, researchers should focus on key ecological processes and perform long-term tracking surveys at the disturbed treelines. It should be noted that the Tibetan Plateau not only hosts the Northern Hemisphere’s highest natural treelines but also diverse disturbed treelines. The Tibetan Plateau could be an ideal location to investigate different contributions of disturbance and climatic change in driving treeline dynamics. In particular, it would be interesting to investigate how fire or grazing shapes treeline structure, and how interactions between disturbances and climatic change drive treeline shift on the Tibetan Plateau.