Impact of plot shape and size on the evaluation of treeline dynamics in the Tibetan Plateau

Key message

Field survey methods influence the assessment of treeline structure and inferences on reconstructed treeline dynamics.

Abstract

Numerous field studies have described the structure of alpine treeline ecotones encompassing the forest limit and treeline to infer their dynamics in response to climate warming. However, the inferred treeline dynamics may be biased due to the selection of different plot sizes and shapes. Rectangular large plots including the whole treeline ecotone, i.e., encompassing the forest limit and the treeline, and square small plots located at current treeline have been widely used. Nevertheless, little is known about how large a plot must be to capture the main features of treeline structure and dynamics. Here, we investigate this question at Smith fir treelines located in the Sygera Mountains, southeastern Tibetan Plateau. Six rectangular large treeline plots (30 × 150 m) were sampled and compared with six square small treeline plots (30 × 30 m). Six rectangular plots with lengths shorter than the treeline ecotone span (100–135 m) were also sampled and compared with the other two plot types. Dendrochronology was used to reconstruct the recruitment dynamics of treelines, which were related to summer mean minimum temperatures. Rectangular large plots better captured the main features of recent treeline dynamics such as the abundance of recruits from the 1950s onwards and the establishment of old trees. Therefore, large plots allowed reaching more robust conclusions on treeline dynamics as compared to small plots. On the other hand, smaller rectangular plots revealed similar findings to those inferred from large rectangular plots but with a much lower survey cost. We propose using smaller rectangular plot with its longest side being shorter than the ecotone span as the most reliable and practical method to characterize alpine treeline dynamics.