Abstract
The long-term stability of alpine treeline positions and increased stem density are frequently reported by recent studies; however, whether a denser treeline forest is relevant to competitive tree–tree interactions remain unclear. Herein, we mapped and surveyed individual trees in two undisturbed Smith fir (Abies georgei var. smithii) treeline plots (with a size: 30 m × 200 m; plot NE1: 4477 m, NE2: 4451 m) near Ranwu Lake (RW) on the southeastern Tibetan Plateau. The surface pattern method and spatial point pattern analysis were used to detect the spatial distribution patterns of three size classes (seedlings, juveniles, adults) and spatial associations between the pairwise size classes. We also compared our results to the spatial patterns of the five other treeline forests (Deqin, Linzhi, Changdu, Yushu, Aba) reported from the Tibetan Plateau. Young trees dominated the two fir treeline plots. Both positive and negative spatial autocorrelations for all of the trees were detected in two study plots. Intraspecific facilitation and competition coexisted at the fir treelines in three forest regions (RW, Linzhi, Aba) characterized by a mild moist climate, whereas intraspecific facilitation dominated the other three forest regions (Changdu, Deqin, Yushu), which featured seasonal climatic stress or high disturbance pressure. Thus, increased stem density at alpine treeline can be linked to competitive interactions in relatively favorable environmental conditions. Overall, the spatial patterns of the treeline population are mainly shaped by the combination of thermal and moisture conditions and are also modulated by non-climatic variables (e.g., disturbance history and microtopography).
Highlights
The spatial patterns of trees provide valuable clues for revealing the linkages between forest dynamics and their underlying ecological processes [1]
The opposite tree–tree interactions that occur at small scales have jointly driven the structure of fir treeline populations near Ranwu Lake on the southeastern TP
Intraspecific facilitation may play a critical role in improving seedling growth and establishment, whereas intraspecific competition may lead to niche separation as the trees become mature
Summary
The spatial patterns of trees provide valuable clues for revealing the linkages between forest dynamics and their underlying ecological processes [1]. The warming-induced tree densification of the treeline ecotone has been found in Europe, North America, and Asia when the moisture conditions are not limiting [10,18,19,20,21] Such a mismatch between the two ecological processes (increasing tree recruitment vs slow upward advances of treeline position) at treeline ecotones could lead to intraspecific interactions [5,9,22,23], even though microsite facilitation from neighboring plants is generally considered to be one of the major drivers of structuring population dynamics in stressful environments such as in alpine treelines [7,24,25]. Both facilitative and competitive interactions among trees might coexist in slowly advancing treelines
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