关键词: 空间点格局, Ripley L函数, 空间关联性, 零模型, 生境异质性

Abstract: The spatial distribution pattern and association of plant species at different developmental stages can reflect the status and growth trend of plant populations, and also can uncover the underlying ecological processes. We investigated the population of Abies georgei in a 1 hm² dynamic plot, which was the dominant species in the subalpine cold temperate coniferous forest in ShangriLa Potatso National Park. We analyzed the diameter class structure of A. georgei, and the spatial distribution pattern for each of the three different developmental stages of A. georgei population and their spatial associations using Ripley’s L function. We found that: (1) The diameter class structure of A. georgei population presented a form of pyramid. The individual number of saplings (1 cm≤DBH<5 cm), mid-sized trees (5 cm≤DBH≤15 cm), and adults (DBH≥15 cm) accounted for 47.42%, 31.55%, and 21.03% of the total abundance, respectively, indicating a growing population with a good natural regeneration status. (2) The spatial distribution of A. georgei populations differed among different developmental stages. Results from the analysis with complete spatial randomness null model showed that saplings and mid-sized trees tended to be significantly aggregated at 0-45 m and 0-35 m scales respectively, and that adults were randomly distributed at all the scales. We used the heterogeneous Poisson process null model to eliminate the effects of habitat heterogeneity and found that saplings, mid-sized trees and adults were randomly distributed at all the scales. This result indicated that habitat heterogeneity was the main reason for the aggregation of A. georgei population at different developmental stages. (3) Saplings showed positive spatial correlation with mid-sized trees at neighborhood scale. Both were negatively correlated with adults at the neighborhood scale, but such negative correlation turned to be weakened and uncorrelated with increasing spatial scale. The potential negative density-dependence effect may result in the shift of spatial distribution from spatial aggregation for saplings and mid-sized trees to randomness for adults. The aggregated spatial pattern and the resulting positive correlation between saplings and mid-sized trees may be both resulted from habitat filtering. The negative spatial correlation between adults and saplings/mid-sized trees might be caused by the potential asymmetrical competition of adults with mid-sized trees and the attack of specific pathogens and pests on saplings.

Key words: spatial point pattern, Ripley’s L function, spatial association, null model, habitat heterogeneity.