A spatially explicit census reveals population structure and recruitment patterns for a narrowly endemic pine, Pinus torreyana

摘要

We conducted a census of the rare pine, Pinus torreyana ssp. torreyana, to determine (a) its population size and whether it is stable, growing, or declining; (b) the spatial variation in population structure; (c) the spatial patterning of trees in different life stages; and (d) the environmental factors that are related to seedling recruitment. Trees were classified into four stage classes: adult (>160cm tall with cones); sub-adult (>160cm without cones); saplings (30-160cm); and seedlings (<30cm). Stem diameter was measured for adults and sub-adults, and height for saplings and seedlings. Stands were defined by spatial clustering of the tree map. Univariate and bivariate point pattern analyses were used to explore spatial patterns for adult and juvenile trees and identify potential stand development processes such as density dependence, dispersal limitations, and patchy recruitment. Logistic regression was used to analyze seedling establishment and survival in relation to environmental variables derived from digital maps. Earlier studies reported little or no recruitment. We mapped 5,394 individuals, and tree size had "reverse J-shaped" distribution suggestive of a recruiting population. However, population structure was variable among stands. The predominant spatial pattern detected for adult and juvenile trees was clustering at lag distances <10m. Bivariate pattern analysis did not suggest repulsion between adult and juvenile size classes. Seedlings tended to be found close to adults and on certain soil types. This suggests that the clustered patterns resulting from patchy recruitment of juveniles persist over time.