Spatial Structure and Associations in a Pinus canariensis Population at the Treeline, Pico del Teide, Tenerife, Canary Islands

Abstract

Variation in spatial pattern and its consequences for intraspecific associations, regeneration, and allometric relationships in a population of Pinus canariensis was studied along an altitudinal gradient at the treeline on the west-northwest slope of Pico del Teide (Mount Teide) in Tenerife, Canary Islands. We sampled 11 plots (20 by 20 m), situated 30 m a.s.l. apart along a 4380 m transect which ran from the treeline at (2020 m a.s.l.) to the forest interior (1720 m a.s.l.). In each plot, we recorded height, diameter at breast height (DBH) and location of all tree stems (>130 cm in height), number of seedlings and saplings, and percent cover of vegetation layers. Stand basal area and maximum tree height decreased with altitude. The spatial distribution of P. canariensis was characterized by increasing regularity with tree size irrespective of altitude. The decrease in intensity of clumping with tree-size and the uniform pattern in older stages indicated intraspecific resource competition. The tree response to increasing altitude was largely associated with restricted height growth. At the treeline, the trees similar in diameter to those of lower-altitude sites were substantially shorter. The seedling and sapling density decreased significantly with increased altitude and increased in stands with increased basal area and canopy cover. Tree density along the altitudinal transect was similar, but trees at high-altitudes were shorter with smaller crowns, thus creating less favorable conditions for seedling and sapling survival. The positive or facilitating relationship between canopy trees and seedlings and saplings suggests that environmental factors (here stressful conditions) perform a key role in seedling establishment. Our results suggest that competitive relationships among canopy trees play a key role in generating stand structure.