Sorbus aucuparia regeneration in a coarse-grained spruce forest – a landscape scale
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What is the spatial range of regeneration of the fleshy-fruited tree Sorbus aucuparia (rowan) in a coarse-grained spruce stand on a large landscape scale? Does the spatial distribution and size of stands of different ages affect the probability of rowan regeneration? What are the consequences of the dynamics of dominant coniferous tree species for the dynamics of admixture rowan?
A sub-alpine spruce forest in the Tatra Mountains, Poland.
We mapped all mature rowans in a 203-ha area and counted the rowan seedlings and saplings on a grid of evenly distributed plots. In plots, the age and diameter of trees were measured. Patches of homogenous stands were distinguished and each rowan tree and each plot was assigned to one of four stand categories: dense small-crowned stands, dense large-crowned stands, sparse large-crowned stands and sparse stands near the upper forest limit. Areas above the upper forest limit formed a separate fifth category.
The distribution of rowan trees was clumped. Most of them grew in dense spruce stands up to 135 yr old and near the upper forest limit. Substantially fewer rowan trees were in sparse spruce stands of nearly 200 yr old. Seedlings and saplings occurred at high density (mean 24.8 individuals 100 m−2) only up to 40 m from trees bearing fruits, and at much lower density at longer distances. In consequence of the clumped distribution of adult trees and the short range of seed dispersal, most of the old spruce stands were outside the range of abundant regeneration of rowan.
The presence of fine- vs coarse-grained mosaics of coniferous stands of different ages can strongly influence population processes in a rowan population on a large landscape scale. Extensive disturbances resulting in large homogenous patches of coniferous stands, the long lifespan of a single generation of spruce and spatial limitation of rowan seed dispersal seem responsible for the small contribution this broad-leaved species makes to sub-alpine forests. A high share of rowan can be expected in forests with fine-grained mosaics of stands, where small patches of young and old stands are inter-mixed, assuring delivery of seeds to stands of each category.