Single-Tree Influence on Understorey Vegetation in a Norwegian Boreal Spruce Forest

A model for tree influence on understorey vegetation in boreal spruce forests is developed according to the principles of ecological field theory. Single-tree influence at a point in space was modelled as the product of two factors: (1) the size (diameter at breast height) of the tree relative to that of the largest tree encountered in the area; a parameter specifies the weight given to large trees, and (2) the point's distance from the centre of the stem (one parameter specifies the zone of influence of a tree in crown radius units, another parameter specifies the relative weight given to closeness to, versus distance from, the tree stem). The total tree influence at the point is calculated from single-tree influence by a multiplicative model. Optimal choice of model parameters was found by maximizing the eigenvalue of a constrained ordination (CO) axis, obtained by use of the tree influence index as the only constraining variable. Two CO methods, Redundancy Analysis (RDA) and Canonical Correspondence Analysis (CCA), were applied to one vegetation data set with two subsets from a boreal spruce forest in SE Norway. The eigenvalue of CO axes constrained by the optimized tree influence index differed between the two CO methods, amounting to 9-10% of total inertia with RDA and 3% with CCA. The higher eigenvalue-to-total-inertia ratio with RDA was interpreted as due mainly to the low species turnover (low β-diversity) along the tree influence gradient, as demonstrated by the monotonic responses of a majority of species to tree influence. The linear species response model in RDA was therefore more appropriate in this case than the unimodal model of CCA. Vascular plants and cryptogamic species differed with respect to optimal parameters in the tree influence model. These differences were in accordance with throughfall precipitation and time in hydrated state as the most important determinants of cryptogamic species performance, and soil moisture and incoming radiation as the most important factors for vascular plants. Some applications of the tree influence model are proposed.