Morphological and chemical responses of mountain birch leaves and shoots to winter browsing along a gradient of plant productivity

Most investigations of the morphological and chemical responses of birches (Betula pendula and B. pubescens) to natural and simulated moose browsing have been made in environments where resources for growth are not severely limited. One subspecies, the mountain birch (B. pubescens ssp. tortuosa), forms the tree-line in northwestern Europe and is common in subarctic environments such as the tree-line zone in northern Finland. We report on the responses by this birch subspecies to simulated browsing at four sites ranging from locally, very productive sites ro tree-line sites where environmental conditions were poor. After simulated brow-sing (clipping of winter twigs), trees responded by producing larger shoots with larger leaves which contained high amounts of water, K, Ca, and N compared with untreated plants. The concentrations of Na and protein-precipitating compounds were lower in leaves on clipped trees. In feeding experiments, snails (which together with large browsers are regarded as generalist feeders) tended to prefer leaf discs from clipped trees although the results were not significant overall. The level of natural invertebrate herbivory also tended to be higher in clipped trees. We conclude that the general responses of these northern birches to simulated browsing are similar to those previously described for birches at more productive, southern sites. We propose that birch responses depend on internal physiological changes that occur after apical dominance has been broken as a result of a reduction in the number of growing points (sinks) which compete for nutrients. such as nitrogen.