How Herbivore Browsing Strategy Affects Whole-Plant Photosynthetic Capacity

If a browse damage index indicates that a tree has been 50% browsed by herbivores, does this mean half the leaves are entirely eaten or are all the leaves half eaten? Were the affected leaves old or young? Large or small? In sunshine or shade? Understanding what effect browsing will have on the photosynthetic capacity and the plant's survival ability clearly requires a greater understanding of browsing strategy across the canopy than can be given by a single index value. We developed stochastic models of leaf production, growth and consumption using data from kamahi (Weinmannia racemosa) trees in New Zealand which have been browsed by possums (Trichosurus vulpecula), to ascertain which of six feasible browsing strategies possums are most likely to be employing. We compared the area distribution of real fallen leaves to model output in order to select the best model, and used the model to predict the age distribution of leaves on the tree and thus infer its photosynthetic capability. The most likely browsing strategy that possums employ on kamahi trees is a preference for virgin (i.e. previously unbrowsed) leaves, consistent with the idea that browsing increases the production of chemical plant defences. More generally, our results show that herbivore browsing strategy can significantly change the whole-plant photosynthetic capability of any plant and hence its ability to survive, and therefore, herbivore damage indices should be used in conjunction with more detailed information about herbivore browsing strategy.