Ungulates as drivers of tree population dynamics at module and genet levels

Forest ungulates affect woody plant species in many ways, e.g. by removing shoots and leaves, stripping bark, fraying, trampling, defecation and urination. High densities of ungulates can have a profound impact on plant populations, forest structure and ecosystem processes. We focus here on plant responses at two levels of population organisation in woody plants: modules and genets. Modules are defined as leaves and winter shoots, and genets as the original zygotes. Our main concern is with boreal and temperate deciduous forests, but examples from other systems are included where appropriate. Our aims are: (1) to describe and evaluate the effect of ungulate browsers on the number and other characteristics of plant population units (modules and genets), and (2) to identify consequences of the resulting changes in module and genet populations for woody plants, the browser and the forest ecosystems. At the module level, the ungulate browsers can easily change not only the number, but also the size and quality of modules. The browser–module system is more dynamic than we commonly believe, and there is a strong seasonal effect of browsing. At medium to high ungulate densities we can expect different food selection processes and food intake rates of ungulates compared to those found at low densities. At the genet level, browsers affect the number of genets of preferred plant species, as well as their size and other qualities. Reductions in the abundance of one plant species can lead to increases in other species, followed by cascading effects involving many different biota. If species increasing in abundance differ in important respects from those being replaced (e.g. if nitrogen-fixing or deciduous trees replace non-fixing or coniferous species, respectively) there may be dramatic long-term changes in ecosystem function. Both module and genet populations respond to ungulate activities, but over somewhat different time scales: the module responses being more short-term, with apparently more dynamic components. We need to better understand the dynamics of trees at both the module and genet levels, especially at high ungulate densities, and to monitor the responses of the woody plants as well as the ungulates. A better understanding of the interactions between population dynamics of ungulates and woody plants is also required.