Generation of spatial patterns in boreal forest landscapes

Boreal forests are composed of a few plant species with contrasting traits with respect to ecosystem functioning and spatial patterning. Early success ional deciduous species, such as birch and aspen, disperse seeds widely, do not tolerate low light and nitrogen availabilities, have rapidly decaying litter, and are highly preferred by herbivores. These later succeed to coni fers, such as spruce and fir, which disperse seeds locally, tolerate low li ght levels and low nitrogen availability, have litter that decays slowly, a nd are unpalatable to most mammalian herbivores. Although there are also ea rly successional conifers, such as jack pine and Scots pine, the aspen-birc h-spruce-fir successional sequence is the most common over much of North Am erica, and (without fir) in Fennoscandia and Siberia. The course of success ion in these forests is controlled partly by seed dispersal and selective f oraging by mammalian herbivores. Both of these processes are spatially dyna mic, but little is known about how their spatial dynamics may affect ecosys tem processes, such as nitrogen cycling or productivity. We present spatial ly explicit models that demonstrate the following: (a) Spatially explicit s eed dispersal results in more clumped distribution of tree species and pers istence of greater paper birch biomass than uniform seed rain across the la ndscape. Such results are consistent with current spatially explicit popula tion models of dispersal and coexistence. (b) With localized seed dispersal , the concentrations of available soil nitrogen are distributed in larger p atches with sharp transitions from low to high nitrogen availability near p atch edges, In contrast, with a uniform seed rain, the distribution of soil nitrogen availability was more uniform and "hotspots" were more localized. Thus, the spatial pattern of an ecosystem process (nitrogen cycling) is de termined by seed dispersal and competition for light among competing popula tions. (c) A dispersing herbivore, such as moose, that selectively forages on early successional deciduous species with high quality litter, such as a spen or birch, and discriminates against late successional conifers, such a s spruce or fir, imposes higher-order repeated patterns of plant species an d biomass distribution on the landscape. Thus, seed dispersal and herbivore foraging correlate properties in adjacent patches but in different ways, a nd different spatial patterns emerge. Other processes, such as insect outbr eaks, fire, and water flow, also may correlate properties between adjacent patches and result in additional patterns.