What governs the boundaries between grassy and woody ecosystems? Three theoretical models may explain the presence of grassy patches in woody biomes: succession, assuming patchy disturbance and facilitation controlled by abiotic site conditions; alternative stable states, assuming stochastic community assemblage and competitive exclusion; and herbivore mediation, assuming effective but patchy herbivores. If successional processes create meadows, grasses facilitate tree establishment and growth, and trees grow differently across abiotic gradients. If meadow and forest are alternative stable states, grasses competitively exclude trees, and tree growth is unrelated to abiotic conditions. If herbivores mediate meadows, herbivory reduces tree growth and is patchily distributed.
To test these hypotheses, I planted 200 Pseudotsuga menziesiiseedlings across 10 meadows in the Pacific Northwest—a very productive temperate rainforest with small, dispersed meadows—for one year. I manipulated grass-tree interactions by removing competitors, and herbivory by excluding browse, fully crossing these treatments. I manipulated site conditions by planting in five persistent meadows on Siuslaw National Forest and five non-persistent meadows on the Grand Ronde Reservation. I stratigraphically measured pH, conductivity, and C:N in meadow and forest at each site. I also observationally recorded woody recruitment and browse along transects from meadow into forest at each site.
Results/Conclusions
Trees survived more often and grew taller in non-persistent meadows, but gained more new woody biomass when protected from herbivory. Seedlings exposed to browse in competition removals were heavily browsed. Persistent meadow soils had higher conductivity, and often lower pH, than adjacent forest soils. Woody plant recruitment was highest at the meadow edge and at non-persistent meadows. Observed browse did not significantly alter woody establishment.
Competition with herbs does not affect tree survival or growth, suggesting that meadows are edaphic climax communities instead of an alternative stable state in this system. Trees respond to differences in abiotic conditions, and may acidify soil with litter. Persistent meadows may have coarser soil than adjacent forest, resulting in water stress limiting tree establishment. Site conditions appear to control successional interactions within and among trophic levels.