Question: Invasion of woody species into grasslands is a global phenomenon. This is also topical in semi-natural temperate grasslands that are no longer profitable for agricultural management. Trees and grasses interact through harsh root competition, but below-ground processes have been neglected in the dynamics of semi-natural grasslands. Trees are thought to have a competitive advantage in resource-rich and heterogeneous soils. We tested whether soil resource quantity and heterogeneity differ between paired temperate semi-natural grasslands and forests (former grasslands), and whether this was caused abiotically by varying soil depth or biotically by fine roots.
Location: Thin-soil calcareous alvar grasslands with over-grown parts (young Pinus sylvestris forests) in W. Estonia.
Methods: The quantity and spatial heterogeneity of soil resources (moisture and nutrients), soil depth, and root parameters (mass, length and specific length) were measured in 1-m transects of 11 samples in 26 paired grasslands and forests. The quantity and heterogeneity of soil resources were compared between vegetation types and related to soil depth and root parameters.
Results: Soil resources were lower and more heterogeneous in forests than in grasslands. The invasion of woody species was enhanced abiotically by deeper soil. Root mass was larger in the forests, but root length was longer in the grasslands. Both root mass and specific root length were more heterogeneous in the forests. Forest root length was negatively correlated with transient soil moisture patches and positively correlated with more persistent nutrient-rich patches. No such relationship was found in grasslands.
Conclusions: Abiotic soil heterogeneity (local deep-soil patches) supports woody species invasion, but the trees themselves also biotically make soils more heterogeneous, which further enhances woody species invasion. Large trees use soil resources patchily, making soils biotically poorer and more heterogeneous in resources. The dynamics of temperate semi-natural grasslands are strongly linked to below-ground ecological processes, and high soil heterogeneity can be both the cause and the outcome of woody species invasion.