N : P stoichiometry and habitat effects on Mediterranean savanna seasonal root dynamics

Abstract

Mediterranean grasslands are highly seasonal and co-limited by water and nutrients. In such systems, little is known about root dynamics which may depend on individual plant properties and environment as well as seasonal water shortages and site fertility. Patterns of root biomass and activity are affected by the presence of scattered trees, grazing, site management, and chronic nitrogen deposition, all of which can affect nutrient ratios and potentially cause development of nitrogen : phosphorus (N : P) imbalances in ecosystem stoichiometry. In this study we combined observations from minirhizotrons with root measurements from direct soil cores and ingrowth cores, along with measures of above-ground biomass to investigate seasonal root dynamics and root : shoot ratios in a Mediterranean tree–grass “savanna”. We investigated responses to soil fertility, using nutrient manipulation (N∕NP addition) and spatial microhabitat treatments between open-pasture and microhabitats under the tree canopy. Root dynamics over time were also compared with indices of above-ground growth drawn from proximal remote sensing. Results show distinct differences in root dynamics and biomass between treatments and microhabitats. Root biomass was higher with N additions, but did not differ from the control with NP additions in early spring. By the end of the growing season root biomass had increased with NP in open pastures but not higher than N added alone. In contrast, root length density (RLD) in pastures responded stronger to the NP than N-only addition, while beneath trees root biomass tended to be higher with only N. Even though root biomass increased, the root : shoot ratio decreased under nutrient treatments. Timing of root and shoot growth was reasonably well paired, although in autumn root growth appeared to be substantially slower than “regreening” of the system. We interpret these differences as a shift in community structure and/or root traits under changing stoichiometry induced by the fertilization. We also consider seasonal (phenology) differences in the strength and direction of effects observed.