Counteracting gradients of light and soil nutrients in the understorey of Mediterranean oak forests
The forest canopy modifies the availability of resources (light, water, and soil nutrients) in the understorey. In this paper we analyze the relationships between woody canopy density, litter accumulation, and topsoil N and P availability in the understorey of two oak forests: one in southern Portugal and the other in southern Spain. Both forests persist on low-nutrient soils, particularly poor in P. We hypothesize that direct and indirect effects of the canopy overstorey cause opposite gradients in the availability of essential resources (light and key soil nutrients) in the understorey. In both studied forests we found significant relationships between the overall canopy density, light availability, topsoil litter accumulation, and the availability of N and P, which frequently limit plant growth. Path analysis (by Shipley’s d-sep method) showed that the available data were consistent with the proposed causal model. The average values of soil variables at the end quartiles of the light-availability gradient were compared. Results showed large differences in litter accumulation (~30×) and available-N and -P topsoil concentrations (~3×) in the Spanish forest (with the wider environmental gradient). Furthermore, P increased from the “very low” range to the “low” or even the “optimum” range of availability (according to standard plant growth criteria), which suggests potential effects on the growth of the understorey plant species. We conclude that the counteracting gradients of the essential resources – light and nutrients – in the forest understorey resulted from direct and indirect effects of the canopy overstorey, respectively. We suggest that these counteracting effects of the woody canopy on essential resources of different nature must be considered when interpreting the patterns of understorey plant populations and communities.