Scale-dependence of environmental effects on species richness in oak savannas

In order to investigate how scale (grain size) affects the relationships between species richness and environmental drivers (Such as stress and disturbance), we collected 12 nested quadrats (from 0.25 m(2) to 1023 m(2)) from seven remnant oak savannas located in the floodplain of the Chippewa River in western Wisconsin, USA. Large and small-scale richness were not significantly correlated, suggesting that small-scale richness is not strongly controlled by sampling effects of the local species pool. Linear and curvilinear regressions between species richness and disturbance, canopy cover, biomass, and soil organic matter were dependent on sampling scale (grain size). Disturbance by fire was strongly related to richness at small scales, while free canopy cover was strongly related to richness at larger scales. While there was some evidence suggesting the transition from disturbance to canopy effects occurs between 10 and 100 m(2), the transition was not particularly abrupt. The results cast doubt on the general importance of local species pools in affecting small-scale richness as well as our ability to make generalizations that do not explicity include scale.