Groundwater depth as a constraint on the woody cover in a Neotropical Savanna

Aims To identify the role of soil properties and groundwater depth on the structure of a fire-protected neotropical savanna. We aimed to address: i) What is the contribution of soil properties and groundwater depth to the physiognomic variation? ii) Are these factors associated with patterns in woody structure? iii) Are open physiognomies maintained by shallow groundwater? Methods We measured soil properties, tree basal area, density, richness, and monitored groundwater depth in two types of grassland and three types of savanna during two years. We also investigated vegetation dynamics over three decades using remote sensing. Results There were no differences in soil properties between physiognomies, except for a greater soil organic matter content in flood-prone grasslands. Woody structure attributes were related to groundwater depth and clay content at coarse spatial scales (1 ha) and to groundwater depth and organic matter at fine spatial scales (100 m). Open savannas and grasslands remained unchanged and occurred where the groundwater depth was lower than 4 m. Conclusion Soil fertility did not drive the spread of closed physiognomies. Low clay content and shallow groundwater depth tended to decrease the woody cover, favoring the occurrence of open savannas and grasslands.