Do groundwater dynamics drive spatial patterns of tree density and diversity in Neotropical savannas?

VILLALOBOS-VEGA, R.; SALAZAR, A.; MIRALLES, W; HARIDASAN, M.; FRANCO, A.C; GOLDSTEIN G

JOURNAL OF VEGETATION SCIENCE

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2014 vol. 25 p. 1465 - 1473

1100-9233

AbstractQuestions: What are the temporal and spatial variations of groundwater depthalong topographic gradients in Neotropical savannas? Are patterns of tree densityand species diversity along topographic gradients in Neotropical savannascontrolled by water table depth? Do soil and groundwater nutrient concentrationsdiffer along topographic gradients in Neotropical savannas?Location: Fire-protected savannas of central Brazil.Methods: Over 16 mo, we monitored temporal and spatial variations ingroundwater levels using automated submersible pressure transducers installedin ten wells located along two topographic gradients (five wells per gradient) of950 m and 1703 m in length, representing elevations of 47 and 37 m a.s.l.,respectively. We located the wells according to changes in vegetation physiognomiesfrom woody savannas at high elevations, to open shrubby grasslands atlow elevations. Along each topographic gradient we determined soil andgroundwater nutrient concentrations as well as richness, density, basal diameterand height of trees within two plots of 14 9 14 m (392 m2) adjacent to eachwell.Results: Along the two gradients, groundwater levels exhibited larger fluctuationsat lower than at higher elevations where the water table was deeper. Richness,density and diversity of trees decreased significantly at lower elevationswhere soils were waterlogged during the wet season. Soil pH and soil concentrationsof carbon, nitrogen and manganese decreased significantly as elevationincreased along the topographic gradients, but soil nutrient concentrations ofphosphorus, aluminium and iron did not change with elevation. Groundwatersamples contained only trace amounts of nutrients and were poorly correlatedwith elevation along the topographic gradients.Conclusions: In Neotropical savannas, the minimum distance between the soilsurface and water table depth (reached during the wet season) and the relativelylarge fluctuations in groundwater limit tree density and diversity at low elevationsas savanna trees cannot cope with extended waterlogging during the wetseason and with low soil water availability during the dry season. Thus, variationsof tree density and diversity along topographic gradients are more relatedto spatial and temporal variations in water table depth than to soil and groundwaternutrient variations in Neotropical savannas