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

Abstract

Questions What are the temporal and spatial variations of groundwater depth along topographic gradients in Neotropical savannas? Are patterns of tree density and species diversity along topographic gradients in Neotropical savannas controlled by water table depth? Do soil and groundwater nutrient concentrations differ along topographic gradients in Neotropical savannas? Location Fire‐protected savannas of central Brazil. Methods Over 16 mo, we monitored temporal and spatial variations in groundwater levels using automated submersible pressure transducers installed in ten wells located along two topographic gradients (five wells per gradient) of 950 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 physiognomies from woody savannas at high elevations, to open shrubby grasslands at low elevations. Along each topographic gradient we determined soil and groundwater nutrient concentrations as well as richness, density, basal diameter and height of trees within two plots of 14 × 14 m (392 m2) adjacent to each well. Results Along the two gradients, groundwater levels exhibited larger fluctuations at lower than at higher elevations where the water table was deeper. Richness, density and diversity of trees decreased significantly at lower elevations where soils were waterlogged during the wet season. Soil pH and soil concentrations of carbon, nitrogen and manganese decreased significantly as elevation increased along the topographic gradients, but soil nutrient concentrations of phosphorus, aluminium and iron did not change with elevation. Groundwater samples contained only trace amounts of nutrients and were poorly correlated with elevation along the topographic gradients. Conclusions In Neotropical savannas, the minimum distance between the soil surface and water table depth (reached during the wet season) and the relatively large fluctuations in groundwater limit tree density and diversity at low elevations as savanna trees cannot cope with extended waterlogging during the wet season and with low soil water availability during the dry season. Thus, variations of tree density and diversity along topographic gradients are more related to spatial and temporal variations in water table depth than to soil and groundwater nutrient variations in Neotropical savannas.

Groundwater recharge decrease with increased vegetation density in the Brazilian cerrado

“…Villalobos‐Vega et al . () studied 10 monitoring wells with water table depths ranging from 0.18 to 15.56 m in an undisturbed cerrado and also found little WTFs in regions with deep water tables. They found that these regions tend to exhibit greater tree abundance and diversity than sites with shallow water tables.…”

The water balance components of undisturbed tropical woodlands in the Brazilian cerrado

“…This may happen because the flux of water reaching the groundwater zone may be in balance to groundwater outflow (water recharge is equaled the outflow). One of the first studies of groundwater dynamics in the undisturbed cerrado was conducted by Villalobos-Vega et al (2014) from 11 monitored wells with water tables ranging from 0.18 to 15.56 m. The authors found little water table change in regions with deep water tables (up to 15.56 m), and in some wells the recharge water took up to 5 months to reach the groundwater table. They also concluded that the water table depth has a strong influence on variations in tree density and diversity, i.e., regions with deep water tables such as the IAB Figure 5.…”

“…Also, hydrological fluxes may vary in a typical catchment due to influences of topographic location (e.g., whether close to a stream or upland, whether in a depression or not), which affect soil water contents. Villalobos-Vega et al (2014) concluded that water table depth has a strong influence on variations in tree density and diversity, i.e., regions with deep water tables such as the IAB site (35 m) tend to exhibit greater tree abundance and diversity than sites with a shallow water table. Therefore, if there is variation in the vegetation characteristics we can expect changes in the hydrological fluxes on different parts of a catchment.…”

No evidence of intrinsic spatial processes driving Neotropical savanna vegetation on different substrates

“…), climate (precipitation, temperature, and seasonality), and variations in water table depth ( e.g ., Villalobos‐Vega et al . ). A continental‐scale study in African savannas indicated that woody community structure responds to mean annual precipitation and soil texture, which are modified by the effects of fire, domestic livestock, human population density, and cultivation intensity (Bucini & Hanan ).…”

“…, Villalobos‐Vega et al . ). Thus, both climate and substrate factors related to resource availability are considered essential determinants of species distributions and structures in such savannas (Cole , Solbrig , Campo‐Bescós et al .…”