Effects and response of the Cerrado ground-layer to frost along the canopy cover gradient.

Frost effects on savanna plant communities have been considered as analogous to those from fire, both changing community structure and filtering species composition. However, while frost impacts have been well-studied for the woody component of savannas, it is still poorly explored for the ground-layer community. Here, we investigated effects of frost in the Cerrado along a gradient of tree cover, focusing on ground-layer plant species, near the southern limit of the Cerrado in Brazil. We aimed to elucidate if the pattern already described for the tree layer also extends to the ground layer in terms of mimicking the effects of fire on vegetation structure and composition. We assessed how damage severity differs across species and across the tree-cover gradient, and we examined the recovery process after frost in terms of richness and community structure along the canopy cover gradient. Frost caused immediate and widespread dieback of the perennial ground-layer, with greatest impact on community structure where tree cover was lowest. However, frost did not reduce the number of species, indicating community resilience to this natural disturbance. Although frost mimicked the effects of fire in some ways, in other ways it differed substantially from fire. Unlike fire, frost increases litter cover and decreases the proportion of bare soil, likely hindering crucial processes for recovery of plant populations, such as seed dispersal, seed germination and plant resprouting. This finding calls attention to the risk of misguided conclusions when the ground layer is neglected in ecological studies of tropical savannas and grasslands.

The distinct roles of water table depth and soil properties in controlling alternative woodland-grassland states in the Cerrado.

Open grassy vegetation and forests share riparian zones across the Neotropical savannas, characterizing alternative stable states. However, factors determining the occurrence and maintenance of each vegetation type are yet to be elucidated. To disentangle the role of environmental factors (soil properties and groundwater depth) constraining tree colonization of wet grasslands in the Cerrado, we assessed tree establishment during the early seedling and sapling stages and the influence of these factors on leaf gas exchange and leaf water potential of tree saplings. Three functionally distinct tree species were studied: (1) flood-tolerant species characteristic of gallery forests, (2) flood-intolerant species characteristic of seasonally dry savannas, and (3) generalist species found in both gallery forests and seasonally dry savannas. Savanna species was constrained by waterlogging, especially at the sapling stage, with restricted stomatal conductance and leaf water potential, resulting in low carbon assimilation, decreased plant size, and high mortality (above 80%). The gallery forest and the generalist species, however, were able to colonize the wet grasslands and survive, despite the low seedling emergence (below 30%) and sapling growth constrained by low gas exchange rates. Soil waterlogging is, therefore, an effective environmental filter that prevents savanna trees from expanding over wet grasslands. However, colonization by trees adapted to a shallow water table cannot be constrained by this or other soil properties, turning the wet grasslands dependent on natural disturbances to persist as an alternative state, sharing the waterlogged environments with the gallery forests in the Cerrado region.