Elucidating water use in native and exotic grass communities in the Cerrado

Abstract

Grasses are one of the main components of the herbaceous vegetation that maintains the functioning of neotropical savannas and grasslands. The Cerrado, savanna with the greatest richness of plant species in the world, presents the herbaceous stratum dominated by a great diversity of native grasses. It is known that, as they have roots mostly in the surface layers of the soil, open savannah and grassland vegetations allow a greater recharge of the water table when compared to forests. However, for water to reach the water table, it needs to cross the surface layer of soil, where most of the roots are interacting. To date, studies that seek to understand how different species affect soil water and the importance of the diversity of this herbaceous component are still scarce. This knowledge gap has hampered conservation actions and led to frequent misunderstandings in Cerrado open ecosystem restoration projects. For example, it is common to use a few fast-growing species in restoration (to ensure rapid coverage), but these need a greater amount of water due to greater transpiration during the photosynthesis process. On the other hand, in more diverse communities dominated by typical Cerrado species with slow growth, water use may have a different pattern, keeping soil moisture more constant over time. Thus, understanding the dynamics of water use by native grasses and what are the different attributes related to possible distinct patterns in different species is crucial for projects that aim to restore not only the structure, but also the functioning of grassland and savanna ecosystems. Therefore, the objective of this research project is to understand how functional attributes and species richness influence water use in experimental communities of native and exotic Cerrado grasses. The following hypotheses will be tested: (I) Communities composed of species with fast-growing attributes deplete soil surface water more quickly; (II) The consumption of soil water in communities invaded by exotic grass species will be higher in relation to communities with only native species; (III) The reduction of surface water in the soil throughout the dry season is gradual in communities more diverse in relation to monocultures because the greater biomass of roots, due to functional complementarity, increases the retention of water in the soil. For this, soil moisture sensors were installed in the dry season of 2021 in 40 plots, 9 controls (without any sown species), 6 monocultures, 22 with eight species and 3 invaded by exotic grasses. With these data, the stability of soil moisture during the rainy season and water consumption after the end of the rains will be calculated. The following functional attributes related to the use of water by native and exotic species that occur in the vicinity of the sensors will also be measured: stomatal conductance, leaf water potential at dawn and midday, residual water loss (gmin). With this study, we aim to demonstrate how different species affect topsoil moisture and water use changes when species grow in monocultures versus polycultures with multiple interacting species. Additionally, we also intend to demonstrate the change in the hydrological cycle when there is the invasion of exotic species. This knowledge will help to build a theoretical basis to support projects to restore vital ecosystem services, such as the maintenance of water resources. In short, the project seeks to emphasize the importance of conserving the diversity of native species, thus being able to apply this knowledge in open ecosystem restoration projects, in order to contribute to the objectives of thematic project # 19/07773-1. (AU)