Stress responses of native and exotic grasses in a Neotropical savanna predict impacts of global change on invasion spread

Abstract

Communities subject to stress, including those with low invasibility, may be dominated by exotic generalist species. African grasses are aggressive invasive species in Neotropical savannas, where their response to abiotic stress remains unknown. We assessed the role of waterlogging and canopy closure on the presence, abundance and reproductive tillering of African and native grasses in a Neotropical savanna in southeastern Brazil. We obtained abundance and reproductive tillering data of exotic (Melinis minutiflora, Melinis repens and Urochloa decumbens) and common native grasses in 20 sites. We also determined the groundwater depth, soil surface water potential and canopy cover at these sites. The grass species generally had a low frequency and performed poorly where soil remained waterlogged throughout the year, except for two native species. Most native species were exclusive to either well-drained savannas or better drained wet grasslands. However, two species (Loudetiopsis chrysothrix and Trachypogon spicatus) occurred in both vegetation types. Two exotic species (M. minutiflora and M. repens) were less common but demonstrated reasonable performance in wet grasslands, possibly due to their root system plasticity. Furthermore, U. decumbens had a lower occurrence, density and reproductive tillering at these sites, but was successful at sites where the groundwater level was slightly deeper. Although the favourable water regime in the savannas increases their invasibility in general, resistance to invasion by African grasses may be greater at microsites with high canopy closure, where these species showed lower performance and did not affect the abundance of co-occurring native grasses. In summary, the Brazilian savanna becomes more susceptible to the spread of African grasses when disturbances decrease canopy closure or lower rainfall associated with climate change reduces the average groundwater depth and consequently releases invasive species from soil waterlogging in grasslands.

Abandoned pastures cannot spontaneously recover the attributes of old‐growth savannas

“…Exotic grass cover, in turn, decreased linearly over time and was estimated to disappear from secondary savannas within 36 years. This is because the C4 grasses present at the study sites (U. decumbens and U. humidicola) are shade intolerant (Klink & Joly, 1989;Xavier, Leite, & da Silva-Matos, 2016), and the regenerating systems were estimated to exhibit forest canopy by the end of this period.…”

Differential effects of soil waterlogging on herbaceous and woody plant communities in a Neotropical savanna

“…In seasonally dry ecosystems, high water tables may even favour the occurrence of herbaceous species with poor tolerance to water deficit, as it may increase the moisture at the superficial soil layer by capillary action (Moore 1939). In effect, there is no annual period of water deficit in most of our study sites subjected to seasonal waterlogging (Xavier et al 2017). Therefore, we hypothesize that conditions associated with high water tables support specific herbaceous species, and competitive ability is expected to be important for their persistence (Grime 1988).…”

“…Although we initially aimed to sample an equal the number of sites within each of these three savanna categories, we found that woody cover varies widely within each savanna category (Leite et al 2018). Likewise, we found that grassland types initially categorized based on visual differences in their flooding regime were in effect distributed along a hydrological gradient, including distinct annual periods of soil waterlogging and standing water (Xavier et al 2017). Considering these limitations on identifying a priori physiognomies or vegetation types at our study site and our aim of assessing community responses to water table depth, our study focus on assessing diversity patterns of woody and plant communities occurring along a continuum of woody cover and hydrological regimes.…”

“…The local climate is Cwa (Köppen) and the average annual rainfall and temperature during the study period were 1,613 mm and 20.6 °C, respectively (IFSP 2018). There is typically a favourable water balance during the rainy season and a short period of water deficit in the dry season (Xavier et al 2017). The prevalent soil is an entisol with at least 90% sand (Leite et al 2018 (Coutinho 1978).…”

Fisioecologia comparativa de gramí­neas nativas e invasoras em duas fitofisionomias de um fragmento de cerrado

“…Por outro lado, no arranjo obtido pelas relações entre variáveis expressas por unidade de massa seca foliar, foi observada uma tendência de aumento no desempenho fotoquímico de invasoras em presença de dossel, enquanto que em nativas a tendência é novamente inversa.A partir dos resultados, sugere-se que nativas e invasoras diferiram quanto ao microclima luminoso ideal para o desempenho máximo de uso da luz, e que a estrutura foliar estaria associada com essa diferença. Estudos em fisionomias de cerrado mostram que a abundância de invasoras pode estar positivamente correlacionada com a disponibilidade de luz(Pivello et al, 1999b), e que o grau de fechamento do dossel pode afetar diferentemente o desempenho reprodutiva de gramíneas nativas e invasoras(Xavier et al, 2016). Ainda que este estudo não tenha incluído altos graus de fechamento de dossel como fonte de variação, as implicações dos padrões de desempenho de uso da luz na aptidão de nativas e invasoras devem ser consideradas.…”