The ground layer community of cerrado sensu stricto facing changes in light and litter triggered by woody encroachment: species dynamics, germination and functional responses

Woody plant encroachment refers to the increase in density and cover of woody species in vegetation with an expressive herbaceous-grassy layer. This process is going rapidly in South America savannas, where many Brazilian savanna areas have undergone losses of herbaceous-shrubby species in encroached sites. Thus, environmental changes caused by woody encroachment may be filtering these species. Such changes comprise understory light reduction and higher litter deposition in encroached sites; these two factors combined can induce narrowing of the soil surface temperature alternation and red and far-red ratio (R:FR) decrease. By means of field manipulation, this study investigated the role of light reduction and litter layer increase caused by woody encroachment in diversity, structure and physiology of savanna ground layer community. Furthermore, seed regeneration aspects of savanna ground layer species were evaluated according to alterations in temperature alternation, R:FR ratio and litter layer thickness. We found that the litter had only small effects in floristic and functional diversity of savanna ground layer community as well as structural and functional parameters. However, litter layer presence drastically reduced seedling emergence of savanna ground layer species. In the other hand, shading was an important factor for savanna ground layer structure and floristic. Shading caused significant reductions in floristic and functional richness as well as in density, cover and aboveground biomass over time. Among growth forms, herbaceous plants were most strongly and rapidly affected by shading. Moreover, savanna ground layer species were capable of some adjusts related to light capture improvement in shaded conditions, such as higher leaf specific area values and chlorophyll and carotenoid content. However, these adjustments were not enough to allow the photochemical and carbon assimilation performance maintenance. Regarding seed regeneration aspects, besides the large litter effects mentioned before, reductions in R:FR and temperature alternation regime also negatively affected some species. In conclusion, results indicate that environmental changes caused by woody encroachment represent strong environmental filters shaping ground layer community assembly. These changes act limiting both survival and regeneration of savanna ground layer species in encroached situations. (AU)