Changes in irradiance and soil properties explain why typical non-arboreal savanna species disappear under tree encroachment

Abstract

Savanna vegetation maintains its openness and its diverse plant composition because of frequent fire events; however, when these are suppressed, encroachment is caused by increases in the tree density. In the neotropical savanna (cerrado of Brazil), typical forest trees are invading savanna areas, altering abiotic conditions and affecting the persistence of their exclusive species. Here we studied changes in abiotic conditions, species richness and diversity of a non-arboreal community (herbs, vines, grasses, subshrubs and shrubs) in a gradient of encroachment at a site where fire has been suppressed for more than 50 years in south-eastern Brazil. Encroached communities were more shaded and possessed a wetter and richer soil (higher contents of organic matter and P) compared with the typical savanna. These abiotic changes were related to a less rich and less diverse plant community in encroached savanna compared with typical savanna. The most important abiotic variables driving such changes were photosynthetically active radiation (PAR) reaching the understorey and soil P content: communities with lower species richness and diversity had lower PAR incidence and higher soil P content. Our results suggest that non-arboreal savanna species may be under serious threat of extinction given the expected ecological changes caused by the widespread expansion of forest on the savannas in the absence of fire.

Leaf anatomical traits of non-arboreal savanna species along a gradient of tree encroachment

“…As the TS species showed thicker leaves, with anatomical traits that are advantageous to survive under elevated light irradiances (Rossatto et al 2015;Rossatto & Franco 2017), this may imply that they cannot thrive under shaded conditions, which in turn can explain why the diversity of such species drastically decrease from the TS to the FS (Pinheiro et al 2016). The disappearance of such typical savanna species may occur because their set of anatomical leaf traits are not specialized in capturing low and diffuse light; this can drastically affect the carbon balance, probably leading plants to carbon starvation (Sevanto et al 2014).…”

“…Plants in dense savanna tended to show intermediate values of leaf thickness. This may be explained by the fact that in the dense savanna, plants from typical and forested savanna communities can co-exist due to the variability of available light niches (Pinheiro et al 2016).…”

“…To compare leaf anatomical strategies and specific leaf area of non-arboreal plants along the gradient of tree encroachment, we selected species in each physiognomy, taking into account species surveyed by Pinheiro et al (2016), but also sampling species found to be common and characteristic of each encroachment situation during field observations. We collected samples from fully expanded leaves (of third or fourth node from the apex for herbs, shrubs and sub-shrubs, and of the eighth or ninth node for climbers), without disease signals, and at least from three or preferably six individuals per species (n = 3-6).…”

“…In this region, tree encroachment has transformed typical savanna areas into forested savanna. For this study, we selected a tree density gradient, encompassing three situations: a) typical savanna (TS): a discontinuous tree layer (785 trees ha ) with an elevated canopy cover (LAI = 1.90 ± 0.12) (Pinheiro et al 2016).…”

“…Th is is only possible because some typical forest tree species can establish in savanna sites during fi re absence (Silva et al 2008;Hoff mann et al 2009;Geiger et al 2011). Th e recruitment of forest species contributes to canopy closure, decreasing the total amount of light radiation reaching the understory (Pinheiro et al 2016) and increasing the availability of nutrients (as P) in the superfi cial soils under encroachment (Blaser et al 2014;Pinheiro et al 2016;Rossatto & Rigobelo 2016). Changes in irradiance have been proven as a strong environmental filter (more than soil P) affecting the occurrence of nonarboreal savanna species, since its decrease drastically reduces species richness and diversity in encroached savanna sites (Ratajczak et al 2012;Pinheiro et al 2016).…”