Shade alters savanna grass layer structure and function along a gradient of canopy cover

Abstract

Aim: In savannas, a grass-dominated ground layer is key to ecosystem function via grass-fire feedbacks that maintain open ecosystems. With woody encroachment, tree density increases, thereby decreasing light in the ground layer and potentially altering ecosystem function. We investigated how light availability can filter individual grass species distributions and whether different functional traits are associated with response to a shade gradient in a landscape experiencing woody encroachment. Location: Savanna-forest mosaic in the Cerrado domain, southeastern Brazil. Methods: Along an encroachment gradient of increasing tree leaf area index (LAI) and shade, we determined how changing light availability alters grass diversity and ground layer structure relative to grass cover and grass functional traits (photosynthetic pathway, underground storage organs, bud protection and traits related to grass shape, size and leaf dimensions). Results: Increasing shade led to a decrease in grass cover and grass species richness, and also compositional and functional changes. We found that where tree LAI reached 1, grass cover was reduced by 50% and species richness by 30%. While C 4 grass species abundances decreased with increasing shade, the opposite pattern was true for C 3 grasses. There were only small differences in light preferences among C 4 subtypes, with phosphoenolpyruvate carboxykinase (PCK) species tolerating slightly more shaded conditions. Persistence of some C 4 species under more shaded conditions was possible, likely due to an ability to store starch reserves via underground storage organs. Conclusions: Woody encroachment changes diversity and structure of the grassy layer that is critical to the functioning of savanna ecosystems, highlighting the dependence of the diverse grass layer on open and sunny conditions. Our results suggest a threshold of tree cover close to LAI ≈ 1 as being critical to cerrado grassy layer conservation.

Shade alters the growth and architecture of tropical grasses by reducing root biomass

“…Crucially, reduced belowground allocation could result in reduced competitive ability and resprouting capacity relative to both fire and grazing that in turn would increase the likelihood of a species being filtered out of an ecosystem. Indeed, such substantial diminishment of belowground reserves observed here may explain the abrupt exclusion of grasses from the ground layer with increasing tree cover observed in field studies (Charles-Dominique et al, 2018;Pilon et al, 2020;Pinheiro et al, 2016).…”

“…By comparing the growth and biomass allocation of two species of Brachiaria in full sunlight and shaded to 30% light availability, Dias‐Filho (2000) showed that plants had higher specific leaf area and allocated significantly less biomass to root and more to leaf tissue than high‐light plants. While C 4 grass species can persist in low light levels utilizing carbon resources stored in underground structures (Pinheiro et al, 2016; Taylor et al, 2010; Zimmermann et al, 2010), it is only possible for a limited time before the plant must enter a carbon deficit (de Moraes et al, 2016; Pilon et al, 2020; Pinheiro et al, 2016). Crucially, reduced belowground allocation could result in reduced competitive ability and resprouting capacity relative to both fire and grazing that in turn would increase the likelihood of a species being filtered out of an ecosystem.…”

“…These ecosystems support frequent fire and tend to occur worldwide where feedbacks between disturbance, water availability, soil characters, and vegetation traits promote variability in tree cover (Hoffmann, Jaconis, et al, 2012). C 4 grasses dominate in these tropical open environments and where increasing tree cover reduces the abundance of C 4 grass biomass and C 4 grass diversity (Pilon et al, 2020; Scholes, 2003; Scholes & Archer, 1997). That is, in tropical grassy biomes, shade cast by trees acts as an environmental filter reducing light availability for C 4 grass species whose photosynthetic machinery is dependent on high light environments (Black, 1971; Hatch, 1992).…”

“…That is, in tropical grassy biomes, shade cast by trees acts as an environmental filter reducing light availability for C 4 grass species whose photosynthetic machinery is dependent on high light environments (Black, 1971; Hatch, 1992). A diversity of field studies observe abrupt thresholds in tree cover and tree leaf area index where grasses are excluded (Charles‐Dominique et al, 2018; Hoffmann, Geiger, et al, 2012; Pilon et al, 2020; Pinheiro et al, 2016). Remarkably, there have few tests of how changing light availability impacts grass growth, allometry, flammability, and biomass allocation, knowledge required to disentangle the mechanism for the abrupt exclusion of grasses with increasing tree cover.…”

“…Although shade can increase herbaceous biomass productivity under individual tree crowns (Belsky, 1994; Belsky et al, 1993), it can also lead to the exclusion of C 4 grass species (Parr et al, 2014). However, the tree cover thresholds at which C 4 grass exclusion occurs is both regionally variable and little investigated (Charles‐Dominique et al, 2018; Pilon et al, 2020), while tree cover thresholds related to reduced fire spread has been shown to be around 40% (e.g., Staver et al, 2011). C 3 and C 4 grasses have been shown to have similar morphological responses to light availability but responses closely related to photosynthesis such as growth rate and biomass have been shown to be greater for C 4 grasses than C 3 grasses (Kephart et al, 1992; Sage & McKown, 2005).…”