Photosynthetic responses of understory savanna plants: Implications for plant persistence in savannas under tree encroachment

Abstract

Savannas are ecosystems where plants are expected to have leaf-level adaptations conferring tolerance to high irradiances, which characterizes this type of open canopy vegetation. The neotropical savannas of Brazil (Cerrado) harbour a large diversity of heliophilous herbaceous species, which are threatened by the current invasion of those open canopy habitats by forest trees, increasing tree density and canopy cover, thereby strongly reducing light availability. In this study, we measured leaf pigment concentrations, specific leaf area and light response curves of photosynthesis to determine whether herbaceous species of open savanna habitats were less tolerant to shade than the ones that were more characteristic of forested savanna. Relative to species of forested savannas, herbaceous species of open savannas had heavier leaves with higher concentration of carotenoids. They also achieved higher photosynthetic rates, reaching light compensation and saturation at higher irradiances. Based on measurements of light compensation points and of prevailing irradiances in forested savannas, leaves of most herbaceous species of more open savannas would be strongly light-limited in sites with a dense woody layer. In summary, we provide evidence that forest tree encroachment of savannas would select for leaf trait combinations in the understory vegetation that would favor shade tolerance and, depending on the degree of leaf acclimatization to shade of individual species, could strongly constrain persistence of the current understory vegetation.

Responses of Neotropical Savannah Plant Species to Abiotic Stresses: A Structural and Functional Overview

“…Finally, the occurrence of invasive tree species in open areas of the Neotropical savannah has altered the microclimate conditions, affecting grasses and shrubs and consequently the dynamics of fire occurrence in the Neotropical savannah [124,220]. This cascade of events will certainly have a negative impact on the plant and animal communities of the Neotropical savannah, changing the local landscape.…”

Fire‐sensitive species dominate seed rain after fire suppression: Implications for plant community diversity and woody encroachment in the Cerrado

“…These changes in turn can enhance recruitment of tree seedlings through facilitative effects, such as amelioration of high irradiance and temperatures (Hoffmann, 1996;Salazar et al, 2012b). With greater shading, fire-tolerant species are likely to decrease reproductive output due to reduced photosynthetic light availability (Rossatto, Araújo, Silva, & Franco, 2018). Some fire-tolerant plants require fire to stimulate flowering and fruiting, which would further reduce reproductive output under fire suppression (Araújo, Amaral, Bruna, & Vasconcelos, 2013).…”

Woody plant encroachment constrains regeneration of ground‐layer species in a neotropical savanna from seeds

“…These environmental shifts induced by woody plant encroachment seem to act as filters for the persistence of typical savanna species and may affect their survival since they may alter both resource availability (Carlos & Rossatto 2017;Rossatto et al 2018) and aspects related to the regenerative ability (Hoffmann et al 2004;Abreu et al 2017). Savanna species persist and regenerate through seed germination and recruitment, vegetative spread, and/or resprouting after disturbance (Pilon et al 2021a).…”

“…Shading caused by increases in canopy cover due to encroachment may be a factor that hinders germination of savanna species (Pinheiro et al 2016), as typical ground-layer savanna species are adapted to thrive under high radiation levels (Rossatto et al 2018), and most are positively photoblastici.e., seed germination is triggered by light exposure (Zaidan & Carreira 2008;Marques et al 2014). Additionally, canopy-filtered light as a result of encroachment reduces the availability of light and produces low red:far-red ratios (R:FR) at ground level (Salazar et al 2012).…”