Understory plant communities and the functional distinction between savanna trees, forest trees, and pines

Abstract

Although savanna trees and forest trees are thought to represent distinct functional groups with different effects on ecosystem processes, few empirical studies have examined these effects. In particular, it remains unclear if savanna and forest trees differ in their ability to coexist with understory plants, which comprise the majority of plant diversity in most savannas. We used structural equation modeling (SEM) and data from 157 sites across three locations in the southeastern United States to understand the effects of broadleaf savanna trees, broadleaf forest trees, and pine trees on savanna understory plant communities. After accounting for underlying gradients in fire frequency and soil moisture, abundances (i.e., basal area and stem density) of forest trees and pines, but not savanna trees, were negatively correlated with the cover and density (i.e., local-scale species richness) of C4 graminoid species, a defining savanna understory functional group that is linked to ecosystem flammability. In analyses of the full understory community, abundances of trees from all functional groups were negatively correlated with species density and cover. For both the C4 and full communities, fire frequency promoted understory plants directly, and indirectly by limiting forest tree abundance. There was little indirect influence of fire on the understory mediated through savanna trees and pines, which are morefire tolerant than forest trees. We conclude that tree functional identity is an important factor that influences overstory tree relationships with savanna understory plant communities. In particular, distinct relationships between trees and C4 graminoids have implications for grass-tree coexistence and vegetation-fire feedbacks that maintain savanna environments and their associated understory plant diversity.

Contrasting sapling bark allocation of five southeastern USA hardwood tree species in a fire prone ecosystem

“…In general, however, our findings on bark thickness and non-bark traits were consistent with those of previous studies (Harmon 1984, Jackson et al 1999, Hoffmann et al 2003, Lawes et al 2011a, Schwilk et al 2013, Graves et al 2014. These correlated suites of fire-adapted traits are found in a diversity of species in fire-prone communities worldwide, including Pinus (Landers 1991, Keeley and Zedler 1998, Schwilk and Ackerly 2001, Veldman et al 2013) and other Quercus (Abrams 1992, Kane et al 2008, Engber and Varner 2012, Veldman et al 2013, and reflect the legacies of past and current fire regimes.…”

Cambios florísticos inducidos por la frecuencia de fuego en el Chaco Serrano.

“…Una respuesta diferencial de las formas de crecimiento a los efectos del fuego ha sido reportada previamente para el Chaco Serrano de las laderas occidental de las Sierras Chicas (Verzino et al, 2005). El aumento de la riqueza de herbáceas junto con la reducción de especies leñosas en respuesta al fuego ha sido extensamente reportado para muchos ecosistemas del mundo, postulando al fuego como un factor ecológico clave para la conservación de la biodiversidad en pastizales, sabanas y matorrales (e.g., Veldman et al, 2013 Nuestros resultados son parcialmente congruentes con esto, debido a que ninguna forma de crecimiento mostró un aumento de su riqueza. Sin embargo, las especies graminoides fueron resilientes a los efectos de diferentes frecuencias de fuego y las especies herbáceas solo ante una frecuencia baja.…”

“…Los cambios fisonómicos inducidos por la frecuencia de fuego sugieren que al no recuperarse la estructura arbórea se estaría afectando directamente la composición de especies. Además, estas comunidades estructuralmente diferentes podrían presentar divergencias funcionales en cuanto a rasgos de regeneración post-fuego (resistencia al fuego y reclutamiento de plántulas), de asimilación de recursos (Veldman et al, 2013;Carbone & Aguilar, 2016), y posiblemente en las interacciones bióticas con animales y en su éxito reproductivo (Enright et al, 2015;Carbone, 2017). El escenario no quemado mostró especies indicadoras que típicamente se desarrollan en el sotobosque con una reducida intensidad lumínica (e.g., Lorentzianthus viscidus y Dicliptera squarrosa), o que necesitan en mayor medida de alguna especie leñosa como elemento de soporte para desarrollarse (e.g., Araujia odorata y Tillandsia capillaris).…”

Clarifying the confusion: old-growth savannahs and tropical ecosystem degradation

“…With chronic fire exclusion, mesic savannahs are replaced by low-diversity forests (i.e. forest-encroached savannahs; [7,[54][55][56]). There is also growing evidence that woody encroachment is accelerating globally [57,58] (but see [59]) as elevated atmospheric carbon dioxide concentrations shift the balance between C 4 grasses and woody species with C 3 photosynthesis [60].…”

“…But in reality, across vast areas of the tropics, savannahs and forests form relatively stable mosaics of fire-dependent and fire-sensitive ecological communities [12]. This is in part because savannah-forest boundaries are reinforced by differences in the flammability of savannah and forest fuels [61], functional differences between savannah and forest trees [3,55], and seasonality of fire (i.e. savannahs often burn during seasons when forest fuels are too moist) [51].…”