An idealized model for tree–grass coexistence in savannas: the role of life stage structure and fire disturbances

Abstract

Summary1. We discuss a simple implicit-space model for the competition of trees and grasses in an idealized savanna environment. The model represents patch occupancy dynamics within the habitat and introduces life stage structure in the tree population, namely adults and seedlings. A tree can be outcompeted by grasses only as long as it is a seedling. 2. The model is able to predict grassland, forest, savanna and bistability between forest and grassland, depending on the different characteristics of the ecosystem, represented by the model's parameters. 3. The inclusion of stochastic fire disturbances significantly widens the parameter range where coexistence of trees and grasses is found. At the same time, grass-fire feedback can induce bistability between forest and grassland. 4. Synthesis. These results suggest that tree-grass coexistence in savannas can be either deterministically stable or stabilized by random disturbances, depending on prevailing environmental conditions and on the types of plant species present in the ecosystem.

Tree–grass co-existence in savanna: Interactions of rain and fire

“…In state-and-transition models multiple stable states are possible, and the rangeland dynamics is explained through transitions (reversible or irreversible and abrupt) due to disturbances between steady states. The existence of multiple stable states has been pointed out by Dublin (1995), van de Koppel et al (1997), Anderies et al (2002), van Langevelde et al (2003), D'Odorico et al (2006), Hanan et al (2008), Okin et al (2009), Baudena et al (2010).…”

“…Fire has been explicitly included in many savanna models (Hochberg et al, 1994, Anderies et al, 2002, van Langevelde et al, 2003, D'Odorico et al, 2006, Beckage et al, 2009) and has often been -5-described as dependent on the availability of fuel load, that in arid ecosystem is given by dead grass. Hanan et al (2008) and Baudena et al (2010) have considered explicitly the demography of trees underlining the asymmetry of the fire effects between saplings and adult trees. This asymmetry can allow the survivorship of trees population at low densities thanks to mechanisms of 'storage effect' that compensate the loss of young trees due to demographic bottlenecks with the very low mortality of adult trees (Warner and Chesson, 1985).…”

Bimodality of woody cover and biomass across the precipitation gradient in West Africa

“…A large number of experimental and modeling studies have focused on the tree-grass co-existence from an ecological perspective (e.g., Higgins et al, 2000;Sankaran et al, 2005;Baudena et al, 2010), thereby mainly focusing on cover and far less…”

“…Our approach of age classing modeling, in which the vegetation increases its biomass while keeping the same structure, is similar to models using tree seedlings and adult trees (e.g., Scheiter and Higgins, 2009;Baudena et al, 2010). Baudena et al (2010) found that tree seedlings compete for the same water as grasses, while adult trees can outcompete grass, as it has deeper roots.…”

“…Baudena et al (2010) found that tree seedlings compete for the same water as grasses, while adult trees can outcompete grass, as it has deeper roots. A similar mechanism is found in our model: young species compete against bare soil processes, while more adult trees have higher root depths having more water resources.…”