Conceptual Models of Tree and Grass Coexistence

Interactions between the coexisting lifeforms in savanna communities are complex and over the last 40 years, a range of conceptual or theoretical models has been proposed to explain tree and grass mixtures. Contrasting models have all been supported by empirical evidence for particular sites, but no single model has emerged that provides a generic mechanism explaining coexistence. Models can be classified into several categories. Competition-based models feature spatial and temporal separation of resource usage by trees and grasses that minimizes competition and enables the persistence of both lifeforms. Alternatively, demographic-based models have been described, where mixtures are maintained by disturbance, resulting in bottlenecks in tree recruitment and/or limitations to tree growth and grasses can persist. Table 1 provides a summary of these models. Root-niche separation models suggest that there is a spatial separation of tree and grass root systems, with grasses exploiting upper soil horizons and trees developing deeper root systems. Trees rely on excess moisture (and nutrient) draining from surface horizons to deeper soil layers. Phenological separation models invoke differences in the timing of growth between trees and grasses. Leaf canopy development and growth in many savanna trees occurs prior to the onset of the wet season, often before grasses have germinated or initiated leaf development. As a result, trees can have exclusive access to resources at the beginning of the growing season, with grasses more competitive during the growing season proper. Given their deeper root systems, tree growth persists longer into the dry season, providing an additional period of resource acquisition at a time when grasses may be senescing. This spatial and temporal separation of resource usage is thought to minimize competition, enabling coexistence. Other competition models suggest that density of trees becomes self-limiting at a threshold of PAM and PAN and is thus unable to completely exclude grasses. These models assume that high rainfall years favor tree growth and recruitment, with poor years favoring grasses, and high interannual variability of rainfall maintaining a relatively stable equilibrium of trees and grasses over time.

Alternatively, savannas can be viewed as meta-stable ecosystems (narrow range of stabile states) with a dynamic structure over time. Demographic-based models suggest that determinants of tree demographics and recruitment processes ultimately set the tree:grass ratios (Table 1 ). Fire, herbivory, and climatic variability are fundamental drivers of tree recruitment and growth, with high levels of disturbance resulting in demographic bottlenecks that constrain recruitment and/or growth of woody components and grass persistence results. At high rainfall sites, in the absence of disturbance, the ecosystem tends toward forest. High levels of disturbance, particularly fire, can push the ecosystem toward a more open canopy or grassland; this ecosystem trajectory is more likely at low rainfall sites.

There is observational and experimental data to support all of the above models and it is highly likely that savanna structure and function results from the interaction of all processes. In many savannas, root distribution is spatially separated with mature trees exploiting deeper soil horizons as the competitive root-niche separation model predicts. Root partitioning favors tree growth in semiarid systems where rainfall occurs during periods when grass growth is dormant; rainfall can drain to deep layers supporting tree components. By contrast, in

Table 1 Conceptual models explaining the coexistence of trees and grasses in savanna ecosystems in equilibrium (tree:grass ratio relatively stable at a given site), nonequilibrium (tree: grass ratio variable) or disequilibrium (disturbance agents essential for the maintenance of tree:grass coexistence)

Competition-based

Mechanisms of coexistence

Spatial and temporal niche separation of resource usage enables both life forms to coexist

Root-niche separation Tree and grasses exploit deep and shallow soil horizons

Phenological separation Temporal differences in leaf expansion and growth, trees have exclusive access to resources at beginning and end of growing season, grasses competitive during growing season

Balanced competition Trees are the superior competitor but become self-limiting for a given rainfall and unable to exclude grasses

Competition-colonization Rainfall variability results in a tradeoff between tree and grass competition and colonization potential. Higher than mean rainfall favours tree growth, lower than mean favours grasses

Primary determinants PAM, PAN

Demographic-based

Mechanisms of coexistence

Climatic variation and disturbance impacts on tree demography Extremes of climate and disturbance influence tree germination and/or establishment and/or transition to mature size classes enabling coexistence At low rainfall sites, tree establishment and growth occurs only in above average rainfall periods At high rainfall sites, high fuel production maintains frequent fire to limit tree dominance

Primary determinants PAM variability, PAN, fire regime, herbivory

Secondary determinants Fire regime, herbivory semiarid savanna where rainfall and growing seasons coincide, investment in deep root systems could result in tree water stress, as rainfall events tend to be sporadic and small in nature, with little deep drainage. In this case, surface roots are more effective at exploiting moisture and mineralized nutrients following these discrete events. In these savannas, tree and grass competition for water and nutrients would be intense. In mesic savanna sites, root competition between both trees and grass roots in upper soil layers is apparent, contrary to predictions of niche-separation models. Mesic savannas of north Australia (rainfall >1000 mm) are dominated by evergreen Eucalyptus tree species, and during the wet season these trees compete with high growth-rate annual grasses for water and nutrients in upper soil layers (0-30 cm). However, by the late dry season, tree root activity has shifted to subsoil layers (up to 5 m depth) and herbaceous species have either senesced or are physiologically dormant. These root dynamics suggest that grasses are essentially drought avoiders but are able to compete with trees during the wet season. This system serves as an example where both root-niche and phenological separation are occurring.

Tree-to-tree competition is also significant, as suggested by the strong relationship observed in most savanna regions between annual rainfall and indices of tree abundance, be it tree cover (Figure 5), tree basal area (area occupied by tree stems), or tree density. As PAM

decreases, tree abundance declines. Competition models also fail to consider impacts of savanna determinants on different demographics of a population, such as recruitment, seedling establishment, and tree sapling growth. Root-niche or phenological separation models largely consider impacts acting on mature individuals, whereas demographic models include impacts of climate variability and disturbance on critical life-history stages (e.g., seedling establishment and accession to fire-tolerant size classes). Demographic models assume that savanna tree dynamics are central to savanna ecosystem functioning and that savanna trees are the superior competitors under most conditions; grass persistence only occurs when determinants act to limit tree abundance. It is clear that competition, both within and between savanna life forms, occurs and that tree abundance is moderated by climate variability and disturbance. A more comprehensive model would integrate both competition and demographic theories to yield a model in which competitive effects are considered for each life-history stage.

The complexity inherent in these models is evident when savanna structure is correlated with any of the environmental determinants. Figure 5 describes the relationship between tree cover and mean annual rainfall, in this case a surrogate for PAM. Tree cover data are shown for African and Australian savanna sites. The figure shows a large scatter of tree cover possible at any given rainfall, especially for the African sites. For African savanna,

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Figure 5 Relationship between mean annual rainfall (MAP) and tree cover for African and Australian savannas, with rainfall setting a maximal climate-determined woody cover. Other factors such as available nutrient, fire frequency and herbivory determine woody cover at any given site. Modified from Sankaran M, Hanan NP, Scholes RJ, et al. (2005) Determinants of woody cover in African savanna. Nature 438: 846-849 (Macmillan Publishers Ltd), with Australian tree cover data from R. J. Williams, unpublished data.

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Figure 5 Relationship between mean annual rainfall (MAP) and tree cover for African and Australian savannas, with rainfall setting a maximal climate-determined woody cover. Other factors such as available nutrient, fire frequency and herbivory determine woody cover at any given site. Modified from Sankaran M, Hanan NP, Scholes RJ, et al. (2005) Determinants of woody cover in African savanna. Nature 438: 846-849 (Macmillan Publishers Ltd), with Australian tree cover data from R. J. Williams, unpublished data.

rainfall sets an upper limit on tree cover, with the relationship linear until approximately 650 mm rainfall with little increase in tree cover observed above this threshold (Figure 5). Points below the line represent savanna sites with a tree cover determined by PAM plus the interaction of other determinants to reduce tree cover below the maximum possible for a given rainfall. At semiarid savanna sites (<650 mm rainfall), it is likely that rainfall limits tree cover and canopy closure, permitting grass coexistence. At rainfalls >650 mm, tree canopy closure may be possible, with disturbance limiting woody dominance. For Australian savanna, there is a simpler relationship evident, with a linear increase in tree cover with annual rainfall and less scatter. Australian savannas also have a reduced tree cover (and biomass) for a given rainfall when compared to African systems (Figure 5). This suggests that while PAM is determining tree cover, other factors such as fire frequency or PAN are also playing a role. Australian savanna soils (PAN) may be systematically poorer than African soils or fire frequency higher, limiting tree cover and productivity.

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