Jacques Gignoux

Determinants of woody cover in African savannas

Savannas are globally important ecosystems of great significance to human economies. In these biomes, which are characterized by the co-dominance of trees and grasses, woody cover is a chief determinant of ecosystem properties. The availability of resources (water, nutrients) and disturbance regimes (fire, herbivory) are thought to be important in regulating woody cover, but perceptions differ on which of these are the primary drivers of savanna structure. Here we show, using data from 854 sites across Africa, that maximum woody cover in savannas receiving a mean annual precipitation (MAP) of less than ∼650 mm is constrained by, and increases linearly with, MAP. These arid and semi-arid savannas may be considered ‘stable’ systems in which water constrains woody cover and permits grasses to coexist, while fire, herbivory and soil properties interact to reduce woody cover below the MAP-controlled upper bound. Above a MAP of ∼650 mm, savannas are ‘unstable’ systems in which MAP is sufficient for woody canopy closure, and disturbances (fire, herbivory) are required for the coexistence of trees and grass. These results provide insights into the nature of African savannas and suggest that future changes in precipitation may considerably affect their distribution and dynamics.

DEMOGRAPHY OF A SAVANNA PALM TREE: PREDICTIONS FROM COMPREHENSIVE SPATIAL PATTERN ANALYSES

Existing statistical methods for spatial pattern analysis now permit precise analyses of patterns and, given the appropriate interpretation keys, the formulation of testable hypotheses about the underlying processes. From a comprehensive analysis of the spatial pattern of a plant population and its environment, we infer scenarios of links between demographic processes, plant spatial patterns, and environmental heterogeneity. The palm tree Borassus aethiopum has easily identifiable demographic stages, a root foraging strategy enabling adults to reach distant nutrient sources, and a marked senescence starting with the onset of reproduction. We analyzed map data for palm individuals (in three different life history stages plus two sexes for adults) and for nutrient-rich patches (clumps of other tree species and termite mounds) in three vegetation types (facies) of a humid savanna of West Africa (Lamto, Ivory Coast). Spatial analyses were based on Diggles nearest neighbor functions F and G and on Ripleys K function. The main results were as follows: (1) juveniles and seedlings are aggregated, while adults have a random pattern or are more loosely aggregated; (2) all stages except female adults are spatially associated with nutrient-rich patches, but association distances increase with stage in the life cycle; and (3) seedlings are associated with female adults, whereas the association of juveniles at longer distances is not clear-cut. We propose from these results a parsimonious scenario linking spatial pattern and mortality pattern during the life cycle. The initial pattern of seedlings (close to maternal trees) results from low dispersal distance. Later stages (older seedlings and juveniles) are mostly restricted to nutrient-rich patches through nutrient shortage away from these patches (environment-induced mortality) and form dense clumps of immature palms. Competition on nutrient-rich patches then favors the few juveniles that manage to survive farther from these patches (density-dependent mortality). Finally, the last surviving juvenile of a clump suddenly experiences almost no competition with conspecifics, due to the long distance between clumps of juveniles, and owing to its root-foraging ability, it can now recruit to the adult stage, subject only to senescence. Pattern variations among savanna facies are consistent with this scenario.

Alternative fire resistance strategies in savanna trees

Abstract Bark properties (mainly thickness) are usually presented as the main explanation for tree survival in intense fires. Savanna fires are mild, frequent, and supposed to affect tree recruitment rather than adult survival: trunk profile and growth rate of young trees between two successive fires can also affect survival. These factors and fire severity were measured on a sample of 20 trees near the recruitment stage of two savanna species chosen for their contrasted fire resistance strategies (Crossopteryx febrifuga and Piliostigma thonningii). Crossopteryx has a higher intrinsic resistance to fire (bark properties) than Piliostigma: a 20-mm-diameter stem of Crossopteryx survives exposure to 650°C, while Piliostigma needs a diameter of at least 40 mm to survive. Crossopteryx has a thicker trunk than Piliostigma: for two trees of the same height, the basal diameter of Crossopteryx will be 1.6 times greater. Piliostigma grows 2.26 times faster than Crossopteryx between two successive fires. The two species have different fire resistance strategies: one relies on resistance of aboveground structures to fire, while the other relies on its ability to quickly re-build aboveground structures. Crossopteryx is able to recruit in almost any fire conditions while Piliostigma needs locally or temporarily milder fire conditions. In savannas, fire resistance is a complex property which cannot be assessed simply by measuring only one of its components, such as bark thickness. Bark properties, trunk profile and growth rate define strategies of fire resistance. Fire resistance may interact with competition: we suggest that differences in fire resistance strategies have important effects on the structure and dynamics of savanna ecosystems.

The influences of tree biology and fire in the spatial structure of the West African savannah

Using a spatially explicit cellular automaton model we explore the effects of tree demography, fire-induced mortality, and seed dispersal on the spatial spread of a single tree species in a humid savanna at Lamto in West Africa. The model system is described by six parameters and consists of a grass-surrounded square grid of connecting cells, each being either inhabited by grass alone or by grass and an individual tree. In the baseline numerical simulations the tree can only recruit seedlings in immediately adjacent cells. These seedlings may perish from annual grass fires in their first year of life if they are not protected from the advancement of the fire by neighbouring reproductively mature trees (...)

Treegrass : a 3D, process-based model for simulating plant interactions in tree-grass ecosystems

Abstract The function and dynamics of savanna ecosystems result from complex interactions and feedbacks between grasses and trees, involving numerous processes (i.e. competition for light, water and nutrients, fire, and herbivory). These interactions are characterised by strong relationships between vegetation structure and function. Given the heterogeneous structure of savannas, modelling appears as a convenient approach to study tree–grass interactions. Most current models that describe carbon and water fluxes are not spatially explicit, which restricts their ability to simulate plant interactions at small scales in heterogeneous ecosystems. We present here a new 3D process-based model called TREEGRASS. The model aims at predicting, in heterogeneous tree–grass systems, plant individual radiation, carbon and water fluxes at a local spatial scale. It is run at a daily time-step over periods ranging from one to a few years. The model includes (i) a 3D mechanistic submodel simulating radiation and energy (i.e. transpiration) budgets; (ii) a soil water balance submodel, and (iii) a physiologically based submodel of primary production and leaf area development. The ability of TREEGRASS to predict the seasonal courses of grass dead and leaf mass, soil water content and light regime as observed in the field has been tested for grassy and shrubby areas of Lamto savannas (Ivory Coast). Simulations showed that the spatial distribution of primary production can be strongly affected by the spatial vegetation structure. Potential applications involve predicting net primary production and water balance from the individual to the ecosystem and from the day to the annual vegetation cycle (e.g. effects of tree spatial patterns on carbon and water fluxes at the ecosystem level).

TREE LAYER SPATIAL STRUCTURE CAN AFFECT SAVANNA PRODUCTION AND WATER BUDGET: RESULTS OF A 3-D MODEL

The spatially explicit and mechanistic model TREEGRASS was used to test the effects of tree density, tree spatial distribution, and tree size distribution on radiation absorption, net primary production (NPP), and water fluxes for a West African savanna. The model has previously been shown to adequately predict radiation absorption, production, and soil water balance in humid savannas. Using field measurements quantifying the spatial variations in grass NPP, the model was found to predict correctly the effect of tree abundance on grass NPP. Model simulations showed that annual photosynthetically active radiation (PAR), absorption efficiency, NPP, and transpiration of the tree layer increased with increasing tree density and decreased with increasing tree aggregation; the grass layer showed the opposite trends. Total NPP (tree plus grass) remained stable for all tree densities and decreased slightly for aggregated tree distributions. Total transpiration increased with tree density and was not affected by t...

Predator foraging behaviour drives food‐web topological structure

1. The structure and dynamics of prey populations are shaped by the foraging behaviours of their predators. Yet, there is still little documentation on how distinct predator foraging types control biodiversity, food-web architecture and ecosystem functioning. 2. We experimentally compared the effects of model fish species of two major foraging types of lake planktivores: a size-selective visual feeder (bluegill), and a filter feeder (gizzard shad). The visual feeder forages on individually captured consumer prey, whereas the filter feeder forages on various prey simultaneously, not only consumers but also primary producers. We ran a 1-month mesocosm experiment cross-classifying a biomass gradient of each predator type. We analysed the effect of each fish on food-web architecture by computing major topological descriptors over time (connectance, link density, omnivory index, etc.). These descriptors were computed from 80 predator-prey binary matrices, using taxa mostly identified at the species level. 3. We found that the visual feeder induced more trophic cul-de-sac (inedible) primary-producer species, lower link density and connectance, and lower levels of food-web omnivory and generalism than the filter feeder. Yet, predator biomass did not affect food-web topology. 4. Our results highlight that top-predator foraging behaviour is a key functional trait that can drive food-web topology and ultimately ecosystem functioning.

Seed shadows, survival and recruitment: how simple mechanisms lead to dynamics of population recruitment curves

According to the Janzen-Connell hypothesis, seedling recruitment around tropical trees is more likely away from parent trees because of density- or distance-dependent predation or pathogen attack on seeds and seedlings. This was expected to lead to a more regular distribution of conspecific adults than would be expected by chance, and to favour coexistence. We first show theoretically that, even if yearly survival increases only slightly with distance to parent trees, an outward shift of seedling recruitment curves with time is very likely simply because seedlings live more than one year before recruiting to the juvenile stage. We tested this hypothesis for a humid savanna, dioecious palm tree, Borassus aethiopum, for which three discrete stages were defined by clear morphological traits. We found that (1) individuals of the second seedling stage are found on average further from their mother than individuals of the first seedling stage, and juveniles are found even further away (relative outward shifts between the three successive stages), and that (2) the older a female is, the further away its seedlings are (temporal outward shifts of distributions of seedlings). Both yearly recruitment (transition between two stages) and survival of seedlings are distance dependent and not density dependent. A matrix population model was used to demonstrate that, during the reproductive part of female palm life cycle, the cumulative effects of these distance-dependent variations in yearly recruitment and survival rates are sufficient to explain qualitatively the observed outward shifts.

Demography of a savanna palm tree in Ivory Coast (Lamto): population persistence and life-history

AFKIXACT. Burassus aetlziokurn is a dioecious palm tree. of African S~V~~IXLS. A stage-classified matrix population model has been parametrizcd with field data (Lamto reserve, Ivory Coast). It enabled the study of the persistence of the population, and analysis ol’ its sensitivity to diff’crcnt vital rates. Age of’ palms in each stage were estimated to complete the description of the palm life-history. The main results arc: (1) The studied populations arc very close to the equilibrium but the stable stage distribution (predicted by the model) and the observed distribution arc significantly different indicating a former change in the vital rates. (2) Reproduction seems to be highly delayed (first reproduction on average at 114 y), while the estimated duration of the reproductive part of the life-cycle is relatively short (22 y).

Positive effect of seed size on seedling survival in fire-prone savannas of Australia, Brazil and West Africa

All plant species face a fundamental reproductive trade-off: for a given investment in seed mass, they can produce either many small seeds or few large seeds. Whereas small seeds favour the germination of numerous seedlings, large seeds favour the survival of seedlings in the face of common stresses such as herbivory, drought or shade (Leishman et al. 2000). One mechanism explaining the better survival of large-seeded species is the seedling size effect (SSE) (Westoby et al. 1996): because seeds with large reserves result in bigger seedlings, seedlings from large-seeded species would have better access to light and/or to reliable water supply than seedlings from small-seeded species.