Jean-Claude Menaut

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.

Tree and grass rooting patterns in an African humid savanna

. Spatial and temporal soil partitioning between roots of the two savanna plant components, i.e. trees and grasses, were investigated in a West African humid savanna. Vertical root phytomass distribution was described for grass roots, large (> 2 mm) and fine (< 2 mm) tree roots, in open sites and beneath tree canopies. These profiles were established monthly over one year of vegetation growth. Natural 13C abundance measurement was used to determine the woody/herbaceous phytomass ratio in root samples. Tree and grass root distributions widely overlapped and both were mostly located in the top 20 cm of the soil. Grass root phytomass decreased with depth whereas woody root phytomass peaked at about 10 cm depth. No time partitioning was detected. These structural results do not support the hypothesis of soil resource partitioning between trees and grasses and are thus consistent with functional results previously reported.

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.

Structure, Long-Term Dynamics, and Demography of the Tree Community

In this chapter, we study the population and community dynamics of the major tree species of the most common savanna type in Lamto, the Andropogoneae shrub savanna (see Sect. 5.2). The most frequent tree species are: Borassus aethiopum, Bridelia ferruginea, Crossopteryx febrifuga, Cussonia arborea and Piliostigma thonningii . Of these, only the palm tree Borassus can be considered as a true tree species since it can reach 20 m in height; the other species are smaller and never grow over 10-12 m. These five species will be the main subject of our study since they comprise 90% of tree individuals. Another species, Annona senegalensis, is very common in Andropogoneae savannas, and although it can develop a real (small) tree morphology, it never grows over 3 m in burned savannas. Other less frequent, true tree (up to 15-20 m in height) species will sometimes be examined in this chapter: Terminalia shimperiana, Zanthoxylum zanthoxyloides, Lannea barteri, Vitex doniana, Pterocarpus erinaceus and Ficus sur. These species are usually rare in the savanna, but can be locally abundant in fire-protected sites like rocks and dense tree clumps: Terminalia shimperiana often dominates savanna woodlands. Tree dynamics is driven by competition for resources and fire. As fire intensity varies considerably in space (Sect. 4.5), as nutrient availability is low in the savanna except in some rich patches (Sect. 4.3), and as the seed dispersal distances of trees are small ([12]; Gignoux, unpublished data), the population dynamics of trees is linked to the spatial structure of the ecosystem. For these reasons, we examine in this chapter tree population dynamics in relation to population spatial structure.