Demography of a savanna palm tree: Predictions from comprehensive spatial pattern analyses

Existing statistical methods for spatial pattern analysis now permit precis e analyses of patterns and, given the appropriate interpretation keys, the formulation of testable hypotheses about the underlying processes. From a c omprehensive analysis of the spatial pattern of a plant population and its environment, we infer scenarios of links between demographic processes, pla nt spatial patterns, and environmental heterogeneity. The palm tree Borassu s aethiopum has easily identifiable demographic stages, a root foraging str ategy enabling adults to reach distant nutrient sources, and a marked senes cence starting with the onset of reproduction. We analyzed map data for pal m individuals tin three different life history stages plus two sexes for ad ults) and for nutrient-rich patches (clumps of other tree species and termi te mounds) in three vegetation types (facies) of a humid savanna of West Af rica (Lamto, Ivory Coast). Spatial analyses were based on Diggle's nearest neighbor functions F and G and on Ripley's K function. The main results wer e as follows: (I)juveniles and seedlings are aggregated, while adults have a random pattern or are more loosely aggregated; (2) all stages except fema le adults are spatially associated with nutrient-rich patches, but associat ion distances increase with stage in the life cycle; and (3) seedlings are associated with female adults, whereas the association of juveniles at long er distances is not clear-cut. We propose from these results a parsimonious scenario linking spatial patte rn and mortality pattern during the life cycle. The initial pattern of seed lings (close to maternal trees) results from low dispersal distance. Later stages (older seedlings and juveniles) are mostly restricted to nutrient-ri ch patches through nutrient shortage away from these patches (environment-i nduced mortality) and form dense clumps of immature palms. Competition on n utrient-rich patches then favors the few juveniles that manage to survive f arther from these patches (density-dependent mortality). Finally, the last surviving juvenile of a clump suddenly experiences almost no competition wi th 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 con sistent with this scenario.