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.