|SREL Reprint #1896|
A null model for neighborhood models of plant competitive interactions
Neighborhood (spatial) models of plant populations have assumed implicity that competition
between adjacent individuals is the primary determinant of observed dynamics. In contrast.
non-spatial models now routinely test this assumption against the null hypothesis that individual
differences in intrinsic growth rate alone can explain observed patterns in a widely-used
neighborhood model (Pacala and Silander 1990). competition is modelled as a direct effect of
neighbors on tareet plant biomass which in turn affects plant fecundity. We have developed a
statistical null model for Pacala and Silander's (1990) neighborhood model for plant monocultures
that explicitly incorporates variation in plant growth rate, and permits a test of the hypothesis that
neighborhood competition controls plant ground area occupied (= rosette diameter) and biomass.
An additional alternative hypothesis to our model is that neighbors can affect fecundity
indepetidentl\- of their effects on plant diameter and biomass. We tested these three hypotheses -
(1) no effect of competition (null): (2) neighborhood competition directly affecting plant diameter
and biomass, and indirectly affecting fecundi", (Pacala and Silander 1990): (3) neighborhood
competition directly affecting plant diameter. biomass. and fecunditv - by replicating, with
appropriate controls. Silander and Pacala's (1985) experiments with the rosette-forming crucifer.
Arabidopsis thaliana. Neighborhood competition in dense stands resulted in simultaneous changes
in plant diameter, biomass. and fecundity relative to plants grown in the absence of competition.
There also were apparent effects of neighbors on plant fecundity independent of their effects on
plant size alone. The null model was not supported. but Pacala and Silander's model did not fit the
data as well as the model that incorporated direct effects of neighbors on plant fecundity.
SREL Reprint #1896
Ellison, A.M., P.M. Dixon, and J. Ngai. 1994. A null model for neighborhood models of plant competitive interactions. Oikos 71:225-238.