Sn. Martens et al., SCALES OF ABOVEGROUND AND BELOW-GROUND COMPETITION IN A SEMIARID WOODLAND DETECTED FROM SPATIAL PATTERN, Journal of vegetation science, 8(5), 1997, pp. 655-664
Semi-arid woodlands are two-phase mosaics of canopy and inter-canopy p
atches. We hypothesized that both aboveground competition (within cano
py patches), and below-ground competition (between canopy patches), wo
uld be important structuring processes in these communities. We invest
igated the spatial pattern of trees in a Pinus edulis-Juniperus monosp
erma woodland in New Mexico using Ripley's K-function. We found strong
aggregation of trees at scales of 2 to 4 m, which indicates the scale
of canopy patches. Canopy patches were composed of individuals of bot
h species. Crown centers of both species were always less aggregated t
han stem centers at scales less than canopy patch size, indicating mor
phological plasticity of competing crowns. In the smallest size classe
s of both species, aggregation was most intense, and occurred over a l
arger range of scales; aggregation decreased with increasing size as i
s consistent with density-dependent mortality from intraspecific compe
tition. Within canopy patches, younger trees were associated with olde
r trees of the other species. At scales larger than canopy patches, yo
unger trees showed repulsion from older conspecifics, indicating below
-ground competition. Hence, intraspecific competition was stronger tha
n interspecific competition, probably because the species differ in ro
oting depth. Woodland dynamics depend on the scale and composition of
canopy patches, aggregated seed deposition and facilitation, above- an
d below-ground competition, and temporal changes in the spatial scale
of interactions. This woodland is intermediate in a grassland-forest c
ontinuum (a gradient of increasing woody canopy cover) and hence we ex
pected, and were able to detect, the effects of both above-and below-g
round competition.