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Spatial Dependence of Tree Biomass in an
Old-Growth Forest
Franco ~ i o n d i and
'
Donald E. ~ y e r s *
Abstract.-Geostatistical models were fitted to aboveground tree biomass and
decadal biomass increment of an old-growth, naturally-seeded stand of southwestern
ponderosa pine (Pinzrsponderosa Dougl. ex Laws. var. sco-pulorurn) whose density
increased over the twentieth century. Our objective was to determine the effects of
crowding on spatial dependence of tree biomass, especially compared to our
previous results concerning tree size and basal area increment (Biondi et al. 1994).
Despite increased crowding, tree biomass maintained the same type of spatial
dependence from 1920 to 1990. An isotropic Gaussian variogram was the model of
choice to represent spatial dependence at all times. Tree biomass was spatially
autocorrelated over distances no greater than 30 m, a measure of average patch
diameter in this forest ecosystem. Since patch diameter remained constant through
time, tree density increased by increasing the number of pine groups, not their
horizontal dimension. Spatial dependence of tree biomass was greater than that of
biomass increment. Increased crowding over time altered spatial dependence of
biomass increment: even though the isotropic Gaussian variogram was still the
model of choice, its range doubled from 1920-30 to 1980-90, while the amount of
spatial dependence was halved. The latter finding indicates that decadal-scale
accumulation rates over the twentieth century became less dependent on mutual tree
position. These results confirmed our previous findings based on tree size and basal
area increment. Simulation models and area estimates of stand dynamics in oldgrowth forests may be improved by including geostatistical components to
summarize ecological spatial dependence.
REFERENCES
Biondi, F., D.E. Myers and C.C. Avery 1994. Geostatistically modeling stem size and
increment in an old-growth forest. Canadian Journal of Forest Research 24: 13541368.
' Dend~ochrvnologist,Scripps Institution of Oceanography, Unrversiy of Califor~~~a-Son
D~ego.La Jolla. CA 920934215, lIS.4
Professo~Department of Mathematics Universigj of Arizona. Tucson, AZ 85721. lJSA