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Much of the
terrestrial biosphere can be viewed as a continuum of plant
communities ranging from grassland to forest that vary in the
relative proportions of herbaceous (grass-like or non-woody) and
woody plants (Belsky and Canham 1994). Many studies and models for
vegetation dynamics focus on either end of the continuum: at
grasslands or at forests. Yet a great majority of plant
communities lie in an intermediate position along this conceptual
gradient of increasing cover by woody plants. Predicting the
responses of these communities of mixed life-forms to changes in
variability in climate and land use is important for addressing
many applied problems and is also a major priority for global
change research.
The
responses of plant communities to climate and land use and is
dependent on an improved understanding of physiological responses
to light and water use, as well as to CO2 and nutrients. Along the
continuum from grassland to forest, the relative importance of
below-ground resources (e.g., water) is hypothesized to decrease
while that of above-ground resources (i.e., light) is hypothesized
to increase.
For
communities that are intermediate along the continuum, competition
for both below-ground resources (e.g., water) and above-ground
resources (i.e., light) is expected to influence plant community
structure. We tested this hypothesis by evaluating the spatial
patterns of piñon and juniper stems and crowns at the Mesita del
Buey study site (Martens et al. 1997).
We used
Ripley’s K, a spatial statistic that indicates whether a pattern
of points is regular, random, or aggregated in spatial
distribution (and similarly distinguishes between attraction and
repulsion for two sets of points [e.g., piñon and juniper]. Our
analyses supported the hypothesis. Evidence for competition for
light was provided by a shift in crown positions relative to stem
position such that crowns became less aggregated. Evidence for
competition for water was provided by a trend towards regularity
in stems at scales of 8 m or more, a finding that is consistent
with other work that we have conducted on plant water use
(Breshears et al. 1997a).
Thus, near
the intermediate portion of the continuum, we believe both below-
and above-ground competition are operating. Variance in a variety
of ecosystem properties may be high near the intermediate portion
of the continuum because both processes are operating and, perhaps
more importantly, because both canopy and intercanopy patches,
which differ in many properties, represent a substantial portion
of the total cover. We propose that the grassland-forest continuum
can be understood more fully by evaluating it in terms of its two
functional units: the canopy patches of woody plants (trees and
shrubs) and the intercanopy patches that separate them (or, in a
more dense stand lie within them—gaps).
The patches
beneath the canopies of woody plants (trees and shrubs) differ in
many fundamental ways from the intercanopy patches that separate
them (or, in more dense stands, lie within them). We view these
canopy and intercanopy patches as functional units for
understanding processes operating at hillslope scales and across
landscape gradients within the grassland-forest continuum. Our
approach to understanding processes along the continuum has been
to quantify the heterogeneity between canopy and intercanopy
patches and the connectivity between them, allowing us to develop
to relationships that translate patch-scale processes to larger
spatial scales.
At our
primary study site, the Mesita del Buey Piñon-Juniper Site, where
there is roughly equal coverage by canopy and intercanopy patches,
we have quantified many aspects of heterogeneity and connectivity
between the two patch types.
Almost ever
major component of water, energy, and nutrient fluxes, differ
substantially between canopy and intercanopy patches, as we have
documented at our site and others have documented elsewhere. Yet
there are important modes of connectivity between these two patch
types. This canopy / intercanopy heterogeneity and connectivity,
in conjunction with physiological relationships - particularly
those related to plant water use, can be integrated to unify
current theories in semiarid ecology.
Related
Publications
Belsky, A.
J., and C. D. Canham. 1994. Forest gaps and isolated savanna
trees. BioScience 44: 77-84.
Breshears,
D. D., O. B. Myers, S. R. Johnson, C. W. Meyer, and S. N. Martens.
1997a. Differential use of heterogeneous soil moisture by two
semiarid woody plant species: Pinus edulis and Juniperus
monosperma. Journal of Ecology 85: 289-299.
Martens, S.
N., D. D. Breshears, C. W. Meyer, and F. J. Barnes. 1997. Scales
of above-ground and below-ground competition in a semi-arid
woodland detected from spatial pattern. Journal of Vegetation
Science 8: 655-664
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