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Boreal Mixedwood Forests May Have No "Representative" Areas: Some Implications for Reserve Design

S. G. Cumming, P. J. Burton and B. Klinkenberg
Ecography
Vol. 19, No. 2 (Jun., 1996), pp. 162-180
Published by: Wiley on behalf of Nordic Society Oikos
Stable URL: http://www.jstor.org/stable/3683337
Page Count: 19
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Boreal Mixedwood Forests May Have No "Representative" Areas: Some Implications for Reserve Design
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Abstract

We tested the shifting mosaic steady state theory in a $\approx 73\ 600\ {\rm km}^{2}$ landscape within the boreal mixedwood region of Alberta, Canada. The theory predicts that, at some spatial scale, mean vegetation attributes are temporally stable. It follows that contiguous sub-regions (stable mosaics) exist whose attributes are similar to regional values. The scale at which this stability occurs may be interpreted as the minimal size for an ecological reserve designed to represent the region. We argue that the appropriate attributes to test the theory in this case are the age and size structures of various forest stand populations. Using forest inventory data, we searched for stable mosaics ("representative areas") at multiple spatial scales, using two different spatial decompositions of the study area. Areas were compared using 2-dimensional similarity metrics based on goodness-of-fit statistics. We found that the age and size structures of the entire study area could not be replicated at any smaller scale, which disconfirms the theory for the boreal mixedwood. Each of five compositionally distinct populations of forest stands have characteristic age and size structures, and patterns of spatial variability, which we relate to aspects of the regional ecology. Stand age and size structures have independent spatial variability. We observed multi-scaled heterogeneity in forest composition, which appears to arise partly from infrequent fire episodes when many large fires occur. These episodes are probably not uniform in their effect on forest structure. We evaluate two methods for constructing reserve systems in non-equilibrium landscapes, using small collections of disconnected subunits. Small areas can readily be built that more closely match landscape properties than any contiguous area of reasonable size. The degree of representativeness and total size of the constructed areas is sensitive to the size of the constituent pieces; in the mixedwood, there is a natural scale of $\approx 1\ 500\ {\rm km}^{2}$ which is near-optimal. Reserves constructed in this way are unlikely to be temporally stable. We therefore propose a dynamic strategy for maintaining reserve systems over time within a managed landscape. Under this "floating reserve" strategy, portions of the system would be periodically replaced in response to the aging of components, unexpected large scale disturbance, or refinements in conservation objectives.

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