Representation of an alpine treeline ecotone in SPOT 5 HRG data
Publication from Digital
Ross A. Hill, Granica K., Geoff M., Smith, Schardt M.
Remote Sensing of Environment , 2007
An ecotone is a zone of vegetation transition between two communities,
often resulting from a natural or anthropogenic environmental gradient.
In remotely sensed imagery, an ecotone may appear as an edge, a boundary
of mixed pixels or a zone of continuous variation, depending on the
spatial scale of the vegetation communities and their transition
zone in relation to the spatial resolution of the imagery. Often
in image classification, an ecotone is either ignored if it falls
within a width of one or two pixels, or part of it may be mapped
as a separate vegetation
community if it covers an area of several pixel widths. A soft classification
method, such as probability mapping, is inherently appealing for
mapping vegetation transition. Ideally, the probability of membership
each pixel has to each vegetation class corresponds with the proportional
composition of vegetation classes per pixel. In this paper we investigate
the use of class probability mapping to produce a softened classification
of an alpine treeline ecotone in Austria using a SPOT 5 HRG image.
Here the transition with altitude is from dense subalpine forest
to treeless alpine meadow and herbaceous vegetation. The posterior
probabilities from a Maximum Likelihood algorithm are shown to reflect
the land-cover composition of mixed pixels in the ecotone. The relationships
between the posterior probability of class membership for the two
end-member classes of ‘scrub and forest’ and ‘non-forest vegetation’
and the percentage ground cover of these vegetation classes (enumerated
in 15 quadrats from 1:1500 aerial photographs) were highly significant:
r2=0.83 and r2=0.85 respectively ( pb0.001, n=15). We identify thresholds
(alphacuts) in the posterior probabilities of class membership of
‘scrub and forest’ and ‘non-forest vegetation’ to map the alpine
treeline ecotone as a transition zone of five intermediate vegetation
classes between the end-member communities. In addition, we investigate
the representation of the ecotone as a ratio between the posterior
probabilities of ‘scrub and forest’ and ‘non-forest vegetation’.
This displays the vegetation transition without imposing subjective
boundaries, and has greater emphasis on the ecotone transition rather
than on the end-member communities. We comment on the fitness for
purpose of the different ways investigated for representing the alpine
treeline ecotone.
Keywords: Soft classification; Probability; Ecotone; Alpine treeline; SPOT HRG