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R.A. Diaz-Varela et al. 2010. Quantitative assessment of temporal dynamics in altitudinal-driven ecotones
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Analysis of land cover dynamics by spatial overlay of multi-temporal land cover maps showed
that forest and scattered tree formations tend to expand their areas, while tundra was more stable
in time. However, the application of analyses addressing ecotone dynamics more specifically
proved that the uppermost location of these formations underwent significant changes and
showed a general trend to advance in altitude.
Ecotone dynamics typically correspond to a balance between colonization and extinctions,
which in the case of mountain altitude-driven ecotones can be formalised as advances or retreats
on the slopes. Therefore, to identify unambiguously if the changes correspond to an effective
advance or retreat in altitude, we performed a directional analysis of the altitudinal shifts of
extreme outposts along the maximum slope path. Results showed that most of the ecotone
dynamics corresponded to advances or overpasses of outposts. Forest line retreats were virtually
absent, corresponding at almost all outpost changes to advances in altitude. Tree and tundra
lines showed a certain amount of retreats concentrated at the top of their current altitudinal
distribution, but had a much higher rate of advance than retreat. The median values for the forest,
tree and tundra line advances corresponded to decadal upward shifts of 26, 17 and 15 m
respectively. Absolute altitudinal shift (considering together advances as positive values and
retreats as negatives in the same frequency distribution for each ecotone) had similar median
values of 25, 13 and 11 m respectively.
Spatial pattern analysis at the ecotone locations provided complementary information for a
better understanding of their dynamics. Thus forest and tree lines tended to show a more
compact arrangement in 2003 than in 1954, with a significant decrease in islets, indicating a
decrease in long-distance dispersion and suggesting a stabilisation of its advance in the form of
massive colonisation along wide advance fronts. Alternatively, the tundra line increased the
number of islets, pointing towards an increase in long-distance colonisation. This fact, along
with the intense dynamics deduced from the results of advance and retreat paths, points out this
ecotone as prone to undergo significant future changes in the case of the continuing of present
trends in environmental dynamics.
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