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R.A. Diaz-Varela et al. 2010. Quantitative assessment of temporal dynamics in altitudinal-driven ecotones
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of herbaceous formations, dwarf scrubs and thickets take place. Woodlands with variable
density of scattered and often stunted individuals of pine (Pinus sylvestris, P. mugo), spruce
(Picea abies) and larch (Larix decidua) occur between this treeless area and dense forest
formations. These species also dominate the high altitude forests, while the lower parts are
covered by different tree formations dominated by deciduous broadleaved species. Hay
meadows are the most common agricultural land, occupying only a small surface at valley
bottoms, while arable land is virtually absent in the area.
2. Methodology
We started with the stereoscopic analysis and visual interpretation of aerial photos from 1954
and 2003 to generate a raster map with the distribution of close forest, scattered trees and alpine
tundra for each of the dates. We then used a spatial overlay technique to analyse the temporal
change of the three classes. The overlay outputs were arranged in a 44 contingency matrix
using as input the 1954 (columns) and 2003 (rows) raster maps, and considering 4 classes,
namely: “Dense forest”, “Scattered trees”, “Tundra” and “Other”. Overall Kappa and Kappa
indices for each land cover class (Cohen 1960) were used as estimators of the rate of change, as
suggested in other studies (Calvo-Iglesias et al. 2006).
We then applied an algorithm developed by Diaz Varela et al. (2010) to identify the uppermost
pixel of targeted ecotones for each of these maps. The method used to delineate forest line, tree
line and tundra line ecotones follows the general definition of tree line by Körner (1999) and
aims at performing the automatic discrimination of elements (outpost) with an uppermost
extreme position on a certain slope. Outposts corresponded to the uppermost pixels of each of
the three land cover classes and they were used to represent and map the ecotones. That is, for
the land cover class “forest”, outposts define the forest line; the “dense forest” plus “scattered
trees” class outposts define the tree line and the “tundra” class outposts define the tundra line.
An outpost is herein defined according to this example regarding trees: an operator in the field
will define a tree as belonging to the tree line if he encounters no other tree while walking
upward in the direction of maximum slope for as long as possible (i.e. until a summit is reached).
Therefore, the trajectory from a given point will move upward following a maximum slope path
until it encounters another element of the same class or finishes at a summit large enough to
define a slope or series of slopes with similar exposure at a certain scale (cf. fig 2). Extending
this concept from single trees to a the maps of forests, trees and tundra, the algorithm was
implemented in ITT-IDLTM V. 6.3 to label the cells of a binary land cover map (in this case
tree – no tree) as belonging to the ecotone or not (see Diaz-Varela et al. 2010 for a more
thorough description of the method).
The temporal evolution of the different ecotones was analysed by extending the algorithm to a
diachronic map. For the identification of ecotone altitude advances we ran upward flow paths
from any upslope outpost pixel for 1954 until a 2003 upslope outpost pixel or a summit was
reached. Altitude retreats were computed with the same scheme but using 2003 and 1954
ecotone pixels as starting and end points, respectively. Therefore, where the ecotone location in
1954 appeared in a higher position than in 2003 along a given maximum slope line, it was
possible to trace and quantify the retreat or negative altitudinal shift.
To characterise morphologically the spatial pattern of the outposts for the three ecotones in the
period of reference and to assess changes in vegetation dispersion strategies, we used the
GUIDOS software (Graphical User Interface for the Description of image Objects and their
Shapes). This software was recently developed for morphological spatial pattern analysis
(MSPA) of forest functional connectivity in the context of biodiversity studies (Vogt et al. 2008,
Vogt et al., 2009). The output of the MSPA is a raster layer where patch cells are assigned to
Forest Landscapes and Global Change-New Frontiers in Management, Conservation and Restoration. Proceedings of the IUFRO Landscape Ecology
Working Group International Conference, September 21-27, 2010, Bragança, Portugal. J.C. Azevedo, M. Feliciano, J. Castro & M.A. Pinto (eds.)
2010, Instituto Politécnico de Bragança, Bragança, Portugal.