Walia Special Edition on the Bale Mountains (2011) - Zoologische ...

Observers located each point by GPS, and four subsamples (N/E/S/W) were taken at each
location. Sampling density was equivalent to that on rodent trapping grids, with 5m between
subsamples. For each plant species intersected by the sampling rod, observers recorded the maximum
height, plant part (leaf, stem or flower) and colour (to indicate seasonal changes in nutritional
quality). Substrate (earth, rock or water), date, time and observer were also recorded.
Vegetation was surveyed by observers without formal botanical expertise, limiting the
number of taxa which could be identified to species level. Surveys occurred in both wet and dry
seasons, so the flower structures necessary for identification of grasses and some herbaceous species
were not always present. All grass-like plants were therefore split into two categories only: wetland
sedges and grasses. Similarly, no attempt was made to distinguish between different mosses or
lichens.
Ground-truthing and statistical analysis
The land cover type represented by each class was determined using qualitative habitat descriptions,
supplemented by visual examination of aerial photographs, false-colour composites, 199 groundtruthing
photographs and familiarity with areas gained during field surveys between April 2003 and
April 2005. To assess how related clusters of classes in the distance dendrogram differed from one
another, pairwise statistical comparisons were made at each node for which sufficient data existed.
Differences in plant species composition and substrate were assessed using contingency tables.
General Linear Models (GLMs) were used to test for differences in altitude, slope, aspect, and
percent vegetation cover, and mean sward height was also compared after accounting for species
composition and altitude. Tests used all classes and all species for which enough data were available;
sample sizes varied depending upon which classes were being compared.
The combination of mountainous terrain, low sun elevation (52.1 degrees) and sun azimuth
(139.6 degrees) at the time of satellite image capture caused large differences in illumination
between southeast and northwest slopes. This meant that classes on steeply sloping ground may
have differed from each other only in the degree of illumination, rather than in species composition
or density of vegetation. Therefore we calculated the incidence angle of the sun’s rays to describe
the relative illumination of each pixel, and test for differences in average illumination between
classes. The following formula was taken from Dozier and Frew (1990):
θ i = arccos(cosθ 0 *cosS + sinθ 0 *sinS*cos(Φ 0 − A))
where θ = illumination angle, θ = solar elevation, S = pixel slope, Φ = solar azimuth and A = pixel
i 0 0
aspect.
We investigated seasonal changes in photosynthetic activity by categorising plants into ‘alive’
(flowering or green) or ‘dead/dormant’ (brown or yellow). This was hard to determine for some
pubescent species such as Helichrysum sp.; the status of these species was assigned as ‘unknown’
unless there was evidence of flowering.
<strong>Walia</strong>-<strong>Special</strong> <strong>Edition</strong> on the Bale Mountains 101