Plant Diversity and Fire - Fire in Mediterranean Ecosystems - page 314

Box 11.1 ( cont. )

Slope orientation potentially has an effect on recorded richness. It has been
suggested that community diversity is maximized when the long axes of rect-
angular plots are oriented perpendicular to the elevational contours (Stohlgren
et al. 1998 ). Although elevational changes in species composition are well
documented, there is no evidence of such an effect at the local 0.1-ha scale.
Indeed, it has been suggested (Keeley & Fotheringham 2005 ) that on steep
slopes in semi-arid MTC regions, plots parallel to the contour might actually
increase diversity as species tend to follow drainage patterns up and down the
slope, with greater species turnover along the contour as populations of differ-
ent species encounter different microscale drainage patterns.
Interpreting differences in species-area curves at the regional level is compli-
cated by one factor not usually encountered at the local level. Community-scale
species-area curves generally measure area along a plane parallel to the ground
surface, whereas larger-scale area measurements use map projections, which
on level terrain can be equated with surface area, but on steep terrain, map
projections underestimate the actual ground surface area. Thus, when one
considers the role of landscape heterogeneity, any increases in diversity in
mountainous regions must be partially the result of the additional ground
surface area hidden in such area estimates. Some forestry studies of community
diversity have used slope-corrected measurements, which creates the same
sampling artifact, and thus they are sampling diversity in areas larger than
the actual area parallel to the ground surface.

Two of the southern hemisphere MTC communities differ structurally from
those in the north. Many of the prominent South African Cape and southwest
Australian ecosystems exist on highly infertile substrates (see Fig. 1.5 ) that do not
lend themselves to tall-stature closed-canopy shrublands. Shrubs of different
stature contribute to multiple strata interspersed with a greater diversity of growth
forms, often including various Restionaceae (see Fig. 7.7 ) and other herbaceous
perennials (Naveh & Whittaker 1980 ). Community diversity is relatively high and
there are only modest increases in species richness after fire ( Table 11.2 ). Chilean
shrublands occur on more fertile substrates and, without disturbance, will form
closed-canopy shrublands and woodlands with little understory. However, due to
the absence of fire-stimulated species, they do not exhibit a postfire increase in
diversity (S. Keeley & Johnson 1977 ; Armesto & Gutierrez 1978 ) as observed in
other MTC communities.
The role of vegetation structure in controlling diversity is evident even within
regions. In eastern Mediterranean Basin shrublands, open-canopy disturbed sites
have more than three times greater alpha diversity than closed-canopy associ-
ations ( Table 11.2 ). Although South African fynbos and Australian heathlands