Environmental Engineering Reference
In-Depth Information
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Fig. 14.6
Small-scale spatial distribution of the amount of bare soil covering planting holes (as shown by the grey
scale, in %; darker areas had more bare soil), and of seedling survival 1 year after planting. Crosses (+) and circles
(•) are dead and alive seedlings, respectively. There is a significant negative relationship between the amount of
bare soil and the survival (logistic regression;
P
< 0.001). Elaborated from Maestre
et al.
(2003b).
establishment of woody shrubs (Maestre
et al.
2003a).
Therefore, the balance between competition and
facilitation is complex and drawing generalizations
about it is unadvised at this point. Biotic crusts, such
as lichens, cyanobacteria, algae and mosses thriving
in surface soil, may also improve nutritional status and
growth of vascular plants and facilitate their estab-
lishment (Belnap
et al.
2001).
There are other factors affecting heterogeneity in
biotic and abiotic conditions in dry areas that may be
relevant to restoration success. Large-scale changes
in exposition, bedrock, slope, etc. can substantially
affect the outcome of restoration. But subtle small-
scale changes in soil properties and microtopography
can also be important. Patches of plant survival in
apparently homogeneous areas are frequently asso-
ciated with small differences in soil depth, stoniness,
texture or nutrient availability (Maestre
et al.
2003b;
Fig. 14.6). As plant responses to these factors may not
be linear it is very important to identify thresholds
that may explain such contrasts in plant perform-
ance. Nevertheless, small-scale heterogeneity may
often not be discernible by direct visual observation.
Thus, other indicators are clearly needed to bring
patchiness into management and restoration plans
and programmes.
As our knowledge on the biotic and abiotic drivers
of seedling establishment progress, we will be able to
incorporate them in routine restoration practices.
However, degraded ecosystems may not be structured
enough to provide favourable sites for establishment.
Or we may want to further promote factors favour-
ing seedling establishment. In these cases we can use
a wide array of ecotechnological tools and techniques
(see below), which in many cases mimic biotic and
biotic interactions.
Soil preparation
Soil-preparation techniques have been developed
to improve water supply to planted seedlings and
ameliorate soil physico-chemical properties. Runoff
harvesting aims to intercept runoff and redirect water
to the planted seedling, and can be performed by
subsoiling (deep regolith drilling) or by the creation
of small runoff collection areas up-slope to direct water
to the plantation hole (microcatchments). Successful
results have been obtained in arid areas of several
countries (e.g. see Whisenant 1999). In arid areas
low-infiltration surfaces allow runoff concentration in
vegetated patches and increase the productivity of the
whole system (Hillel 1992). Compacted soils show
poor aeration, low water permeability and high resist-
ance to root penetration, thus reducing the effective
soil depth for roots. Thus subsoiling/ploughing has also
been used for reducing soil compaction.
Terracing has traditionally been used in Med-
iterranean countries for agricultural purposes and
later for reforestation (ICONA 1989). The effectiveness
of this technique varies greatly with available soil depth,
