Environmental Engineering Reference
In-Depth Information
Seedling q uality
How to characterize seedling quality is subject of much
debate. Seedlings should be able to withstand unfa-
vourable growing conditions (transplant shock, sea-
sonal water stress and drought cycles), and take
advantage of short favourable climatic periods to
achieve sustained growth. Seedling quality has been
substantially improved over the past decade (Cortina
et al
. 2006). Most approaches are based on nursery
manipulations simultaneously affecting several plant
morpho-functional traits. Seedling size together with
other, mostly visually assessed, characteristics are used
to defi ne acceptable stocks. However, above-ground
size may not be a good indicator of seedling quality
under dryland conditions. Indeed, the relationship
between seedling morpho-functional traits and estab-
lishment success is not clear, as seedling fate may be
ultimately driven by the complex set of interactions
allowing fast and deep rooting. The few long-term
studies undertaken to date suggest that the positive
effects of enhanced seedling quality may endure for a
decade after planting (Cortina
et al
. 2006 ). Fortunately,
many sophisticated
nursery
practices are now available
to manipulate seedling traits, although there is still
much scope for improvement in this area, particularly
through the use of alternative, biodegradable and recy-
cled materials as containers, as well as substitutes for
standard soil mixes. Already, however, seedling quality
is probably no longer a major limiting factor for the
establishment of common species in the Mediterra-
nean and in the other MCRs, provided that current
knowledge is applied.
tussock steppes (
Stipa tenacissima
), common in south-
ern Spain and throughout northern Africa, is a clear
example of this, as tussock microsites have higher
organic matter, higher water availability, lower tem-
peratures and lower soil penetration resistance than
intertussock patches. These environmental modifi ca-
tions facilitate the development of bryo-lichenic com-
munities, and introduced woody plants (Maestre
et al
.
2003a). In other systems, the nurse plant can be a
spiny shrub, as it protects planted seedlings from
grazing (G รณ mez
et al
. 2001). In the Sierra Nevada,
southern Spain, 4 years after planting, seedlings of
Quercus
,
Pinus
and
Acer
spp. planted under shrubs
had, on average, rates of survival three times higher
than those planted in open microsites (Castro
et al
.
2002). In contrast, under semi-arid conditions,
Pinus
halepensis
may not be capable of facilitating the estab-
lishment of woody shrubs (Maestre
et al
. 2003b ). The
outcome of plant-plant interactions depends on the
level of stress and on the morpho-functional attributes
of benefactor and benefi ciary species. Uncertainty in
the results of plant-plant interactions represents a
major challenge for the use of facilitative interactions
in restoration programmes.
Heterogeneity
in biotic and abiotic conditions in
dry areas may be relevant to restoration success. Large-
scale changes in slope aspect and bedrock can substan-
tially affect the outcome of restoration. Further, subtle
small-scale changes in soil properties and microto-
pography can also be important. Patchiness of plant
population survival and persistence in apparently
homogeneous areas are frequently associated with
small differences in soil moisture, soil depth, stoniness,
texture or nutrient availability (Figure 11.2). As plant
responses to these factors tend to be nonlinear, it is very
important to identify thresholds that may explain such
differences in plant performance. A wide array of eco-
technological tools and techniques, which in many
cases mimic abiotic conditions and biotic interactions,
is currently available (see below). But it is important
to note that the effect of local biotic and abiotic con-
ditions on plant performance is commonly much
greater than the effects of the various ecotechnological
tools employed by restorationists (Cortina
et al
. 2011 ).
Microsite s election and n urse p lants
The recognition of the importance of microhabitat
heterogeneity - particularly the role of positive plant-
plant interactions and the role of 'safe sites' - has con-
tributed to the elaboration of a new paradigm for
ecological restoration in MCRs. Facilitative interac-
tions are especially important under semi-arid condi-
tions, where isolated vegetation patches act as
'resource islands', mainly in terms of shade and soil
fertility (Maestre & Cortina 2004).
Facilitation
may
show a unimodal response to
stress
, with a maximum
at intermediate levels and declining both at the lowest
and highest levels of stress (Maestre & Cortina 2004),
but the relationship between facilitation and stress is
not simple. The patterned landscape of semi-arid
Soil p reparation and a mendment
Various soil preparation techniques have been devel-
oped to improve water supply to planted seedlings and
ameliorate soil physico-chemical properties in degraded
