Environmental Engineering Reference
In-Depth Information
2
using known reference systems, such as known
or attributed original vegetation and semi-
natural, humanized landscapes;
3
fully exploiting the potential of native species,
ecotypes and provenances.
• To increase ecosystem resilience, especially in rela-
tion to the most threatening disturbances, such as
wildfires, extreme drought events and gradual drift
to hotter and drier climates (Pausas & Vallejo
1999, Pausas 2004).
• To promote self-regenerating systems that will be
as independent as possible from further subsidies,
and to ensure ecosystem sustainability and health
for both natural and semi-cultural systems (such as
the dehesas, described in Box 14.1).
Box 14.1 Dehesas
Over five centuries or more, people in southern
Spain, Portugal, Italy and parts of north Africa have
fashioned and maintained a two- or multi-tiered,
semi-natural ecosystem from within the matrix pro-
vided by natural woodlands (Joffre
et al.
1988, 1999,
González-Bernáldez 1992). These silvopastoral or
agrosilvopastoral systems make multiple use of trees
(shade, fruits, bark, firewood, etc.) and the herbaceous
stratum. Grazing by domestic livestock of annual and
especially perennial herbs and grasses is invariably an
important element in
dehesa
(Spanish) and
montado
(Portuguese) management; concurrently shrubs and
small trees are regularly removed. However, in
Sardinia, where the system is called
pascolo arbolato
,
and in north Africa the shrubs are usually present.
Of particular interest are the 2.2 million ha of cork
oak (
Quercus suber
)-dominated open woodlands in
north-western Africa and south-western Europe where
periodic harvests of natural cork provide an import-
ant cash supplement to the annual revenues derived
from animal husbandry and, in southern Iberia, from
sown crops such as cereals.
Dehesas are thus artificially opened and managed
woodlands that are simplified compared to natural
woodlands. They have the virtue of mimicking nat-
ural Mediterranean ecosystems and are thus highly
attractive alternatives, from both ecological and
economic perspectives, as compared to other dry-
farming or irrigated systems where all or most trees
are eliminated. Unfortunately, these formations are
widely threatened with extinction (e.g. Mellado
1989), either through intensification, or extensifica-
tion of their usage, both of which tend to be dele-
terious for the resilience and productivity of the
systems. They require new inputs and new ideas to
promote natural regeneration and to re-introduce lost
species while maintaining economic viability in an era
of profound socio-economic change. Many of them
require active restoration following a careful diagno-
sis and cooperative dialogue with all stakeholders.
14.2 Disturbance and land-use changes
14.2.1 Ancient history of human impacts
There are natural drivers for degradation in the
Mediterranean area (Grove & Rackham 2000); how-
ever, human impacts are long-standing features of the
Mediterranean Basin. Environmental degradation in the
Mediterranean is ancient (Thirgood 1981, Wainwright
1994). Palaeolithic people used fire deliberately to
facilitate hunting and food gathering (Stewart 1956,
Trabaud 1998) and, since then, millennia of severe
pressure resulting in burning, cutting and grazing non-
arable lands, and clearing, terracing, cultivating and
later abandoning arable portions, have created a vast
array of strongly human-modified landscapes. Still
today we see a kaleidoscopic tapestry of managed cul-
tural and semi-cultural landscapes and agroecosystems
which reflects the historical, cultural and legislative
diversity of the region.
As compared to the Persian, Greek, Egyptian and
Carpathian civilizations, the Roman empire was one
vast city-building enterprise requiring huge amounts
of wood (Hughes 1982). In Spain, the human popu-
lation was already close to the theoretical carrying
capacity of traditional Mediterranean agroecosystems
during the second century
AD
(Butzer 1990). In the
Middle Ages, the Arab empire also made large inroads
into the forests of the north African littoral and
mountains, and those of Iberia. The recognition of land
degradation and the declaration of the intention to pro-
mote reforestation by the various ruling regimes also
came relatively early, and developed especially since
the Modern age (Marsh 1871). In short, we can see
the Mediterranean Basin as a moving mosaic where
various land uses have moved in space and in time.