Geology Reference
In-Depth Information
previously protected from erosion processes. The
changes in vegetation productivity, described in
the previous paragraphs, would also lead to
changes in ground cover by vegetation residue,
with additional impacts from changes in micro-
bial activity driven by temperature and soil mois-
ture availability (Kundzewicz
et al
., 2007).
The impacts of climate change upon soil
organic matter and structural properties are more
difficult to estimate, since the processes involved
are complex and not completely understood (e.g.
Dawson & Smith, 2007). A number of studies
along climatic gradients (e.g. Lavee
et al
., 1998;
Boix-Fayos
et al
., 2001; Sarah, 2006) have demon-
strated how soil structural stability is related to
soil organic matter content, which varies with
temperature and soil moisture in a non-linear
way, although with high spatial heterogeneity
due to relationships with vegetation cover pat-
terns. The IPCC report points to climate change
impacts on soil carbon dynamics, however, due
to a combination of changes to vegetation pro-
ductivity and an increase in soil respiration; this
could lead to a global decrease in soil organic car-
bon content, with soil becoming a net source of
CO
2
during the early stages of climate change
until a carbon equilibrium is eventually re-
established (Fischlin
et al
., 2007). While there is
significant uncertainty in the magnitude of this
process, it could lead to a decrease in soil struc-
tural stability in many regions.
systems, due to either overexploitation or a reduc-
tion in the carrying capacity for these systems
(Puigdefábregas, 1998; Fernández, 2002; Herrmann
& Hutchinson, 2005).
As described above, climate change could lead
to decreased water availability and increased
physical constraints on ecosystem productivity
in drylands, especially in subtropical regions and
Mediterranean-type ecosystems in the mid lati-
tudes, increasing their vulnerability to desertifi-
cation (Fischlin
et al
., 2007). An increase in
climatic aridity can surpass a threshold where
available water cannot support full vegetation
canopy cover; ecosystems adopt strategies to har-
vest water and nutrients by adopting a pattern of
vegetated and bare patches, with the latter acting
as runoff and sediment sources for the former,
leading to an increase in runoff generation and
soil erosion when compared with drylands above
the threshold (Bergkamp
et al
., 1999; Imeson &
Prinsen, 2004; Ludwig
et al
., 2005). This process
can be self-reinforcing, as reduced biological
activity in the bare patches promotes an increase in
runoff generation and a decrease in soil structural
stability, resulting in greater erosion and poorer
conditions for vegetation support in these patches
(Imeson & Lavee, 1998; Yair & Kossovsky, 2002).
This process can be exacerbated by soil ero-
sion, due to a reduction in the soil's capacity to
support vegetation when compared with non-
eroded soils in a similar climate as a result of
nutrient losses and, in severely eroded soils, to a
decrease in the soil water-holding capacity (Arora,
2002; Bakker
et al
., 2004; Boer & Puigdefábregas,
2005). Intensive agricultural, forestry and grazing
practices are common in many dryland regions,
usually leading to increased soil erosion and land
degradation; high market prices, government sub-
sidies and other socio-economic factors can extend
these practices to unsuitable regions, such as mar-
ginal areas with steep slopes and low water avail-
ability, and maintain them even after the onset of
land degradation and consequential decreases in
crop yield (Martínez-Fernández & Esteve, 2005;
Audsley
et al
., 2006; Vogiatzakis
et al
., 2006).
Furthermore, disturbances common in drylands
such as severe droughts and wildfires can increase
15.2.5
Crossing desertification thresholds
The potential of climate change to aggravate
desertification processes in drylands illustrates
how the different impacts of climate change on
soil erosion drivers can combine and reinforce
each other and lead to severe problems of land
degradation. Desertification can be defined as the
degradation of biophysical and socio-economic
conditions in dry regions, leading to land degrada-
tion, reduced vegetation productivity and human
abandonment (Thornes, 1998; Fernández, 2002).
This process usually occurs when biophysical
conditions (e.g. soil and climate) are insufficient
to support existing natural and socio-economic
