Agriculture Reference
In-Depth Information
I
NTRODUCTION
The observation and enjoyment of the structural complexity of semiarid landscapes has
stimulated a considerable amount of research aimed at describing and cataloguing the
processes that support such variability. As it is well known to ecologists, the description of
the structure of the landscape is fundamentally a matter of scale. For instance, landscapes
dominated by semiarid Mediterranean steppes can be described using at least two biologically
meaningful spatial scales, namely a coarsed-grained scale defined by the slopes as a fine-
grained scale defined at the individual plant level within slopes. One source that justifies and
connects both scales is the behavior of hydrological fluxes and its relation to the abundance
and distribution of soil. Indeed hydrological fluxes can be regarded as landscape-modelling
forces that condition the disposition of the vegetation in the terrain and the ecophysiological
performance of individual plants. A fine example of such interactions can be found in
semiarid Mediterranean steppes dominated by
Stipa tenacissima
L. - a perennial, rhizomatous
and tussock grass, which represent a prominent vegetation type in the Mediterranean basin
(Le Houérou 2001). These ecosystems are characterized by being subjected to drastic
environmental conditions, notably hot summers with severe rain shortages that cause an
important light and water stress to the vegetation (Joffre et al. 1999). Characteristically,
rangeland Mediterranean soils are often shallow, discontinuous, poorly structured and with
rock outcrops, which represent critical constraints for the settlement and even the restoration
of the vegetation in this region (Vallejo et al. 2000). Such environmental conditions result in
low plant productivity, low plant cover and high susceptibility (sometimes an actual trend) to
degradation and desertification (Cortina et al. 2004). The amount of bare-ground areas on
S.
tenacissima
grasslands represent an important total figure in the Iberian Peninsula (between
40 - 82%, Maestre et al. 2007). The dynamic interaction between these areas and vegetated
clumped patches is one important trait of these ecosystems. For instance, Puigdefábregas and
Sánchez (1996) found that interaction occurs at different scales. At the micro-site scale, the
overland flow of water and sediments is retained in the uphill part (or terrace) of the
S.
tenacissima
tussock. This soil under the tussock has more infiltration (Cerdà 1997),
abundance and stability of organo-mineral of aggregates (Puigdefábregas et al. 1999; Bochet
el al. 1999), soil moisture (Puigdefábregas and Sánchez 1996), and organic matter content
(Sánchez 1995; Bochet et al. 1999; Maestre et al. 2001) than bare soil areas, which helps
create “resource islands” in this ecosystems (
sensu
Reynolds et al. 1999) that facilitate the
establishment of other vascular plants (Maestre et al. 2001; Maestre 2006). In turn, the
intensity of the accumulation of soil sediments influences the mortality of stems and the
branching rate, which ultimately has a negative effect on the space occupation of the tussock
(Sánchez and Puigdefábregas 1994). At the stand scale, the spatial pattern of
S. tenacissima
tussocks depends on topographic conditions because slope angle and length, catchment area
influence the circulation of sediments (Puigdefábregas and Sánchez 1996). Thus, in hillsides
with moderate sediment circulation the spatial pattern of the tussocks results in contour
vegetated lines parallel to the hillside that allow the accumulation of sediment. However,
under conditions of circulation of sediment such as those promoted by high slope angles or
slope-length the tussock structure is broken, forming downhill strips. On the other hand,
recent studies (Maestre and Cortina 2006) have proved that bare soil condition is a key factor
to consider in order to understand the physiological performance of the
S. tenacissima
stands.
