Noise-induced patterns in environmental systems - Noise-induced Phenomena in the Environmental Sciences - page 246

( Jones et al. , 1994 ; Gilad et al. , 2007 ). For example, the improvement of conditions
existing in the microenvironment underneath the canopy of so-called nurse plants
( Nigering et al. , 1963 ; Kefi et al. , 2007 ) favors the establishment and growth of
other plants (e.g., Garcia-Moya and McKell , 1970 ; Burke et al. , 1998 ; Aguiar and
Sala , 1999 ). Vegetation cover may decrease the amplitude of temperature fluctuations,
reduce the exposure to solar radiation, wind desiccation, and soil erosion, or prevent
soil-crust formation ( Eldridge and Greene , 1994 ; Smit and Rethman , 2000 ; Greene
et al. , 2001 ). Moreover, plant individuals located in the middle of vegetated patches
are protected against fires and grazing.
As noted, it has been also found that water or nutrient (or both) availability is
higher in the areas located under the canopy of existing plant individuals than in
the surrounding bare soil ( Charley and West , 1975 ). These nutrient-rich areas are
known as fertility islands or resource islands ( Schlesinger et al. , 1990 ). Mechanisms
commonly invoked to explain the formation of these heterogeneous distribution of
resources include the ability of the canopies to trap nutrient-rich airborne soil particles,
the accumulation of sediments transported by wind and water, the sheltering effect
of vegetation against the erosive action of wind and water, and the presence of
nitrogen-fixing species ( Garcia-Moya and McKell , 1970 ; Charley , 1972 ; Archer ,
1989 ; Schlesinger et al. , 1990 ; Breman and Kessler , 1995 ; Li , 2007 ). Sometimes
fertility islands lead to the formation of aperiodic vegetation patterns in the form
of stable spatial configuration corresponding to isolated vegetation patches (spots),
usually called localized structures or localized patches ( Lejeune et al. , 2002 ).
Other examples of ecosystem engineering relevant to pattern formation include the
ability of deep-rooted plants to facilitate shallow-rooted species by increasing surface-
soil moisture through “hydraulic-lift” mechanisms ( Richards and Caldwell , 1987 ),
the reduction in fire pressure resulting from the encroachment of woody vegetation
at the expenses of grass fuel (e.g., Anderies et al. , 2002 ; van Langevelde et al. ,
2003 ; D'Odorico et al. , 2006b ), and the ability of alpine or subalpine vegetation and
desert shrubs to maintain warmer microclimate conditions and reduce frost-induced
mortality.
Similar facilitative mechanisms exist also in wetland environments, including salt
marshes, where vegetation may prevent salt accumulation by limiting soil evaporation
(shading effect), riparian corridors, and wetland forests, where vegetation can favor
the aeration of anoxic soils through soil drainage by plant uptake and transpiration
( Wilde et al. , 1953 ; Chang , 2002 ; Ridolfi et al. , 2006 ). It has also been found that on
cobble beaches, dense stands of spartina alterniflora occupying the lower intertidal
zone can protect other plant communities from intense wave action ( Bruno , 2000 ;
van de Koppel et al. , 2006 ).
On the other hand, competitive or inhibitory effects typically occur within a longer
range. Competition for water and nutrients is generally exerted by means of the root
system ( Aguilera and Lauenroth , 1993 ; Belsky , 1994 ; Breman and Kessler , 1995 ;
Breshears et al. , 1997 ; Martens et al. , 1997 ; Couteron and Lejeune , 2001 ). In fact, the