Environmental Engineering Reference
In-Depth Information
Though most plant-plant facilitation experiments have focused on salt marshes, some
have investigated cobble beaches or estuarine marshes and very few have investigated coastal
forests or mangrove forests in particular (Huxham et al. 2010; Table 1). Moreover, most
studies have assessed interspecific facilitation and largely disregarded intraspecific
facilitation (i.e., positive density dependence). Negative density dependence, which can be
caused by strong competition for resources, habitat overlap for individuals, increasing
susceptibility to infection by pathogens or detection by consumers, is regarded as a key
organization rule in traditional population dynamics. However, when the benefits of
conspecific group living outweigh the effects of resource competition or other negative
density dependence mechanisms, positive density dependence may occur, particularly in high
stress environments (Goldenheim e al. 2008; Fajardo & Mclntire 2011). This type of positive
density dependence is very useful in restoration management of coastal wetlands (Halpern et
al. 2007). We recommend researchers consider both interspecific and intraspecific facilitation
and compare the relative role of inter- and intraspecific facilitation in shaping coastal wetland
populations and communities to avoid underestimating the role of plant-plant facilitation.
M ECHANISMS OF P LANT -P LANT F ACILITATION
IN C OASTAL W ETLANDS
The major mechanism for species facilitation among plants in coastal wetlands is the
reduction of abiotic or biotic stress (Figure 1). Biotic stresses, such as herbivory, could be
reduced by neighboring plants decreasing the probability of encounters between target plants
and animal consumers, thus defending target plants from animal consumption (Alberti et al.
2008; Daleo & Iribarne 2009). Abiotic stresses often reduced through plant-plant facilitation
include high salinity, flooding, and nutrient limitation stress.
Salinity stress may be reduced through the presence of neighboring plants shading their
surrounding soil, thus decreasing water evaporation rate and preventing salt accumulation in
the soil. Alternatively, neighboring plants could absorb salt from the soil and store it in their
tissues or excrete it from their salt glands, thus reducing the salinity of their ambient
environment (Bertness et al. 1992). An additional mechanism could be increased proline
production in target plant cells enabled by nitrogen supplementation from neighboring plants
(Levine et al. 1998). Flooding stress may also be reduced by neighboring plants. Flood-
tolerant plants could ameliorate anoxic substrate conditions by enhancing soil oxygen levels
through rhizosphere oxidation. Alternatively neighboring plants could lift the soil to decrease
waterlogging (Fogel et al. 2004). Limiting nutrient stress could be decreased through plant-
plant facilitation when neighboring plants enrich the soil with nutrients (Levine et al. 1998).
Facilitating tolerance of other biotic and abiotic stresses such as sea waves, pollination or
dispersal stress has also been proposed as mechanisms of species interaction (Table 1).
However, the cumulative effects of co-occurring environmental stresses, particularly abiotic
and biotic stresses on species interactions, which may play an important role in community
assembly are still poorly understood (Bulleri et al. 2011).
Search WWH ::




Custom Search