These introductions have had a strong negative impact on local soil attributes, as
well as on the population, community, and ecosystem levels (Gordon 1998).
14.1.1 Impact of Casuarina on Soil Attributes
Casuarina is an actinorhizal species that is able to grow in nutrient-poor soils,
mainly because of its ability to fix N 2 in the root nodules through a mutualistic
symbiosis with the actinomycete Frankia, and by its capacity to develop mycorrhizae (Gordon 1998). Because of this, Casuarina plantations may largely modify
soil characteristics, especially in coastal dunes.
Sand dune soils have high leaching rates, with a resulting low level of nitrogen.
When nitrogen fixers form large stands, local nitrogen cycling may be altered
significantly (Vitousek 1986), resulting in higher phosphorous and nitrogen concentrations in leaves as well as enriched soil-nutrient levels (Versfeld and van
Wilgen 1986). This enriched soil has been observed in Casuarina stands too. Leaf
and branch litter beneath Casuarina trees can be 5–10 cm thick (Fernald and
Barnett 1991), with organic matter accumulating at rates that are similar to those
recorded in boreal and subalpine forests (Mailly and Margolis 1992; Izquierdo
et al. 2005).
14.1.2 Impact of Casuarina on Vegetation
The high rates of litterfall accumulation and nutrient accumulation beneath
Casuarina stands can have contrasting effects at the community level. On the one
hand, it has been observed that a higher nutrient content in the soil (organic matter
and nitrogen) can actually facilitate natural regeneration and thus catalyze succession in deforested and degraded sites (Parrotta 1995). However, contrasting
patterns have also been observed by Bond (1993); Duever et al. (1986); Gordon
(1998); and Abdel Wahab (1980). These authors found that Casuarina plantations
can also suppress recruitment of other species, although the mechanisms causing
inhibition or shifts in recruitment patterns have not been investigated in detail.
Evidence suggests that because Casuarina trees reduce light availability, and have
higher evapotranspiration rates than the native vegetation (Gordon 1998), they are
likely to modify competitive interactions and can out-compete native species
(Abdel Wahab 1980; Gordon 1998), by suppressing seedling growth and establishment. Because very few plants grow beneath the canopy of Casuarina, Bond
(1993) has named this tree a ‘‘keystone weed’’.
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well as on the population, community, and ecosystem levels (Gordon 1998).
14.1.1 Impact of Casuarina on Soil Attributes
Casuarina is an actinorhizal species that is able to grow in nutrient-poor soils,
mainly because of its ability to fix N 2 in the root nodules through a mutualistic
symbiosis with the actinomycete Frankia, and by its capacity to develop mycorrhizae (Gordon 1998). Because of this, Casuarina plantations may largely modify
soil characteristics, especially in coastal dunes.
Sand dune soils have high leaching rates, with a resulting low level of nitrogen.
When nitrogen fixers form large stands, local nitrogen cycling may be altered
significantly (Vitousek 1986), resulting in higher phosphorous and nitrogen concentrations in leaves as well as enriched soil-nutrient levels (Versfeld and van
Wilgen 1986). This enriched soil has been observed in Casuarina stands too. Leaf
and branch litter beneath Casuarina trees can be 5–10 cm thick (Fernald and
Barnett 1991), with organic matter accumulating at rates that are similar to those
recorded in boreal and subalpine forests (Mailly and Margolis 1992; Izquierdo
et al. 2005).
14.1.2 Impact of Casuarina on Vegetation
The high rates of litterfall accumulation and nutrient accumulation beneath
Casuarina stands can have contrasting effects at the community level. On the one
hand, it has been observed that a higher nutrient content in the soil (organic matter
and nitrogen) can actually facilitate natural regeneration and thus catalyze succession in deforested and degraded sites (Parrotta 1995). However, contrasting
patterns have also been observed by Bond (1993); Duever et al. (1986); Gordon
(1998); and Abdel Wahab (1980). These authors found that Casuarina plantations
can also suppress recruitment of other species, although the mechanisms causing
inhibition or shifts in recruitment patterns have not been investigated in detail.
Evidence suggests that because Casuarina trees reduce light availability, and have
higher evapotranspiration rates than the native vegetation (Gordon 1998), they are
likely to modify competitive interactions and can out-compete native species
(Abdel Wahab 1980; Gordon 1998), by suppressing seedling growth and establishment. Because very few plants grow beneath the canopy of Casuarina, Bond
(1993) has named this tree a ‘‘keystone weed’’.
218
P. Moreno-Casasola et al.