Linking Restoration and Ecological Succession (Springer ... - Inecol

58 David A. Wardle and Duane A. Peltzer
are found in the root nodules of exotic plants from those found in native plant
species, but it is unknown to what extent these novel root mutualists drive the
high N inputs by invasive nonnative plants. Many non-N-fixing invasive species
have higher growth rates, foliar nutrient contents, or litter inputs than do the
native vegetation of the habitat being invaded, and can therefore also increase N
availability in invaded ecosystems (e.g., Herman and Firestone 2005, Lindsay
and French 2005).
Although most attention has focused on how plant invaders elevate N availability
in ecosystems, there is increasing evidence that invaders also influence
P availability and therefore the stoichiometry of systems. For example, the
widespread invading shrub Buddleja has much higher foliar P than do other
plant species dominating primary succession in both Hawaii (Matson 1990)
and New Zealand (Bellingham et al. 2005). Similarly, Pinus contorta invading
temperate grasslands increases soil P availability and rates of P cycling
(Chen et al. 2003). Although the impacts of contrasting plant species on soil
nutrient status are well documented (reviewed in Binkley and Giardina 1998,
Wardle 2002, Ehrenfeld et al. 2005), the long-term implications of altered nutrient
fluxes or stoichiometry for successional processes, ecosystem properties,
and restoration are largely unknown. An unresolved issue is whether enhanced
nutrient availability or terrestrial eutrophication caused by invaders has longterm
negative impacts on late successional species composition, diversity, and
successional trajectories (i.e., through promoting early successional species
that are more nutrient-demanding). Results from ecosystem models suggest
that nutrient inputs early in succession can have large and persistent effects
on long-term productivity (e.g., Rastetter et al. 2003, Walker and del Moral
2003), exert differential impacts on later successional species (Bellingham
et al. 2001), and influence species coexistence or persistence (Miki and Kondoh
2002).
Managers and restoration ecologists have sought to mitigate nutrient inputs
from invaders by applying soil impoverishment treatments to reduce soil fertility
levels (e.g., Alpert and Maron 2000, Wilson 2002, Corbin and D’Antonio 2004).
This is typically accomplished by adding a carbon source to soils that can
increase the soil microbial biomass and induce short-term reductions in nutrient
availability to plants (e.g., Morghan and Seastedt 1999, Blumenthal et al. 2003,
Yelenick et al. 2004). For example, additions of sucrose at 160 g m −2 yr −1 to
sagebrush-prairie vegetation in Colorado increased plant species richness by an
average of 2.1 species per 0.25 m 2 plot after 8 years of treatment (McLendon and
Redente 1992). In contrast, Peltzer and Wilson (unpublished data) found that
addition of C as sawdust at a rate of 400 g C m −2 yr −1 over 5 years to a prairie
grassland restoration project in western Canada did not significantly decrease
soil N or increase plant diversity. Adding C to soil is an extremely intensive
undertaking and is likely to result in only short-term nutrient reduction. Overall,
the evidence that soil impoverishment using carbon additions as a method for
shifting the balance of species composition from exotic- to native-dominated
systems or increasing plant diversity is weak (Corbin and D’Antonio 2004),
and hence has not proven to be realistic for restoration management.
The impacts of invaders on the soil microbial community and soil fauna are
less well understood than their impacts on nutrient availability. However, interactions
between invasive plants and soil biota have been receiving increased
recent attention in the literature for at least three reasons: soil communities