Passive restoration
Passive ecological restoration involves removing human induced degrading
pressures from a site with minimal remediation. In many cases, it is
presumed that non-target species will expand without human intervention,
however many passive restorations have observed weeds decline with
sufficient native recruitment (Sinkins & Otfinowski, 2012; Valkoet al ., 2017). Notable vegetation shifts often occur within 10
years of rest (Smallbone, 2014), however, some may take several decades
to reach similar species richness as the remnant sites, and even then,
the composition of vegetation can significantly differ (van de Merwe &
van Rooyen, 2011).
Successful passive restoration involves careful grazing management and
the ability for target species to recruit and establish. Rapid recovery
of a degraded Hungarian alkaline grassland was observed (within one
year) in sites directly adjacent a natural grassland, and within six
years, all sites where restored regardless of proximity to the remnant
site (Valko et al ., 2017). In this example, the dispersal of
native plant propagules was promoted by livestock roaming between
natural and degraded sites (Valkko et al ., 2017). Grazing animals
were also observed to provide an important service in maintaining
species richness for highly productive Themeda grassland in south-east
Australia (Schultz et al ., 2011). Grasslands often require
disturbances such as grazing or fire to maintain species richness and
grazing animals remove excess phytomass in order to generate niche space
for rarer species.
While grazing plays an important role in maintaining highly productive
grasslands, those suffering extensive degradation or of lower
productivity often benefit from the complete removal of grazing
livestock. Grazing exclusion is a cost-effective tool for passive
restoration, particularly if native species are well represented. A
long-term (20 and 30 years) grazing exclusion zone was developed in the
steppe grasslands of China, which observed an increase in perennial
grass cover, as well as higher density bud banks of these favourable
grasses when compared to the grazing sites (Zhao et al ., 2019).
The long-term (40 years) removal of cattle from northern fescue prairies
in Canada was effective for reducing some invasive plants, but not
others, including Poa pratensis , which in some areas occupied up
to 90% of the canopy (Sinkins & Otfinowski, 2012). Generally,
grassland species have short-lived seedbanks and if desirable species
are rare, or the site is isolated from remnant patches, the seedbank
will continue to diminish (Bossuyt & Hermy, 2003). Further, the
recruitment of native species in degraded temperate grasslands is rare,
as seedlings often fail to survive as a result of competitive weed
interactions (Morgan, 2001; Lenz & Facelli, 2005). This demonstrates
that isolated sites dominated by aggressive weeds may not be suitable
for passive restoration. While passive restoration has proven successful
under specific conditions, in sites where invasive plants have dominated
for several decades and biotic and abiotic thresholds have been crossed,
active intervention will be required.