Revegetation Guidelines for Western Montana - Global Restoration ...

Appendix C. Understanding succession to direct successful revegetation
Revegetation can be most successful when it works with successional processes to direct
communities toward a desired plant community. Three components can influence the direction of
succession (defined below) and can be modified to direct predictable successional transitions. These
are:
1. Site availability (disturbance) – plays a central role in initiating and altering successional
pathways. Site availability can be a designed disturbance such as seedbed preparation to
produce seed safe sites or herbicide applications for weed removal to open niches for
occupation by desired species. Site availability is important for the persistence of many
native species, but can also facilitate noxious weed invasion (Kotanen 1997);
2. Species availability (colonization) – the intentional alteration of seed availability by
influencing seed banks and propagule pools of desired plants and weeds and the regulation
of safe sites for desired plant germination and establishment. Weed seed banks can be
depleted through attrition if seed production is prevented or significantly reduced each
growing season. For example, Olson et al. (1997) found the number of spotted knapweed
seeds in the soil was reduced after three years of intensive sheep grazing directed at buds
and flower heads, resulting in decreased weed density.
3. Species performance – the manipulation of the relative growth and reproduction of plants
in an attempt to shift the plant community in the desired direction. Domestic sheep can shift
a plant community toward desired grasses by selectively grazing forbs. In contrast, cattle can
shift a plant community toward forbs (e.g. weeds) by selectively grazing grasses. Herbicide
applications can alter resource availability and increase desired species performance through
competitive weed removal. In other words, soil resources become available for neighboring
desired plants through careful herbicide treatment.
Pioneer species (e.g. annual forbs or early seral species) are usually the first plant types to begin
growth on a disturbed site. These pioneer species are eventually replaced by later seral species such
as grasses that are replaced over time by shrubs and trees. The replacement of early seral species,
such as forbs, by mid-seral species, such as grasses, and eventual replacement by late-seral species,
such as shrubs, to a “climax state”, is plant succession. Noxious weeds act as pioneer species but
interfere with or arrest succession before it reaches the mid- or late-seral stage most landowners
hope to attain (Munshower 1994). In response, the development of a plant community that is more
mature than the classic pioneer stage can help ensure that noxious weeds do not become
established at the disturbed site with proper weed management.
The first manipulation to a site to make it capable of supporting later seral species is topsoiling, if this
layer is absent. The process of providing or replacing salvaged topsoil upon the subsoil strata can
move the successional process from the primary level to secondary succession given topsoil is
generally “mature” enough to support mid-seral stage plants. Seeding later-successional species can
further accelerate the process of plant succession. However, in some cases, the topsoil may lack the
maturity needed to support late successional or climax communities. Such plant communities
require mature soils with intact and complex nutrient cycles, essential mycorrhizal associations, and
proper surface litter distribution and soil microtopographies that can be easily damaged following
disturbance (Munshower 1994). Care in selecting the proper species that compliment site soil
maturity is recommended. Also, the introduction of early successional species can direct changes in
soil properties that would facilitate later successional species.
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