Agriculture Reference
In-Depth Information
cess of a species, whether a plant is density dependent to its own or another
species can determine the manner in which the new plant community evolves.
The impact of the invasive plants on ecosystem function might then be exam-
ined by the degree of niche overlap among the co-existing species as deter-
mined by their responses to separate and combined densities. Potentilla recta
is a highly successful plant species that effectively displaces native vegetation
in certain habitats of northeastern Oregon. However, little empirical evidence
exists about the mechanism by which such displacements occur.
To clarify the competitive relationship of P. recta and other plant species
over a range of habitats two experiments were established. In the first experi-
ment the competitive relationship of P. recta , P. gracillis (a native Potentilla ),
and Pseudoroegneria spicata (a native grass) is being examined in an addition
series experiment [82, 83]. Various densities of each species are planted alone
and in mixtures and by analyzing the effects of the resulting species ratios on
mortality, biomass accumulation and reproduction, the interaction among the
species can be determined. The second ongoing study is a diallel experiment
[82, 83], where intra- and inter-specific combinations of the three species are
planted in a variety of habitats (e.g., wet meadow, dry meadow, and open-
canopy forest) and elevations. Data from both experiments will be used to
determine yield ratios and competitive ability among the species, and should
illuminate plant-plant interactions for disturbed and undisturbed areas and site
conditions.
Landscape model
Modeling weed infestations has been attempted in numerous studies during the
last decade with varying success [38]. Predictive models thus far have gener-
ally used analytical diffusion equations in an attempt to apply any weed spe-
cies to any area [84, 85]. However, both biological (species) and environmen-
tal (habitat) factors form the baseline to determine the invasion risk and sub-
sequent spread of non-native plant introductions. Modeling to assess the risk
and spread of plant invasions should be at this interface of a species' biology
and the speed at which it can occupy habitats as modified by environment, land
uses and disturbances [38, 66]. Development of a prototype cellular automata-
type model to project geographic spread of P. recta and other invasive plant
species in the PNW (Fig. 2) is underway following the theoretical papers of
Caswell and associates [66, 67, 86]. This landscape model couples a popula-
tion matrix that describes population growth (e.g., Tab. 3) with an integrodif-
ference equation [66] to account for species seed dispersal wave speed (Fig. 5)
at which an exotic plant population, such as P. recta , might expand.
Requirements for the model are life stage parameters to calculate the dom-
inant eigenvalue for population growth of the exotic species (Tab. 3) and a
determination of propagule dispersal distance or wave speed over the life cycle
of the plant [65]. When these calculations are combined with a GIS risk assess-
Search WWH ::




Custom Search