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Project description

Effects of invasive woody plant species in California: a nascent protocol for an impact assessment. (01XN020)
Program Exotic Pests and Diseases Research Program
Principal
investigators
M. Rejmanek, Evolution and Ecology, UC Davis
C.L. Elmore, Vegetable Crops, UC Davis
Host/habitat Wildland; Grassland; Forest
Pest Woody Species; Silver Wattle Acacia dealbata; Tree of Heaven Ailanthus altissima; Scotch Broom Cystisus scoparius; Blue Gum Eucalyptus globulus; Fig Ficus carica; French Broom Genista monspessulana; Myoporium Myoporium laetum; Himalayan Blackberry Rubus armeniacus; Chinese Tallow Tree Sapium sebiferum; Scarlet Wisteria Tree Sesbania punicea; Gorse Ulex europaea
Discipline Weed Science
Review
panel
Natural Systems
Start year (duration)  2001 (Three Years)
Objectives Rapidly assess relative impact of invaders.

Develop a model procedure for risk assessment to prioritize eradication and control efforts.

Measure how invasive woody species effect the light environment by relating canopy openness and leaf area index to basal area, density, and age as a measure of their relative effects.

Measure how invasive woody species affect the edaphic environment by comparing soil moisture, nutrient dynamics and nutrient pools in invaded versus comparable uninvaded habitats.

Experimentally separate invasive effects on aboveground versus belowground environment by the use of a native phytometer and rank the species in terms of their relative competitive performance.

Compare plant community composition and structure in invaded versus comparable uninvaded habitats.

Final report We evaluated the dependence of understory plant diversity on canopy openness of stands of the invasive woody species in California. Among the shrub species, there is a highly significant negative dependence of species diversity on canopy openness for all species. Arundo, Ficus, and Rubus have virtually no plant species in their understory. The understory of Ailanthus is the most exotic species, rich and diverse, but has the lowest native richness and diversity. Eucalyptus understory has the highest native richness and diversity and the lowest exotic richness and diversity. For both richness and diversity, the threshold of canopy openness when species start occurring is lower for exotic species (3.0%) than for native species (7.5%). This is surprising since exotic plant invaders are thought to demand more light.

Plants propagated as cuttings of the native Mimulus aurantiacus (monkeyflower) were used as a phytometer to assess the impact of exotic woody species with lowest canopy openness—Ailanthus altissima and Ficus carica. All 40 plants under Ficus carica died and only 12% of plants under Ailanthus altissima survived the whole growing season. Surprisingly, both survival and relative growth rate of survivors were significantly higher in a parallel artificial shading experiment where light intensity was similar to Ailanthus understory. Only 5% of control plants in a semiopen natural environment died.

We compiled a list of invasive exotic woody species and their related noninvasive (or much less invasive) exotic woody species commonly planted in California for use in germination trials and the seedling growth study. Germination of untreated seeds of invasive and less invasive woody species in the phylogenetically related pairs exhibited a complex pattern. Only five of eight pairs of invasive species germinated faster consistently under both light and dark conditions. The growth study provided clearer results. Invasive species had significantly higher seedling relative growth rates (RGR) than phylogenetically related less invasive species. Seedling RGR may be used as a predictor of invasive species and also to test horticultural species for promotion as use as alternatives to invasive species.

We finished harvesting the plants from the Acacia RGR study of 11 species (nine of which are naturalized in California) under three watering regimes, but we are still analyzing the data.

Third-year
progress
We evaluated the dependence of understory plant diversity on canopy openness of stands of invasive woody species in California. Among the shrub species, there is a highly significant negative dependence of species diversity on canopy openness for all species. Arundo, Ficus, and Rubus have virtually no plant species in their understory. The understory of Ailanthus is the most exotic-species rich and diverse, but has the lowest native richness and diversity. Eucalyptus understory has the highest native richness and diversity and the lowest exotic richness and diversity. For both richness and diversity, the threshold of light availability when species start occurring is lower for exotic species (3.0%) than for native species (7.5%). This is a surprising result because of the general belief that exotic plant invaders are more light demanding.

After extensive consulting of nursery catalogues and discussions with land managers, master gardeners, and other experts, we compiled a list of invasive exotic woody species and their related non-invasive (or much less invasive) exotic woody species commonly planted in California for use in our seedling growth study and germination trials. All exotic woody species included in our original proposal will be used in this experiment. We completed the first harvest of all plants and will soon analyze the data. These data should help managers choose alternative horticultural species instead of known invasives and potentially invasive species. Interestingly, it is not always the invasive species that has higher germinability.

Second-year
progress
Thus far, our results demonstrate the mechanism by which some woody species in California are affecting our communities: altering the light environment. By comparing canopies of Cytisus scoparius and a native counterpart, Arctostaphylos viscida, we found that the openness of Cytisus was significantly lower for part of the year. This indicates that Cytisus is altering the light environment. While, Tamarix and native Populus andSalix species were not significantly different in their mean openness, there still was a trend for Tamarix to have half the openness. For tree species, the openness of Ficus carica is significantly lower than native vegetation, and Ailanthus altissima has a more narrow range of openness compared to native vegetation. Native bioindicators grown under Ailanthus demonstrate a significant decrease in growth, compared to a shading treatment in a greenhouse.

By comparing the mean canopy openness of tree and shrub species, we have developed a tool to rank them according to their relative impact on the light environment. Additionally, we have found that a useful tool for ranking tree species is to compare the slopes of canopy openness versus total basal area.

The soil environment for the two invasive nitrogen-fixers, Robinia pseudoacacia and Cytisus, indicates there are significant differences between invaded and non-invaded sites. The soil under Cytisus had significantly lower phosphorus, and moderately significantly higher total nitrogen compared with native soil. The soil under Robinia did not differ in phosphorus or total nitrogen, but had significantly higher organic matter and organic carbon compared to native soil.

First-year
progress
Plots were established in stands of Ailanthus altissima, Eucalyptus globulus, Robinia psuedoacacia, Cystisus scoparius and Rubus armeniacus. Density measurements were taken and releves were done for each plot. The releves indicate that the dominant herbaceous vegetation below these invading stands is nonnative. In stands of C. scoparius and R. armeniacus there was little understory growth, which is most likely due to the high amount of shading of R. armeniacus and the build up of litter below C. scoparius.

When stands could not be located, light measurements were taken for smaller groups of individuals, including Arundo donax and Ficus carica. Preliminary results indicate that F. carica and Rubus armeniacus have very high levels of shading, whereas Robinia pseudoacacia and Ailanthus altissma have the lowest levels of shading. The remaining species have intermediate shading.

A good deal of time was spent searching for sites of invasion either from literature that indicated the location, or through correspondence with various organizations and local botanists. For example, Ficus carica was said to be extensively invading the Dye Creek Preserve. However, only a few trees in isolation were located. Additionally, Sapium sebiferum and Sesbania punicea were said to be invading the American River Parkway, but with a local expert we could only locate individuals scattered among the vegetation. Currently, we are researching the use of fisheye photography for tree and shrub canopy light measurements as our data indicate that plant canopy analyzers allow for a very short period of time to take the measurements without error.

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