Ecology of the Possum (Trichosurus Vulpecula Kerr) in the Karori Water Supply Reserve, Wellington, Sounds Ecological District

<p>The abundance and condition of possums in various areas of the Karori Reserve, Wellington - Sounds Ecological District was assessed. Possums were most abundant in the upper part of the reserve in an area dominated by bishop and radiata pines with an undergrowth of native species and in an area dominated by gorse and flax shrublands. In the lower part of the reserve, possums were less abundant and there a number of characteristics were observed: possums had a higher body weight for age, their reproductive output was higher and the population was marked by a higher proportion of females and juveniles. This was interpreted as a "compensatory response" characteristic of possum populations maintained at low densities. Possum stomachs were sampled in spring and their contents analysed in order to investigate the importance of Fuchsia excorticata and other plant species to possum diet. In the lower part of the reserve the leaves of Fuchsia excorticata and the flowers of Cytisus scoparius made up more than 50 percent of the possums' forage. In the upper part of the reserve exotic species such as the flowers of Cytisus scoparius and Ulex europeans as well as the leaves of native species such as Aristotelia serrata, Pseudopanax arboreus and Coprosma robusta made up the bulk of possum forage. Exotic plants accounted for a significant proportion of the spring possum diet in the reserve. Whereas most leaves originated from native plants, the exotic species contributed mainly carbohydrate-rich flowers and pollen. The canopy condition of two possum-susceptible tree species Fuchsia excorticata and Pseudopanax arboreus was assessed early in autumn over the years 1993, 1995 and 1996. Over these years, 47 percent of marked fuchsia trees were completely defoliated and most trees are unlikely to recover even if possums are eradicated from the reserve. All defoliated fuchsia trees were located in the upper part of the reserve. On the other hand, 37 percent of marked fuchsia trees exhibited only a little possum damage. Most of these trees were located in the lower part of the reserve where possum density was lower. Increasing possum densities could therefore be correlated with an increasing level of possum damage to fuchsia canopies. Even though Pseudopanax arboreus leaves and petioles were encountered in 10 percent of possum stomachs, the canopies leaves and petioles were encountered in 10 percent of possum stomachs, the canopies of assessed trees did not seem to be greatly affected. It appeared that due to the high abundance of this plant species in the reserve the effects of possum browse were spread over the whole reserve to levels which could be tolerated by an individual Pseudopanax arboreus tree.</p>

Ecology of the Possum (Trichosurus Vulpecula Kerr) in the Karori Water Supply Reserve, Wellington, Sounds Ecological District

<p>The abundance and condition of possums in various areas of the Karori Reserve, Wellington - Sounds Ecological District was assessed. Possums were most abundant in the upper part of the reserve in an area dominated by bishop and radiata pines with an undergrowth of native species and in an area dominated by gorse and flax shrublands. In the lower part of the reserve, possums were less abundant and there a number of characteristics were observed: possums had a higher body weight for age, their reproductive output was higher and the population was marked by a higher proportion of females and juveniles. This was interpreted as a "compensatory response" characteristic of possum populations maintained at low densities. Possum stomachs were sampled in spring and their contents analysed in order to investigate the importance of Fuchsia excorticata and other plant species to possum diet. In the lower part of the reserve the leaves of Fuchsia excorticata and the flowers of Cytisus scoparius made up more than 50 percent of the possums' forage. In the upper part of the reserve exotic species such as the flowers of Cytisus scoparius and Ulex europeans as well as the leaves of native species such as Aristotelia serrata, Pseudopanax arboreus and Coprosma robusta made up the bulk of possum forage. Exotic plants accounted for a significant proportion of the spring possum diet in the reserve. Whereas most leaves originated from native plants, the exotic species contributed mainly carbohydrate-rich flowers and pollen. The canopy condition of two possum-susceptible tree species Fuchsia excorticata and Pseudopanax arboreus was assessed early in autumn over the years 1993, 1995 and 1996. Over these years, 47 percent of marked fuchsia trees were completely defoliated and most trees are unlikely to recover even if possums are eradicated from the reserve. All defoliated fuchsia trees were located in the upper part of the reserve. On the other hand, 37 percent of marked fuchsia trees exhibited only a little possum damage. Most of these trees were located in the lower part of the reserve where possum density was lower. Increasing possum densities could therefore be correlated with an increasing level of possum damage to fuchsia canopies. Even though Pseudopanax arboreus leaves and petioles were encountered in 10 percent of possum stomachs, the canopies leaves and petioles were encountered in 10 percent of possum stomachs, the canopies of assessed trees did not seem to be greatly affected. It appeared that due to the high abundance of this plant species in the reserve the effects of possum browse were spread over the whole reserve to levels which could be tolerated by an individual Pseudopanax arboreus tree.</p>

Soil texture and other site-level factors differentially affect growth of Scotch broom (Cytisus scoparius) and Douglas-fir (Pseudotsuga menziesii) seedlings in the western Pacific Northwest

The invasive shrub Scotch broom (Cytisus scoparius (L.) Link) is a pervasive threat to regenerating Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) stands in the Pacific Northwest, USA. Field observations indicate that the susceptibility of areas to Scotch broom invasion and dominance can vary by site. We selected ten sites throughout the western Pacific Northwest that spanned a gradient of soil textures and other factors to test the site-specific susceptibility of Douglas-fir to overtopping by Scotch broom. We expected to find that the ability of Scotch broom to dominate a site was mediated by site-level factors, particularly those influencing soil water – the most limiting factor to growth in the region. We found Scotch broom and Douglas-fir were inversely affected by site-level factors. In general, Douglas-fir absolute height growth rates were more competitive with those of Scotch broom on fine-textured soils than on more coarsely textured soils. We also found Douglas-fir to have a more dramatic response to increasing down woody material than Scotch broom. Scotch broom height growth approached an asymptote at 3 m. Sites with fast-growing Douglas-fir were able to surpass this height six to seven years after planting and appear likely to avoid suppression by Scotch broom.

Scotch broom (Cytisus scoparius) germination and growth responses to light: implications for logging debris retention after forest harvesting

Scotch broom [Cytisus scoparius (L.) Link] is a large nonnative, leguminous shrub that threatens native plant communities by rapidly invading recently disturbed sites, competing vigorously for soil water and nutrients, and imparting soil legacy effects that inhibit native plants. In the Pacific Northwest, logging debris retention after forest harvesting prevents or slows C. scoparius invasions. A series of studies were conducted to determine potential mechanisms by which logging debris modifies the light environment to limit germination and growth of C. scoparius. In laboratory studies, seed germination did not vary significantly (P > 0.05): (1) between presence and absence of light for several temperature regimes, (2) when exposed to red (660-nm wavelength) versus far-red (730-nm wavelength) light, and (3) across a range of red/far-red light (R/FR) ratios. These results indicate that modification of the light environment by logging debris or plant canopies has little or no influence on C. scoparius germination. In a study to simulate effects of variable mass of logging debris, “heavy” debris (2 kg m−2) caused biologically relevant reductions in photosynthetic photon flux density (PPFD) and R/FR relative to conditions under “light” debris (1 kg m−2). Cytisus scoparius germination did not differ significantly between simulated heavy and light debris; however, values of seedling root and shoot biomass under heavy debris were 16% and 71% of those observed under light debris, respectively. These results indicate that heavy debris limits biomass of C. scoparius seedlings, particularly roots, by reducing both PPFD and R/FR, which increases seedling vulnerability to summer drought or other stressors. Retention of heavy logging debris after forest harvesting has potential application on sites likely to be invaded by C. scoparius, as well as those sites with seedbanks containing C. scoparius.