Tree Recruitment Limitation by Introduced Snowshoe Hares, Lepus americanus, on Kent Island, New Brunswick

Species introductions often have negative consequences for native plant and animal communities of islands. Herbivores introduced to islands lacking predators can attain high population densities and alter native plant communities by selective consumption of palatable plants. We examined the legacy of the 1959 introduction of Snowshoe Hares (Lepus americanus) to Kent Island, New Brunswick, by reconstructing a history of tree recruitment on Kent Island and on nearby Outer Wood Island, which lacks Snowshoe Hares. Tree-ring records show pronounced recruitment peaks associated with farm abandonment in the 1930s for Kent Island and in the 1950s for Outer Wood Island. Following the introduction of Snowshoe Hares to Kent Island, tree recruitment plummeted and has remained low ever since. In contrast, trees continued to establish throughout the latter 20th century on Outer Wood Island. The high rates of seedling mortality on Kent Island associated with Snowshoe Hare browsing coupled with high rates of canopy tree mortality threaten to degrade severely the forest of this important seabird nesting sanctuary

Development of an intense positron source using a crystal--amorphous hybrid target for linear colliders

In a conventional positron source driven by a few GeV electron beam, a high amount of heat is loaded into a positron converter target to generate intense positrons required by linear colliders, and which would eventually damage the converter target. A hybrid target, composed of a single crystal target as a radiator of intense gamma--rays, and an amorphous converter target placed downstream of the crystal, was proposed as a scheme which could overcome the problem.This paper describes the development of an intense positron source with the hybrid target. A series of experiments on positron generation with the hybrid target has been carried out with a 8--GeV electron beam at the KEKB linac. We observed that positron yield from the hybrid target increased when the incident electron beam was aligned to the crystal axis and exceeded the one from the conventional target with the converter target of the same thickness, when its thickness is less than about 2 radiation length. The measurements in the temperature rise of the amorphous converter target was successfully carried out by use of thermocouples. These results lead to establishment to the evaluation of the hybrid target as an intense positron source.Comment: 17pages, 10figure