No abstract

Predictions of snag longevity, defined here as the probability of snag survival to a given age, are key to designing silvicultural regimes that ensure their availability for wildlife and form an important component of carbon flow models. Species, diameter at breast height, stand density, management regime. and agent of tree mortality were assessed for their effect on snag longevity in a long-term siivicultural study on the Penobscot Experimental Forest in central Maine. Snag recruitment and fall data from USDA Forest Service inventories between 1981 and 1997 were analyzed using parametric survival analysis. A Weibull model fit the data best, indicating a significant lag time followed by rapid . fall rates. Half-times varied among species. with Thuja occidentalis L. having the longest (10 years) and Picea species the shortest (6 years). Snag longex~ity was signilicantly greater with increasing diameter and decreased with increasing stand density. Agent of mortality and silvicultural treatment were also significant. Two models were developed for estimating probability of snag survival over time. one that included predictor variables unique to the silvicultural systems study on the Penobscot Experimental Forest and one using predictor variables available in most standard inventories. Snag survival models can easily be incorporated into comprehensive forest dynamics models to facilitate estimates of wildlife habitat structure and carbon flow. R b m B : La prediction de la longevit6 des chicots, dhfinie ici c o m e Ctant la probabilite de survie dles chic06 est un Clement cl6 de la conception des regimes sylvicoles qui en assurent la disponibilitk pour la faune et constitue une composante importante des modkles de flux de carbone. L'espitce, le dinmktre 2 hauteur de poitrine. la densite du peuplement, le r6gin1e d'itn~Cnagement ed la cause de mortalit6 des arbres ont 6t6 Cvalu6s pour connaftre lew effet sur la long6viti5 des chicots dans le cadre d'une etude sylvicole h long terme B la For& exp6rimeatale de Penobscot situke dam le centre du Maine, aux ~t a t s -~n i s .Les donnCes d'inventaue de 1981 A 1997 du USDA Forest Senice sur le recrutelaeht et la chute des chicots ont 6t6 Ctudihes h I'aide d'une analyse parametrique de survie. Un nnoditle de Weibull s'est le mieux ajust6 aux donnCea ce qui indique qu'il y a une importante periode de latence suivie par un taux de chute rapide. La demi-vie des chicots variait selon I'espsce, avec la plus longue (10 am) associh B Thuja occidentaZis L. et la plus courte (6 ans) & Picea spp. La longkvit6 des chicots etait significativement plus forte lorsqne leur diamktre augmentait et plus faible lorsque la densit6 du peuplement augmentait. La cause de mortalitt? et le traitement sylvicole etatent Cgalemen~ importants. Deux modeles ont C d dCvelopp6s pour estimer la probabilite de suwie des chicots en fonction du ;temps. Le premier inclut des variables de prediction propres aux systkmes sylvicoles de la Fort% exp6ri-mentale de Penobscdt et le deuxikme utilise des variab...

A method is presented for modeling the cavity formation and collapse induced by high-speed impact and penetration of a rigid projectile into water. The approach proposes that high-speed water-entry is characterized by a cavity that experiences a deep closure prior to closure at the surface. This sequence in the physical events of the induced cavity dynamics is suggested by the most recent high-speed water-entry experimental data, by results from numerical experiments using a hydrocode, and by an understanding of the fundamental physics of the processes that govern surface closure. The analytical model, which specifies the energy transfer for cavity production as equivalent to the energy dissipated by velocity-dependent drag on the projectile, provides accurate estimates for variables that are important in characterizing the cavity dynamics, and reveals useful knowledge regarding magnitudes and trends. In particular, it is found that the time of deep closure is essentially constant and independent of the impact velocity for a given projectile size, while the location of deep closure has a weak dependence on impact velocity. Comparison of these analytical results with experimental results from the literature and with results from numerical simulations verifies the analytical solutions.

Neotyphodium endophytes in introduced agronomic grasses are well known to increase resistance to herbivores, but little is known of interactions between Neotyphodium endophytes and herbivores in native grass populations. We investigated whether endophytes mediate plant-herbivore interactions in a native grass species, Festuca arizonica in the southwestern United States, in two ways. First, to test the prediction that the presence and frequency of endophyte-infected (E+) plants should increase with increasing herbivory, we determined endophyte frequencies over a 4-year period in six natural Arizona fescue populations. We compared Neotyphodium frequency among plants growing inside and outside long-term vertebrate grazing exclosures. Second, we experimentally tested the effects of Neotyphodium infection, plant clone, and soil nutrients on plant resistance to the native grasshopper Xanthippus corallipes. Contrary to predictions based upon the hypothesis that endophytes increase herbivore resistance, levels of infection did not increase in plants subjected to grazing outside of exclosures relative to ungrazed plants within exclosures. Instead, endophyte frequencies tended to be greater inside the exclosures, where long-term vertebrate grazing was reduced. The grasshopper bioassay experiment corroborated these long-term patterns. Survival of grasshoppers did not differ between infected (E+) and uninfected (E-) plants. Instead, mean relative growth rate of grasshoppers was higher on E+ grasses than on E- ones. Growth performance of newly hatched grasshopper nymphs varied among host plant clones, although two of six clones accounted for most of this variation. Our results suggest that Neotyphodium-grass-herbivore interactions may be much more variable in natural communities than predicted by studies of agronomically important Neotyphodium-grass associations, and herbivory is not always the driving selective force in endophyte-grass ecology and evolution. Thus, alternative hypotheses are necessary to explain the wide distribution and variable frequencies of endophytes in natural plant populations.

Fungi are frequently found within insect galls. However, the origin of these fungi, whether they are acting as pathogens, saprophytes invading already dead galls, or fungal inquilines which invade the gall but kill the gall maker by indirect means, is rarely investigated. A pathogenic role for these fungi is usually inferred but never tested. I chose the following leaf-galling-insect/host-plant pairs (1) a cynipid which forms two-chambered galls on the veins of Oregon white oak, (2) a cynipid which forms single-chambered galls on California coast live oak, and (3) an aphid which forms galls on narrowleaf cottonwood leaves. All pairs were reported to have fungi associated with dead insects inside the gall. These fungi were cultured and identified. For the two cynipids, all fungi found inside the galls were also present in the leaves as fungal endophytes. The cottonwood leaves examined did not harbor fungal endophytes. For the cynipid on Oregon white oak, the fungal endophyte grows from the leaf into the gall and infects all gall tissue but does not directly kill the gall maker. The insect dies as a result of the gall tissue dying from fungal infection. Therefore, the fungus acts as an inquiline. Approximately 12.5% of these galls die as a result of invasion by the fungal endophyte.