Melissa Savage

The role of climate in a pine forest regeneration pulse in the southwestern United States

Abstract:The role of historical influences in patterning forest landscapes was explored in a case study of forest structure change in the American Southwest. A group of ponderosa pine trees was destructively sampled and year of germination identified in order to assess the strength and nature of the climate signal in influencing ponderosa pine germination in this century. A novel methodology for estimating year of germination in woody species by identifying the root-shoot boundary appears to be promising. Both rare seasonal and interannual climatic factors and a unique set of circumstances associated with anthropogenic disturbances played a role in shaping a germination pulse early in the 20th century. A cohort originating in 1919 captured available space and, barring major disturbance, will dominate forest structure at the site for centuries. Such rare germination events support the view that forest communities are essentially dynamic and non-equilibrial over the long-term.

The role of anthropogenic influences in a mixed-conifer forest mortality episode

Anthropogenic influences on recent tree mortality in mid-montane mixed conifer forests of southern California, USA, and northern Baja California, Mexico, were investi- gated. The Pinus jeffreyi-Abies concolor phase of the mixed- conifer montane community was sampled at three sites, char- acterized by: (1) low levels of air pollution and long-term fire suppression (Holcomb Valley in the San Berardino Moun- tains, California); (2) chronic, severe air pollution and long- term fire suppression (Barton Flats in the San Bernardino Mountains, California); and (3) little air pollution and no fire suppression (La Corona Arriba in the Sierra San Pedro Martir in Baja California, Mexico). Similar percentages of dead standing trees, around 14 %, were found at both San Bernardino sites, but a comparatively low level, 4 %, was found in the Mexican forest. Anthropogenic effects, in particular fire sup- pression, may play an important role in enhancing the impact of natural stresses on the dynamics of mixed conifer forests.

DIVERSITY AND DISTURBANCE IN A COLORADO SUBALPINE FOREST

Diversity of understory plant species in a disturbed subalpine forest in the Colorado Rockies was examined to explore the hypothesis that intermediate levels of disturbance generate and maintain high levels of diversity. In 1974, hurricane-force winds created a blowdown with three types of canopy conditions—severely disturbed, moderately disturbed, and lightly disturbed. Data on tree-stand structure were taken in order to characterize the gradient of canopy disturbance. Species diversity of understory plants was estimated using both alpha diversity and beta diversity. The stand canopy with intermediate damage had higher numbers of both understory species and individuals, as well as higher beta diversity compared with other sites. This suggests that intermediate levels of disturbance severity may offer more microhabitats, at least temporarily, for a wider range of species. [Key words: Colorado Rocky Mountains, diversity, intermediate disturbance, subalpine forest.]

WIND DISTURBANCE AND DEVELOPMENT OF A NEAR-EDGE FOREST INTERIOR, MARYS PEAK, OREGON COAST RANGE

This study describes the structural characteristics of a developing forest interior in a nearly pure stand of noble fir near the summit of Marys Peak (1249 m) in the Oregon Coast Range. Our objective was to determine how catastrophic wind storms may have contributed to the structural changes that occur as a forest edge becomes part of the forest interior. Our results suggest that wind disturbance hastens the development of forest-interior characteristics through the creation of canopy gaps and the promotion of gap-phase regeneration. On Marys Peak, the incidence of gap creation and gap size appears to be related to site exposure and the size and age of trees in the developing forest interior. The high frequency of gap formation, as indicated by different-aged cohorts, suggests a rapid turnover of the forest canopy relative to stand age and the longevity of noble fir. We hypothesize that the combination of a high frequency of wind-related disturbance, gap-phase regeneration, and rapid growth of noble fir f...