Overstory Structure and Community Characteristics of Oregon Ash (Fraxinus latifolia) Forests of the Willamette Valley, Oregon Public Deposited

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  • As species of ash trees become increasingly threatened worldwide by exotic pests and pathogens, it is important to develop descriptions of their ecologies that help guide the conservation and restoration of forests in which they are a major component. Oregon ash (Fraxinus latifolia) is a dominant tree species in wetland forests of the Willamette Valley, Oregon. It is potentially threatened by the emerald ash borer, an exotic forest insect that is expanding in range in North America. The arrival of the emerald ash borer in the western United States is expected to cause high rates of Oregon ash mortality, which will likely highly alter structural and compositional characteristics of regional riparian and wetland forests. To investigate the structural development and plant community dynamics of Willamette Valley Oregon ash forests, I quantified species structure and composition for both overstory and understory communities along gradients of stand age and soil moisture. Community Type Wetland Index (CTWI) scores were calculated for each plot by multiplying plot-level species abundances by the national wetland indicator value for each species, and were used as an approximation of local soil moisture regimes. Ash forests and early seral savannas were sampled in 102 plots within 11 wetlands between Lane and Washington Counties, Oregon. Early seral wetland savannas were dominated by herbaceous vegetation and had highly variable ash and other tree species recruitment. Young ash forests initiated on wetland sites that were formally maintained as savannas via anthropogenic disturbance. Structural development in these forests generally followed previously described post-disturbance stages: stem exclusion, understory initiation, young multi-strata, and old growth. As stands aged, tree densities decreased and mean tree diameter increased. Mean stand densities (for trees >5 cm at 1.3 m height) ranged from 1727 trees per hectare in stem exclusion stands to 348 in old growth stands, while mean tree diameter increased from 12 to 32 cm between those same stages. Furthermore, understory vegetation shifted from herbaceous to shrub and understory tree species. Oregon ash had the greatest relative dominance of all tree species at all stages of structural development. Oregon white oak (Quercus garryana) was the only other tree species commonly found in forest canopies, though it was typically far less abundant than Oregon ash. Plots in which oak was present had a greater depth to soil mottling, indicating a lower depth to the annual high water table. Oregon ash was also the most commonly regenerating tree species in all stages of structural development, and nearly 70% of ash regeneration was vegetative. Other tree regeneration was typically restricted to understory species, especially cascara (Rhamnus purshiana) in forests and hawthorn (Crataegus spp.) in savannas. Forest structural diversity increased between each stage of development. In total, 216 plant species were identified. Of these, 147 were found in forest plots. Community composition differed significantly between forests and savannas, and between "young" (stem exclusion and understory initiation) plots and "old" (young multi-strata and old growth) plots. Nonmetric multidimensional scaling (NMS) indicated that, for all wetland plots, the environmental variables most highly correlated with community composition were CTWI scores and canopy cover. For forest plots only, NMS indicated that CTWI scores and stand density were the environmental variables most related to patterns in community composition. Species richness was not correlated with canopy cover, but was negatively correlated with CTWI scores. Exotic species cover was negatively correlated with canopy cover, but was not correlated with CTWI scores. Exotic species composition was dependent on both canopy cover and CTWI scores. On wet sites, reed canary grass (Phalaris arundinacea) was particularly abundant, and was associated with low species richness. On drier sites, common exotic species included trees and shrubs such as blackberries (Rubus spp.), exotic roses (Rosa spp.), single-seeded hawthorn (Crataegus monogyna), and common pear (Pyrus communis) as well as several grass and forb species. The results of this study suggest that, although overstory tree communities are characterized by low diversity, Oregon ash forests are important for landscape-scale structural and plant community diversity in Willamette Valley wetlands. However, overstory tree communities lack diversity; Oregon ash appears to be both an early seral, colonizing species in open wetlands and a climax species that is capable of regenerating under a closed canopy. Few other trees are present regionally that are capable of maintaining a forested state in wetlands. Furthermore, closed canopy Oregon ash forests may suppress exotic species spread, and in the absence of management, overstory ash mortality may lead to increased exotic plant cover. Regardless of future forest health threats, intact Oregon ash forests, like other regional wetland-types, are currently few and limited in extent and should be considered a conservation priority.
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