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>An evaluation of understory vegetation dynamics, ecosystem resilience and state and transition ecological theory in an eastern Oregon ponderosa pine forest.
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An evaluation of understory vegetation dynamics, ecosystem resilience and state and transition ecological theory in an eastern Oregon ponderosa pine forest.
Relatively recent increases in ponderosa pine abundance have effected unprecedented changes to ecosystem structure and function. Efforts to restore ponderosa pine systems are often focused on the manipulation of tree structure and the re-introduction of a more natural fire regime. Successful restoration should also incorporate understory components but information addressing changes in understory species is minimal for Pacific Northwest ponderosa pine forests. Moreover, state and transition models (STM) derived from the concepts of non-equilibrium ecology may be appropriate for characterizing ecosystem dynamics in modern ponderosa pine forests, however this approach has not previously been used. The focus of this research was to evaluate ecosystem dynamics within a hypothesized STM framework and to experimentally assess the existence of model components with particular emphasis on understanding understory species dynamics.; The analyses of vegetation, soil, and environmental attributes measured in a eastern Oregon ponderosa pine forest indicated that increased ponderosa pine occupancy modified the under-canopy environment through alterations in light, nitrogen availability, and soil temperature and was related to reductions in understory species diversity caused by a major shift in understory character away from the dominance of perennial bunchgrasses. Light availability appeared to be the most influential driver in understory species distribution. Similar results were observed in a greenhouse experiment that identified high light intensity as significant in generating increased Festuca idahoensis Elmer growth, biomass and vigor. A seed bank assessment to evaluate recovery potential from diminished understory conditions indicated that understory species dominant in open ponderosa pine forests do not form a persistent seed bank and can not be relied upon as a tool for their recovery.; These data described ecosystem dynamics in accordance with the hypothesized STM model. Loss of ecosystem resilience associated with a fire loss threshold can initiate additional degradation in the understory component, however, pine influenced alterations to the under-canopy environment did not appear irreparable and restoration practices that reduce pine occupancy should enhance the conditions for perennial bunchgrass growth. Successful restoration will require the re-introduction of desired understory species seeds or other viable plant material when considering stands that have lost the understory component.
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