Chaparral history, dynamics, and response to disturbance in southwest Oregon : insights from age structure Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/kw52jd525

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  • Fuels management on Bureau of Land Management lands in SW Oregon, USA, is motivated by the needs to reduce fire hazard and restore ecosystems thought to be impacted by fire suppression. Chaparral is one of the most characteristic vegetation communities of SW Oregon's interior valleys, and extensive acreages within this system are targeted annually for fuels treatment. However, this community is also poorly understood. Little is known about its response to fire suppression, and the assumptions justifying the need for fuels reduction and on which treatment prescriptions are based are extrapolations from other ecosystems. I studied patterns in chaparral age structure of two obligate-seeding shrubs, Arctostaphylos viscida C. Parry and Ceanothus cuneatus (Hook.) Nutt., for insights into the influences of environment and the two dominant disturbances in SW Oregon, fire and grazing, on population structure and dynamics. My 31 low to mid-elevation study sites represent four chaparral 'types' and a range in times since fire (22 yr to > 114 yr). My study area is near the northernmost extension of the California chaparral vegetation type, and results indicate that SW Oregon chaparral populations diverge from studied California chaparral populations in several fundamental ways. All stands unburned for > 30 yr were uneven-aged due to prolific recruitment in the absence of fire and appreciable fire survivorship. Relatively high shrub densities were associated with even-aged sites, but these sites were all young (< 30 yr) and density declined the longer stands were unburned. No relationships were detected between site environment or disturbance history and age structure in general, or the relative amount of fire-free recruitment or fire survivorship in particular. Results suggest that most, if not all, chaparral stands would over time develop uneven-aged structure across the range of environments represented in the study area. Current fuels treatment prescriptions in SW Oregon chaparral are predicated on the assumption that fire suppression has replaced naturally high-frequency, low-severity fires with less frequent and higher-severity fires due to an unnatural accumulation of fuels. However, it appears that this model is not appropriate for chaparral vegetation. Chaparral has apparently experienced a fire regime that is distinct from that of adjacent vegetation communities, and fire suppression does not appear to have altered overall chaparral structure or fire severity. Current fuels treatments are unlikely to reproduce stand structures observed in mature chaparral or in post-wildfire stands. Rather, fuels treatments are likely to alter the trajectory of chaparral development that appears to have been in place for the last century. Results clearly illuminate that chaparral encompasses a wide range of structures and responses to environment and disturbance, and imply that precepts of chaparral ecology and management should be tailored on a geographic basis. Results also underscore the need for management objectives to recognize differences among vegetation communities in complex environments.
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