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Browsing by Author "Romme, William H., advisor"

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    Characterize southwestern United States pinon-juniper woodlands: seeing the "old" trees for the "young" forest
    (Colorado State University. Libraries, 2008) Jacobs, Brian Francis, author; Romme, William H., advisor
    Southwestern U.S. piñon pine and juniper woodlands are often represented as an expanding and even invasive vegetation type, a legacy of historic grazing and culpable in the degradation of western rangelands. Yet the extent and dynamics of piñon-juniper communities pre-dating intensive Euro-American settlement activities are poorly known or understood, while the intrinsic ecological, aesthetic, and economic values of old-growth woodlands are often overlooked. Historical changes in piñon juniper include two related, but poorly differentiated, processes: recent tree expansion into grass or shrub dominated (i.e., non-woodland) vegetation and thickening or infilling of savanna or mosaic woodlands pre-dating settlement. My work addresses the expansion pattern, modeling the occurrence of "older" savanna and woodland stands extant prior to 1850, in contrast to "younger" piñon juniper growth of more recent, post-settlement origin. I present criteria in the form of a diagnostic key for distinguishing "older", pre-Euro-American settlement woodlands from "younger" (post-1850) stands, and report results of predictive modeling and mapping efforts within the Four Corners states (i.e., Arizona, Colorado, New Mexico, and Utah) of the American southwest in piñon juniper types characterized by Pinus edulis and three associated junipers (Juniperus osteosperma, J. monosperma, J. scopulorum). Selected models suggest a primary role for soil moisture in the current distribution of "old" versus "young" piñon juniper stands. Pre-settlement era woodlands are shown to occupy a discrete ecological space, defined by the interaction of effective (seasonal) moisture with landform setting and fine-scale (soil-water) depositional patterns. "Older" stands are generally found at higher elevations or on skeletal soils in upland settings, while "younger" stands (often dominated by one-seed juniper, Juniperus monosperma) are most common at lower elevations or in productive, depositional settings. Areas of the southwestern U.S. with strong monsoonal (summer moisture) patterns appear to have been the most susceptible to historical woodland expansion, but even here the great majority of extant piñon juniper has pre-settlement origins (although widely thickened and infilled historically) and old-growth structure is not uncommon in appropriate upland settings. Modeling at broad regional scales can enhance a general understanding of piñon juniper ecology, while predictive mapping of local areas has potential to provide products useful for land management.
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    Disentangling fire, climate, forest structure, and land-use history interactions in Mexico's northern Sierra Madre
    (Colorado State University. Libraries, 2012) Meunier, Jed, author; Romme, William H., advisor; Knight, Richard L., advisor; Brown, Peter M., committee member; Fiege, Mark, committee member
    The 20th century was a period of profound changes in climate, land-use, forest structure, and fires throughout much of western North America and few montane forests continue to function under historical influences of climate variations and uninterrupted fire regimes. Yet, if we are to manage for resilient forests, understanding these linkages is critical and will depend on both pre-1900 and 20th century observations. My research takes advantage of a unique opportunity in northern Mexico to study forest and fire dynamics before a century of fire exclusion. My research documented a shift in climate - fire relationships in the late 19th century toward an overwhelming importance of antecedent moisture, unlike that seen previously for > 200 years. Tree recruitment peaks were tied to local processes, not broad-scale climate conditions. Antecedent wet conditions that promote fire occurrence suggests that in arid regions of the Southwest, anomalously wet years, still functioning under frequent fire occurrence, may further limit tree recruitment. The importance of fire induced mortality in shaping stand structure underscores the spatial variability of forests and helps explain even-age patches in forests as an artifact of patch survival of seedlings that recruit into the overstory.
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    Disturbance impacts on understory plant communities of the Colorado Front Range
    (Colorado State University. Libraries, 2009) Fornwalt, Paula J., author; Romme, William H., advisor
    Pinus ponderosa - Pseudotsuga menziesii (ponderosa pine - Douglas-fir) forests of the Colorado Front Range have experienced a range of disturbances since they were settled by European-Americans approximately 150 years ago, including settlement-era logging and domestic grazing, and more recently, wildfire. In this dissertation, I explored the impacts of these disturbances on understory plant communities. I investigated the long-term effects of settlement-era logging and grazing on forest understories by comparing understory composition at a historically logged and grazed site to that of a site that was protected from past use. I found little to no evidence of long-term logging and grazing impacts on understory richness, cover, and composition in upland forests. Long-term changes in richness, cover, and composition due to past logging and grazing were somewhat apparent in riparian forests, however, where these activities were likely the most intense. I analyzed data collected before (1997) and after (2003-2007) the 2002 Hayman Fire to examine wildfire effects on understory communities. Some declines in species richness and cover were observed immediately following fire, but by 2007, richness and cover often exceeded prefire conditions, even in severely burned areas. Fire-induced changes in community composition were apparent in all postfire years; regardless of fire severity, these changes were primarily due to new species recruitment, particularly short-lived native forbs, rather than due to a loss of prefire species. While exotic richness and cover generally increased as fire severity and time since fire increased, they remained low at the end of the study, and have not yet interfered with the recovery of the native understory community. I conducted a literature review to examine the mechanisms through which Front Range understory species establish after fire (i.e., by sprouting, establishing from soil-stored seed, and/or establishing from offsite seed). I found that postfire establishment mechanisms for many species are poorly understood, although some broad patterns did emerge. Short-lived forbs appear to establish postfire primarily through soil-stored seed, while sprouting is the most common postfire establishment mechanism for long-lived forbs, graminoids, and woody plants. Many species have multiple postfire establishment mechanisms, which helps to ensure their continued presence after fire.