In mountains throughout western North America, large, mixed-severity fires produce a mosaic of low and high tree mortality. Following wildfire, plant communities may recover to their pre-fire state, or may remain altered in composition and structure. In this study I quantified the extent to which fire severity influenced post-fire vegetation composition and structure in comparison to pre-fire states in the Santa Catalina Mountains, Arizona, USA. I used a stratified random design that sampled plots across ecological units and fire severity classes. Tree diameter at breast height (DBH), shrub cover, and seedling and sapling density was recorded by species in five plant communities: oak/pinyon/juniper woodland on hills landscapes of mixed lithology; Madrean pine-oak forest/woodland on granite, gneiss, or metasedimentary rock; Madrean oak/conifer/manzanita on rock outcrops; ponderosa pine forest on granite, gneiss, and similar rocks; and mixed conifer forest on metasedimentary landscapes. Subsets of these data were then used to reconstruct overstory vegetation present when the Bullock (2002) and Aspen (2003) fires occurred. Data from a 1984 pre-fire study was used to substantiate the overstory reconstruction and to determine shrub understory components. I tested the hypothesis that tree mortality was a determinant of post-fire shrub cover, and calculated post-fire importance values (IVs) of tree and shrub components. Ordination and non-metric multidimensional scaling (NMS) of IVs confirmed that overstory reconstruction aligned with 1984 field surveys. Tree mortality was a predictor of post-fire shrub cover, but only with certain species in specific ecological units. Ordinations indicated that tree composition in post-fire plots has diverged from that in pre-fire plots in all but the oak/ pinyon/juniper community. Ordination of shrub components indicated novel configurations of post-fire communities, including association of pre-fire mixed conifer elements with oak woodland elements. The intermixing of tree species in mid- and higher-elevation communities with those historically confined to lower elevation community types suggests that recent fires has disrupted vegetation inertia and initiated novel ecological change. The re-structuring observed within these community types are in agreement with projections that disturbance and climate change will interact to facilitate the spread of lower elevation species to higher elevation zones. Fire x climate interactions may therefore trigger long-lasting changes to ecosystem structure in ways not predicted by models of fire-effects or climate-effects occurring in isolation from each other.
| Identifer | oai:union.ndltd.org:arizona.edu/oai:arizona.openrepository.com:10150/337365 |
| Date | January 2014 |
| Creators | Maghran, Lauren A. |
| Contributors | Falk, Donald A., Falk, Donald A., Archer, Steven, Malusa, Jim |
| Publisher | The University of Arizona. |
| Source Sets | University of Arizona |
| Language | en_US |
| Detected Language | English |
| Type | text, Electronic Thesis |
| Rights | Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. |
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