Overstory-Understory Relationships along Forest Type and Environmental Gradients in the Spring Mountains of Southern Nevada, USA Scott R. Abella & James C. Hurja & Douglas J. Merkler & Charles W. Denton & David G. Brewer # Institute of Botany, Academy of Sciences of the Czech Republic 2011 Abstract Isolated forested mountains in deserts have numerous ecological and societal values, but land-management practices (e.g., fire-regime alteration) and climate change can affect forest composition. We analyzed tree overstory-understory relationships on 123 sites in the Spring Mountains within the Mojave Desert near Las Vegas, Nevada, USA to assess three hypotheses. We hypothesized that: the tree species comprising understories are less tolerant of fire than species in overstories, reflecting land- management practices of fire exclusion; mid-elevation forests have the lowest overstory: understory similarity because this zone could have maximum species mixing; and overstory:understory similarity is correlated with environmental gradients (consisting of 14 topographic and soil variables). We found that Pinus monophylla comprised greater relative canopy cover in understories of juniper (32% relative cover) and pinyon- juniper (78%) forests than it did in overstories of these forests (0% and 53%). Similarly, fire-intolerant Abies concolor had 6-fold greater understory than overstory cover in forests with overstories dominated by the fire-tolerant Pinus ponderosa. Overstory:understory Sørensen similarity averaged 43%-77% among six forest types, and there was little support for the supposition that similarity was lowest in mid- elevation forests. Distributions of individual overstory and understory species more Folia Geobot (2012) 47:119–134 DOI 10.1007/s12224-011-9108-7 S. R. Abella (*) School of Environmental and Public Affairs, University of Nevada Las Vegas, Las Vegas, NV 89154-4030, USA e-mail: scott.abella@unlv.edu J. C. Hurja Humboldt-Toiyabe National Forest, U.S. Forest Service, 4701 N. Torrey Pines Dr., Las Vegas, NV 89130, USA D. J. Merkler Natural Resources Conservation Service, 5820 S. Pecos Road, Las Vegas, NV 89120, USA C. W. Denton : D. G. Brewer Ecological Restoration Institute, Northern Arizona University, Flagstaff, AZ 86011-5017, USA