Journal of Biogeography (J. Biogeogr.) (2005) 32, 2083–2102 ORIGINAL Understorey plant community structure ARTICLE in lower montane and subalpine forests, Grand Canyon National Park, USA Daniel C. Laughlin1*, Jonathan D. Bakker1 and Peter Z. Fule´1,2 1Ecological Restoration Institute and 2School of ABSTRACT Forestry, Northern Arizona University, Aim Our objectives were to compare understorey plant community structure Flagstaff, AZ, USA among forest types, and to test hypotheses relating understorey community structure within lower montane and subalpine forests to fire history, forest structure, fuel loads and topography. Location Forests on the North Rim of Grand Canyon National Park, Arizona, USA. Methods We measured understorey (< 1.4 m) plant community structure in 0.1-ha plots. We examined differences in univariate response variables among forest types, used permutational manova to assess compositional differences between forest types, and used indicator species analysis to identify species driving the differences between forest types. We then compiled sets of proposed models for predicting plant community structure, and used Akaike’s information criterion (AICC) to determine the support for each model. Model averaging was used to make multi-model inferences if no single model was supported. Results Within the lower montane zone, pine–oak forests had greater understorey plant cover, richness and diversity than pure stands of ponderosa pine (Pinus ponderosa P. & C. Lawson var. scopulorum Engelm.). Plant cover was negatively related to time since fire and to ponderosa pine basal area, and was highest on northern slopes and where Gambel oak (Quercus gambelii Nutt.) was present. Species richness was negatively related to time since fire and to ponderosa pine basal area, and was highest on southern slopes and where Gambel oak was present. Annual forb species richness was negatively related to time since fire. Community composition was related to time since fire, pine and oak basal area, and topography. Within subalpine forests, plant cover was negatively related to subalpine fir basal area and amounts of coarse woody debris (CWD), and positively related to Engelmann spruce basal area. Species richness was negatively related to subalpine fir basal area and amounts of CWD, was positively related to Engelmann spruce basal area, and was highest on southern slopes. Community composition was related to spruce, fir and aspen basal areas, amounts of CWD, and topography. Main conclusions In montane forests, low-intensity surface fire is an important ecological process that maintains understorey communities within the range of natural variability and appears to promote landscape heterogeneity. The presence of Gambel oak was positively associated with high floristic diversity. Therefore management that encourages lightning-initiated wildfires and Gambel oak production may promote floristic diversity. In subalpine forests, warm southern slopes and areas with low amounts of subalpine fir and CWD were positively associated with high floristic diversity. Therefore the reduction of CWD *Correspondence: Daniel Laughlin, Ecological and forest densities through managed wildfire may promote floristic diversity, Restoration Institute, Northern Arizona University, PO Box 15017, Flagstaff, AZ 86011, although fire use in subalpine forests is inherently more difficult due to intense USA. E-mail: [email protected] fire behaviour in dense spruce–fir forests. ª 2005 Blackwell Publishing Ltd www.blackwellpublishing.com/jbi doi:10.1111/j.1365-2699.2005.01357.x 2083 D. C. Laughlin, J. D. Bakker and P. Z. Fule´ Keywords Akaike’s information criterion, Kaibab Plateau, model selection, natural variability, North Rim, Pinus ponderosa, Populus tremuloides, Quercus gambelii, spruce–fir, time since fire. are strongly influenced by fire (Crawford et al., 2001; Scho- INTRODUCTION ennagel et al., 2004), forest structure and tree species compo- Management of montane and subalpine forests requires an sition (Naumburg & DeWald, 1999; Chipman & Johnson, understanding of the range of natural variability of understo- 2002), fuel load (Laughlin et al., 2004) and topography (Zobel rey plant community structure. ‘Natural variability’ is the et al., 1976). range of ecological conditions found in intact systems, and in Montane forest communities in the south west have evolved North America is often regarded as the range of conditions in an environment that historically burned every 2–20 years by within the ecosystem prior to Euro-American settlement low-intensity surface fires (Swetnam & Baisan, 1996; Moore (Landres et al., 1999). Various approaches have been used to et al., 1999). Dendrochronological analyses of our study sites quantify the natural variability of tree species in south-western have not detected any stand-replacing fires in the montane forests, in particular, dendrochronological methods have been zone before Euro-American settlement (Fule´ et al., 2003a), used to determine overstorey reference conditions (Fule´ et al., although stand-replacing fires appear to have occurred in 1997; Mast et al., 1999). However, less information is available northern Rocky Mountain ponderosa pine forests (Ehle & concerning the range of natural variability of understorey plant Baker, 2003; Pierce et al., 2004). Subalpine plant communities communities. in North America are adapted to an evolutionary environment One useful approach used to examine the variability of characterized by short growing seasons and a disturbance understorey communities has been to measure relict sites that regime of infrequent, high-intensity crown fires (Agee, 1993; have been relatively undisturbed since Euro-American settle- Kipfmueller & Baker, 2000; Johnson et al., 2001; Fule´ et al., ment (Gildar et al., 2004). Unfortunately, the vast majority of 2003b). In both systems, fire has strong direct effects on upland forests in the western USA have been affected by understorey plant community structure by altering site con- commercial logging, overgrazing by domestic livestock (Belsky ditions and microclimate. Fire also affects forest structure and & Blumenthal, 1997), and the exclusion of surface fires (Agee, fuel loads which, in turn, may directly influence the under- 1993). However, Grand Canyon National Park (GCNP) contains storey plant community. the largest unharvested (Warren et al., 1982), minimally grazed, Plant community structure may be indirectly related to fire and occasionally burned (Fule´ et al., 2003a) forest ecosystem in history through the effect of fire on forest structure: established Arizona, and therefore provides valuable sites for measuring overstorey trees outcompete many herbaceous species for natural variability. We focused our study on lower montane and resources (Riegel et al., 1995). In montane and subalpine subalpine forests (sensu Marr, 1961; Brown & Lowe, 1980). The systems, plant production and species richness are generally lower montane zone (2200–2350 m, hereafter ‘montane’) is positively related to light availability mediated through abun- dominated by ponderosa pine (Pinus ponderosa P. & C. Lawson dances of conifer and hardwood tree species (Daubenmire, var. scopulorum Engelm.) and Gambel oak (Quercus gambelii 1943; Oosting & Reed, 1952; Ellison, 1954; Langenheim, 1962; Nutt.) forest. The upper montane zone (2350–2600 m) consists Fonda & Bliss, 1969; Reynolds, 1969; Despain, 1973; Ffolliott, of dense ponderosa pine and mixed conifer forests that have been 1983; Tapia et al., 1990; Stromberg & Patten, 1991; Moore & heavily invaded by shade-tolerant and fire-intolerant trees such Deiter, 1992; Reich et al., 2001). It has been shown that species as white fir [Abies concolor (Gord. & Glend.) Lindl. ex Hildebr.] richness is higher in conifer stands with greater light availab- (Fule´ et al., 2004; Mast and Wolf, 2004); the plant community ility (Chipman & Johnson, 2002); plant production in aspen structure within this zone was examined by Huisinga et al. forests decreases as conifer abundance increases; and clear- (2005) and was not included in this analysis. The subalpine zone cutting spruce–fir stands in Colorado increases forage pro- (2600–2900 m) is a diverse mixture of Engelmann spruce (Picea duction (Kranz & Linder, 1973; Regelin and Wallmo, 1978; engelmannii Parry ex Engelm.), subalpine fir [Abies lasiocarpa Mueggler, 1985). Hardwood species, such as aspen and (Hook.) Nutt.], aspen (Populus tremuloides Michx.) and mixed Gambel oak, might have a positive effect on herbaceous conifer forests with ponderosa pine on southern slopes. plant communities relative to conifers by increasing litter Natural variability of understorey plant community struc- quality through greater litterfall nitrogen (Daubenmire, 1953; ture is determined by complex interactions of biotic and Klemmedson, 1987, 1991; Reich et al., 2001), and by changing abiotic factors. Our use of the phrase ‘plant community the quality of light in the understorey. structure’ is inclusive, including plant species composition, Plant community structure may also be indirectly related to richness and abundance (Tilman, 1982; Krebs, 1994). In fire history through fire’s reducing effects on fuel loads. upland forest ecosystems of western North America, it has Accumulated litter can intercept light, alter soil microclimates, been proposed that understorey plant community assemblages form physical barriers to plant emergence (Facelli & Pickett, 2084 Journal of Biogeography 32, 2083–2102, ª 2005 Blackwell Publishing Ltd Plant community structure in