Understorey plant community structure in lower montane and subalpine forests, Grand Canyon National Park, USA

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Abstract:

Abstract Aim 

Our objectives were to compare understorey plant community structure 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 permutationalmanovato 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 and forest densities through managed wildfire may promote floristic diversity, although fire use in subalpine forests is inherently more difficult due to intense fire behaviour in dense spruce–fir forests.

Keywords: Akaike's information criterion; Kaibab Plateau; North Rim; Pinus ponderosa; Populus tremuloides; Quercus gambelii; model selection; natural variability; spruce–fir; time since fire

Document Type: Research Article

DOI: http://dx.doi.org/10.1111/j.1365-2699.2005.01357.x

Affiliations: Ecological Restoration Institute

Publication date: December 1, 2005

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