PARENT SESSION
Oral Session #48: Vegetation Change and Response.
Presiding: D. Zobel
Tuesday, August 6. 1:00 PM to 4:45 PM. Mesquite Room, Radisson.

Long-term (85 year) understory vegetation change in Pinus ponderosa stands of northern Arizona.

BAKKER, JONATHAN^*,1, MOORE, MARGARET¹, SPRINGER, JUDY¹, CROUSE, JOSEPH¹, ¹ Northern Arizona University, Flagstaff, AZ

ABSTRACT- Long-term dynamics of herbaceous vegetation are difficult to quantify because of the absence of long-lived plant structures such as woody stems. We use 48 permanent plots (each 1.5 x 3 m) to document changes in the forest understory during 85 years of fire exclusion. Thirty-two plots were 'natural' (uncultivated) and sixteen were 'denuded' (cultivated) when the plots were established in 1914. The plots are located in four sites on the Fort Valley Experimental Forest near Flagstaff, AZ. Fire was excluded from the plots since 1876 and livestock grazing was excluded since 1910. On each plot, plant species were ranked by cover in 1919 and again in 1999. Species richness declined by 40% between 1919 and 1999. Community analysis of the ranked data indicated that sites differed significantly in both 1919 and 1999. The vegetation on a subset (n = 10) of the natural plots was mapped in 1914 and again in 1997. On the mapped plots, plant density declined significantly and functional group and species dynamics changed. In particular, graminoids (Carex spp., Elymus elymoides, and Muhlenbergia montana) increased as a proportion of total plant density while legumes (Lotus wrightii and Oxytropis lambertii), shrubs (Ceanothus fendleri), and other species (Artemisia spp., Eriogonum alatum, Penstemon spp., and Senecio spp.) declined. Since Pinus ponderosa ecosystems are adapted to frequent fire regimes, these changes are likely due to long-term fire exclusion and subsequent changes in forest structure.

KEY WORDS: pinus ponderosa, vegetation change, fire exclusion, understory