Seasonal effects of fire on microbiotic crusts in shortgrass steppe.
FORD, P.L.* 1,2 and G.V.JOHNSON 3
Rocky Mountain Research Station, Albuquerque, NM USA 1
University of Arizona, Tucson, AZ USA 2
University of New Mexico, Albuquerque, NM USA 3
Microbiotic crusts are considered fragile and easily damaged by disturbance. Several studies have suggested fire disturbance destroys microbiotic crusts, while other studies have suggested that season of fire may mitigate the severity of crust damage. However, empirical evidence for this later hypothesis is lacking. Our objective was to experimentally test the seasonal effects of fire on microbiotic crusts in shortgrass steppe. Our study took place on 12 2-ha plots located in northeastern New Mexico in the southern Great Plains. Our experimental design was completely randomized, with 3 treatments, and 4 replicates per treatment. Treatments were dormant-season fire in April, growing-season fire in July, and unburned. Response variables for crusts were: 1) percent crust cover; 2) nitrogen fixation rate; and 3) chlorophyll content. Nitrogen fixation activity was measured using acetylene as a substitute substrate for nitrogen gas. Growing-season fire occurred July 1997 when vegetation was green and moisture was high. Dormant-season fire occurred April 1997, when moisture was low and fine fuel was cured. Sampling took place from March 1997 through October 1999. Cyanobacteria were the main components of our crust samples. Results indicate microbiotic crust made up an average of 7% of total ground cover on our site. Crust cover never significantly differed among treatments. Acetylene reduction significantly differed among treatments, X2 = 7.5, p = 0.02, as did chlorophyll content of crusts, X2 = 6.03, p = 0.04. The Dunn Procedure, with an adjusted alpha level of 0.008 and critical value z = 2.41, indicated dormant-season fire treated crusts had significantly lower rates of acetylene reduction than unburned crusts (z = 2.65). Growing-season fire treated crusts did not significantly differ from unburned (z = 0.97) or dormant-season fire treated crusts (z = 1.81). The Dunn Procedure, again with an adjusted alpha level of 0.008 and critical value z = 2.41, indicated dormant-season fire treated crusts had significantly lower chlorophyll content than unburned crusts (z = 2.45). Chlorophyll content of growing-season fire treated crusts did not significantly differ from unburned (z = 1.07) or dormant-season fire treated plots (z = 1.37). Growing-season fire appeared to reduce the impact of fire on microbiotic crusts.
Keywords: fire, microbiotic crust, nitrogen fixation
This abstract is being presented at: 10:30 AM in session:
Poster Session #12: Disturbance Ecology.