Scale, ecosystem resilience, and fire in shortgrass steppe

Abstract

Consideration of scale and ecosystem resilience is integral to any conceptual model of the effects of disturbance on ecosystems. Organisms, populations, communities, and ecosystems are differentially affected by disturbance based on the scale at which they occupy the landscape. Scale of observation influences perceptions about ecosystem resilience. There is no single correct scale at which ecological phenomena should be studied, and management decisions require the interfacing of phenomena that occur on very different scales of space and time. Fire disturbance affects a variety of ecosystem factors including nutrient cycling, species diversity, and population and community dynamics. My experimental research on fire in shortgrass steppe examined the effects of fire and season of fire on various components of shortgrass steppe at multiple spatial and temporal scales and organizational units. My experimental design was completely randomized, with 3 treatments, and 4 replicates per treatment. Treatments were dormant-season fire, growing-season fire, and unburned. Response variables were (1) ground cover; (2) microbiotic crust nitrogen fixation, and chlorophyll a content; and (3) species richness, abundance, and relative abundance of small mammals and arthropods. Microbiotic crust cover never differed significantly among treatments for all periods, however, acetylene reduction and chlorophyll a content of crusts differed significantly among treatments. Dormant-season fire-treated crusts had significantly lower rates of acetylene reduction than unburned crusts, while growing-season fire-treated crusts did not differ significantly from unburned or dormant-season fire-treated crusts. Dormant-season fire-treated crusts had significantly lower chlorophyll content than unburned crusts, while growing-season fire-treated crusts did not significantly differ from unburned or dormant-season fire treated-crusts. Initially, growing-season fire significantly reduced grass cover compared to unburned and dormant-season fire. Approximately 30 months later there were no significant differences in grass cover among treatments. Bare ground response was basically the inverse of grass cover response. The only significant differences in litter cover between treatments occurred immediately after the growing-season fire. Arthropod species richness differed significantly among treatments; growing-season fire plots had a significantly higher number of beetle species. However, overall beetle abundance did not significantly differ among treatments. Significant differences were never detected in overall rodent species richness or abundance among treatments.