|HOME SCHEDULE AUTHOR INDEX SUBJECT INDEX|
An ecosystem in transition: What are the mechanisms driving shrub expansion in temperate grasslands?
Heisler, Jana*,1, Briggs, John1, 1 Arizona State University, Tempe, AZ 85287-1601, USA
ABSTRACT- Fire is an integral component of the disturbance regime in temperate grasslands and impacts the native grass and shrub species through both its direct and indirect effects. During the last century, fire suppression in these grasslands has been accompanied by an increase in shrub cover. While this shift in growth form dominance has been well documented, short-term mechanistic studies are integral in understanding the mechanism driving this transition from grassland to shrubland. In 2001, an experimental study was initiated at Konza Prairie to assess the ways in which fire acts as a constraint to and/or facilitator of shrub expansion. Briefly, the experimental design included fire, nitrogen addition, and litter as main effects and pairwise comparisons between shrub islands allowed us to uncouple and subsequently quantify the impact of the fire event itself from that of the post-fire microclimate. In shrub islands (species Cornus drummondii) exposed to spring fire, 100% aboveground mortality was observed, but by ca. 60 days post-fire, a pulse of sprouting enabled stem density to return to pre-burn levels. Light availability, and subsequently warmer soil temperatures, further stimulated this sprouting response with burned islands and burned + litter islands increasing in stem density by six-fold and four-fold, respectively. By comparison, unburned islands (controls) increased by only two-fold. While ANPP for C. drummondii in all islands was similar, resource allocation differed between burned and unburned islands, with new sprouts investing in stem tissue to rapidly regain height and pre-burn stems producing greater foliar tissue. These results, when paired with a recent long-term study of fire frequency and patterns of shrub expansion, suggest that fire events stimulate rapid increases in C. drummondii stem density, which may lead to faster rates of shrub expansion, especially when followed by fire-free intervals during which shrubs may increase in total cover.
Key words: woody encroachment, grassland, fire, resource availability