Return of fire to a free-flowing desert river: Effects on vegetation

Changes in disturbance regime often lead to changes in plant community structure. After a long period in which fuel loads were sparse, fire recently has occurred with high  frequency in the ungrazed riparian zone of the Upper San Pedro River (Chihuahuan Desert, Arizona).  We studied four accidental fires that occurred during 1994-2003. Woody vegetation and landscape structure were contrasted between burned sites and matched spatial controls, and before and after one fire.  Herbaceous vegetation was sampled in multiple years producing a chronosequence of time since fire.  Riparian fire was associated with reductions in woody plant species diversity and canopy cover.   For herbaceous vegetation, fire caused a short-term (2 year) pulse of diversity, driven by annual species, similar to patterns observed after fire in Chihuahuan desert uplands and after floods in riparian zones.  Fire also caused persistent increase in herbaceous cover, mediated in part by the reduction in tree canopy cover.  By converting riparian woodlands to grasslands and savannahs, fire appears to be shifting vegetation structure of the Upper San Pedro floodplain closer towards conditions present during past centuries, when fire was frequent in the upland desert grasslands and embedded riparian corridor.   These patterns constitute a reversal of trends, observed in many ecoregions, for grasslands to shift to wooded vegetation types following grazing-linked fire suppression.