24 April 2013 (mongabay.com) – Rainforests that have been affected by even low-intensity fires are far more vulnerable to invasion by grasses, finds a new study published in special issue of the journal Philosophical Transactions of the Royal Society B. The findings are significant because they suggest that burned forests may be more susceptible to subsequent fires which may burn more intensely due to increased fuel loads.
The research is based on an eight-year study involving experimental burning of three 50-hectare plots in the Amazon rainforest. Each plot underwent a different "fire treatment" — no burning, burning every year, and burning every three years. While the fires were small, their impact was significant. Both types of burned plots experienced invasion of native and non-native grasses from adjacent pasture lands. The plot that was burned on a three-year cycle, actually had the highest total area invaded by grasses (10 percent of its area).
The areas that had the highest fuel loads were those dominated by exotic grasses often used to seed cattle pasture. Naturally, areas with higher fuel loads are at risk for more intense and more damaging fires.
The study however found that grass invasion was generally limited to forest edge areas, a result the authors attribute to difficulties in seed dispersal, herbivory, and lower light availability in deeper forest zones. Nonetheless, the researchers said the findings support the argument that parts of the Amazon rainforest may become increasingly susceptible to transitioning toward a drier, grassier ecosystem if forest fragmentation and recent incidents of severe drought continue.
"Major increases in grass invasion occurred following intense fires associated with drought events," the authors write. "This result suggests that fire could mediate the predicted climate-driven substitution of large portions of Amazonian forest by grass-dominated ecosystems. With the increases in air temperatures and decreases in precipitation predicted for the Amazon, fires are expected to become more frequent and intense, killing proportionally more trees." [more]