Effects of Fire Disturbance and Grass Removal on Tree Growth in Mesic Savannas

Abstract

The coexistence of trees and grasses in the savanna landscape has confounded scientists for decades. In contemporary research, the mechanisms for codominance are usually explained through two different hypotheses depending upon annual precipitation level: rooting-niche-based and disturbance-based. The rooting-niche hypothesis states that grasses and trees have different root depths, and thus occupy different competitive niches. The disturbance-based hypothesis argues that life disturbances, such as fire, are what limit tree growth and survival, thus maintaining the grass-tree coexistence. A key principle in savanna ecological models is that the fire regime is a major determinant of vegetation cover where late-dry season fires are more damaging to trees than early fires. This is indirectly linked to the tree-grass coexistence, as less grass would provide less competition for resources, meaning small trees may grow faster in order to escape fire. Thus, such disturbances as fire and grass removal may have the ability to alter the growth rate and height of juvenile and adult trees. This study analyzed the effects of fire disturbance and grass elimination on tree growth in a mesic savanna in Mali, West Africa, for a period of two years. This study divided two study sites into eight experimental plots, four were subjected to different grass reduction treatments (clipping, hoeing, herbicide, and grazing), three were subjected to fire treatments (early, middle, and late-season), and one plot was set as the control variable. This study addresses three key hypotheses:

1. Grass removal will increase the rate of juvenile tree growth in terms of height and growth rate.
2. Late fires will slow tree growth rate the most, followed by mid-season and then early season fires.
3. Tree deaths will be highest on late-season fire plots followed by mid-season and then early-season.

All trees under grass removal disturbances and early-season fires had an increased growth rate as compared to the control trees. Late-season fires may trigger some trees to grow faster to overcome flames, but they also cause the most tree deaths. Trees of each size class–small juvenile, large juvenile, and adult-grow more rapidly with grass removal. The findings suggest that early burning and grass reduction practices have applications for increasing woody vegetation in savanna landscapes, which would increase carbon sequestration. However, it must be noted that local savanna inhabitants rely on grass cover for animal fodder and other uses; thus, converting grasslands to woodlands may have negative side effects.