The response of sub-adult savanna trees to six successive annual fires in the Guinean savannas of West Africa.

In a six year experimental field study, N’Dri and colleagues demonstrate how burning during the annual long dry season can be managed to maintain a target density of trees, with implications for use in the other humid savannas.

Fires in humid savannas are set by humans for different management purposes. In the Guinean savannas of West Africa, successive annual fires are common, with stakeholders generally agreeing that fires must be used to reduce bush encroachment. However, it is also desirable to maintain some density of trees in the landscape for many other management objectives.

The effects of any management fire depend on the responses of established sub-adult trees. Thus, in our study, we monitored annually the survival, recovery, and growth of 1,765 sub-adult trees under fires set in three different periods of the long dry season: early-dry season (EDS), mid-dry season (MDS) and late-dry season (LDS).

Sub-adult trees resisted being top-killed under EDS when they were ∼1-∼2 m in height under both MDS and LDS fires. The threshold height where sub-adult trees could transition to an adult stage (be recruited in a subsequent year) was ∼3 m for EDS and ∼ 3.3 m for MDS fires, but no height was great enough for sub-adult trees to be recruited to adult stages under LDS fire.  

Fire 1
Each burning season. EDS: early-dry season fire (November), MDS: mid-dry season fire (January); LDS: late-dry season fire (March). MDS fires are the most common management fires in our study area. The intensity and flame heights of the MDS and LDS fires were similar, but the EDS fires were less intense and with lower flame heights 

Our study shows clearly that successive EDS and MDS fires can enhance tree density and that successive LDS fires alone reduce tree density. From a management perspective, we recommend incorporating occasional late-dry season (LDS) fires within the existing practice of annual mid-dry season (MDS) fires in order to reduce the recruitment of sub-adult trees to adult stages.

Our work is also applicable to the broader Guinean savannas, where in some areas there is a projected a tree cover increase of 1–10% by 2050 (assuming no changes in management or conditions). Within these areas of heavy bush encroachment we suggest that LDS fire be used triennially, as only sub-adult trees that were able to resist direct damage for three successive years were growing under EDS and MDS fires (but not LDS fires).

Finally, we demonstrate a new method of managing tree density in Guinean savannas and in other humid savannas, whether the goal is to increase, decrease, or maintain tree densities. e.g. LDS fires could be used triennially where the management aim is to reduce tree density, and alternatively, successive EDS and MDS fires could be used to enhance tree density

Read the full paper The response of sub-adult savanna trees to six successive annual fires: An experimental field study on the role of fire season in Journal of Applied Ecology

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