Fire season affects size and architecture of Colophospermum mopane in southern African savannas

Published source details Kennedy A.D. & Potgieter A.L.F. (2003) Fire season affects size and architecture of Colophospermum mopane in southern African savannas. Plant Ecology, 167, 179-192.
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Summary

Lightning and human induced fires have often been considered by land managers to exert similar effects on African savanna vegetation, however, if they occur in different seasons as is frequently the case, their effects may differ. In the savanna regions of southern Africa, 'natural' lightning fires tend to occur at the start of the wet season (October and November) while anthropogenic fires are usually lit in the dry season, between July and August. A long-term field experiment initiated in the Kruger National Park in 1952 was used to investigate whether this seasonal divergence affects tree abundance, spatial pattern, size and architecture, with regards in particular one dominant and ecologically important tree, mopane or turpentine tree Colophospermum mopane.

Study area: The study was carried out in the Kruger National Park (19,485 kmÂ²), South Africa. The park contains many tree and shrub species of which C.mopane is one of the most abundant. The significance of this small tree is extremely high; mopane scrubveld is a favoured habitat of many rare large mammals; the trees are browsed by vertebrate herbivores and the leaves are of great value as forage for ungulates in times of drought; it is also the host plant of the economically-important 'mopane worm' Gonimbrasia belina, a highly-valued protein source among indigenous people.

Experimental design: In 1952, in four landscapes (approx. 12 km apart) within the park, a series of 180 × 360 m plots were laid out in a randomised block design. In 1954, fires were applied to these plots under a strictly controlled burning protocol. Each fire treatment was replicated three times in each of the four areas, with treatments ongoing thereafter (over 44 years). The use of long term studies is important as the long-term effects of exposure to fire may differ greatly from short-term effects.

This present study, concentrated on one of the four study areas, the 'Letaba Experimental Burning Plots' located in mopane shrubveld habitat. Tree distribution patterns, size and architecture were compared following three fire treatments were compared:

i) Burning during the mid-dry season once every two years (simulating anthropogenic fires);

ii) Burning in the early-wet season once every two years (simulating lighting induced fire);

iii) No burning.

Mopane assessment: In May and July 1998, the abundance, spatial pattern, size and architecture of mopane trees were assessed in three replicates of each of the three treatments. For each tree selected to be sampled within a series of transects, the following were recorded: tree height; maximum canopy depth; maximum canopy diameter; total number of stems alive and dead; number of coppiced and uncoppiced stems; and average stem circumference of live stems.

After 44 years of prescribed burning treatments, average densities of the locally-dominant C.mopane were 638 trees/ha in mid-dry season fire (antropogenic) plots, 500 in early-wet season (lightening) plots, and c.370 in the fire exclusion plots. Trees in burnt plots had aggregated distributions while those in unburnt plots were randomly spread.

Significant differences were recorded in a range of morphological parameters including tree height, canopy diameter, average stem circumference and number of stems.

Average tree height was tallest in the no burn plots (250 cm), 170 cm in the early-wet season fire plots and lowest in the mid-dry season burn plots (c. 155 cm); thus fire induced a shift in population structure from mainly tall shrubs in the no burn plots to mainly short shrubs in the mid-dry season fire plots, with early-wet season fire plots falling in between.

Average maximum canopy diameter and maximum canopy depth were reduced in the mid-dry season fire plots (diameter - c.180 cm; depth - 55 cm) compared with early-wet season fire season fire plots (diameter - c.205 cm; depth - 95 cm) and no burn plots (diameter - 280 cm; depth 145 cm).

Average stem circumference was lowest in the mid-dry season fire plots (34 mm) compared with early-wet season fire season fire plots (c.45 mm;) and no burn plots (124 mm). The incidence of resprouting also differed significantly between treatments, with burnt trees containing a high proportion of coppiced stems.

Conclusions: The authors consider that the differences resulting in tree size and architecture between under the mid-dry season ‘anthropogenic fire’ in July and August, cannot substitute for the natural early-wet season ‘lightning fire’ regime. Tree height was the most noticeable aspect of C. mopane morphology affected by fire. The mid-dry season burns induced a savanna with shorter, scrubbier shrubs than that generated by early-wet season fires.

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