5. DISCUSSIONS
Based on the findings of this study and following the wood
anatom ical classification of Worbes (1989), Fichtler (2003) and Schöngart
(2006) , the presence of annual tree-ring in tropical African species such as
A. leiocarpa, D. microcarpum, D. abyssinica, I. doka and P. erinaceus
is undoubtable. A dry period of at least two months with less than 50 mm
of rain would be required to expect annual rings in tropical tree-species
(Worbes, 1999). However, Fichtler et al. (2003) reported that even
small annual variation in rainfall occurring under everwet conditions may
trigger ring formation. This confirms the role of abiotic factor triggering on
tree growth such as the precipitation (Worbes, 1995; Fichtler et al.,
2004; Shöngart, 2006). Furthermore early studies and especially those
of Coster ( 1927) well illustrated the connection between the formation of
annual tree rings and seasonal precipitation. He found that trees of the same
species might form clear and annual rings under monsoon climates, while the
same species form distinct and irregular rings under almost everwet
conditions.
Concerning the growth performance, the results of Worbes
(1989) proved that whenever the pioneer species like A. leiocarpa grow
faster, those of the understory like D. abyssinica grow slowly and
constantly during all life time. He also demonstrated t hat within one species,
the mean annual growth can show a variation according to tree age. His findings
in 2003 proved also that the lowest values of diameter growth rates are
observed for understorey species whereas the highest values were noticed in the
main canopy and in emergent species. The young trees of A. leiocarpa
grew under good light conditions and showed therefore a good ring
increment. This high difference of growth within the same species could be
explained by the mean annual rainfall, index of the soil fertility and the
variation in disturbance following areas. However, the analysis of these
ecologic factors combined with the results about the ring increment of A.
leiocarpa from the two studied sites (Tables 2, 3 & 4) proved
that in this case, age of trees would be the main variable that caused the
variation observed. About D. microcarpum species, the trees grow fast
the first year with an annual ring increment of about 8 mm. The rate of growth
decreases the next years. In fact, the light is on e of most important limiting
factor on tree growth. According to Shöngart (2006), pioneer species are
highly light demanding for germination and growth, and are generally short
lived. The non -pioneer species have low light requirements and are able to
surv ive in the dark forest understorey, growing at low rates. The juvenile
trees of A. leiocarpa were found in gap dynamics (canopy
opening) and D. abyssinica trees were observed under
closed forest where less than 2 % of light penetrates.
According to Hennenberg et al. (2006), the use of
fires in Comoé National Park justifies a dramatic decrease of wild
animals. The results of his study proved that fire is more used in savanna area
than in closed forest. He demonstrated that the forest boundary was sometimes
treated by fire to about 30 m which was the consequence of using of wide forest
boundary of 60 m. This helps us to accept our hypothesis about closed forest
protecting against fire.
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