Impact of forest management on insect abundance and damage in a lowland tropical forest in southern Cameroon

Published source details Watt A.D., Stork N.E., McBeath C. & Lawson G.L. (1997) Impact of forest management on insect abundance and damage in a lowland tropical forest in southern Cameroon. Journal of Applied Ecology, 34, 985-998.
Share Tweet Email

Summary

The rate of deforestation in West Africa during the 1980s was estimated at around 2% (12,000 km²) per annum, and in Cameroon 0.4-1% (800-1,500 km²) annually. Pratical methods to regenerate forest cover are needed. Research in the Mbalmayo Forest Reserve in southern Cameroon has examined contrasting silvicultural techniques for regeneration of degraded forest with West African tree species such as Terminalia ivorensis. These techniques included complete clearance, where all existing trees are felled and these and other vegetation is removed from planting plots; and partial clearance methods (manual/mechanical clearance and line planting), where tree-felling resulted in a 50% canopy cover reduction. This study investigated arthropod abundance and damage caused to planted T.ivorensisi trees by insects under these two systems within the Forest Reserve.

Study area: The study was undertaken at three sites (Ebogo, Bilik and Eboufek) within the Mbalmayo Forest Reserve (11º25’-11º31’E – 3º23’-3 º31’N). The Reserve (9,200 ha) comprises an area of semi-deciduous forest that has been partially logged several times, parts are farmed and little undisturbed forest remains.

Treatments: The following treatments were applied to nine 1-ha plots (four at Ebogo planted with T.ivorensisi in 1987; five at Bilik planted with T.ivorensisi in 1988):

i) Uncleared forest control;

ii) Partial manual clearance - ground vegetation and some small trees removed using machete and chainsaws (minimal soil compaction);

iii) Partial mechanical clearance - bulldozer used to clear undergrowth and many large trees (c. 50% canopy reduction, some soil compaction);

iv) Complete clearance – all large trees felled, remaining vegetation bulldozed.

In 1991 a more extensive series of 1 ha plots was established at a third site, Eboufek.

Arthropod damage and abundance: Damage assessments were undertaken in two complete clearance and two line planting plots at Eboufek (November 1992 – March 1994) where trees were younger (thus smaller). The larger trees at Bilik and Ebogo were more suited to knockdown fogging (see below). Four types of damage was assessed:

i) Damage by leaf-chewing insects (visual estimate of leaf area removed)

ii) Leaf miner abundance in a sample of 10 leaves/plant

iii) Damage by gall-forming mites

iv) Presence or absence of shoot borers

Arthropod abundance was assessed by insecticide knockdown fogging at Bilik and Ebogo. Initially a trial was undertaken at Bilik in in November 1991, with subsequent surveys in November 1992, and February and November 1993.

The amount of damage caused by leaf-mining insects, the abundance of gall-forming mites, and the number of T.ivorensisi trees affected by shoot borers were not affected by either of the two silvicultural practices. However, the amount of damage caused by leaf-chewing insects was greater in the line planting plots than the complete clearance plots, but the leaf area removed by insects rarely exceeded 6%.

Arthropod abundance: A mean total of 196 arthropods/m² was recorded, ants (Formicidae) being the most common arthropod group (63%), followed by adult Diptera (10%) and then Hymenoptera other than ants (4%), Homoptera (4%), Thysanoptera (4%), adult Coleoptera (3%) and other groups (13%). Ants, Diptera, Araneae, Thysanoptera, Homoptera, Coleoptera and Orthoptera were significantly more abundant in the partial manual clearance plot than the complete clearance plot in one or both of the study sites.

The authors speculate that because several of these groups are composed mainly of predatory and parasitic species, it is possible that, despite the pest damage results recorded, the long-term likelihood of pest outbreaks occurring on T.ivorensis is lower in partial clearance than complete clearance plantations. However, current lack of knowledge of the species composition of different arthropod groups limits the relevance of these findings.

Conclusions: The degree of forest clearance before establishing T.ivorensis plantations does not markedly affect the amount of damage caused by arthropod pests at the spatial scale at which this study was carried out.

Note: If using or referring to this published study, please read and quote the original paper.