Marc P. E. Parren
C.T. de Wit Graduate School for Production Ecology and Resource Conservation, Wageningen University, 6708 PB Wageningen, THE NETHERLANDS.
In this dissertation the role of lianas in the tropical forest ecosystem in relation to logging activities is analysed. Lianas (woody vines) are an abundant, diverse, and conspicuous growth form in nearly all tropical forests. Foresters mostly see lianas as a nuisance. Cutting of liana stems is an important technique in some forest management practices. Pre-harvest liana cutting is aimed at reduction of logging damage, improved felling precision, enhancement of the growth of the remaining trees, and reduction of the regrowth capacity of lianas. However, studies investigating the effectiveness of pre-harvest liana cutting are scarce. Most studies in SE Asia and in the Amazon Basin established during the last half of the 20th century pointed towards positive effects of liana cutting. They concluded that liana-cutting leads to reduction of logging damage to residual trees reduced size of logging gaps and more rapid development of tree regeneration in logging gaps.
In the past, liana cutting was prescribed over vast areas of the African continent, although a proper evaluation never took place. For this purpose, a large-scale experiment was set up as part of the study reported here, to test liana cutting as a pre-harvest silvicultural treatment. The silvicultural treatment formed part of a larger project that aimed at increasing stand productivity by applying silvicultural treatments to favour desired species. The study was conducted in a logging concession 100 km east of Kribi, Cameroon (30 °N, 100 °E). The concession area covers more than 2,000 km2. The study area was located in the northeastern part near the village Ebom, in a part of the forest where large-scale commercial logging had not happened before. 33 one-hectare research plots, surrounded by a 100-m wide bufferzone, were located in mostly flat terrain. In 5 control plots no logging or silvicultural treatments were applied. The remaining 28 plots were all logged and in 16 of them pre-harvest liana cutting was applied (May 1995). Felling was carried out nine months later (February 1996). Liana cutting was applied in the research plots and in the surrounding buffer zone. Harvest levels were set at one tree per ha over 60 cm dbh, resembling normal exploitation practise in the region.
An important aspect of this study was to assess the influence of physical damage of felling to the remaining vegetation, and the response of the remaining trees to the changed conditions. By harvesting trees the light climate at the felling sites changes drastically. Field measurements on various aspects of responses of individual trees and lianas in relation to logging activities took place at several stages of harvesting, such as during the pre-harvest, actual felling and post-harvest period. An initial inventory of all lianas ≥ 2 cm diameter at breast height (dbh) and trees ≥ 10 cm dbh took place within the 1-ha plots. As a pre-harvest treatment liana cutting was applied in order to assess the role of lianas in the ecosystem and to analyse their response to stress and disturbance. Lianas have a high capacity to re-sprout, but the potential changes in liana composition in response to cutting have received little attention. Re-sprouting capacity and mortality were assessed regularly for a period of 22 months after climber cutting. In this study it was also assessed whether liana cutting is effective in reducing felling damage to residual trees and to reduce logging gap sizes significantly. A six-year chronosequence of logging gaps was used to determine the speed by which lianas develop into logging gaps. The effectiveness of pre-harvest liana cutting in reducing the number of lianas that re-colonise logging gaps was also assessed.
In summary, lianas are abundant and will continue to play a prominent role in West African forest dynamics in coming decades since disturbance regimes can be expected to increase in intensity and frequency. Pre-harvest liana cutting is recommended in the area beneath the canopy of the target tree when lianas are abundant, to reduce felling damage and to avoid problems related to liana tangles in logging gaps after felling. However, it all depends on the logging intensity and frequency. To avoid negative impacts of liana cutting and fire on liana species diversity, it is recommended to apply this treatment only selectively. Spatially, treatments should be limited to zones where lianas are heavily interfering with trees to be felled, thereby taking into account felling direction. Treatments also should be species-specific, by limiting liana cutting to those species, which cause most of the damage. In general, a balance has to be sought between optimising the production potential of the forests and limiting the negative impact of liana cutting on liana populations. We conclude that lianas play an important role in the West African moist forests and that their detrimental effects in relation to logging can be brought under control.