Individual study: Liana cutting and controlled burning as silvicultural treatments for a logged forest near Paragominas, Pará, Brazil
Gerwing J.J. (2001) Testing liana cutting and controlled burning as silvicultural treatments for a logged forest in the eastern Amazon. Journal of Applied Ecology, 38, 1264-1276
In the eastern Brazilian Amazon (and frequently elsewhere in tropical regions), logged forests frequently have patches where liana (woody vine) density is particularly high. In these so-called liana tangles, competition from lianas is thought to significantly reduce tree establishment and growth. To begin to develop a silvicultural strategy for these patches in order to encourage reforestation, the impact of liana cutting and controlled burning on liana density, tree growth and tree regeneration in these liana-dominated patches was investigated in a logged forest.
Study site: This study was conducted 30 km south-east of the town of Paragominas, Pará, Brazil (3° S, 50° W). The study site consisted of 2,500 ha of forest on level terrain that had been selectively logged in 1989-90 (8 years previous to the study). Typically for the region, logging operations had harvested 3–9 trees/ha, and was conducted in a haphazard fashion. Damage caused by road building, tree felling and tree extraction operations generally resulted in a 20% reduction in forest canopy cover, a 40% reduction in the density of trees > 10 cm diameter, and the disturbance of around 30% of the forest floor.
Experimental design: The two treatments (liana cutting and controlled burning) and a control were installed in 40 × 40 m plots in a randomized complete block design of six blocks. Treatments were conducted during October 1997, after more than a month without any measurable rain. Tree diameter growth and mortality, canopy cover, regeneration and liana density were monitored over 2 years post-treatment.
Average mortality following burn treatment was significantly higher for lianas (79%) than for trees (48%), as was the average rate of top-killed stems (42% for lianas vs. 20% for trees). Regrowth with some recruitment from seed resulted in liana densities in the burned plots returning to 70% of the values in the control plots only 2 years post-treatment.
For both lianas and trees, there was a clear trend of increasing mortality with increasing burn contact. Liana mortality ranged from 58% for lightly burned stems to 97% for those that were severely burned.
Canopy light transmittance increased significantly from around 4% in controls to 8% in cut and 12% in burned treatments. These differences persisted over the 2-year study period.
In the absence of any intervention, in the control plots mean tree diameter growth increments were low (1.3 mm/year), suggesting that transition to higher stature forest was occurring only very slowly. Each of the treatments resulted in a more than doubling of average annual tree growth compared to the controls, being 3 mm/year for liana-cut and 2.8 mm/year for burn treatments. The treatments also significantly reduced the occurrence of trees that showed no growth over the study period, from 56% in controls to 30% in liana-cut and 32% in burned treatments.
Conclusions: The results of this study in the eastern Brazilian Amazon suggest that controlled burning resulted in increased tree growth, but rapid recolonization by lianas and the high vulnerability of burned stands to unwanted repeat burns are considered likely to cancel out any of the possible benefits as a silvicultural treatment within liana-dominated patches. Liana cutting, on the other hand, showed promise with cutting all lianas in a multi-species stand resulted in a nearly threefold increase in average annual tree diameter increment.
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