Alternative tree girdling and herbicide treatments for liberation and timber stand improvement in Bolivian tropical forests

Silvicultural treatments that remove non-commercial trees can be very beneficial to the residual stand, but are rarely implemented in tropical forests due to perceptions of high cost. This study evaluated the efficacy and cost of four chainsaw girdling and herbicide treatments in a humid and dry tropical forest of Bolivia. Treatment variables included single-band girdling versus double-band girdling, 50% versus 100% concentration of 2,4-D, and dry versus rainy season application. After 18 months, double-band girdling caused higher mortality of treated trees than single-band girdling (87% versus 76%). There was no difference in the efficacy of different concentrations of 2,4-D. In the humid forest, trees treated in the dry season experienced 97.5% mortality after 18 months, whereas those treated in the rainy season had 90% mortality. In the dry forest, trees treated in the dry season experienced 71.3% mortality after 18 months, whereas those treated in the rainy season had 66.3% mortality. The percentage of trees with callus formation was significantly higher in the humid forest than in the dry forest (22 and 5%, respectively). Differences in the efficacy of treatments for individual tree species were insignificant, with the exception of Acosmium cardenasii, which experienced 53% mortality overall after 18 months. Treatment costs ranged from US$ 0.21 to US$ 1.04, with differences due to the high volumes of herbicide required to apply either two girdles, 100% concentrations, or both. The additional cost of more intensive treatments almost certainly outweighs the benefits of increased efficacy in most circumstances, although application of double-band girdles is recommended for resistant species such as Acosmium, which had two times higher mortality rates with double-band girdles (72% versus 35%).