Strategies for restoring tree seedling recruitment in high conservation value tropical montane forests underplanted with cardamom

Abstract

Question What strategies are most appropriate for restoring tree seedling recruitment whilst avoiding the spread of invasive plant species in high conservation value tropical forests disturbed by planting a shade-demanding crop? Location Knuckles Forest Reserve, Sri Lanka (7°21′–7°24′ N, 80°45′–80°48′ E). Methods An experiment was conducted to test the effects of clipping or removal of established cardamom plants on recruitment of native tree seedlings and spread of non-native plants in a tropical montane forest with abandoned cardamom stands in the understorey. The number and composition of tree seedling emergents, the cover of herbaceous plants and the recovery of cardamom were assessed for 3 yr. Results Tree seedling recruitment was higher in plots from which above-ground cardamom biomass had been removed through slashing (mean ± SE per 5 m2; 28.9 ± 2.70) and those where cardamom plants had been removed completely through uprooting (32.2 ± 3.17), or when dead cardamom leaves and stems were removed with small-scale extraction of pods (22.5 ± 2.16), than in unmanipulated control plots (16.6 ± 1.13) over 15 mo. The species composition of tree seedling emergents did not differ in response to removal of cardamom. However, the cover of herbaceous plants, including the non-native invasive Ageratina riparia, increased in response to removal of cardamom. Recovery of cardamom was higher when the plants had been slashed than when entire plants were uprooted and removed. Conclusion Slashing or uprooting cardamom plants is a potential strategy for restoring tree seedling recruitment in forests with abandoned cardamom stands in the understorey, but these interventions would need to be repeated annually over many years to be successful, and they risk promoting expansion of the cover of herbaceous plants, including non-native species. Hence, this approach would be labour-intensive and costly. An alternative approach to promoting tree seedling emergence and establishment is to clear dead cardamom leaves and stems, and to encourage small-scale extraction of pods from the residual cardamom plants. Harvesting pods reduces the likelihood that the crop will be sustained in situ through natural regeneration, and supplies an income to local communities, which would enhance the social acceptability of the intervention.