Mapping cryptic invaders and invasibility of tropical forest ecosystems : Chromolaena odorata in Nepal

For centuries, people continuously increased the rate of biological invasions and there is no sign of slowing down. From the depth of the Ocean to the crest of Himalayas, they are occupying pristine and semi-natural ecosystems at an alarming rate, threatening human, animal, plant as well as ecosystem health. Efforts to avoid or eradicate them are not achievable except for very few cases. Currently, therefore, their management aims at controlling invaders and mitigating their impact rather than eradication. Limitation of resources forces land managers to carefully plan and prioritize interventions only in areas most severely affected by invaders. Hence, information on the actual and potential distribution of invaders is considered crucial for their management. It has long been recognized that remote sensing (RS) and geographical information systems (GIS) could contribute to help solving this problem. Remote sensing has so far been applied predominantly to invasive species that dominate the canopy of the ecosystem. The large majority of invasive species do however not show up in the canopy and thus remain difficult to detect by remote sensing in a direct and straightforward manner. Techniques for mapping such cryptic invaders have not been developed so far. In this thesis we explored methods to map the distribution of Chromolaena odorata (L.) RM King & H Robinson, one of the world's worst invasive species. This cryptic heliophyte originating from central America invaded the understorey of many tropical forest ecosystems throughout the world. C. odorata is a cryptic invader hidden under the forest canopy in the Terai of Nepal. It occurs predominately in opened up forest, with increased light intensity. The approach to map its distribution was to develop first remote sensing techniques to map forest canopy density and light intensity reaching the understorey and next relate these radiation maps to various aspects of the life history of C. odorata . To set the scene we reviewed in chapter 2 existing methods to map the distribution of invasive species. Next, we explored the quality of four alternative methods to predict forest canopy density (Chapter 3). This comparative study revealed that an artificial neural network best explained canopy density in terms of variance explained and bias. A Landsat ETM+ image processed through a neural network predicted 81% of light intensity reaching the understorey. The resulting radiation map was the environmental data layer that was subsequently used to map the distribution of C. odorata. This study revealed that in the Nepalese Terai C. odorata failed to produce seed below a light intensity of 6.5 mJ m -2 day -1 , and that light intensity determined 93% of the variation in log 10 seed production per plant (Chapter 4). This enabled us to map its distribution in Nepal based on under-canopy light intensities. C. odorata invades new areas by generative reproduction (wind dispersal of pappus-bearing achenes). Once established, clonal propagation through underground corms enhances further expansion of populations. We have discovered (Chapter 5) that the age of corms can be determined using corm rings in cross sections. While individual corms survive for only five years, we obtained evidence that multi-corm genets, which must have been much older, had developed in forests with opened canopy. We furthermore demonstrated that light intensity positively related to the rate of clonal growth. The light dependence of the expansion rate of plants is apparently a key attribute explaining the invasion success of this species. Any disturbances in forest canopy density leading to increased light intensities would ultimately trigger its clonal growth.  The canopy in tropical forest ecosystems in Nepal is severely degraded offering light conditions suitable for C. odorata . The degradation of the forest canopy was attributed to a series of interrelated causes including human dimensions and government policies (Chapter 6). Herbarium records revealed that C. odorata invaded southern Nepal shortly after the initiation of malaria control. We furthermore demonstrated that this was followed in Chitwan district by an influx of migrants, land use changes and degradation of the forests. It was therefore argued that the species invaded because of canopy degradation, which was caused by change in land use and demography and triggered by malaria control. This process of malaria control followed by migration increased human population growth rates that we described for Chitwan district occurred in tropical regions all over Nepal. Hence, we suggest that it must have contributed to the rapid spread of C. odorata in the lowland Terai forest belt. Field observations revealed that C. odorata did not invade the tropical forests in the west of Nepal below 83°45' east longitude. We were unable to explain this distribution pattern with simple climatic indicators. However, a bioclimatic indicator, the length of the growing season predicted the absence from west Nepal remarkably well. This suggests that C. odorata requires a minimum length of the growing period to accumulate sufficient resources to establish and persist. We thus conclude that an agro-ecological modeling approach yielded a better prediction than the commonly used bioclimatic approach (Chapter 7). Since, deforestation and forest degradation are a point of concern for management of both biological invaders and native biota, there is a need to more closely monitor biological conservation areas because of the potentially irreversible impacts of deforestation and forest degradation. In chapter 8 we assess the rate of deforestation and the current degree of forest degradation in the Terai of Nepal. Forest canopy density class was predicted with 82% overall accuracy. Data analysis revealed that the forested area reduced from 21774 km 2 in 1958 to 12649 km 2 in 2000 corresponding to an annual rate of decline of 1.38%. Our analysis further revealed that 70% of the forested area outside conservation areas had canopies with density below 60%, thus confirming widespread degradation. More surprisingly, 50% percent of the forested area inside protected areas had such opened canopies. This indicates that canopy degradation is also very common inside protected areas. These areas play a special role in the conservation of internationally threatened forest communities, for instance Nepalese tropical rain forests. Our analysis revealed that canopy opening prevailed as well in these communities. We argue that from a biodiversity point of view conservation effort should focus on the preservation and restoration of these forest types. The forest degradation maps presented in this chapter could serve as a start to prioritize such interventions. In this thesis, we demonstrated how the impressive developments in computational performance, the rapid growth of remote sensing and GIS technologies for spatial data acquisition and analysis could be used beyond their traditional application in mapping canopy-dominant invasive species. We have shown how a few of these cost-effective mapping techniques can reliably be up scaled to the national level to map the distribution of even those invasive species that do not dominate the canopy of forest ecosystems. This thesis emphasizes the importance of site-specific microclimatic variation and empirical observations of the species' ecology, while applying remote sensing techniques in invasion studies. This could significantly reduce the uncertainties and the degree of "erroneous prediction". Maps displaying seed-producing sites could be used to significantly reduce the costs of controlling C. odorata infestation by providing information on the spatial segregation of source and sink populations. These will support efficient habitat ranking to restore invaded areas and protect non-invaded ecosystems. Such an approach may prove particularly valuable when implementing control measures under circumstances of limited capital and labour. This thesis also showed the necessity to understand the connection between the human historical, socio-economic, and cultural context with the environmental conditions and the ecology of the invader. It facilitates conceptualising the situation and hopefully it also helps in translating research results into appropriate policy measures.