An island consumed by invasive vines
images copyright Benjamin White and Google Earth, all rights reserved
Over the past year, I’ve been working with a Cook Islands NGO, Te Rito Enua, with funding from the Asian Development Bank, to develop a pilot project on participatory GIS as a tool to assist island communities to develop climate adaptation strategies. While there, Mona Matepi, president of TRE, called my attention to the problem of invasive vines on the island of Rarotonga. Three species of woody vines* are colonizing the island forests, causing massive deforestation. The overtop and kill trees, replacing the forest with a solid jungle of vines. Since Rarotonga is dependent upon surface water for its entire supply, and since vines were killing the trees in its forested watershed, it seems like a non-trivial issue. Nobody knows how the vines will affect water supply. Will they reduce surface water supply through evapotranspiration? Will they hold the soils as well as the trees they are replacing? How will they respond to the more frequent cyclones and droughts that climate models predict? And, if they are a problem, how can they be controlled? Many questions to answer - our challenge right now is to find support for research into the issues and the options available. If no one does anything, there’s a chance, and its not a tiny one, that there could someday be a humanitarian crisis that would have severe implications for one of the dwindling number of robust Polynesian cultures remaining.
I asked University of Maryland doctoral candidate Benjamin White, a remote sensing specialist, for advice on how to illustrate the extent of the vine infestation. The island is rugged and steep, difficult to map on foot. But I was able to take some measurements using a handheld GPS unit. Ben offered to have a go at classifying the vines using my field observations as training data. Commercial remote sensing imagery provider GeoEye donated high-resolution (4m and 1m) satellite images. Ben developed a sophisticated neural net classifier, and processed the images as R/G/IR reflectance, reflectance-based NDVI, principal components, mean texture and a quick reflectance to “dense vegetation” classification. The final result was uploaded to Google Earth for visualization purposes; Google Earth data is not useful for this kind of application, but overlaying the classification results on a Google Earth image (Figure 3) gives a context in terms of location and topography. Additional satellite imagery could provide complete ground coverage and (subject to availability) time series to measure change in land cover.
I’m hoping that the image will drive home how bad the problem is, and mobilize some support for Te Rito Enua and the Cook Islands government to get a handle on the vine problem.
Heartfelt thanks go to Ben White and the University of Maryland Geography Department, GeoEye, and the Asian Development Bank for support.
* the vines are Cardiospermum grandiflorum, Mikania micrantha, and Merremia peltata.
Climate change and invasives
The link between global warming and the spread of invasive species is real. But authorities responsible for food security and natural resource management are either unaware of the linkages between invasiveness and climate change, or are aware of the linkages and view that the science as inconclusive. Not enough attention is being given to the potential risks to food systems, water supply, energy production and biodiversity as a result of climate change. And no climate model considers the impact of weeds on crop yield in the face of climate change.
Plants can respond to climate change in several ways; temperature, precipitation, available light, and CO2 levels all affect plant growth patterns. Plants are adapted to different environmental conditions, and the composition of species will change according to the combination of climatic factors. 90% of all living matter consists of plant life, so a perturbation in plants due to climate has potentially broad ramifications for ecosystem services and life support systems.
Presently, 96% of all plant species lack optimal CO2. All plants do not respond equally to elevated levels of CO2, however. Plants with C4 photosynthesis are more efficient users of existing levels of CO2 and will not respond as well to elevated CO2 levels as will plants with C3 photosynthesis. Initial evidence suggests that in elevated atmospheric CO2 levels, C3 weeds could be preferentially selected, potentially resulting in weed species dominance and concomitant reduction in crop yields. Response to CO2 is independent of nitrogen requirements, meaning that more efficient users of nitrogen may be better able to take advantage of elevated atmospheric CO2. Elevated atmospheric CO2 levels will favor vegetative reproduction (rhizomes, runners or stolons, suckers, bulbs corms etc) over sexual reproduction through seeds and spores; weedy vines can be expected to become an increasing problem.
Rising minimum winter temperatures are expected to reduce the range of some species and expand the range of others. In temperate climates, this will favor invasive weed species.
CO2 increases biomass of some invasive weedy plants. In temperate regions, the range of invasive weedy plants will expand. The implications of more invasive plants over a wider range include:
• potential for increased evapotranspiration
• potential for increased fuel loads and risk of wildfire
• reduction in crop yields due to increased competition
loss in biodiversity due to increased competition, changes in wildlife habitat affecting climate-sensitive species
Not only can CO2 result in reduced crop yield and water loss due to weeds, but the ability to control weeds is itself impaired. The efficacy of glyphosphate, an important agricultural herbicide for weed control, is reduced as CO2 increases. Mechanical control will be problematic when conditions favor vegetative propagation that can be enhanced through mechanical disturbance.
Adaptive management is needed. Models must be developed for land managers and new management strategies produced in consultation with stakeholders. Early warning systems can aid in effective responses to biological invasions, but investment in control and management of invasive weed species is necesssary. In some cases, control of such species could include biomass energy applications, creating new opportunities. All this requires additional investment in science, management tools, and public information.
Much attention has been given to hazard reduction and disaster response in view of changing climatic conditions. With the exception of the role of ecological resilience as a mitigating factor in natural disaster, the biological dimensions of climate change have been largely ignored. But the biological dimensions extend far beyond the response to acute episodic events such as storms, floods, fire and drought. The biological dimensions that are chronic and persistent, in the form of changing plant communities and plant behaviors, have the potential to undermine food security, health and water supply. To be comprehensive, adaptation measures must better address impacts on plants.
(photo: invasive vines causing deforestation of the interior of Rarotonga, Cook Islands. Photo credit: John Waugh, use with attribution authorized).
