Crops dedicated to bioenergy production have been widely criticised for competing with food production and biodiversity conservation for limited land and water resources. One way to address these conflicts is to shift bioenergy production to low-grade lands that have low biodiversity value. This, however, requires improvements in plant water-use efficiency that are beyond conventional C3 and C4 crops. Human activities currently appropriate 20 - 40% of terrestrial net primary productivity through farming, land-use change, and fire. This 'diverted energy' is overwhelmingly re-routed through the C3 and C4 photosynthetic pathways, leaving the 'third' CAM pathway almost completely unexploited for commercial purposes.
CAM is a relatively under-studied, yet highly successful pathway that has evolved on multiple occasions in response to low and intermittent water availability. Succulent CAM tissues allow plants to maintain water-homeostasis and facilitate a carbon acquisition strategy of 'drought avoidance'. This is distinct from 'drought tolerance', which is typically observed in arid- and semi-arid C3 and C4 plants that show the capacity to endure low cell water potential or in extreme cases, desiccation. Integrated over a 24-hour period, typical WUE (defined as the ratio of mmol CO2 fixed to mol H2O lost) values for C3 plants are 0.5-1.5, C4 plants 1.0 – 2.0, and 4.0-10 for plants displaying CAM. Under ideal conditions, some species of Agave and Opuntia average 43 Mg ha-1 yr-1 above ground dry mass productivity which is comparable to agronomic C4 species and C3 herbaceous species and trees. These attributes suggest, in theory at least, the capacity for high productivity low-grade lands, resilience to climate change, and the potential to alleviate land and water resource conflicts.
In this paper, we present a global-scale geospatial model of CAM productivity for two bioenergy candidates: Agave tequilana and Opuntia ficus-indica. Yield simulations were performed under present-day conditions and in the year 2070 under the four representative concentration pathway scenarios presented in the IPCC's 5th Assessment Report. Our simulations suggest that the CAM pathway could meet ‘extreme’ bioenergy demand scenarios without significantly contributing to deforestation and encroaching on lands dedicated to food production. We also show that CAM could play an important role as an adaptive measure against the negative impacts of climate change.
Research was published in:
Nick A. Owen, Kieran F. Fahy, Howard Griffiths (2015) Crassulacean Acid Metabolism offers sustainable bioenergy production and resilience to climate change. GCB Bioenergy. DOI: 10.1111/gcbb.12272