Doctoral Student: Cecilia Chavana-Bryant
Position: DPhil Student
Title: Relationships between leaf life-cycle and canopy shadow patterns on Earth Observation-derived indices in Amazonian forests
Supervisors: Professor Yadvinder Malhi and Dr France Gerard (Centre for Ecology and Hydrology)
I joined the ECI as a DPhil student in October 2008. I am a forest ecologist with a BSc Hons in Ecology and an MSC in Botanical Conservation. I have been interested in forest structure and dynamics since I was an undergraduate. This was reflected in the choice of subjects for my BSc and MSc dissertations. My undergraduate dissertation investigated the role of forest gaps in creating environmental heterogeneity and in shaping forest composition and structure in a mature broadleaf forest. For my MSc dissertation, I used a variety of methods -dendrochronology and dendroclimatology, historical grazing records, historical aerial photography and GIS- to report on a 150-year history of recruitment, growth, expansion and forest structure at Wistman's Wood, one of three small and isolated old-growth oakwood remnants in upland Dartmoor. The results from these five sources of information revealed a currently expanding forest with a changing history of regeneration and growth due to grazing, competition and, most significantly climate change. This study was done in collaboration with Natural England, who awarded me with a 'Priority site research grant'.
For my doctorate research, I will be working in tropical forests, more specifically the Amazon. This work will involve using field data, modelling and remote sensing to investigate tropical phenology (the timing of leaf flushing and abscission). Watch the video below to find out more about my field work.
The phenological dynamics of terrestrial ecosystems reflect the response of the Earth's biosphere to inter- and intra-annual dynamics of the Earth's climatic and hydrological regimes (Myneni, 1997). Current modeled phenology is based on observations of vegetation in the temperate zones and accurate representation of phenology of the tropical zones is long overdue. A number of recent studies based on satellite remote sensing have reported seasonal variation in the phenology of the Amazon rainforest, with enhanced "greenness" in the dry season (Huete et al., 2006), and further enhanced greenness during the drought of 2005 (Saleska et al., 2007). These studies have been interpreted as evidence of resilience of tropical rainforests to seasonal and interannual drought. The studies have been entirely satellite-based however, where “greenness” is expressed through vegetation indices (NDVI or EVI) or vegetation index based Leaf Area Index estimates, and thus far there has been little corroboration with on-the-ground observations of the phenology of tropical forests. It is very likely that leaves vary in their spectral reflectance properties as they age, and where their life cycle is strongly synchronised it is likely to be linked to the seasonal variation in climate and/or hydrology. Hence, there is a distinct possibility that the seasonal variation in vegetation indices (VI) is driven by the leaf aging as well as by the shedding or appearance of new leaves. The fundamental objective of my researchl is to investigate the influence that age-related variation in the spectral reflectance properties of leaves may have on apparent "greenness" of a tropical forest canopy.
This research will contribute to a better mechanistic understanding of tropical phenological dynamics and help to clarify the origin of the changes in the remotely sensed vegetation indices. It will exploit ongoing measurements of vegetation phenology being conducted by Oxford-supported field researchers at two filed sites in French Guiana and Peru; conduct targeted field campaigns on leaf reflectance, physical and physiological properties; and utilise a vegetation canopy model to scale to whole vegetation canopies and to the wider Amazon region.
- Chavana-Bryant, C. and Franco, M. (in preparation) Long-term regeneration patterns and conservation status of a remnant old-growth oak forest: Wistman’s Wood, Dartmoor. Environmental Conservation.
- Chavana-Bryant,C. and Franco, M. (in preparation) The influence of temperature and precipitation on ring-width growth of Quercus robur at Wistman’s Wood, Dartmoor. Global Change Biology.