Impacts of leaf age on the spectral and physiochemical traits of trees in Amazonian forest canopies

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Cecilia Chavana-Bryant / Amazonian Canopy Research from Jake Bryant on Vimeo.

Academic Profile

Cecilia is a forest ecologist with a BSc Hons in Ecology and an MSc in Botanical Conservation. She has been interested in forest structure and dynamics since she was an undergraduate. This was reflected in the choice of subjects for her BSc and MSc dissertations. Her undergraduate dissertation investigated the role of forest gaps in creating environmental heterogeneity and in shaping forest composition and structure. For her MSc dissertation, she used a variety of methods -dendroecology, 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, Devon, UK. 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 Cecilia with a 'Priority site research grant'.

For her doctoral research, Cecilia worked at two Amazonian primary tropical forest sites in Peru and French Guiana. She spent two years climbing some of the tallest trees in the Amazon (up to 65m tall) to study how leaf age and other leaf phenological processes impact the spectral properties of tropical forest canopy communities as captured by remote sensing. Natural leaf ageing is a fundamental driver of change in morphological, biochemical and spectral leaf traits, thereby, regulating both ecosystem processes (plant growth, energy and nutrient cycling) and remotely-sensed canopy dynamics. Cecilia's doctoral research represents the first comprehensive analysis of the morphological, biochemical and spectral leaf traits of canopy and emergent tropical trees during natural (in situ) leaf ageing. Her research applied an interdisciplinary approach that included leaf trait data, modelling and remote sensing together with multiple scales of analysis (leaf, individual tree crown and canopy community) to generate insights into the effects of natural leaf ageing on our current understanding of tropical leaf trait variation, chemometric models used to spectrally predict leaf traits, and together with other leaf phenological processes, on remotely-sensed vegetation indices (VIs) commonly used to monitor canopy dynamics in tropical evergreen forests. This research makes significant contributions towards a better mechanistic understanding of tropical leaf phenology and helps to clarify the origin of seasonal dynamics observed in remotely sensed VIs of tropical forest canopy communities.

Awards

  • 2016 - 1,500: Presenter travel award for EGU. CEH, UK
  • 2013 - US$1,000: Presenter travel award. AGU, USA
  • 2009-2012 - 200,000: Fieldwork spectroscopy equipment and support grant. NERC Field Spectroscopy Facility, Edinburgh, UK
  • 2009-2011 - 32,000: Various fieldwork and equipment awards from UK institutions
  • 2010 - €12,000: Nouragues Ecological Scientific Station researcher grant. CNRS, French Guiana, France
  • 2008 - Doctoral scholarship & 28,000 fieldwork budget. NERC (TROBIT project: NE/D005469/1), UK
  • 2008 - 2,000: Invited researcher grant. IBISCA, Auvergne, France
  • 2006 - 600: Priority Site Research Grant. English Nature, UK

Current Research

For her doctoral research, Cecilia is working in tropical forests, more specifically the Amazon. This work involves using field data, modelling and remote sensing to investigate tropical phenology (the timing of leaf flushing and abscission).

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. 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, and further enhanced greenness during the drought of 2005. 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. Furthermore, leaves vary in their physiochemical and 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 observed 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 Cecilia's research is to investigate the impacts of leaf age on the spectral and physiochemical traits of canopy trees in Amazonian evergreen rainforests and their implications for Earth Observation-derived indices.

This research makes significant contributions towards a better mechanistic understanding of tropical phenological dynamics and helps to clarify the origin of the changes in remotely sensed vegetation indices. It exploited ongoing measurements of vegetation phenology being conducted by Oxford-supported field researchers at two field sites in French Guiana and Peru; conducted targeted field campaigns on leaf reflectance, physical and physiological properties; and utilised modelling to scale to whole vegetation canopies and to the wider Amazon region.

Current Teaching

Publications

  • Chavana-Bryant, C., Malhi, Y., Wu, J., Asner, G.P., Anastasiou, A., Enquist, B.J., Cosio Caravasi, E. G., Doughty, C. E., Saleska, S. R., Martin, R. E. and Gerard, F. F. (2016) Leaf aging of Amazonian canopy trees as revealed by spectral and physiochemical measurements. New Phytologist.
  • Wu, J., Chavana-Bryant, C., Prohaska, N., Serbin, S. P., Guan, K., Albert, L. P., Yang, X., van Leeuwen, W. J. D., Garnello, A. J., Martins, G., Malhi, Y., Gerard, F., Oliviera, R. C. and Saleska, S. R. (2016), Convergence in relationships between leaf traits, spectra and age across diverse canopy environments and two contrasting tropical forests. New Phytol.
  • Ribeiro, S.P., Da Silva, M.B.Jr., Tagliati, M.C. & Chavana-Bryant, C. (2011) Vegetation traits and herbivory distribution in an Australian subtropical forest. Memoirs of the Queensland Museum. Nature 55(2): 481-493. Brisbane. ISSN 0079-8835
  • Chavana-Bryant, C., Gerard, F.F., Enquist, B.J., Patrick Bentley, L., Salinas Revilla, N., Cosio Caravasi, E.G., Anastasiou, A., Martin, R.E., Asner, G.P. and Malhi, Y. (in revision) Assessing the role of leaf age as a fundamental driver of intraspecific leaf trait variation within Amazonian canopy trees. New Phytologist.
  • Chavana-Bryant, C., Malhi, Y., Anastasiou, A., Enquist, B.J., Cosio, E.G. and Gerard, F.F. (forthcoming) Leaf age effects on the spectral predictability of leaf traits in Amazonian canopy trees.
  • Chavana-Bryant, C., Malhi, Y., Anastasiou, A., Cosio, E.G., George, C.T. and Gerard, F.F. (forthcoming) Remotely sensed seasonal canopy dynamics in the tropics: a riddle of many scales.
  • Gerard, F.F., Weedon, G.P., George, C.T., Hayman, G. and Chavana-Bryant, C. (forthcoming) Leaf phenology amplitude derived from MODIS NDVI and EVI: Synchrony of leaf phenology for Meso- and South America.
  • Doughty, C.E., Santos-Andrade, P.E., Blonder, B., Shenkin, A., Chavana-Bryant, C., Diaz, S., Salinas, N., Enquist, E., Martin, R., Asner, G.P. and Malhi, Y. (in review) Can leaf spectroscopy predict leaf and forest traits along a Peruvian tropical forest elevation gradient? Journal of Geophysical Research - Biogeosciences.
  • 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.
  • 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.