Project Funding: 2013 - 2016

NRA: 2012 NASA: Terrestrial Ecology   

Funded by NASA

Abstract:
Mixed tree-grass and shrub-grass vegetation associations are one of the most spatially extensive and widely distributed forms of terrestrial vegetation on earth. They constitute significant fractions of all continents, except Antarctica, in tropical, subtropical and temperate bioclimatic regions. While global tree-grass systems are diverse in their phylogeny, physiology and plant morphology, they share the key structural characteristic of woody plants distributed in the landscape at densities low enough to allow significant growth of herbaceous plants (mostly grasses) underneath and between them. Despite the importance of tree-grass systems in earth system processes and human well- being, they are not well represented in our remote sensing and modeling capabilities. Ecosystems characterized by horizontally and vertically complex tree-grass mixtures are inherently difficult to measure with remote sensing and difficult to represent in ecosystem and earth system models. We contend in this proposal, however, that emerging data on slowly varying canopy structure (tree cover) provides us with a key constraint in the estimation of rapidly (i.e. seasonally) varying tree and grass leaf area index (LAI) and fractional PAR interception (fPAR). This tree-grass separation in remote sensing data has not, to our knowledge, been attempted before at regional and continental scales using MODIS data. We propose to capitalize on prior TE-sponsored research to develop and test a tree-grass separation methodology, validated using field data, and applied for Africa during the MODIS (2002-2012) era. We anticipate that the methodology thus developed will later be used to also partition VIIRS LAI and fPAR aggregates. These tree and grass biophysical parameters will be of direct use as a validation and data-assimilation stream for our savanna dynamic vegetation model (the Tree-Grass Vegetation Model, TGVM). TGVM models explicitly the growth and demographics of trees and grasses in response to climate, disturbance and management. The new tree-grass biophysical datasets will advance the testing and application of TGVM in both diagnostic and prognostic modes to explore how tree-grass systems are responding to human management and climate change. This project will (i) refine methodologies for separation of tree and grass LAI and fPAR biophysical parameters using MODIS products for all Africa, (ii) validate results using field measurements from our own and collaborator field sites across Africa, (iii) generate and distribute Africa-wide tree-grass component biophysical parameters for 2002-2012, at 8-day and 1 km resolutions, (iv) analyze and synthesize the new LAI and fPAR products for publication, and (v) use the tree-grass data with our tree-grass vegetation model (TGVM) to transform our understanding of, and ability to simulate, the future provision of ecosystem goods and services in tree-grass systems. Over the past two decades NASA has made considerable investment in development and deployment of the MODIS platform and MODIS data products. This â"successor" proposal seeks to leverage that investment using the LAI-fPAR product to diagnose and better understand the separate and distinct role of woody and herbaceous components in mixed tree-grass ecosystems.

Publications:

Anchang, J. Y., Prihodko, L., Kaptue, A. T., Ross, C. W., Ji, W., Kumar, S. S., Lind, B., Sarr, M. A., Diouf, A. A., Hanan, N. P. 2019. Trends in Woody and Herbaceous Vegetation in the Savannas of West Africa. Remote Sensing. 11(5), 576. DOI: 10.3390/rs11050576

Axelsson, C. R., Hanan, N. P. 2017. Patterns in woody vegetation structure across African savannas. Biogeosciences. 14(13), 3239-3252. DOI: 10.5194/bg-14-3239-2017

Axelsson, C. R., Hanan, N. P. 2018. Rates of woody encroachment in African savannas reflect water constraints and fire disturbance. Journal of Biogeography. 45(6), 1209-1218. DOI: 10.1111/jbi.13221

Griffith, D. M., Lehmann, C. E. R., Stromberg, C. A. E., Parr, C. L., Pennington, R. T., Sankaran, M., Ratnam, J., Still, C. J., Powell, R. L., Hanan, N. P., Nippert, J. B., Osborne, C. P., Good, S. P., Anderson, T. M., Holdo, R. M., Veldman, J. W., Durigan, G., Tomlinson, K. W., Hoffmann, W. A., Archibald, S., Bond, W. J. 2017. Comment on "The extent of forest in dryland biomes". Science. 358(6365). DOI: 10.1126/science.aao1309

Hanan, N. P., Tredennick, A. T., Prihodko, L., Bucini, G., Dohn, J. 2013. Analysis of stable states in global savannas: is the CART pulling the horse? Global Ecology and Biogeography. 23(3), 259-263. DOI: 10.1111/geb.12122

Hanan, N. P., Tredennick, A. T., Prihodko, L., Bucini, G., Dohn, J. 2015. Analysis of stable states in global savannas - a response to Staver and Hansen. Global Ecology and Biogeography. 24(8), 988-989. DOI: 10.1111/geb.12321

Kahiu, M. N., Hanan, N. P. 2018. Estimation of Woody and Herbaceous Leaf Area Index in Sub-Saharan Africa Using MODIS Data. Journal of Geophysical Research: Biogeosciences. 123(1), 3-17. DOI: 10.1002/2017JG004105

Kahiu, M. N., Hanan, N. P. 2018. Fire in sub-Saharan Africa: The fuel, cure and connectivity hypothesis. Global Ecology and Biogeography. 27(8), 946-957. DOI: 10.1111/geb.12753

Kaptue, A. T., Hanan, N. P., Prihodko, L., Ramirez, J. A. 2015. Spatial and temporal characteristics of rainfall in A frica: S ummary statistics for temporal downscaling. Water Resources Research. 51(4), 2668-2679. DOI: 10.1002/2014WR015918

Kaptue, A. T., Prihodko, L., Hanan, N. P. 2015. On regreening and degradation in Sahelian watersheds. Proceedings of the National Academy of Sciences. 112(39), 12133-12138. DOI: 10.1073/pnas.1509645112

Lehmann, C. E. R., Anderson, T. M., Sankaran, M., Higgins, S. I., Archibald, S., Hoffmann, W. A., Hanan, N. P., Williams, R. J., Fensham, R. J., Felfili, J., Hutley, L. B., Ratnam, J., San Jose, J., Montes, R., Franklin, D., Russell-Smith, J., Ryan, C. M., Durigan, G., Hiernaux, P., Haidar, R., Bowman, D. M. J. S., Bond, W. J. 2014. Savanna Vegetation-Fire-Climate Relationships Differ Among Continents. Science. 343(6170), 548-552. DOI: 10.1126/science.1247355

Yan, D., Zhang, X., Yu, Y., Guo, W., Hanan, N. P. 2016. Characterizing land surface phenology and responses to rainfall in the Sahara desert. Journal of Geophysical Research: Biogeosciences. 121(8), 2243-2260. DOI: 10.1002/2016JG003441

2015 NASA Carbon Cycle & Ecosystems Joint Science Workshop Poster(s)

• Partitioning savanna tree and grass leaf area index (LAI) from MODIS aggregates in Sub-Saharan Africa   --   (Milkah Njoki Kahiu, Niall Hanan)   [abstract]

More details may be found in the following project profile(s):

• NASA Terrestrial Ecology (TE) Project Profile