title: Mapping gains and losses in woody vegetation across global tropical drylands.
abstract: Woody vegetation in global tropical drylands is of significant importance for both the interannual variability of the carbon cycle and local livelihoods. Satellite observations over the past decades provide a unique way to assess the vegetation long-term dynamics across biomes worldwide. Yet, the actual changes in the woody vegetation are always hidden by interannual fluctuations of the leaf density, because the most widely used remote sensing data are primarily related to the photosynthetically active vegetation components. Here, we quantify the temporal trends of the nonphotosynthetic woody components (i.e., stems and branches) in global tropical drylands during 2000-2012 using the vegetation optical depth (VOD), retrieved from passive microwave observations. This is achieved by a novel method focusing on the dry season period to minimize the influence of herbaceous vegetation and using MODerate resolution Imaging Spectroradiometer (MODIS) Normalized Difference Vegetation Index (NDVI) data to remove the interannual fluctuations of the woody leaf component. We revealed significant trends (P < 0.05) in the woody component (VODwood) in 35% of the areas characterized by a nonsignificant trend in the leaf component (VODleafmodeled from NDVI), indicating pronounced gradual growth/decline in woody vegetation not captured by traditional assessments. The method is validated using a unique record of ground measurements from the semiarid Sahel and shows a strong agreement between changes in VODwoodand changes in ground observed woody cover (r2= 0.78). Reliability of the obtained woody component trends is also supported by a review of relevant literatures for eight hot spot regions of change. The proposed approach is expected to contribute to an improved assessment of, for example, changes in dryland carbon pools.
authors: Department of Geosciences and Natural Resource Management, University of Copenhagen, 1350, Copenhagen, Denmark.
authors: Feng
authors: 0000-0002-9686-2769
authors: F
authors: Tian
authors: Department of Geosciences and Natural Resource Management, University of Copenhagen, 1350, Copenhagen, Denmark.
authors: Martin
authors: M
authors: Brandt
authors: ARC Centre of Excellence for Climate Systems Science & Climate Change Research Centre, University of New South Wales, Sydney, 2052, NSW, Australia.
authors: Yi Y
authors: YY
authors: Liu
authors: Department of Geosciences and Natural Resource Management, University of Copenhagen, 1350, Copenhagen, Denmark.
authors: Kjeld
authors: K
authors: Rasmussen
authors: Department of Geosciences and Natural Resource Management, University of Copenhagen, 1350, Copenhagen, Denmark.
authors: Rasmus
authors: R
authors: Fensholt
mesh_terms: D057486:Carbon Cycle; D017753:Ecosystem; D004784:Environmental Monitoring; D065928:Forests; D010944:Plants; D015203:Reproducibility of Results; D063809:Satellite Imagery; D012621:Seasons
keywords: deforestation; drylands; nonphotosynthetic woody component; remote sensing; shrub encroachment; trend analysis; wood biomass; woody vegetation