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 (VOD ) in 35% of the areas characterized by a nonsignificant trend in the leaf component (VOD modeled 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 VOD and changes in ground observed woody cover (r = 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.

Woody Vegetation Die off and Regeneration in Response to Rainfall Variability in the West African Sahel

“…After a serious reduction in canopy cover was reported in the 1990s [6], recent studies report that woody vegetation in Sahelian regions with a sparse human population shown signs of increase [11,34]. The pastoral zone of Senegal has been identified as the area with the largest increase in woody cover within the Sahel, and this study aimed at further exploring these findings.…”

Increased vegetation growth and carbon stock in China karst via ecological engineering

“…Contrary to LAI, vegetation optical depth (VOD) is sensitive to the water content of all vegetation, including the green and non-green parts (that is, branches and stems of trees) 42 . VOD is thus less prone to saturation and less sensitive to inter-annual variations in climatic conditions.…”

“…VOD is thus less prone to saturation and less sensitive to inter-annual variations in climatic conditions. The signal is primarily driven by aboveground biomass which is commonly dominated by woody plants 13,42 . The long-term VOD data set (1992-2012) is retrieved from satellite passive microwave observations with a spatial resolution of 0.25° × 0.25° (ref.…”

Estimating aboveground woody biomass change in Kalahari woodland: combining field, radar, and optical data sets

“…This region provides the natural resources for the livelihoods of greater part of the Namibian population, yet its spatial and temporal dynamics and associated four vegetation change processes (i.e. deforestation and woodland degradation, woody encroachment, and woodland regrowth) remain poorly quantified (Strohbach 2001;Mendelsohn and El Obeid 2002;Wingate et al 2016;Mendelsohn and el Obeid 2005a;Tian et al 2016). Hence, we use field measurements of AGB in combination with high spatial resolution multi-temporal maps L-band SAR and dry season Landsat vegetation index composites for two periods (2007 and 2015) to define the extent, intensity and severity of these change processes.…”

“…A long standing debate remains as to the sustainability of these land-uses; it is widely believed that commercial farming better manages resources, while communal farming is resulting in a typical "tragedy of the commons" situation (Hardin 1968;Strohbach 2001). Permanent vegetation loss and woody encroachment with the consequent loss of economically important herbaceous layer species, are the principal environmental concerns faced by both farming sectors and these processes are also believed to be impacting conservations areas (Tian et al 2016;Ward 2005;Wingate et al 2016;Ward and Ngairorue 2000). Changes in AGB resulting from deforestation or degradation are often limited to approximately 2 t ha -1 due to lack of mechanical means, while woody encroachment occurs at regional scales (approximately 10,000 ha -1 ) (Wingate et al 2016;De Klerk 2004b).…”