Abstract
We present an efficient method for monitoring woody (i.e., evergreen) and herbaceous (i.e., ephemeral) vegetation in Mediterranean forests at a sub pixel scale from Normalized Difference Vegetation Index (NDVI) time series derived from the Moderate Resolution Imaging Spectroradiometer (MODIS). The method is based on the distinct development periods of those vegetation components. In the dry season, herbaceous vegetation is absent or completely dry in Mediterranean forests. Thus the mean NDVI in the dry season was attributed to the woody vegetation (NDVIW). A constant NDVI value was assumed for soil background during this period. In the wet season, changes in NDVI were attributed to the development of ephemeral herbaceous vegetation in the forest floor and its maximum value to the peak green cover (NDVIH). NDVIW and NDVIH agreed well with field estimates of leaf area index and fraction of vegetation cover in two differently structured Mediterranean forests. To further assess the method’s assumptions, understory NDVI was retrieved form MODIS Bidirectional Reflectance Distribution Function (BRDF) data and compared with NDVIH. After calibration, leaf area index and woody and herbaceous vegetation covers were assessed for those forests. Applicability for pre- and post-fire monitoring is presented as a potential use of this method for forest management in Mediterranean-climate regions.
Highlights
Monitoring the dynamics of different vegetation components of the forest is essential to identify and understand trends in vegetation structure and ecosystem functioning
Because understory vegetation in Yatir is comprised mostly of ephemeral herbaceous species, this supports the assumption that herbaceous vegetation in the understory is the main contributor to the seasonal change in NDVIEcos
We proposed a phenology-based method that uses the distinct timing of ephemeral herbaceous and woody foliage development to separate time series of the Normalized Difference Vegetation Index (NDVI) into their respective contributions
Summary
Monitoring the dynamics of different vegetation components of the forest is essential to identify and understand trends in vegetation structure and ecosystem functioning. To distinguish between woody and herbaceous contributions, Roderick et al [20] used time series analysis on NDVI They aimed to map woody and herbaceous cover across Australia by applying a moving average method to separate between the long inter-annual trend and the seasonal (intra-annual) components of the time series. They assumed that woody vegetation in Australia is mainly evergreen. We present an efficient method to distinguish between NDVI from woody (i.e., evergreen) and herbaceous (i.e., ephemeral) vegetation in Mediterranean woodlands at a sub-pixel scale using phenology-based time series analysis. We demonstrate the potential applications of this method for forest management and pre/post-fire monitoring
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