Forest Age Structure Modeling with LiDAR Measurements and Landsat Data

Forests of different ages exhibit very different characteristics. The aging of forests will affect their timber productivity, wildlife habitat diversity, tree species composition, and susceptibility to disturbance events. Moreover, the net sequestration or loss of CO₂ by forests after disturbance follows a predictable pattern determined by age and other site factors. Therefore forest stand age is a critical parameter in determining both the current state and potential of forest ecosystem carbon sequestration. Consequently, continuous forest age estimation will be of great importance for climate change studies, conservation and ecosystem management. In existing studies, age is usually estimated as the time since last major disturbance, based on the assumption of a very distinct disturbance and regeneration activity such as a clear-cut followed by a plantation establishment. However many natural disturbances do not kill all the trees in a forest stand. Regeneration may be a slow process that new trees span a range of ages over several decades. In addition, spatial resolutions of most existing age maps are around 1km, which is most appropriate for large-scale studies but could not reflect spatial details.

Therefore the objective of this study is to map forest age structure by taking disturbance intensity into consideration to avoid the arbitrary assumption mentioned above. Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) data recently being available offers great opportunity to quantify forest structure at large scales. ICEsat-2 LiDAR metrics and Landsat surface reflectance data are used to represent site conditions, allowing higher resolution of age estimation. Forest Inventory and Analysis (FIA) National Program ground measurements are used to derive reference data for training and validation of the model. In this presentation, we will discuss 1) the effectiveness and uncertainties of the modeling approach, 2) forest age compositions in southeastern United States, 3) comparison of forest age structures across ecoregions.