Forest Canopy Gap Dynamics: Quantifying Forest Gaps and Understanding Gap-level Forest Regeneration

Science Center Objects

Floodplain forest is a major and critical component of the Upper Mississippi River System (UMRS). The current and future condition of floodplain forest on the UMRS is receiving increased attention from managers as the forest appears to be declining and not regenerating. A primary concern is invasive reed canarygrass (Phalaris arundinacea), Dutch elm disease (Ophiostoma ulmi and O. novo-ulmi), and emerald ash borer (Agrilus planipennis). A secondary concern is changing patterns of flooding length and severity that may be suppressing floodplain forest regeneration.  To map, analyze, and understand trends in floodplain forest loss on the UMRS, an R script was developed to map forest canopy gaps across the UMRS floodplain using existing lidar data and Long-term Resource Monitoring (LTRM) program data. These data will be analyzed to help managers make floodplain forest management decisions.

The USGS Upper Midwest Environmental Sciences Center is working with the U.S. Army Corps of Engineers to identify forest canopy gaps in floodplain forests along the Mississippi and Illinois Rivers to help understand the factors inhibiting floodplain forest regeneration and how these factors might be alleviated.


To locate the canopy gaps, lidar data was analyzed to create a canopy height model for the areas of interest (Figure 1A). Areas with a canopy height of -0.05 to 10 meters were considered a canopy gap when the area had a portion with a minimum 18-meter diameter (Figure 1B). Lidar-detected forest canopy gaps were then randomly selected for field testing and data collection to determine forest health metrics in and surrounding the canopy gap (Figure 1C).


The data collected from these sample plots will assist resource managers in determining which factors are inhibiting or promoting floodplain forest regrowth and how to address the issue. With the acquisition of new lidar, a change analysis will be possible to further explain the factors controlling floodplain forest survival and regrowth within the Mississippi and Illinois Rivers floodplains.

Figure 1: Tile A showing the lidar point cloud along the eastern shore of Railroad Island in Pool 13 with gaps delineated.

(Public domain.)