Detecting and comparing static and dynamic gaps in a western Amazonian tropical forest landscape

Abstract:

Spatial and temporal variation in forest structure is a central aspect of tropical forest carbon dynamics. A major determinant of forest structure is the size and frequency of natural forest disturbance events. The resulting forest gaps are direct expressions of tree mortality, damage, and regrowth, and play integral roles in community structure and forest carbon storage. Static forest gaps, or those found at a single timeframe, express the time-integrated pattern of both past disturbance and forest regrowth. Dynamic gaps, or those appearing between two measurement periods, reflect committed carbon losses to the atmosphere. Studies of tropical forest disturbance often examine only static gaps to make inferences about annual carbon dynamics. Our goal was to determine whether the use of static gaps rather than dynamic gaps is appropriate for quantifying annual carbon flux. Using airborne LiDAR data collected by the Carnegie Airborne Observatory at two time periods (2011 and 2012) and a new gap detection technique, we compared static and dynamic gaps over a 9000 ha lowland tropical forest in southeastern Colombia. We find static gaps are present across the landscape at 1.6 gaps ha-¹, while dynamic gaps are present at 0.5 gaps ha-¹. Moreover, 60% of these dynamic gaps are connected to pre-existing gaps (i.e., gap edge expansion), with new isolated gaps making up just 0.2 gaps ha-¹. The lower density of dynamic gaps suggests studies using static gaps may overestimate the amount of tropical forest carbon lost annually to natural disturbance.