Forests of the Future — Equivalent Clearcut Area (ECA)
Forest disturbance has significant effects on B.C. ecosystems that last for decades. The effects of forest disturbance are cumulative over space and time, and have implications for ecological functions, hydrological responses, and public safety. These facts serve as evidence for the importance of equivalent clearcut area (ECA), a unit of measurement relating the area and timing of disturbance at the watershed scale. It accounts for areas that have been harvested, burned, or otherwise impacted.
ECA is widely used as an indicator to quantify forest disturbances as it not only covers all disturbance types but also considers the subsequent recovery of these disturbed areas through space and time. The three most common forest disturbances that we have identified are harvesting/logging, forest fires, and Mountain Pine Beetle infestations. The combined impacts of these three factors, as well as any other disturbances, such as permanent linear infrastructure, may cause substantial changes to the hydrology in a given watershed, including adverse impacts to water quality and quantity, as well as aquatic habitats.
Forests influence the water cycle through rainfall interception, evapotranspiration, and soil infiltration and storage. Therefore, forest disturbance can have a significant impact on the hydrologic regimes of watersheds. This is mainly because the infiltration and runoff of precipitation and melted snowpacks will be different.
How Does It Work?
First, a disturbance happens at a specific time, and the area in which it occurred is determined. Therefore, if we want to look at the cumulative disturbance at any point in time, we can apply recovery curves to account for the regrowth at a specific period to examine each disturbance that occurred.
Hydrologic recovery curves examine decades of data and look for recurring trends in recovery to help determine how recovery occurs and how long it takes. They are functions of tree growth and/or regrowth, including height, root systems, and canopy breadth. For example, if trees have been planted in the examined area, what kind of changes have occurred after five years? By examining the changes that occur over the decades following disturbance, we can utilize ECA to quantify the cumulative disturbance at any point in time.
Foundry Spatial automates the calculation of ECA by combining decades of satellite imagery, the automated detection of forest disturbance from year to year, and integrating the timing and location of a disturbance with recovery curves and detailed watershed mapping. This combination allows for a seamless evaluation of the large landscapes that ECA measures.
Pilot Project in Tulameen
We focused our study in the Regional District of Okanagan Similkameen and the Tulameen River watershed. Our methods grouped land cover change to watershed units by year and type, applied recovery curves to disturbance by type and age, and summarized cumulative disturbance by watershed unit. This research is our response to the knowledge gap that exists due to the lack of accessible, quantitative data describing the cumulative amount of forest disturbance in watersheds around British Columbia. In the end, we were able to mathematically separate the impacts of climate change and forest disturbance in an independent watershed upstream of a long term monitoring location. The results of this initial pilot project were compared against an evaluation of ECA in the same watershed following a manual process, with very similar final results.
Implications and Applications
Natural disasters such as flooding and wildfires can be prevented and/or mitigated with the right information. Currently, governments and forestry companies do not have access to the valuable information that ECA provides, thus leading to decision making that lacks a sustainable, long-term perspective. As a key metric for planning future forest activities, ECA enables users to determine how much time it will take for forests to recover and the increased chances of specific natural disasters occurring given the lack of forest. Understanding and utilizing this powerful information allows decision makers to evaluate all the possible scenarios from a sustainable lens and reach conclusions that mitigate catastrophic events.
Zhang M, Wei X. The effects of cumulative forest disturbance on streamflow in a large watershed in the central interior of British Columbia, Canada. Hydrology and earth system sciences. 2012 Jul 11;16(7):2021–34.