Managing forests for old-growth attributes better promotes the provision of ecosystem services than current age-based old-growth management

Old-growth forests with complex structural attributes and large trees are rapidly transformed to more homogenous secondary forests through logging, reducing ecosystem services such as carbon storage, water provision and biodiversity. In British Columbia (BC), Canada, a century of logging resulted in strong pressures in the perceived dichotomy of conserving or logging the remaining old-growth. There is an urgency to determine if the current conservation policy for old-growth forests (e.g. old-growth management areas OGMAs) primarily based on stand age is adequate to protect old-growth structural attributes and their ecosystem services (ES) while leaving opportunities for timber harvesting within timber harvesting tenures. We applied a systematic conservation planning tool (PrioritizR) to design and evaluate attribute-based old-growth reserves as an alternative to the current age-based OGMAs, in a Community Forest (123,695 ha), managed primarily for timber. Old-growth, timber, carbon, tree diversity, and water services were mapped using Aerial Laser Scanning (ALS), and field measurements were used to ground truth with the random forest machine-learning algorithm. Then, PrioritizR was used to identify optimal reserve's networks for age, old-growth attribute, carbon, water, and a combination thereof. We found that the attribute-based old-growth reserves had significantly higher ESs provisioning than the age-based OGMAs. In addition, tradeoffs with timber harvesting were reduced when we simultaneously prioritized old-growth attributes and water values. Finally, while timber harvesting affects the provision of ESs by up to 11.8% ES loss for each 1% timber harvested (-283,150 m3 of timber), an increase in the area used for old growth conservation did not affect priority areas for timber harvesting until 22.6% of the study area was set aside (-30,316 ha) (more than threefold increase of the current OGMAs' area, 8,611 ha). Such results indicate that the conservation of old-growth via attribute-based OGMAs can help maintain the provision of multiple ESs in the landscape, including sustainable timber harvesting.

Automated summary

This study found that attribute-based old-growth reserves have higher ecosystem services provisioning compared to age-based OGMAs. Additionally, tradeoffs with timber harvesting were reduced when old-growth attributes and water values were simultaneously prioritized.