Role of forest thinning techniques towards nature-based climate solutions

Forest ecosystems provide many essential services such as climate regulation and carbon storage, which are important for many industries and for global Earth system health. However, forest ecosystems are endangered by ongoing resource exploitation and climate and land cover changes which could lead to the destruction of large quantities of forest carbon stocks and stand inventory. Nature-based climate solutions are gaining traction in recent years, particularly forest thinning techniques like variable retention harvesting (VRH) which promotes forest growth, biodiversity, and ecosystem function. Still, they require an intensive assessment of their contribution to forest structure and enhanced carbon dioxide (CO₂) sequestration, but traditional inventory-based forest monitoring practices are time-, cost-, and labour-intensive and impractical at a national scale. In this study, we implement a comprehensive methodology of forest monitoring that uses a combination of field measurements, digital hemispherical photography, spectroscopic analysis, and unmanned aerial vehicle (UAV)-derived data to derive canopy structure, light environment, and soil biogeochemistry. We evaluated the impact of four different VRH treatments on the leaf area index (LAI), canopy openness, photosynthetically active radiation (PAR) absorbance, biomass, and soil carbon and nitrogen content of an 84-year-old red pine (Pinus resinosa) plantation forest in Southern Ontario, Canada. The VRH treatments included 33% dispersed crown retention (33D), 33% aggregated crown retention (33A), 55% dispersed crown retention (55D), and 55% aggregated crown retention (55A). Our findings show that the VRH treatments were major controls or drivers of seasonal variation in LAI, canopy openness, PAR absorbance, biomass, and soil carbon and nitrogen content. Our study suggests that the dispersed crown retention of 55% basal area is the ideal forest thinning technique to enhance CO₂ sequestration and preserve forest structure and light environment. This study provides insight into the interactions between forest ecosystem dynamics and silvicultural interventions, which is indispensable for improving our understanding of nature-based climate solutions. It will also help outline the framework for monitoring forest structure and CO₂ sequestration on large spatiotemporal scales.