Using Locally Developed Allometric Equations Improves Biomass Estimation in Managed Forests
This study demonstrates that locally developed allometric equations significantly improve biomass estimation in intensively managed lodgepole pine forests by accurately capturing crown structural changes caused by pre-commercial thinning, which generalized national equations fail to account for and consequently underestimate.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Forests are not just collections of trees; they are complex, living systems where every branch and leaf plays a role in the planet's health. Scientists rely on measurements of forest biomass—the total weight of all the living plant material—to understand how much carbon trees store, how productive a forest is, and how much fuel is available to carry a fire through the canopy. To get these numbers without cutting down every single tree, researchers use mathematical formulas called allometric equations. These formulas act like a translator, converting easy-to-measure features, such as the width of a tree's trunk and its total height, into an estimate of its total weight. For decades, scientists have used broad, national versions of these formulas, which work well for forests that have grown naturally without human interference. However, many forests today are actively managed. Foresters often thin out young stands of trees, removing competitors to give the remaining ones more room to grow. This changes the shape of the trees, but it was unclear whether the old, general formulas could still accurately weigh these newly shaped giants.
In a study conducted in the foothills near Whitecourt, Alberta, researchers set out to test whether these standard formulas hold up in managed forests. They focused on lodgepole pine, a common tree in the region, growing in stands that had undergone pre-commercial thinning. This practice involves cutting down young, competing trees before they are large enough to be sold as timber, allowing the remaining trees to expand their crowns and grow faster. The team selected nine different sites where this thinning had taken place seven to eight years prior. At each site, they established plots that included both thinned areas and unthinned control areas. To get the most accurate data possible, they felled 52 trees across these plots—27 from the unthinned areas and 25 from the thinned ones. Back in the lab, they meticulously separated every branch and needle, weighing the foliage, small twigs, and larger branches to determine the exact biomass of each tree.
With this precise data in hand, the researchers developed a new set of local equations specifically for these managed stands. Unlike the national formulas, which rely mainly on trunk width and height, the new local models incorporated the percentage of the tree that was covered in live branches. This detail proved crucial. The study found that trees in the thinned plots had significantly more foliage and branch weight than unthinned trees of the same trunk width. Because the thinning reduced competition for sunlight, these trees had retained more of their lower branches and grown fuller crowns. The local equations, which accounted for this extra greenery, predicted the weight of these trees with high accuracy.
When the researchers applied the standard national equations to the same trees, the results were surprisingly different. For the unthinned, densely packed trees, the national formulas worked just fine, matching the local predictions closely. However, in the thinned stands, the national formulas consistently underestimated the amount of biomass. On average, they calculated about 14 percent less foliage and 7 percent less branch weight than what was actually there. The national models failed to capture the structural changes caused by the thinning treatment. They seemed to assume that a tree with a certain trunk width would have a smaller crown, not realizing that the managed tree had been released from competition and had grown much larger and fuller.
The implications of this discrepancy are significant. If forest managers and climate scientists use the standard formulas for thinned forests, they will systematically underestimate how much carbon these forests are storing. Furthermore, from a fire safety perspective, this underestimation is dangerous. Canopy fuel loads are a critical factor in predicting how high a fire might climb and how fast it might spread. By underestimating the weight of the branches and needles, the national formulas could lead to an underestimation of the fire risk in these managed landscapes. The study concludes that while broad, national formulas are robust for natural, unmanaged forests, they are not sufficient for the intensively managed forests that make up a large part of the timber supply. To get an accurate picture of biomass, carbon, and fire risk in these areas, scientists need to use locally developed equations that recognize how human management reshapes the very architecture of the forest.
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