We keep hearing from the forest industry that old trees need to be logged to make room for young, fast-growing trees.  But old trees are massively important for ecosystem integrity and carbon stocks. When they’re logged, most of the carbon ends up in the atmosphere in short order (very little of it being incorporated into long-lived wood products). It doesn’t matter then how fast-growing the replacement trees are, logging these big old trees will cause a surge in GHG emissions. Here are four important papers that make the case for old trees (full text at links). Abstracts have the most important findings highlighted.

  1. This new very important paper makes the point that allometric modeling that translates survey data into volume data may be biased against big trees, leading to underestimation of their true role. It also got written up by the BBC, that’s here.

Calders, K., et al. (2022). Laser scanning reveals potential underestimation of biomass carbon in temperate forest. Ecological Solutions and Evidence 3(4): e12197.

Quantifying climate mitigation benefits of biosphere protection or restoration requires accurate assessment of forest above-ground biomass (AGB). This is usually estimated using tree size-to-mass allometric models calibrated with harvested biomass data. Using three-dimensional laser measurements across the full range of tree size and shape in a typical UK temperate forest, we show that its AGB is 409.9 t ha−1, 1.77 times more than current allometric model estimates. This discrepancy arises partly from the bias towards small trees in allometric model calibration: 50% of AGB in this forest was in less than 7% of the largest trees (stem diameter > 53.1 cm), all larger than the trees used to calibrate the widely used allometric model. We present new empirical evidence that the fundamental assumption of tree size-to-mass scale-invariance is not well-justified for this kind of forest. This leads to substantial biases in current biomass estimates of broadleaf forests, not just in the UK, but elsewhere where the same or similar allometric models are applied, due to overdependence on non-representative calibration data, and the departure of observed tree size-to-mass from simple size-invariant relationships. We suggest that testing the underlying assumptions of allometric models more generally is an urgent priority as this has wider implications for climate mitigation through carbon sequestration. Forests currently act as a carbon sink in the UK. However, the anticipated increase in forest disturbances makes the trajectory and magnitude of this terrestrial carbon sink uncertain. We make recommendations for prioritizing measurements with better characterized uncertainty to address this issue.

  1. This global study shows big trees continue to accumulate carbon at a meaningful rate.

Stephenson, N. L., et al. (2014). Rate of tree carbon accumulation increases continuously with tree size. Nature 507(7490): 90-93.

Forests are major components of the global carbon cycle, providing substantial feedback to atmospheric greenhouse gas concentrations1. Our ability to understand and predict changes in the forest carbon cycle—particularly net primary productivity and carbon storage—increasingly relies on models that represent biological processes across several scales of biological organization, from tree leaves to forest stands2,3. Yet, despite advances in our understanding of productivity at the scales of leaves and stands, no consensus exists about the nature of productivity at the scale of the individual tree4,5,6,7, in part because we lack a broad empirical assessment of whether rates of absolute tree mass growth (and thus carbon accumulation) decrease, remain constant, or increase as trees increase in size and age. Here we present a global analysis of 403 tropical and temperate tree species, showing that for most species mass growth rate increases continuously with tree size. Thus, large, old trees do not act simply as senescent carbon reservoirs but actively fix large amounts of carbon compared to smaller trees; at the extreme, a single big tree can add the same amount of carbon to the forest within a year as is contained in an entire mid-sized tree. The apparent paradoxes of individual tree growth increasing with tree size despite declining leaf-level8,9,10 and stand-level10 productivity can be explained, respectively, by increases in a tree’s total leaf area that outpace declines in productivity per unit of leaf area and, among other factors, age-related reductions in population density. Our results resolve conflicting assumptions about the nature of tree growth, inform efforts to undertand and model forest carbon dynamics, and have additional implications for theories of resource allocation11 and plant senescence12.

  1. Another global survey, this one showing that the largest 1% of trees in any given area constitute around 50% of aboveground biomass. This has huge implications for forest management for climate and biodiversity. Logging big trees releases huge amounts of carbon (most of which ends up in the atmosphere in short-order, because a very small proportion tends to be turned into truly long-lived wood products).

Lutz, J. A., et al. (2018). Global importance of large‐diameter trees. Global Ecology and Biogeography 0(0): 1 – 16.

Aim: To examine the contribution of large‐diameter trees to biomass, stand structure, and species richness across forest biomes. Location Global. Time period Early 21st century. Major taxa studied Woody plants. Methods We examined the contribution of large trees to forest density, richness and biomass using a global network of 48 large (from 2 to 60 ha) forest plots representing 5,601,473 stems across 9,298 species and 210 plant families. This contribution was assessed using three metrics: the largest 1% of trees ≥ 1 cm diameter at breast height (DBH), all trees ≥ 60 cm DBH, and those rank‐ordered largest trees that cumulatively comprise 50% of forest biomass. Results Averaged across these 48 forest plots, the largest 1% of trees ≥ 1 cm DBH comprised 50% of aboveground live biomass, with hectare‐scale standard deviation of 26%. Trees ≥ 60 cm DBH comprised 41% of aboveground live tree biomass. The size of the largest trees correlated with total forest biomass (r2 = .62, p < .001). Large‐diameter trees in high biomass forests represented far fewer species relative to overall forest richness (r2 = .45, p < .001). Forests with more diverse large‐diameter tree communities were comprised of smaller trees (r2 = .33, p < .001). Lower large‐diameter richness was associated with large‐diameter trees being individuals of more common species (r2 = .17, p = .002). The concentration of biomass in the largest 1% of trees declined with increasing absolute latitude (r2 = .46, p < .001), as did forest density (r2 = .31, p < .001). Forest structural complexity increased with increasing absolute latitude (r2 = .26, p < .001). Main conclusions Because large‐diameter trees constitute roughly half of the mature forest biomass worldwide, their dynamics and sensitivities to environmental change represent potentially large controls on global forest carbon cycling. We recommend managing forests for conservation of existing large‐diameter trees or those that can soon reach large diameters as a simple way to conserve and potentially enhance ecosystem services.

  1. There are innumberable papers out there on the importance of old-gorwth forests. This paper is interesting because it makes the argument that even old and decaying trees are holding a lot of carbon. When these areas are salvage-logged, that carbon ends up in the atmosphere. Also, look at the relative importance of the soil carbon stocks in the overall picture.

Jacob, M., et al. (2013). Significance of Over-Mature and Decaying Trees for Carbon Stocks in a Central European Natural Spruce Forest. Ecosystems 16.

Old-growth forests are important stores for carbon as they may accumulate C for centuries. The alteration of biomass and soil carbon pools across the development stages of a forest dynamics cycle has rarely been quantified. We studied the above- and belowground C stocks in the five forest development stages (regeneration to decay stage) of a montane spruce (Picea abies) forest of the northern German Harz Mountains, one of Central Europe’s few forests where the natural forest dynamics have not been disturbed by man for several centuries. The over-mature and decay stages had the largest total (up to 480 Mg C ha−1) and aboveground biomass carbon pools (200 Mg C ha−1) with biomass C stored in dead wood in the decay stage. The soil C pool (220–275 Mg C ha−1, 0–60 cm) was two to three times larger than in temperate lowland spruce forests and remained invariant across the forest dynamics cycle. On the landscape level, taking into account the frequency of the five forest development stages, the total carbon pool was approximately 420 Mg C ha−1. The results evidence the high significance of over-mature and decaying stages of temperate mountain forests not only for conserving specialized forest organisms but also for their large carbon storage potential.

Global evidence that big trees are hugely important for mitigating climate change

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