Enhanced woody biomass production in a mature temperate forest under elevated CO₂

The terrestrial carbon cycle's role in regulating atmospheric CO₂ and climate change is a major source of uncertainty in climate projections. While increasing atmospheric CO₂ concentrations have led to enhanced CO₂ uptake by plants (the CO₂ fertilization effect), it remains unclear whether this effect persists in older, mature forests. Previous FACE experiments primarily focused on young tree plantations. Mature forests face potential limitations like progressive nitrogen limitation (PNL), where nitrogen becomes sequestered, hindering growth. Elevated CO₂ can exacerbate PNL. Furthermore, older trees have a smaller fraction of live tissue contributing to growth. Net primary productivity (NPP) is a key metric for evaluating forest response to elevated CO₂, but the allocation of additional carbon to long-lived wood versus rapidly-cycling tissues is crucial. Carbon allocated to wood contributes to long-term carbon sequestration, while allocation to leaves and fine roots quickly releases CO₂ back into the atmosphere. Tree mortality rates also influence long-term carbon sequestration, potentially offsetting growth benefits. The 'second generation' of forest FACE experiments seeks to determine whether mature forests respond similarly to elevated CO₂ as younger ones. This study uses the BIFOR FACE facility in central England, a 180-year-old *Quercus robur* woodland, to address this question.

The literature extensively documents the CO₂ fertilization effect in forests, with evidence from various studies indicating increased CO₂ uptake leading to higher growth rates in younger forest plantations. However, this evidence is predominantly from 'first-generation' FACE experiments conducted on young, fast-growing stands. Concerns exist regarding the generalizability of these findings to older, more mature forests. The literature highlights potential nutrient limitations, particularly nitrogen limitation (PNL), in mature forests, which may constrain the response to elevated CO₂. Studies show that elevated CO₂ can accelerate PNL, and that phosphorus limitation might be crucial in forests on old soils. Existing allometric equations for estimating biomass may not accurately reflect the diverse structures of mature stands, leading to potential errors in estimations. Previous studies also suggest a trade-off between growth rate and tree lifespan, further complicating the evaluation of long-term carbon sequestration.

The BIFOR FACE facility in central England (52.801° N, 2.301° W, 107 m above sea level) is a 19-ha *Quercus robur* woodland established in the mid-nineteenth century. Six experimental arrays surround plots of ~30 m diameter. Three arrays received CO₂ enrichment (eCO₂), aiming for 150 ppm above ambient (aCO₂). Tree-ring analysis provided pretreatment data, accounting for initial differences in tree growth among plots. A site-specific allometry, based on terrestrial laser scanning (TLS), was developed to accurately estimate annual wood production from diameter measurements. This addressed inaccuracies associated with applying generalized allometric equations to this site and stand structure. Diameter increments were measured using dendroband sensors and manual dendrometers, ensuring consistency and accuracy. Dry matter increment (DMI) per tree was calculated and adjusted for pretreatment differences. Net primary productivity (NPP) was estimated using data on wood, fine-root, leaf, understory, reproductive tissue, and exudation. Repeated measures analysis of variance was used to assess the effects of eCO₂ on DMI and NPP, accounting for plot differences. Exudation of organic carbon from roots was measured using a modified method described in reference [53].

Tree-ring analysis revealed high variability in tree growth, reflecting the characteristics of mature forest stands. Despite pretreatment differences, basal area increment (BAI) was significantly greater in eCO₂ plots after CO₂ treatment began. After normalizing for pretreatment differences, dry matter increment (DMI) per tree was greater in eCO₂ plots in most years, with a significant overall CO₂ effect (P = 0.028). Over seven years, tree growth was 9.8% greater under eCO₂. The loss of response in 2019 was possibly due to insect defoliation. NPP, estimated for 2021 and 2022, showed a 9.7% and 11.5% increase in eCO₂, respectively, though not statistically significant individually. However, the combined two-year analysis shows a 10.6% increase (P = 0.099). Aboveground *Q. robur* wood production was the largest component of NPP (40-48%), with no significant differences in leaf or fine-root production. Leaf mass per unit area (LMA) was greater, and leaf area index (LAI) was slightly lower in eCO₂. Root exudation of organic carbon was significantly greater under eCO₂ (P = 0.042), indicating increased belowground carbon allocation.

The study's findings demonstrate that even mature, 180-year-old *Quercus robur* trees in a temperate forest exhibit a positive growth response to elevated CO₂. The increased allocation of carbon to woody biomass under eCO₂ highlights the significant role of mature forests in carbon sequestration. The results contrast with findings from other FACE experiments, such as EucFACE, where no such increase was observed. This difference might be attributed to nutrient dynamics, suggesting that nutrient availability plays a critical role in determining forest response to eCO₂. While mature forests are often considered nitrogen-limited, the BIFOR FACE site exhibits relatively high nitrogen deposition, potentially mitigating nutrient constraints. The lack of a significant increase in fine root production is consistent with the apparent lack of nitrogen limitation. Continued monitoring of the BIFOR FACE experiment is crucial to ascertain the long-term effects of eCO₂ on carbon sequestration and to better understand the factors driving the difference in responses across various forest types. The observed increased root exudation indicates a potential shift in belowground carbon dynamics, which requires further investigation.

This study demonstrates that mature temperate forests retain the capacity to enhance woody biomass production under elevated CO₂. The observed increase in woody biomass under eCO₂, coupled with increased root exudation, underscores the importance of mature forests in mitigating climate change. Further research should focus on long-term impacts of eCO₂, nutrient dynamics in mature forests, and the role of root exudation in ecosystem carbon cycling.

The relatively short duration (seven years) of the experiment may not fully capture long-term responses of the forest to elevated CO₂. The statistical significance of NPP increase was marginal in individual years, suggesting the need for more years of data to strengthen this conclusion. The study focused on a single forest type and species; the generalizability of these findings to other forest ecosystems needs further investigation. The exact mechanisms underlying the observed differences in responses between this study and others remain to be explored.