by Rob MacKenzie
Unlike, say, buildings, grown trees don’t ‘just stand there’. Even before we consider the communities supported by mature trees, we should recognise the active nature of the trees themselves. More like whirlpools than buildings, trees continually exchange energy and material with their surroundings throughout their lives. Trees use sunlight to create order out of the chaos of the air and earth; tempting a scientist to call them the ultimate entropy deniers. By appreciating that trees must continually run to stand still, that forests are complex plant-animal-microbe systems rather than simply a bunch of ‘carbon sticks’, we gain a much more complete understanding of how standing forests have a significant influence on the flows of carbon, water, nitrogen, and other nutrients through the Earth system.
When we consider the global CO2 budget, it’s important to note the distinction between emissions, which are largely driven by socio-economics, and sinks, which have a very strong biological component. Only a bit less than half the CO2 emitted into the atmosphere stays there; the rest is taken up by the ocean surface and the terrestrial biosphere. The research consensus is that the land carbon sink is bigger than the ocean sink. Currently, the northern hemisphere is taking up billions of tonnes of carbon, and the most important landscape for this uptake is temperate forest.
In the far North, the vast boreal forest may be taking up 0.5 billion tonnes of CO2 per year but, further south, temperate forest biome is taking up 0.8 billion tonnes, which is equivalent to all the emissions from the EU 27 nations plus the UK, or approximately the amount of CO2 released from deforestation globally. This is a huge number, worth almost £15 billion at today’s carbon trading price and much, much more in reality. The countries of the temperate zone — between the 30th and 50th parallels, very roughly, and encompassing the great majority of what is often called the Global North — are in an ideal position to lead in the adoption of practical measures to maintain and maximise the land carbon sink. Activities to maintain stocks of, and increase flows into, land carbon should recognise the three great forest biomes (tropical, temperate, boreal) and fit policy options to each.
What we know
The land carbon sink is important now; how it reacts in future is critical to our climate projections. The Intergovernmental Panel on Climate Change (IPCC) recognises the CO2 feedback on the land carbon sink as the most important feedback in the carbon cycle. ‘Feedbacks’ are the accelerator and brake pedals of any complex system such as climate. Positive feedbacks accelerate change; negative feedbacks, like that of CO2 on the land carbon sink, resist change. The CO2 feedback on the land carbon sink is often called “CO2 fertilisation” and is currently providing a very significant, but far from total, brake on global warming. CO2 fertilisation cannot now, and never will, offset entirely historical fossil fuel use and save us from global warming — not for the UK, and not for the world. Nevertheless, a key question for our climate projections is whether CO2 fertilisation will continue to provide its current planetary free gift, whether it will provide an even larger brake on climate change as CO2 levels increase further, or whether it will begin to decrease in importance as factors other than the CO2 level begin to dominate forest responses.
Currently our understanding of how temperate forest will respond to elevated CO2 is based largely on the first-generation forest Free-Air CO2 Enrichment (FACE) experiments, for example, the experiment in Sweet Gum in Oak Ridge, Tennessee, or the experiment in Loblolly pine in North Carolina. All of these experiments were in young plantations growing in non-forest soils and so, for those parameters derived from FACE experiments, climate modellers are predicting the response of the world’s forests based on the responses of young plantations, despite the majority of broad-leafed temperate forests being greater than 50 years old.
We know that, as forests age, competition between trees intensifies, and this is expected to result in mature trees exploring more and more of their soil resource to acquire nutrients until the soil and the forest are optimally interlocked. In this case, mature trees would not be able to use additional carbon fixed under elevated CO2 to promote greater amounts of nutrient uptake and therefore may not be able to sustain a growth response to elevated CO2. However, in absorbing carbon, perhaps mature trees, in the temperate forests and elsewhere, find more nutrients to take up, use nutrients more efficiently, or both? To answer this question, we require detailed experiments that link the carbon cycle to nutrient and water cycles in order to improve our climate models. Very few models have carbon coupled to nutrients, and those models are not challenged by data (because there have been no data).
As it grows to maturity, standing forest is not undisturbed; forests continually experience disturbance from fire, flood, disease, storm, and harvest. The ‘natural’ disturbances are increasingly affected by human influence, including the human influence on climate. Strong intergovernmental approaches to climate mitigation can stabilise the random effects of fire, flood, and storm. Strong regulatory measures can control the globalisation of invasive pests and pathogens. Moving harvest ‘closer to nature’ through carbon- and biodiversity-conscious management will ensure that standing forests continue to deliver environmental, social, and economic goods.
The latest research
FACE facilities are the key to understanding ecosystem-level responses under future atmospheric compositions. Several small-scale facilities exist; a number of first-generation forest facilities operated at the turn of the Millennium. Only two large-scale forest FACE facilities operate currently, although plans for a third, in the Amazon, are well advanced. Forest FACE facilities are the largest climate-change experiments in the world (Figure 1); together, they represent a time-machine transporting the plants, animals and microbes of today’s forests into the atmosphere of the 2050s. International agreement to set up and run an Amazon FACE, closely linked to the existing FACE facilities, would send a very clear message from COP26 that world leaders appreciate the substantial evidence gap we are currently bridging very sketchily with models.
Figure 1. The Free-Air CO2 Enrichment facility of the Birmingham Institute of Forest Research: BIFoR FACE, one of the three largest climate-change experiments in the world. Three of the patches surrounded by the metal infrastructure receive elevated CO2 (150 ppm above an ambient level which is currently ~410 ppm). The other three, ‘control’, patches receive unaltered ambient air. Photo courtesy of Prof. Jo Bradwell.
How do the forests respond in FACE? The Australian facility, EucFACE, sited in old-growth eucalyptus forest outside Sydney, have produced a budget for their ‘Mediterranean-type’ climate and environment. Carbon is taken up when extra CO2 is available, as expected, but seems not to be stored in this hot, dry, and phosphorus-poor environment. If there is substantial CO2 fertilisation in standing mature forests, as our models and satellite measurements say there must be, we need to look elsewhere. A tremendous amount has been learnt in producing a carbon budget for EucFACE. The first precious data to challenge the leading climate models have been produced. The core of a forest time machine is in place; now we need to add the other great forested landscapes of the Earth to our investigation.
EucFACE has a sibling facility in cool, moist England: BIFoR FACE. Although the quantitative details are yet to be peer-reviewed for BIFoR FACE, qualitatively the picture is becoming clear (Figure 2). Firstly, the forest under elevated CO2 draws more carbon into the trees (i.e. photosynthesis is increased); wood production is increased; production of leaves and fine roots is increased; and secretions into the soil by the roots are increased. Ecologists say that the Gross Primary Production (GPP) of the forest is increased. This increased living plant mass stimulates microbial life, especially below ground, so that the respiration of CO2 — an inevitable by-product of all life — also increases. Unless significant amounts of old soil carbon are being accessed and used up in this increased respiration (an unlikely but not impossible scenario), the carbon balance overall is that the mature forest in BIFoR FACE is drawing carbon out of the atmosphere and so helping to lessen global warming. However, until detailed budgets are produced and until the experiments have been allowed to progress for a decade or even beyond, we will not be sure the degree to which this carbon uptake by mature forests is persistent.
Figure 2. Qualitative assessment of the effects of elevated CO2 on forest at BIFoR FACE for the carbon cycle (green), water cycle (blue), nitrogen cycle (red). Arrows show the direction of extra flow under elevated CO2 conditions. ‘Plus’ signs indicate an increase under elevated CO2. The inset image shows leaf mines, which are decreased under elevated CO2 for the larvae of some species. Image from Hiclipart.com, adapted by Nine Douwes Dekker and Rob MacKenzie.
TThe forest FACE results make clear that the land carbon sink is not straightforward and is emphatically not just a story about carbon. Our early results from BIFoR FACE show that the essential nutrient, nitrate, is less available under elevated CO2, because the plants and soil microbes are taking up more nitrogen to balance the increased carbon that is now in their diet. A very simple analogy to this requirement for balanced uptake of nitrogen and carbon in forests is the need to balance carbohydrate (mostly carbon) and protein (high in nitrogen) in our own diets if we are to stay healthy.
If we — and our partners working at EucFACE and the putative AmazonFACE — find that mature trees cannot take up more nutrients under elevated CO2 then it would mean that these biomes may have much less potential to take up carbon in the future than we are currently relying on, which would have massive consequences for the fight against climate change. The potential for carbon uptake by terrestrial ecosystems in the future would, in this case, have been severely overestimated. According to one study, accounting for nutrient availability limitations added 15–150 ppm to atmospheric CO2 concentration or, equivalently, 0.14–0.6oC to the global mean surface temperature change at 2100.
If future carbon uptake by mature temperate forests were confirmed to be limited by nutrient availability, the forest-based CO2 removal techniques relied on in the IPCC’s “1.5 degrees” document would be called into question. Much deeper cuts in allowable emissions of CO2 (equivalent to 8-33 years of emissions at current rates) would be required to achieve the target agreed at UN COP219, with profound policy and societal implications. We need to know what future we are facing, and forest FACE facilities (punning name, perhaps, intended) are the only way to probe this future experimentally.
The climate crisis is not the only global environmental challenge: a mass extinction is in progress for not-unconnected reasons. The world’s standing forests are immensely species-rich, particularly when left close to their primeval condition or restored to provide multiple ecosystem services resiliently. Whether storing more carbon or not, accelerated cycling of carbon and nutrients through the forest has profound implications for forest food webs and, hence, biodiversity. Increasing GPP in a forest is like increasing GDP (Gross Domestic Product) in a country: things are bound to change. In BIFoR FACE, for instance, some leaf-mining grubs seem to do less under elevated CO2. These grubs are moth larvae that feed on the trees’ photosynthetic apparatus, the leaf, and are themselves parasitized by wasps. Other work in BIFoR FACE will investigate the implications of the changed nutritional quality of leaf litter on the forest floor food web and the impact of elevated CO2 on decomposition of deadwood by fungi and invertebrates. By recognising fully the carbon utility of standing forest, the international community will also provide essential knowledge to help prevent a biodiversity catastrophe.
So, nature-based solutions to climate change are not all about tree planting. Afforestation can never solve our problems with carbon and climate, even if we reinstated every terrestrial landscape to its most-treed extent. Working with nature strongly implies protection of, and care for, our standing forest, moving beyond the existing recognition of reducing emissions from deforestation and forest degradation in developing countries to value all mature forest. In absorbing carbon, do mature trees in old-growth forests take up more nutrients, use nutrients more efficiently, or both? The answer to this question will unlock the CO2 fertilisation conundrum and make our climate projections much more robust. Whatever the answer, let’s protect, expand, and manage our forests for their true value, not because we wish they would do the hard work of emissions reduction for us.
 Ciais et al 2013. In: Climate Change 2013. The Physical Science Basis. Cambridge Univ Press. (Fig 6.22). https://www.ipcc.ch/report/ar5/wg1/
 The IPCC “1.5 degrees” report: https://www.ipcc.ch/sr15/
 For more information see https://www.researchgate.net/publication/260197077_Carbon_sequestration_Managing_forests_in_uncertain_times and https://theconversation.com/using-forests-to-manage-carbon-a-heated-debate-81363
 Norby, R. J., et al., Model-data synthesis for the next generation of forest FACE experiments, New Phytologist, 2015, DOI: 10.1111/nph.13593. Note that SwedFACE, the boreal forest FACE mentioned in this paper, is not yet underway.
 Hart, K. M., et al. Characteristics of free air carbon dioxide enrichment of a northern temperate mature forest. Glob Change Biol. 2020; 26: 1023– 1037. https://doi.org/10.1111/gcb.14786
 Gardner A, et al., Is photosynthetic enhancement sustained through three years of elevated CO2 exposure in 175-year old Quercus robur? BioRxiv preprint 2020: https://biorxiv.org/cgi/content/short/2020.12.16.416255v1
 Ziegler, C., et al., Quantifying carbon fertilisation of root biomass production from elevated CO2 in mature temperate deciduous forest. BioRxiv preprint 2021: https://www.biorxiv.org/content/10.1101/2021.04.15.440027v1
 See my recent overview talk: https://derby.cloud.panopto.eu/Panopto/Pages/Viewer.aspx?id=8620142d-9d03-49f5-81d0-abc900cf1809 and a short radio interview: http://birminghamtreepeople.org.uk/bifor/