Hi Reddit! I’m Dr Bonnie Waring, an ecologist and Senior Lecturer at Imperial College London’s Grantham Institute for Climate Change and the Environment. I head up The Waring Ecology Lab, which investigates how the ecology of plant and soil microbial communities influences the carbon cycle and its feedbacks on climate change.

What are the pitfalls and benefits of reforestation and afforestation? What role can forests play in tackling climate change? These are the central questions of my current research.

Large-scale tree planting is an increasingly popular component of global efforts to meet climate targets. However, forests are complex ecosystems, and poorly planned planting efforts can actually increase the amount of carbon dioxide in the atmosphere and increase global warming. My research explores how carbon flows between the atmosphere and its two other major reservoirs on land: plants and soils.

I’ll be back to answer questions at 16:00 GMT on Wednesday 18 November (11:00 ET). Ask Me Anything.




Personal web page: https://www.imperial.ac.uk/people/b.waring

Waring Ecology Lab: https://www.waringecologylab.com/

Briefing paper: https://www.imperial.ac.uk/grantham/publications/earth-and-life-sciences/what-role-can-forests-play-in-tackling-climate-change.php

YouTube Q&A: https://www.youtube.com/watch?v=fmmfT54BxIg&feature=emb_title
Q&A: https://www.imperial.ac.uk/news/199473/qa-is-planting-trees-answer-climate/

UPDATE [1PM ET / 6PM GMT]: Thanks very much for your fantastic questions. I’m heading off for now but I’ll be checking back in tomorrow, so please do submit any more questions you may have.

And a big thanks to r/IAmA for hosting this AMA!

Comments: 94 • Responses: 17  • Date: 

MinMunMon8 karma

Are we doomed?

ImperialCollege3 karma

We are not doomed, but we are certainly faced with an enormous challenge that will need to be addressed at all levels of society, by individual, political, and corporate actors. Climate change news can seem overwhelmingly grim, and it’s easy to feel helpless in front of the magnitude of the problem. I like to remind people that we have already successfully addressed quite analogous issues – for instance, another important atmospheric gas, SO2, was a major ecological problem in the United States in the 20th century, contributing to damaging acid rain. Through a cap-and-trade scheme limiting SO2 emissions from coal plants, the US was able to dramatically reduce emissions while seeing an increase in electricity generation over the same period. Addressing climate change will likely require a similar combination of political will, technological innovation, and cooperation from industry.

sovereignbiopolitic6 karma

How can people support biodiversity and/or reforestation in their daily life?

ImperialCollege12 karma

Great question! For addressing all environmental issues (biodiversity loss, climate change, etc.) I advocate a two-pronged strategy.

The first prong is to make a manageable but consistent change in your daily life. For instance, you could use a carbon footprint calculator to identify what contributes most to your personal emissions, and take appropriate action (e.g., take one fewer flight a year, replace one meat-based meal a week with a plant-based alternative, replace an inefficient furnace in your home). In terms of reforestation, you could check the supply chain of household products to ensure that wood, palm oil, coffee, and other forest products are generated in a sustainable way. To contribute to local biodiversity, you could plant native and pollinator-friendly species in your garden.

The other prong of the strategy is to place consistent pressure on your political representatives to take strong action on environmental issues. Responsible large-scale reforestation depends on land management decisions that are strongly influenced by national policy.

We recently published a handy ‘9 things you can do about climate change’ list: https://www.imperial.ac.uk/stories/climate-action/

Ruin19806 karma

How does reforestation done wrong increase greenhouse gasses? I get that randomly planted trees disrupt the existing established ecosystem, especially if theyre not native trees. But I never heard about the increase of carbon dioxide that way.

ImperialCollege12 karma

It does seem counter-intuitive that planting trees could increase emissions or lead to further warming! But there are several ways this could happen. I’ll outline a few below:

LOSSES OF SOIL CARBON: We need to consider carbon not only in the bodies of trees, but also in the soil. Soil organic matter – the stuff that makes dirt brown or black – is full of carbon. In fact, soils contain more carbon than all terrestrial plants and the atmosphere combined! The amount of soil carbon reflects the balance between inputs (dead plant matter) and outputs (CO2 produced by decomposer microbes). Soil carbon ‘hotspots’ occur where decomposition is very slow, either because of the cold (permafrost) or flooding (peats and bogs). When trees are planted on peats, the soil must be drained for the trees to grow well. This triggers a large increase in the rate at which soil organic matter is decomposed, and the losses of CO2 from soil can be much greater than the gains of carbon in plant biomass.

CHANGES IN FIRE REGIME: Ecosystems dominated by grasses, like savannas, have evolved over millions of years to tolerate periodic fires. Planting trees too densely in these ecosystems can increase the severity of fires, and then much of the carbon captured in those trees is emitted as CO2 during a burn.

LOCAL WARMING THROUGH CHANGES IN ALBEDO: At high latitudes (close to the poles), native vegetation is short, shrubby, and covered by snow most of the year. Snow reflects the sun’s energy back into space. As it gets warmer, it’s possible to plant trees at higher latitudes – but these trees are tall and dark, so they absorb the sun’s energy, even during the winter.


Friggens et al. 2020 Glob Change Biol DOI: 10.1111/gcb.15229 https://onlinelibrary.wiley.com/doi/full/10.1111/gcb.15229

Waring et al. 2020 DOI: 10.3389/ffgc.2020.00058


lavium3 karma

Hi Doctor Waring,

You've talked about bad ways to reforest. Which organization (if any) for carbon offsets is doing things the right way? In other words, if I want to offset my emissions (assume that I'm already working to reduce what I can) or donate to a cause, which is the one that is reforesting the correct way?


ImperialCollege2 karma

Great question. There are many organizations, large and small, that do a great job conserving and regenerating forest - rather than call out a few by name, I’ll describe the questions you should ask before donating to a particular group. First, how does the organization select and obtain land on which to plant? This is critical to avoiding what are called ‘leakage’ effects, when negative environmental impacts are displaced from one area to another. As an example, if an organization purchases land that is in active use for cattle pasture, fencing it off in order to plant trees, then the farmers may have to cut down forest elsewhere to graze their cattle. So you want to look out for an organization that is well embedded in the communities where forests will be established. Second, how will the organization select which species to plant? I’d avoid any operations that use a handful of non-native, fast-growing species, as this approach is least likely to achieve environmental co-benefits. Third, what plans are in place to continue monitoring and caring for the trees as they grow? A surprisingly large number of tree-planting projects can fail if the saplings are not looked after and protected from herbivory, etc. Finally – perhaps most importantly – protecting existing forest is more beneficial to the climate than establishing new forest. So I would be careful to select an organization that acknowledges this and balances tree planting activities with strategies to promote conservation and natural regeneration.

mylifewithoutrucola3 karma

Hi Bonnie!

I have read that even though trees store carbon, cutting them releases the carbon and that only very old trees have net positive carbon storage balance.

Could you explain that in simple terms? I feel there may be shortcuts in this reasoning or at least I don't understand it fully.

ImperialCollege9 karma

The amount of carbon in a tree is proportional to its size – each new bit of wood added represents more CO2 absorbed from the atmosphere. In general, the older a tree, the bigger it gets – so old trees store more carbon than young ones. However, the rate at which a tree grows slows down with time, so carbon is accumulated the fastest in younger trees.

When a tree dies, the carbon in its wood may be released back to the atmosphere via a number of different processes. If the tree remains in the forest, the dead wood will be very slowly decomposed by microbes, converting the carbon in the tree back into CO2. (Ecologically, this process is beneficial – it releases nutrients for younger trees to grow, and creates valuable habitat for many species that specialize on dead wood). If the tree is converted into short-lived products (like tissue paper), we can assume that the product will soon decompose in a landfill. However, if the wood is used for very long-lived products (like timber framing for a home), it can continue to sequester carbon for as long as the building lasts. So, cutting a tree doesn’t always mean the carbon is immediately released back to the atmosphere – it depends what products the wood is used for.

thomas_magnum2773 karma

My 10 year old son says he wants to be an ecologist when he grows up to fight climate change. Any advice for him about what role he can play to fight climate change or becoming an ecologist himself?

ImperialCollege5 karma

You must be very proud of your son! I would tell him that some of our most powerful voices on climate action emerge from his generation – people like Greta Thunberg, Xiye Bastida, and Felix Finkbeiner have triggered big political changes.

But he can also make a real difference in his own backyard. For example, if you have a garden or lawn, leave a small patch unmowed so he can watch what plants grow there, and what kinds of insects arrive as the grasses flower and set seed. He will be doing his part to help declining pollinators, and get a chance to do some ecological research of his own! All it takes is a good set of guidebooks, an observant eye, and patience. I wish him the best of luck, and maybe I’ll be lucky enough to work with him in the future!

kisamoto3 karma

In theory, do trees live forever? By that I mean no disease, wildfires or logging etc. but left to grow. Will a tree continue growing (and sequestering carbon dioxide) forever or do trees have a limit before dying?

ImperialCollege13 karma

No, each tree species has its own characteristic lifespan, just like humans do. However, some tree species can live a really long time – some bristlecone pine individuals (Pinus longaeva) are over 4000 years old! This isn’t the norm, though – most trees have a lifespan on the order of a century or maybe two. Some recent work has also shown that the species that grow faster tend to die younger.

Reference: Brienen et al. 2020 Nat Comm 11:4241


hustonat2 karma

Is it possible to quantify the carbon-saving impact of existing forestland? What would go into such a calculation?

ImperialCollege4 karma

It is definitely possible to do so, and it’s what keeps ecologists like me busy! To assess how a particular ecosystem is influencing the climate, we would want to measure the carbon contained in vegetation (leaves, wood, and roots), and in the soil – these are termed the plant and soil carbon pools.

We would also want to measure exchanges of carbon among these pools and with the atmosphere: the rate at which CO2 is taken up by plants in the process of photosynthesis, the release of CO2 through the breathing (respiration) of plants and decomposer microorganisms, and the transfer of dead plant biomass into the soil.

From these data, we can tell how much carbon is contained in the ecosystem, and whether the ecosystem is a net carbon source (emitting CO2 to the atmosphere) or a carbon sink (absorbing CO2 from the atmosphere).

You may wonder why I didn’t mention animals at all in my answer above. Even though animals play very important roles in the ecosystems (e.g. pollinating plants and dispersing their seeds), they represent a very small fraction of the total carbon pool in the ecosystem, so we tend to ignore their contribution to flows of CO2.

text_fish2 karma

Can increasing innercity trees and designing buildings with trees in amongst the architecture play a significant role in tackling climate change, or does reforresting only work on a large scale?

ImperialCollege8 karma

Urban trees certainly do contribute to carbon sequestration. One study estimated that urban trees in the USA (lower 48 states) sequester about 23 million tonnes of carbon per year (0.228% of annual man-made emissions). However, they have many other equally important environmental benefits – they provide local cooling, reducing fossil fuel consumption for air conditioning. They also reduce air pollution and contribute to the aesthetic enjoyment of city residents.

Source: Nowak and Crane 2002 Environmental Pollution 381-389


Carlos-Spiceyweiner42 karma

Are monoculture plantations as effective at carbon sequestration and what not in comparison to a diverse forestry landscape?

ImperialCollege2 karma

The vast majority (well over 90%) of forest plantations are monocultures, reflecting the enormous degree of practical knowledge about the handful of species that are the source of most harvested wood products. However, recent work suggests that tree species mixtures can be more productive and more resilient to disturbance than monocultures.

Osuri, A., Gopal, A., Shankar Raman, T. R., DeFries, R., Cook-Patton, S., and Naeem, S. (2020). Greater stability of carbon capture in species-rich natural forests compared to species-poor plantations. Environ. Res. Lett. 15:034011. https://iopscience.iop.org/article/10.1088/1748-9326/ab5f75/meta

Paquette, A., and Messier, C. (2011). The effect of biodiversity on tree productivity: from temperate to boreal forests. Glob. Ecol. Biogeogr. 20, 170–180. doi: 10.1002/eap.1727 https://onlinelibrary.wiley.com/doi/10.1111/j.1466-8238.2010.00592.x

Liu, X., Trogisch, S., He, J. S., Niklaus, P., Bruelheide, H., Tang, Z., et al. (2018). Tree species richness increases ecosystem carbon storage in subtropical forests. Proc. R. Soc. B 285, 20181240. doi: 10.1098/rspb.2018.2090

phhill2 karma

What trees do you recommend we plant on our own gardens (UK)? Perhaps an example for a small, medium and large garden. Native, non-native, fruit or flower etc

Also, thank you for your positive contributions to the planet

ImperialCollege2 karma

Thanks for your kind words! My expertise is not in horticulture, so I am not the best person to ask about specific species recommendations. However, all trees sequester carbon so I wouldn’t feel pressure to make a selection based on how fast the tree grows, etc.

I would think about the ecological services your tree might perform, and the aspects you might enjoy, like autumn foliage or fruit production, then plant what appeals to you most. From a climate perspective, the most important thing is that the tree lives a long life, so you might as well enjoy your long-term garden residents!

jacosis2 karma

The news about the shrinking Amazon rain forests are disturbing. What would be your suggestions to preserve them? Additionally, being such a complex ecosystem, is it possible to restore the damaged rain forests by human effort?

Thank you so much for all the work you are doing!

ImperialCollege3 karma

The fate of the Amazon is certainly critical to the whole climate system, and to the future of our planet’s biodiversity. Ecologists have shown that forest fragmentation (chopping the forest into smaller, disconnected ‘chunks’) increases the vulnerability of trees to other disturbances associated with climate change, including drought and wildfires. So, keeping the forest intact is critical, although doing so will depend on very complex political and economic drivers.

The good news is that tropical forests can often recover quite well from disturbance, if left to their own devices. In Central and South America, for instance, tree biomass reaches 90% of pre-disturbance values within about 65 years (although biodiversity can take much longer to fully recover, if at all). In certain cases, where the forest has been severely disturbed and the soil degraded, ‘assisted regeneration’ or direct tree planting might be necessary to help the ecosystem recover.

Reference: Brando et al. 2014 PNAS 111: 6347-6352. https://www.pnas.org/content/111/17/6347

Poorter et al. 2016 Nature 530: 211-214

Rozendaal et al 2019 Science Advances https://advances.sciencemag.org/content/5/3/eaau3114

CultAwarenessNetwork1 karma

I'll give you an scenario and you tell me: some land in Argentina, the owner wants to create a forest. What would be the best trees to use for that endevour?

ImperialCollege6 karma

First, I would ask whether you even need to decide what to plant! If the ecosystem’s natural state is forest, it can probably recover within a fairly short period of time without any direct human intervention, unless the scale of deforestation was very large or the soil was very degraded.

Natural regeneration is preferable to direct planting – it’s cheaper and the forest that grows back will likely be more diverse. However, if nature needs a helping hand, then I would suggest planting a mixture of tree species which are native to the area. Work has repeatedly shown that tree species mixtures are more resilient to disturbance than monocultures.

Reference: Liang, J., Crowther, T. W., Picard, N.,Wiser, S., Zhou, M., Alberti, G., et al. (2016). Positive biodiversity-productivity relationship predominant in global forests. Science 354:aaf8967. https://science.sciencemag.org/content/354/6309/aaf8957

radiant_coconut_21 karma

Did global warming pass the point of no return, or do we still have time to fix it?

ImperialCollege6 karma

There will never come a point where we should abandon efforts to address climate change. Any actions we take to mitigate the problem are worthwhile, given the risks to human life and the rest of the biosphere increase exponentially with continued warming. However, the longer we go without making serious reductions in emissions, the more challenging it will be to rapidly transition our society off fossil fuels.

This is why it’s important to pursue an ‘all of the above’ strategy where emissions reductions are coupled with negative emissions technologies to remove CO2 from the atmosphere – and reforestation should be included in that portfolio of strategies.

kisamoto1 karma

Do you think the capacity of reforestation is enough to stop and reverse the climate crisis or will we need to employ other - possibly man-made - efforts such as direct air capture and storage to keep things under control?

Thanks for all the work you do.

ImperialCollege3 karma


Great question! In order to stay below 2°C warming, we must drastically reduce emissions; there is no way that we can continue ‘business as usual’ patterns of fossil fuel use and avoid catastrophic climate change. But, as emissions ramp down, we must also employ what are called ‘negative emissions technologies’ (NETS) to remove CO2 from the atmosphere. Reforestation – whether by planting trees or allowing forests to naturally regenerate – is one of these NETS.

Our best estimates suggest that tree planting could remove up to 100 GtC from the atmosphere – by comparison, human activities generate about 10 GtC per year. So, reforestation would be a big help in easing our transition to a low-fossil fuel economy. And, in comparison with other NETS like direct air capture and storage, forest regeneration is much less expensive and provides other important benefits, like protection of biodiversity. So, reforestation should be a key part of the climate change solution – but, on its own, tree planting can’t solve climate change.

kisamoto1 karma

Interesting - thanks!!

Is the 100 GtC sequestration annually or in total if maximum tree coverage was used? If in total, how long would we take to reach that point?

ImperialCollege3 karma

The 100 GtC figure is how much would be stored at forest maturity. It would take several decades, perhaps up to a century, to reach that point.

kisamoto1 karma

With the growing interest from corporations to "offset" their footprints via tree planting, do you think is a sustainable method or will it encourage mono-cultures and actually increase deforestation (to make money by planting trees to be sold as offsets)?

ImperialCollege4 karma

There is certainly reason to be concerned that poorly planned reforestation projects can do more environmental harm than good. There are several ways this could occur.

First and foremost, no form of carbon offsetting will ever be as effective as a reduction in emissions. Thus, the perception that planting trees can somehow negate fossil fuel emissions and allow ‘business as usual’ is a dangerous one. However, if emissions cannot be avoided, then tree planting programs must meet the fundamental criteria of a good offsetting scheme: additionality (the carbon wouldn’t have been captured without the offset) and permanence (the carbon will remain locked away from the atmosphere at century-long timescales).

Forests are complex living systems, and they face many environmental challenges – drought, warming, pests, destructive storms. Ensuring forest permanence is a high bar, and requires a lot of care and thought on behalf of forest managers. I would be wary of any reforestation project that places a lot of focus on the start of the process (the number of trees and the process of planting them) rather than on the structure and function of the whole forest ecosystem over time.

Finally, we must avoid generating ‘perverse incentives’ to destroy existing habitat to plant new trees. This is really environmentally destructive and does not result in a net enhancement of CO2 uptake by land ecosystems.