On the Bonn Challenge: Tree Restoration and the Climate Emergency

“Plant a tree and save the world” is the short version of the Bonn Challenge of 2011 and the UN Decade of Ecosystem Restoration 2021-2030 (Stanturf and Mansourian 2020), and so here we are with a major ecological challenge for the decade we have just started. Planting trees around the world to restore 350 million hectares of degraded land is the goal, and it is a challenge that ecologists must think clearly about to avoid failure of another grand scheme.

Restoring ecosystems is not easy as we have already learned to our dismay. What began as a relatively simple restoration of old fields used in agriculture, a few hectares of ploughed ground surrounded by forest or grassland, has now morphed into very large areas devastated by forest fires, insect outbreaks, or drought. The largest forest fires in Arizona prior to the year 2000 were 20,000 ha, but after prolonged drought by 2020 they have reached nearly 300,000 ha (Falk 2017). The larger and more severe the fire, the greater the distance seed must disperse to recolonize burnt areas, and hence the recovery from large fires differs dramatically from the recovery from small or patchy fires.

I concentrate here on forest restoration, but always with the caveat in mind that the trees are not the forest – there are a plethora of other species involved in the forest ecosystem (Temperton et al. 2019). The restoration of forest landscapes is driven by the estimate that forest originally covered about 5.9 billion ha of the Earth but at the present time there is about 4 billion ha of forest remaining. Restoration of degraded ecosystems has always been a good idea, and this program can now be tied in with the climate emergency. New trees will remove CO2 from the air as they grow so we can score 2 points with every tree we plant (Bernal and Pearson 2018).  

The scale of plans for the UN Decade of Ecosystem Restoration 2021-2030 are challenging and Stanturf and Mansourian (2020) provide current details country by country. For example, Brazil a country of 836 million ha has pledged to restore 12 million ha (1.44%), with some countries like Spain and Russia so far not pledging any Bonn Challenge restoration. The take-up of actual restoration is uneven globally. The USA has committed to restore 12 million ha to the Bonn Challenge, but Canada has made no formal commitment, although the federal government has proposed to plant 2 billion trees during this decade to counteract climate change.  

Many problems arise with every ecological restoration. Not the least is the time frame of the recovery of damaged ecosystems. Forests recover slowly even when carefully tended, and 100 years might be a partial target for temperate forests. For North American west-coast forests a 400+-year time frame might be a target. Most private companies and governments can not even conceive of this scale of time. For those who think everything should work faster than this, Moreno-Mateos et al. (2020) report a large sample of >600 restored wetlands that recovered to only 74% of the target value in 50-100 years. Schmid et al. (2020) found that the microbial community of a lignite mine in Germany had not recovered to the control level even after 52 years. Ecological time does not always conform readily to industrial time.

Other constraints blur the grand global picture. Restoration with trees should not be done on tropical grasslands because of their inherent biodiversity values (c.f. Silveira et al. 2020 for excellent examples), nor can we restore trees on rangeland that is used for agricultural production lest we engage in robbing agricultural Peter to pay forester Paul (Vetter 2020). These important ecological critiques must be incorporated into country-wide plans for reforestation whose primary aim might be CO2 capture. Again the devil is in the details, as Vetter (2020) clearly articulates.  

The Bonn Challenge remains ongoing, waiting for another review after 2030. Who will remember what was promised, and who will be given the awards for achievements reached? What quantitative goals exactly have been promised, and what happens if they slip to 2050 or 2070?  

Bernal, B., Murray, L.T., and Pearson, T.R.H. (2018). Global carbon dioxide removal rates from forest landscape restoration activities. Carbon Balance and Management 13, 22. doi: 10.1186/s13021-018-0110-8.

Bonnesoeur, V., Locatelli, B., Guariguata, M.R., Ochoa-Tocachi, B.F., Vanacker, V. et al. (2019). Impacts of forests and forestation on hydrological services in the Andes: A systematic review. Forest Ecology and Management 433, 569-584. doi: 10.1016/j.foreco.2018.11.033.

Falk, Donald A. (2017). Restoration ecology, resilience, and the axes of change. Annals of the Missouri Botanical Garden 102, 201-216, 216. doi: 10.3417/2017006.

Moreno-Mateos, D., et al. (2020). The long-term restoration of ecosystem complexity. Nature Ecology & Evolution 4, 676-685. doi: 10.1038/s41559-020-1154-1.

Silveira, F.A.O., Arruda, A.J., Bond, W., Durigan, G., Fidelis, A., et al. (2020). Myth-busting tropical grassy biome restoration. Restoration Ecology 28, 1067-1073. doi: 10.1111/rec.13202.

Stanturf, J.A. and Mansourian, S. (2020). Forest landscape restoration: state of play.
Royal Society Open Science 7, 201218. doi: 10.1098/rsos.201218.

Temperton, V.M., Buchmann, N., Buisson, E., Durigan, G. and Kazmierczak, L. (2019). Step back from the forest and step up to the Bonn Challenge: how a broad ecological perspective can promote successful landscape restoration. Restoration Ecology 27, 705-719. doi: 10.1111/rec.12989.

Vetter, S. (2020). With Power Comes Responsibility – A rangelands perspective on forest landscape restoration. Frontiers in Sustainable Food Systems 4, 549483. doi: 10.3389/fsufs.2020.549483.

3 thoughts on “On the Bonn Challenge: Tree Restoration and the Climate Emergency

  1. Peter Day

    Nice comment about recovery after larger fires differing to that from small ones. It’s not just heat or intensity, but size as well.

  2. Nigel Gardner

    I teach IB Environmental Systems and Societies, have an original background in Agro-forestry, work in Asia, but am British and live in the South of France right in the middle of the Gariggue. So I get to see a lot of intentional restoration and un-intentional restoration.

    Yes the aspect of “which biodiversity” is often over looked and many of my students enter my course believing that planing trees is the answer to everything. From the perspective of the Gariggue, the loss of traditional grazing and cutting with rural depopulation is leading to secondary succession in many areas to mixed evergreen and deciduous oak. Much much lower biodiversity….because we have “gone back to nature”.

    The element that man has played in creating habitat space for diversity is so often overlooked.

    As always a great article that my students always enjoy picking to pieces and challenging their assumptions..

  3. Rachel Holt

    The Bonn Challenge is good – but we don’t have time to wait for these trees to sequester carbon.

    Step 1 is to prevent harvest of primary / old growth forest – which releases huge amounts of carbon from soil and standing biomass. This has an immediate effect on preventing increased GHG release.
    Step 2 is to restore degraded forest lands, which over time will contribute to lower GHG levels.

    Time matters. The focus needs to be also on keeping standing forests standing.


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