Monday 4 January 2016

Afforestation and Reforestation: a mitigation of climate change impacts?

Deforestation has occurred for thousands of years, forests have been cut down for industries, homes, wood fire, logging, cattle and ship building (Brown et al 2014).  Deforestation releases CO2 in the atmosphere and stops CO2 absorption, leading to an increase in climate change impacts.  However, the restoration of trees could be used as a CDR geoengineering method to reduce CO2 emissions, as urged by COP21 agreements to reduce carbon sinks (Bailey 2015).  This can be undertaken by afforestation and reforestation.  Afforestation is the human growth of trees on land, which has not been forested in the past.  Reforestation is the human growth of trees on land, which was previously forested (Figure 1) (Caldeira et al 2013).  The natural absorption of CO2 is stored in trees and reduces atmospheric CO2, hence this geoengineering process could reduce climate change impacts. 

Figure 1: Afforestation and Reforestation Processes
Source


Afforestation and Reforestation Impacts

The increase in tree plantation may be very beneficial as large amounts of CO2 will be absorbed from the atmosphere, provided trees are planted in the correct areas.  Albeit reforestation and afforestation absorb large amounts of CO2  they may negatively impact aquifers, due to high demands of water for tree growth.  Large amounts of trees require large amounts of water consumption for irrigation.  Moreover, evapotranspiration levels will increase leading to an increase in aquifer discharge (Heck et al 2015).  Hence although atmospheric CO2 may be absorbed and reduce climate change impacts, aquifers may deplete due to not enough recharge occurring in arid areas.  Moreover, ecosystems may change due to changes in water availability.  In China, there were abrupt changes in ecosystems due to a high level of tree planting and a low rainfall occurrence of 400-500mm per year (Brown et al 2014).  This lead to a degradation of ecosystems due to not enough water being supplied, questioning the degree of benefits of afforestation and reforestation.  Controversially, precipitation levels should increase by 1-2%, as bare soil will be replaced by trees, for instance in Nigeria.  Abiodum et al (2013) suggest that extreme rainfall events may increase in Nigeria due to a slower monsoon occurrence created by afforestation.  It is suggested that an increase in afforestation may have regional positive impacts of increased rainfall events, yet global warming may increase in regions around the afforested area.  Additionally, there may be an increase in droughts in semi-arid regions, as monsoons decrease and delays in air moisture occur in the region, hence delaying rainfall events.  It is evident that precipitation events will change due to afforestation and reforestation.  However, it is unclear to what degree this may be beneficial or disadvantageous for the hydrological cycle.

Although afforestation and reforestation can reduce atmospheric CO2  a large supply of trees will also demand a high supply of nutrients.  As afforestation and reforestation will occur at large scales, soil depletion will take place (Heck et al 2015).  It is estimated that 50-150kg N/ha/yr will be required for herbaceous plants (Heck et al 2015).  Thus a Nitrogen(N) fixation will be required to replenish the soils.  It is likely that fertiliser use will increase due to an increase in nutrient demand for soil replenishment, hence increasing the costs of afforestation and reforestation (Heck et al 2015). Therefore, questioning the degree of this geoengineering process being advantageous.

Moreover, another negative impact that may occur with afforestation and reforestation may be a decrease in albedo (i.e. solar reflectance) and an increase in roughness to a higher coverage of land with trees (Heck et al 2015).  Hence, if afforestation and reforestation occur on a large-scale, this may increase global temperatures at a local and global scale.  Thus, it may not be as influential as desired as a geoengineering process due to not reducing climate change impacts . 

Furthermore, there may be shifts in biodiversity, a main concern regarding environmental sustainability issues.  To minimise this impact it is essential when planting trees in an area to maintain the local species (Heck et al 2015).  Therefore plantation management is essential to ensure an equal growth of all species in a habitat (Heck et al 2015).  It is very difficult to ensure that no species outcompetes another species causing a shift in food webs or the natural biodiversity of an area.

Lastly, there may be an issue when regarding space.  If trees are planted at large scales for afforestation and reforestation, this creates an opportunity cost which enables the growth of agriculture or urban areas (Heck et al 2015).  Hence, this may lead to a degradation of economic growth and food security (Caldeira et al 2013).  Thus questioning to what degree people and governments would be willing to ‘sacrifice’ land for this geoengineering process to reduce CO2 levels.

Conclusion

Afforestation and reforestation can be considered an easy process to implement, with low costs and relatively low risks due to less technology being required and being such a familiar process.  However, this process may occur at a fast rate and may degrade ecosystems and the water cycle.  Hence it is questionable to what degree this process would be beneficial as a CDR method.  I believe the growth of trees is always beneficial.  However, I think the human induced growth of trees (especially in large-scales) would be very difficult to implement successfully, due to water and biodiversity management issues.  I look forward to you opinion on this CDR method.

12 comments:

  1. Hi Maria, very interesting and detailed post as usual! I'm definitely a supported of afforestation as well, though as you say, we need to be careful to avoid issues with water and biodiversity management. From a biodiversity point of view, I would think that there would need to be consideration of which tree species are used for afforestation and that there would need to be a concerted effort to create a heterogeneous environment, rather than one dominated by a single species. This heterogeneous community may be less effective at removing carbon than a plantation of hardwood trees for example, but would be far superior from an ecological viewpoint. Do you know what species they intend to use for afforestation mostly?

    ReplyDelete
    Replies
    1. Hi Ben! Thank you for your comment! I totally agree that a heterogeneous environment is better in terms of biodiversity. You also raise a very interesting issue in regards to what species are grown. From what I gathered, it is suggested that with afforestation they would try to grow tree species that are native to the area, creating a heterogeneous environment similar to forests nearby. However, I am unsure to what degree this has been implemented in attempts of afforestation. I assume it may vary depending on the knowledge about an area. I hope this answers your question!

      Delete
  2. Interesting post! I do think afforestation is a suitable mitigation technique for environmental impacts such as increasing CO2 levels (and it also is effective against natural hazards such as mass movement and flooding!), but I do worry slightly about ecological changes that may be inflicted from the land change - especially if the trees are homogeneous as Ben says! As species are so specialised to live in certain environmental conditions, don't you think that afforestation could be a danger to these by changing the habitat?
    However I think that reforestation is a great idea and can't seem to find any faults with that strategy - we need more of it in the UK!

    ReplyDelete
    Replies
    1. Hi Celia! Thank you for your opinion it is much appreciated. I believe afforestation and reforestation are a good idea. However, I also lean more towards reforestation as afforestation could have many negative impacts on the environment and also the hydrological cycle. It is expected that the next wars will be on water and with increasing populations and climate change impacts people are demanding far more water, hence questioning to what degree water will be available for afforestation/reforestation and also people.

      Delete
  3. Hi Maria great post! We usually always see planting trees as positive so it was really interesting to read the negative impacts they can have. Do you think reforestation is better and has less negative impacts as its planting where trees were before and so its almost as if your changing the land back to its natural state? As afforestation seems more likely to cause changes to local environment (nutrients, precipitation) as you're changing the land from one without trees to almost a new habitat. So do you think one is better or more viable than the other? And do you think you need both re- and af-forestation to help mitigate climate change?

    ReplyDelete
    Replies
    1. Hi Vasu, thank you for the comment and sorry for the late reply! I agree with you it may be better to restore trees to their natural state, as it creates pre-industrial conditions and it does not encourage any invasive species. So yes I agree with you that reforestation is far more affective than afforestation. However, for this geoengineering process to be effective I believe that reforestation alone would not be enough, as many areas are likely to be urbanised or unable to grow forests at present. Hence afforestation is also needed although it may not be as beneficial! I hope this answer's your question!

      Delete
  4. An interesting post as always! In response to the earlier comment by Ben and your reply, I feel that as well as considering the vegetation composition nearby, there also needs to be a comprehensive understanding of species that migrate in and out the area. Like wind carries light seeds etc species like migratory birds and mammals often play a role in exchanging plants across. If even one of them are, for example, connected to remote island where distinct, vulnerable rare plants are found, it is necessary for us to consider competitive relationship between the species and those that will be planted in afforestation area. However, I also think that it is still very difficult to draw a full picture of such an extensive relationship between species. Here I'm wondering whether afforestation and reforestation can also be undertaken on 'experimental basis' like other geoengineering methods (since the certainty of the success and negative effects on the surrounding ecosystems cannot be predicted by 100%)?

    ReplyDelete
    Replies
    1. Hi Satomi! Thank you for your comment and sorry for the late reply! I completely agree with you, not only vegetation should be restored to similar composition nearby., I also believe that the habitat in general, regarding birds mammals and even insects is essential to have an effective restoration and reducing any negative impacts that may be induced. But yes, I also agree with you, it is very difficult to take all these factors in consideration, however careful monitoring of competitiveness of species and avoiding invasive species is essential.

      Moreover, I also completely agree that afforestation and reforestation should be implemented on an experimental basis for better understanding. However, in many nations afforestation and reforestation tends to occur without people thinking of the negative impacts that may be induced. It is quite a common geoengineering method, as the impacts are regional and in many cases people may not see it as a geoengineering process. If I am not mistaken, I believe I read an article a few days ago, when researching the topic that afforestation measures were undertaken in Nigeria (I think). If you are more interested in the topic I could try and find the article I was reading. I hope this helps you!

      Delete
    2. Hi Maria! Thank you very much for your reply! I completely agree with you over the fact that afforestation and reforestation are often associated with the negative impacts. I feel that their incentives for greening their land are sometimes not just for climate change mitigation but for resource use (e.g. fuel wood and food). Perhaps, these methods are better integrated with a project which economically & financially benefits the locals? If you still have spare to find and share the article with me, I'll really appreciate it! :)

      Delete
    3. I will look into the article in 1-2 days and let you know if that is okay :) in regards to afforestation and reforestation, I agree with you that this method could be more integrated regarding a project better integrated for economic and financial benefits, which may also have some positive impacts on the environment!

      Delete
  5. Hi Maria, a very interesting yet easy to read post. I can see the advantages of local heterogeneous forests but just wondered if trees with silver/white leaves or modified (possibly by genetic engineering)would that potentially improve the albedo, thereby reflecting radiant heat into the climate change scenario.
    regards
    Sheila Humphreys ( Open University student SK 201)

    ReplyDelete
    Replies
    1. Hi Sheila, Thank you for reading my blog post and apologies for the late response. To answer your questions, I do believe that silver/white leaves would possibly improve the albedo, thus increasing reflection of the sun and possibly reducing heat. I believe it would have a similar impact to painting roofs white, another topic I talked about whilst analysing geoengineering. (http://geoengineeringinquiries.blogspot.co.uk/2015/11/colouring-roofs.html) As for modified trees I can not have an opinion as I have not researched the topic, yet it could be possible, even though the costs behind genetically modified trees may be considerably higher than desired.

      Delete