Reafforestation and afforestation refer to to activities where trees are established on lands with no forest cover. The concept of reafforestation is usually used in reference to areas where there was recent forest cover. Reafforestation and afforestation activities, as well as existing forests, can help to reduce the occurrence and intensity of floods.
Reafforestation and afforestation refer to to activities where trees are established on lands with no forest cover. The concept of reafforestation is usually used in reference to areas where there was recent forest cover. For areas without an historical record of forest cover, the planting of trees is referred to as afforestation (IPCC 2000). The activities could also include making a conscious decision to maintain forests on lands that would otherwise be handed over for other types of land development, as part of targeted water management interventions.
Reafforestation and afforestation activities, as well as existing forests, can help to reduce the occurrence and intensity of floods. Many communities are already harnessing the water benefits of forests. Forest areas in the upper watersheds can help retain water and stabilize slopes, thereby reducing the risks and disaster caused by storms. Deforestation is a major cause of land degradation and soil erosion. With soil erosion, its ability to store and retain water diminishes, which contributes to higher risks of flooding, as the soils are no longer able to reduce the rate and volume of runoff that occur in the event of heavy rainfall and storms. While increase in forest cover is unlikely to significantly affect outcomes of strong flood events in big watersheds or strong, low frequency floods in smaller rivers, it can have a high impact on reducing minor to moderate floods in relatively small and medium-sized watersheds.
Trees intercept rainfall and increase infiltration, and the ability of soils in forest areas to store more water and release it through evaporation helps in regulating the water quantity during extreme weather events (CNT & American Rivers 2010). However, intensive reafforestion/afforestation activities may reduce the local total annual runoff and groundwater recharge due to increased water loss through evapotranspiration. Thus, there is a trade-off between a more constant supply of water and a reduction in total available water volume.
Planting tree species particularly adapted to the local climate and hydrology can help to predict the impacts on groundwater recharge with higher certainty and thus provide greater choices of the most appropriate interventions.
Forests can also reduce the likelihood (or frequency) of landslides, mudflows and avalanches, which can cause extensive damage to infrastructure and inhabited areas vulnerable to floods (EC 2011).
Establishing or conserving forests (but also promoting other sustainable land use activities in the watershed) can contribute to improving water quality. Forests improve water quality by reducing sediment in water bodies and trapping or filtering other water pollutants. Along the shores of water bodies, the roots help to stabilize banks against erosion. Forest cover is also an effective way to prevent other pollutants from draining into the watercourse and regulating sediment flow, if distributed throughout the upstream watershed e.g. drinking water supply reservoir (FAO, 2008). Such measures to ensure a high quality drinking water supply have already been put in place in a number of countries across the globe - a third of the world’s hundred largest cities rely on forested protected areas for their drinking water. In fact, well- managed forests often provide clean water at costs lower than those of treatment plants (TEEB 2009).
Forests and riparian buffers in particular, also help to mitigate thermal pollution by providing shade to the streams (see Riparian buffers).
Forests are among GI solutions with the greatest environmental and socio-economic co-benefits. In addition to the immediate benefits that forests have in regulating water quantity and quality, they can also function as carbon sinks, increase pollination for nearby agricultural fields, improve air quality, regulate local climate (including cooling) and help preserve biodiversity. For example, a study from Cascine Park in Florence, Italy, shows that the urban park forest maintained its ability to remove air pollutants over a period of 19 years, removing about 72.4 kg per hectare per year, despite some tree losses due to logging and extreme weather events. Harmful pollutants removed included O3, CO, SO2, NO2, and particulate pollutants, as well as CO2 (TEEB 2011). Increasing forest cover can also open up possibilities for alternative livelihoods and income opportunities through agroforestry, ecotourism and a range of other forest products.
The primary costs of establishing forests include the cost of land, purchasing seeds or saplings and tree planting (Foster et al. 2011). Reafforestation can also take place through natural regeneration. Just like any infrastructure investment, opportunity costs and ongoing maintenance needs should be considered, in addition to the initial cost of investment in GI. The costs of reafforestation/afforestation or forest conservation activities are usually directly dependent on existing alternatives for land use and vary greatly depending on the location.
For example, lands in or near major urban centres are likely to be more costly to use for such purposes than those in more remote areas due to demands for competing land uses. They may also yield larger benefits. In the city of San Diego, USA, it was estimated that in 2002 the stormwater retention capacity of the urban forest was 2 million cubic meters. If this forest were lost, it is estimated that providing the same benefit through built infrastructure would cost approximately USD 160 million (American Forests 2003).
An important cost consideration is also time lag, which can be substantial for reforestation/ afforestation projects, increasing the overall project costs (TEEB 2009). This makes a particularly strong case for forest conservation as one of the priority interventions to maintain water services provided by trees. Trees take time to mature and be able to deliver the full range of services, in contrast to grey infrastructure, which begins operation as soon as it is created. This can also cause afforestation activities to be evaluated negatively, compared to traditional solutions. Forests are also exposed to a number of less predictable risks that can compromise delivery of water benefits and require additional investment – e.g. wildfires, change in ecosystem services due to climate change and pests. The exact costs of GI activities within afforestation or forest conservation are location specific and dependent on a wide range of variables. The data on exact costs is sparse, but a recent database with 127 GI projects from the European Union found that the costs for reafforestation/afforestation projects ranged from USD 1,300 to USD 2,500 per hectare of forest (Naumann et al. 2011).
Where standing forests are a source of delivery of these benefits, and where land development activities threaten the continuity of these, an “action” for these communities to take is to conserve strategic networks of those forests that are most important for the provision of water. Part of that means making sure timber operations are conducted in a way that don’t degrade water resources, choosing alternative land development options, and making a strategic decision to forego potential investments returns to preserve forest cover and associated ecosystem service delivery. Water funds, operating based on the principles of payment for ecosystem services (PES), is just one example of possible approaches to preserve forest ecosystem services.
CNT & American Rivers (2010). Center for Neighbourhood Technology and American Rivers, The Value of Green Infrastructure. A Guide to Recognizing its Economic, Environmental and Social Benefits.
EC (2011). European Commission, Directorate-General Environment, Towards Better Environmental Options for Flood risk management, DG ENV D.1 (2011) 236452.
TEEB (2009). TEEB – The Economics of Ecosystems and Biodiversity for National and International Policy Makers.
TEEB (2011). TEEB – The Economics of Ecosystems and Biodiversity (2011). TEEB Manual for Cities: Ecosystem Services in Urban Management.
Foster, J., Foster, H., Lowe, A., and Winkelman, S. (2011). The Value of Green Infrastructure for Urban Climate Adaptation, The Center for Clean Air Policy, (February).
American Forests (2003). Urban Ecosystem Analysis, San Diego, California. (July).
Naumann, S., McKenna, D., Kaphengst, T., Pieterse. M. and Rayment, M. (2011). Design, implementation and cost elements of Green Infrastructure projects. Final report to the European Commission, DG Environment, Ecologic Institute and GHK Consulting. Available from http://ec.europa.eu/environment/enveco/biodiversity/pdf/GI_DICE_FinalReport.pdf.