Nature reacts on the human impacts with catastrophes, heat, loss of flora and fauna, increasing sea levels as well as deforestation. Forests cover a great deal of the earth's land surface. 31% is covered by forests and trees (Emily E. Adams, 2012). The global net forest loss from years 2000 to 2010 was about 5.2 million hectares per year, the highest one being noted in the 1990s (Emily E. Adams, 2012). In the past centuries, the earth climate changed drastically. The highest deforestation can be found in the tropical rainforests of South America and Africa. But forest and tree loss isn’t necessarily deforestation (Institute, W.R. (n.d) Global Forest Watch). It is only deforestation when the forest and tree loss is very high. Factors such as climate change and the increase of greenhouse gases, desertification, soil and coastal erosion, fewer crops, and flooding, increase deforestation (The Pachamama Alliance). Deforestation describes the loss of trees in a forest ecosystem. In order to sustain the ecosystem, measures have to be taken. One of them being reforestation in a forest or afforestation on a previous soil used for agricultural purposes. Selective logging, road constructions, climate change, harvesting wood for fuel and industrial uses diminish the forests' health and well-being (Emily E. Adams, 2012). Especially climate change influences the health of the forests a lot. The dry summers lead to tree depletion.
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The overly excessive use of agriculture plantations for goods, for common and daily use such as cash coffee, soy, and palm oil harm the forests and increase tree depletion due to fewer water resources. Important nutrients are missing in the soil to guarantee healthy tree growth (Bradford, 2018). Generally, forest depletion and loss mostly show negative effects on environments. Trees reduce high air pollutions and provide oxygen. Trees purify the air by taking up pollutants, such as carbon dioxide, ozone, nitrogen, ammonia, sulphur, etc, which they absorb through their pores and stomata (Top 22 Benefits of Trees, 2018). Forest biodiversity enhances ecosystem functions such as water uptake, tree growth and pest resistance (Ellison et al., 2017). The questions that come with taking those measures are what do these practices do with the soil structures and characteristics, are soil properties being altered and how much water do trees in early growth stages need in order to grow healthy. The amount of water required for reforestation methods varies by species, climate, topography, etc., which makes it nearly impossible to answer this question. The water use requirements that trees have in early growth stages are correlated with the size and productivity of the tree species. “Healthy land is a natural storage for freshwater. If it is degraded, it cannot perform that function.” “Scaling up land rehabilitation is essential for building drought resilience and water security.” (United Nations; Land and Drought (n.d.)).
Figure 1. Describes in a very simple way the hydrological cycle of a forest. Evaporation leads to cloud forming and therefore let rain fall, whereas drier climates that are warm and sunny have a strong sensible heat flux with less evaporation and water availability
But why does deforestation occur in such drastically ways and what does deforestation do with the Earth's hydrological cycles? Forests regulate water supplies (Ellison et al. 2017). Trees are multifunctional, complex organisms that keep the ecosystem stable; this they do by taking up water, purifying the air through photosynthesis and cooling the earth climate globally as well as locally by "releasing water vapour into the atmosphere.” (Pearce et al. 2018), (see Figure 1). Deforestation in tropical forests may affect the climate much greater than anticipated by disrupting and altering the hydrological water cycle. This may result in “posing a substantial risk to agricultural” land-use and practices (Pearce et al. 2018).
Wolosin, M., & Harris, N. (2018, June). TROPICAL FORESTS AND CLIMATE CHANGE: THE LATEST SCIENCE. http://wriorg.s3.amazonaws.com/s3fs-public/ending-tropical-deforestation-tropical-forests-climate-change.pdf
“Healthy forests release volatile compounds that have a cooling effect” on the earth climate (Pearce et al. 2018). Those volatile compounds (VOCs) such as isoprene and terpenes; when these compounds are oxidised by e.g., light energy, ozone or other free radicals they form compounds, unknown as secondary aerosols, that act as nucleation points for moisture to form droplets (clouds). This can be observed within cloud forests. Cloud forests are tropical forests growing on high altitudes/elevations in mountain regions up to 8.000ft. The forests consist of “stunted old trees covered in moss and ferns” (USDA Forest Service/Southern Research Station. (2019, April 17)). The trees, plants and their lichens live in symbiosis; “the lichens intercept water vapour that can supply 75% of the stream water in drier places”(USDA Forest Service/Southern Research Station. (2019, April 17)).
Cloud forests are one to Earth's most biologically diverse ecosystem, providing many species with a habitat. It has been observed that could forest are likely to be negatively affected by climate change, especially greenhouse gases, leading to cloud forest species not being likely to adapt to cloud immersions. “Cloud immersion may increase because of an increase in humidity that will increase over warming oceans, implying thicker clouds forming at lower elevations”(USDA Forest Service/Southern Research Station. (2019, April 17)). The opposite effect of an increase in cloud immersions would be a decrease of immersions based on rising temperatures over the mainland leading to clouds traveling further elevations before cooling off to form clouds. The outcome for cloud forests is unknown, but associated with greenhouse gas emissions altering climatic conditions and therefore leading to alterations in cloud forests and changes within the hydrological cycle of tropical forests. Altitudes can indirectly affect water use efficiency e.g., temperature (climate), quality of light, soil type, etc. but typically at high altitudes, growth rates start to decline, but a high VPD (Vapour Pressure Deficit) would have a greater effect on water use.
Kubin, E., Křeček, J., & Palán, L., (2017) summarises a study on effects of forest practices on Finland's water availability and resources, describing Finland ranked as one of the top countries for water scarcity (Water Poverty Index), due to its climatic occurrence. Finland is at a high risk of “eutrophication of shallow surface waters”(Kubin, E., Křeček, J., & Palán, L. (2017)) associated with the snow melting processes in spring leading to half of the annual runoff. “Forest practices affect forest ecosystems by the cycling of energy, water, and nutrients.”(Kubin, E., Křeček, J., & Palán, L. (2017)) . They can affect the environmental services a forest provides in “boreal head-waters” and water availability (Kubin, E., Křeček, J., & Palán, L. (2017)) .
Land-use changes of recently used agricultural soils disrupt the hydrological cycle, altering rainfall and evaporation, and therefore alter the runoff responses. Afforestation methods are beneficial to regenerate biodiversity, soil properties and the availability of nutrients and water within the soils (Sahin and Hall, 1999).
There is still a lot of controversies whether afforestation methods are considered beneficial to the ecosystem and its services or not as afforestation produces peak discharges. “Seasonal differences were observed suggesting different runoff generating processes, and […] the effect of forest cover on peak discharges became less important as the size of hydrological event increased.”(Nadal-Romero et al., 2016). “Compared with a natural forest, the afforested area recorded greater flows and shorter recession limbs. Afforestation reduces the water yield and the number of floods compared with non-vegetated areas and abandoned lands.”(Nadal-Romero et al.,2016). Afforested soils show a certain ability to hold water in high amounts. Tree and plant roots increase the “permeability” of the soil “favouring infiltration processes in the afforested area.” (Nadal-Romero et al., 2016; Lana- Renault, N., Nadal-Romero, E., López-Vicente, M., & Ojanguren, R. (2018, January)). This is because the roots or vegetation generally “lock” soil, water, and sediments away to prevent soil erosion.
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Afforestation can be associated as a man-made conversion of an ecosystem from agricultural landscape to a forest- both ecosystems would have very different properties, which may be related to an (unnatural) regime shift within either the agricultural or forest ecosystem. The definition of afforestation is that it can restore forests and prevents the biome to be disrupted by soil erosion and flooding (Reid, 2019). Furthermore, afforestation delays and reduces the size of floods (Reid, 2019). Reforestation and afforestation can have negative impacts on species diversity and “related ecosystem services” (Ellison et al., 2017). It is important to have a mixed-species forest to obtain not only the species richness but also to have a more productive forest that is more resilient towards change and that provides “reliable water-related services” (Ellison et al., 2017), but also to prevent the negative effects of a monocultural land or forest. Monocultures can be prone to drought and have a lesser resilience towards pests and invasive species (Ellison et al., 2017).
Evidence can be found that forests either increase water availability (e.g. cloud forests or if they are being used for flood mitigation) or equally that trees reduce water availability- it depends on the situation and the environment surrounding the ecosystem. Ultimately it is very difficult to generalise as both the tree species and the environment they grow in can be very different and result in different outcomes. Ilstedt et al.,2016 describes that groundwater recharge is maximised at an intermediate tree density and cover, meaning moderate tree cover may lead to an increase of groundwater recharge because of “various tree management options” that “can improve groundwater resources” (Ilstedt et al.,2016; Hernández-Morcillo, M., Burgess, P., Pantera, A., & Mirck, J. (2017, November); Ilstedt et al. (2016, February 24)). Afforestation can result in a loss of biodiversity in poorly monitored, due to modification and regime shifts within the forest or in this case agricultural ecosystem, possibly introducing non-native, invasive species the pose a threat to the existing species richness/diversity (Reid, 2019).
“Soil infiltration is improved by trees through litter inputs and roots, promoting higher activity of soil animals. This results in increased soil hydraulic conductivity due to enhanced organic matter content, topsoil aggregation, and macro porosity.”(Ilstedt et al.,2016; Ilstedt, U., Tobella, A. B., Bazié, H. R., Bayala, J., Verbeeten, E., Nyberg, G., … Malmer, A. (2016, February 24)). The nutrient availability and microorganisms within the soils are associated with water availability and groundwater recharge (Ilstedt et al., 2016).
Reforestation benefits the physical, chemical and biological properties of soil, considering “sustained fertility and water supply quantity and quality.” (Ilstedt et al., 2007). Soil water infiltrability one of the most important variables, responsible for groundwater recharge (Ilstedt et al., 2007).
To conclude, reforestation within a forest or afforestation on previously agricultural land-use has different effects on the hydrological cycle. Climate change is a constant driver of changes in all ecosystems and on Earth forcing biomes to change. Reforestation has a lot of benefits to biodiversity and to the hydrological cycle inducing cloud formation by evaporation; the best example for positive effects of reforesting an area is the increase of cloud forming and inducing rainfall. Reforestation is the replanting of trees or plants within an existing forest ecosystem, meaning it is beneficial to the environment to plant trees where the setting is already there. Afforestation has its positives as well as negatives that appear more present than the negatives for reforestation. Afforestation, as mentioned above, is the plantation of trees on agricultural land, which has to transform into a forest.
Therefore, agricultural land undergoes an entire regime shift in biodiversity, land-use and soil properties, but afforestation is not necessarily bad. It provides richer biodiversity and protects the soil from erosion by “locking away” sediments, soil, and nutrients whilst acting as a storage space for water (increased water holding capacity). There have been many studies about the effects of either reforestation or afforestation on water resources, but all of them have in common that, because of climate change, the outcome of water resources and availability may only be assumptions and predictions. The effects of reforestation or afforestation on water resources are difficult to predict as it depends on the climate, altitudes, latitudes and continents and whether water scarcity is an issue. Climate change is an ever-changing process, driven by changes and human activity which make it almost impossible to predict the outcome of effects on wate resources, the fate of cloud forests; that cool the climate (all forests have to be taken into consideration) and biodiversity.
- Ilstedt, U., Malmer, A., Verbeeten, E., & Murdiyarso, D. (2007). The effect of afforestation on water infiltration in the tropics: A systematic review and meta-analysis. Forest Ecology and Management, 251(1-2), 45–51. doi: 10.1016/j.foreco.2007.06.014
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