The effect of acid rain on trees manifests in two ways – upon foliage, and roots (Kimmins, 1997). The symptoms include direct damage to plant tissue (particularly the foliage), reduced density of foliage within the crown, distinct areas of dieback, and whole tree death (Singh & Agrawal, 2007).
Studies into the impact of acid rain upon the foliage of forest trees conclude that necrotic (dead) patches are particularly common, with injuries being primarily to the epidermis (outer-most layer of a leaf). Acid rain therefore may have potentially severe impacts upon leaf structure (Fan & Wang, 2006). The foliage of coniferous species is more prone to the foliar effects from acid rain, likely given conifers do not shed foliage annually (Percy, 1986). Persistent (long-term) damage via acid rain may actually bring about a potential shift away from coniferous forests and towards deciduous forests. Exactly how this will impact upon the boreal (northern) forests however is not fully known, though the observed transition between northern broadleaved forests and boreal coniferous forests is seemingly rapid (Johnson, 1983).
Forest ecosystems either at high altitude or prone to misty conditions can also be damaged by the acidic nature (both sulphur and nitrogen are present) of the water droplets that constitute the clouds (Kimmins, 1997). A study into how such acidic mist impacted upon forest trees identified that growth was rapidly reduced and calcium was directly leached from the foliage, in turn leading to foliar injury (DeHayes et al., 1999).
Turning attention towards the impact upon the root system of a tree, as the forest soil increases in acidity – due to acid rain – seedling germination lowers (Percy, 1986). Therefore, if soils do continue to acidify, the future of forests may be under threat as a result of a possible lack of regeneration. The lowering of soil pH via the accumulation of sulphur and nitrogen ions within the soil (Kimmins, 1997) means important nutrients are leached from the soil, and increases in the abundance of phytotoxic (toxic to plants) heavy metals, such as aluminium, occur. Such changes in the soil chemical characteristics reduce soil fertility, which ultimately has a negative impact on growth and productivity of forest trees both above and below the ground (DeHayes et al., 1999; Singh & Agrawal, 2006; Menon et al., 2007), with the example of a reduction in fine root mass being evident.
The accumulation of toxic heavy metal ions, which may be brought about (at least in part) by acid rain, is also known to have negative impacts upon the ability for decomposers (fungi and insects) of a forest ecosystem to function properly. This leads to imbalances in nutrient cycling, litter decomposition, and productivity of the ecosystem (Pennanen et al., 1998), which in turn can impact upon vegetation life, and may lead to stresses that can increase in severity over the years.
Such soil acidification ultimately can change the entire vegetation composition of a forest – much like how acid rain damage to leaves can alter composition – with one study highlighting how a pine and spruce forest transitioned towards and then into a mixed and birch forest (Koptsik et al., 2001). Further studies have concluded similarly, by identifying that certain tree species will begin to die from ill-health following the change in soil properties (Johnson, 1983; Swaine, 1996; van Breemen et al., 1997). As a result, acid rain can initiate a transition away from forests dominated by particular species, and towards forests dominated by different species. This has impacts for the species that rely on the trees for habitat and food, as well – birds, insects, fungi, lichens, mammals, and bacteria are but just a few examples of the types of organisms that will be impacted.
Soil acidification within forests may therefore be very destructive, in time. However, as germination is only significantly stunted at a pH of 2.0-3.5 (Percy, 1986), such an issue may only be a distant concern for now. Despite this, one study that looked at modelling future soil profiles on a heavily acidified site concluded that pH is unlikely to revert back towards what it once was, even if drivers of acidification (such as acid rain) slow. This is because soil profiles suffer from past inputs into their system (Małek et al., 2005), thereby meaning that the future extents of acid rainfall upon forest soils could be very damaging.
DeHayes, D., Schaberg, P., Hawley, G., & Strimbeck, G. (1999) Acid rain impacts on calcium nutrition and forest health alteration of membrane-associated calcium leads to membrane destabilization and foliar injury in red spruce. BioScience. 49 (10). p789-800.
Fan, H. & Wang, Y. (2000) Effects of simulated acid rain on germination, foliar damage, chlorophyll contents and seedling growth of five hardwood species growing in China. Forest Ecology and Management. 126 (3). p321-329.
Johnson, A. (1983) Red spruce decline in the northeastern US: hypotheses regarding the role of acid rain. Journal of the Air Pollution Control Association. 33 (11). p1049-1054.
Kimmins, H. (1997) Balancing Act: Environmental Issues in Forestry. Canada: UBC Press.
Koptsik, G., Koptsik, S., & Aamlid, D. (2001) Pine needle chemistry near a large point SO2 source in northern Fennoscandia. Water, Air, and Soil Pollution. 130 (1-4). p929-934.
Małek, S., Martinson, L., & Sverdrup, H. (2005) Modelling future soil chemistry at a highly polluted forest site at Istebna in Southern Poland using the “SAFE” model. Environmental Pollution. 137 (3). p568-573.
Menon, M., Hermle, S., Günthardt-Goerg, M., & Schulin, R. (2007) Effects of heavy metal soil pollution and acid rain on growth and water use efficiency of a young model forest ecosystem. Plant and Soil. 297 (1-2). p171-183.
Pennanen, T., Perkiömäki, J., Kiikkilä, O., Vanhala, P., Neuvonen, S., & Fritze, H. (1998) Prolonged, simulated acid rain and heavy metal deposition: separated and combined effects on forest soil microbial community structure. FEMS Microbiology Ecology. 27 (3). p291-300.
Percy, K. (1986) The effects of simulated acid rain on germinative capacity, growth and morphology of forest tree seedlings. New Phytologist. 104 (3). p473-484.
Singh, A. & Agrawal, M. (2007) Acid rain and its ecological consequences. Journal of Environmental Biology. 29 (1). p15-24.
Swaine, M. (1996) Rainfall and soil fertility as factors limiting forest species distributions in Ghana. Journal of Ecology. 84 (3). p419-428.
van Breemen, N., Finzi, A., & Canham, C. (1997) Canopy tree-soil interactions within temperate forests: effects of soil elemental composition and texture on species distributions. Canadian Journal of Forest Research. 27 (7). p1110-1116.
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