Rainforests are a vast ecosystem, thriving in biodiversity. Organisms within a rainforest have adapted to a forests soil and climate in order to survive and flourish. Tropical rainforests are often considered infertile, but this can be down to a number of factors. Fertility is a measure of a soils ability to provide plants with suitable conditions and nutrients to grow healthily.
Reasons for low fertility
The soil type that lies beneath the canopy plays an important role in forest fertility. Latosol rainforest soils are red due to high concentrations of iron (as well as aluminium oxides), e.g., oxisol (ferralsol) and ultisol soils, that are naturally low in fertility due to their high permeability.
Rainforests have a large accumulation of fungi on the forest floor. The humid environment (and these fungi) of a tropical rainforest allows for fast decay of organic matter, therefore their litter and humus layers are nutrient rich. Heavy rainfall periods and high soil permeability leads to leaching of nutrients. Trees, thus often have shallow roots known as buttress roots (Johnson, 2015) that enable quick absorption of nutrients from the soil. Nutrients in the rainforest are largely stored in its vegetation, and not in the roots or soil surrounding the plant.
The rainforests vegetation often limits the intensity of sunlight reaching the forest floor. Microorganisms, such as cyanobacteria, require sunlight to synthesise; therefore, lack of sunlight indirectly reduces soil fertility.
Where do forests get their minerals from?
Plants growing from soils which are nutrient poor are often favoured by other environmental factors. For example, hot regions can supply vegetation with energy over a long period of the year. Plants will adapt to the soil type they are growing from; nitrogen fixing plants can replenish soil nitrogen. The flora present in ancient infertile landscapes have been found to have leaves which are low in phosphorus (Lambers et al., 2011). Woody plants deter herbivores due to their low nutrition levels. Foliage is often depleted of nutrients before it is shed (Orians and Milewski, 2007).
Plants growing from nutrient-poor soils have adapted to these conditions by making more carbohydrates (Orians and Milewski, 2007). Tropical forests have been found to store 49.4 % of carbon in their vegetation and 50.6 % in soil up to a metre deep. Tropical forests store a greater proportion of carbon in their vegetation compared to temperate grasslands and forests (Neufeld and Smith, 2022), but temperate grasslands store nearly twice the amount of carbon in the soil than can be found in the foliage of tropical forests.
Rivers and streams run along the forest floor, eroding away the banks, and transporting minerals in their flow. Parker (1983) states that another large source of nutrients (potassium, sodium, and sulfur) to the forest floor comes from precipitation transferred by throughfall (rain running through the canopy) and stem flow.
Epiphytes are plants which grow within a trees canopy. Instead of taking nutrients from the trees, they gain nutrients from the air, precipitation and fallen debris. Nadkarni (1981) details how rainforest trees develop adventitious roots (outside the soil) which take nutrients from the epiphytes that then become excellent sources of minerals for the tree.
Another way nutrient-poor soils receive minerals is through the process of weathering. Colonies of mineral-weathering microbes, such as Burkholderia, can be found in the rhizosphere areas of the forest (Uroz et al., 2011). These mineral ions are then available for plant roots to absorb. These microbes can secrete chemicals and enzymes that facilitate the breakdown of the insoluble compounds of the soil into minerals that can be absorbed by the tree. If a soil is sterile the plant growth will be poor.
Conclusion
The environmental conditions of a tropical forest help control the cycle of nutrients through the ecosystem. Tropical vegetation has adapted to the availability of nutrients in the soil. Due to a large proportion of nutrients being stored above the soil, it raises the question as to whether soils below many tropical rainforests are suitable for arable agriculture or, are they more of a vital resource for an ecosystem as a diverse rainforest?
References
Johnson, H., J., 2015. Rainforest. Available at: https://www.nationalgeographic.org/encyclopedia/rain-forest/ (Accessed: 21/03/2022)
Lambers, H., Brundrett, M.C., Raven, J.A. and Hopper, S.D., 2011. Plant mineral nutrition in ancient landscapes: high plant species diversity on infertile soils is linked to functional diversity for nutritional strategies. Plant and Soil, 348(1), pp.7-27.
Nadkarni, N.M., 1981. Canopy roots: convergent evolution in rainforest nutrient cycles. Science, 214(4524), pp.1023-1024.
Neufeld, D., and Smith, M., 2022. Visualizing Carbon Storage in Earth’s Ecosystems. Available at: https://www.visualcapitalist.com/visualizing-carbon-storage-in-earths-ecosystems (Accessed: 22/03/2022)
Orians, G.H. and Milewski, A.V., 2007. Ecology of Australia: the effects of nutrient‐poor soils and intense fires. Biological Reviews, 82(3), pp.393-423.
Parker, G.G., 1983. Throughfall and stemflow in the forest nutrient cycle. Advances in ecological research, 13, pp.57-133.
Uroz, S., Oger, P., Lepleux, C., Collignon, C., Frey-Klett, P. and Turpault, M.P., 2011. Bacterial weathering and its contribution to nutrient cycling in temperate forest ecosystems. Research in microbiology, 162(9), pp.820-831.