Serpentine soil is made up of worn ultramafic rocks. These rocks are brought to the earth's surface by either tectonic uplift or erosion of material above the ophiolite. Because of the mineralogy and forces at work in the physical and chemical environment that constitutes metamorphism, the rocks are further classified as serpentine and peridiotic. Serpentine rocks are created as a result of strong deformation caused by crustal movement. This rock, however, develops fractures in which water flows through, the hydration process then changes the mineralogy of rock resulting into formation of serpentine which is a polished, gray-green black rock. This type of soil supports limited biodiversity, and only life forms adapted to unsuitable soil conditions would thrive in serpentine soils.
Factors Contributing to Harsh Plant Survival Conditions of Serpentine Soils
There are several factors which contribute to the harsh conditions of the serpentine soils and these include; reduced ratio of calcium or magnesium, lack of essential nutrients such as nitrogen, potassium and phosphorus and high levels of heavy metals such as magnesium which leads to mineral toxicity (Herath et al.,130). Calcium is essential for the stability and development of plant cell membranes and enzyme activation (Cacho et al., 15135). Nitrogen, phosphorus, and potassium are the most important elements for sustaining plant growth and essential for the production of enzymes, chlorophyll, amino acids and DNA in plants. Mineral toxicity is another factor which contributes to the exclusion of serpentine soils from supporting plant growth as these soils contain high concentration of heavy metals such as nickel and chromium which are toxic to plant life.
Types of Plants that Grow on Serpentine Soils
There are three types of plants that have high affinity to these serpentine soils and these include; endemics, level indicators, and bodenvag or also known as indifferent as it is not restricted to a specific type of substrate. The endemic grow exclusively on serpentine soils, and according to research conducted by Kruckerberg in 1954, it indicated that its due to competition from other plant species that made it not to survive on non-serpentine soils. However, results indicated that this endemic serpentine Streptanthus was able to grow on non-serpentine soils if there was no competition. Local indicators are the plants that survive on non-serpentine soils but use the serpentine soils. These plant species include Jeffrey pine and incense cedar which are restricted to grow in serpentine soils in the north coast range but grow in a variety of habitats in Sierra Nevada Mountains. Other examples such as the nonwoody species which include Douglas thistle, sulphurflower buckwheat, confusing fescue, and bristly jewel flower and spring deathcamas are local serpentine indicators for the Coast Ranges but are not restricted to serpentine habitats in other locations.
The bodenvag or the indifferent species refer to the plant species that can either grow in serpentine or non-serpentine soils in the same location. These plant species can be divided into two types. These include; the plant species that have adapted to serpentine environments and the other non-serpentine species which cannot grow on the serpentine soils and the other type is of the plant species that are genotypically preadapted to grow on both the serpentine and the non-serpentine soils. For instance, the New Idria region of the southern Coast Range is less likely to have indifferent species in the serpentine soils due to its harsh condition.
Plant Adaptations to Survive on Serpentine Soils
Morphological Adaptations
Plant species such as the Serpentine endemics have developed xeromorphic foliage. These are the adaptations that enable them to survive in very harsh and dry environments. The adaptations include; leaves having small surface areas to reduce the loss of water through evaporation and also reduce the area exposed to air. These plants also exhibit tiny hairs on their surface which reduce air and wind flow and also reduce the rate of evaporation. The leaves are also reflective, hardened and appear to be waxy. The stems are also red and bluish with altered pubescence. However, serpentine plants appear to be dwarfed and stunted with many developed roots.
Tolerance Mechanism
These tolerance mechanisms include hyperaccumulation where plant species such as milkwort jewel flower can concentrate heavy metals such as nickel in large amounts which would otherwise be extremely toxic to the biotic life (Hidalgo-Triana, 135). Other endemic serpentine plants can cope with high metal toxicity by blocking its accumulation by the plants. Some of the plant species that exclude or hyper-accumulate the heavy metals can extract some key elements such as calcium from the soil to the plants at very high rates. This hyperaccumulation is important in defense against herbivores and pathogen resistance.
Dominant Occurrence of Serpentine Soil near the Coastline
It is important to demystify why serpentine soils are immensely distributed near the coastlines. While there is less research on this issue, it is essential to trace the phenomenon back to the morphology of serpentine soil, Serpentinite rocks, and their formation from ultramafic rocks. In other words, the parent rock for serpentine soils is serpentinite, which forms from the metamorphosis of ultramafic rocks. An understanding of these varied but related processes would plainly elaborate how serpentine soils are formed and why they dominate the coastline areas. Similarly, it becomes easier to compare variable and conditions under which the soils form.
Serpentinite rocks are formed through chemical reactions of water and peridotite rock, which highly olivine. Additionally, the rock forms near the earth’s surface, especially in areas that experience rapid water circulation that cools the rock strata close to mid-ocean ridges. This formation occurs where the ultramafic rock residue that is found in ophiolites is incorporated in the continental crust that is closer to current and former plate tectonic boundaries (Lewis et al. 2). In this case, subduction is likely to occur where two plates meet. Such phenomenon would alter the geography of a coastal region, where pressure between tectonic plates could lead to an accelerated metamorphosis of ultramafic rocks to form serpentinite rocks. Consequently, the rocks would be subjected various agents of weathering over specific timeframes to form serpentine soils near the coastline.
Another significant reason for the dominance of serpentine soils near the coastline is the availability of water, and other mineral deposits contribute to speeding up the rock formation processes. For example, the presence of immense amounts of magnesium and aqueous silica enhance formation of olivine. In the presence of water, olivine reacts to form serpentine. The process is accelerated by high temperatures in most coastal areas. Therefore, the serpentine distribution would accumulate and develop over time. As the process repeats over more extended periods of time, more forces are exerted on the rocks to make them change further. The outcome remains a distinct ecosystem life forms adapted to serpentine environments.
Other than subduction that occurs when the oceanic and continental crusts meet, the coastline is subjected to collision and friction between moving continental plates (Lewis et al. 3). Such motions and associated vibrations culminate into forces that speed up the rock and soil formation process. Most coastal regions are zones of convergence or divergence of continental plates. Both occurrences would affect the formation of soil and speed up the rock metamorphosis processes. At the same time, tremors may lead to increased pressure and temperature changes, which could alter chemical composition or serpentinite rocks. Such changes are likely to hasten weathering process to form more serpentine rocks near the coastline.
Once these rocks have been exposed to factors such as rainfall, topography and the length of time they are exposed, agents of weathering would then enhance formation of soil that. Furthermore, serpentine soils can be in variety, and these soils are inhabited by different types of plants (Anacker, 220). Despite the fact that these soils host a variety of plant species, their environs support very little plant biomass which makes it very unsuitable for the growth of these plants. Therefore, coastal regions with serpentine soils would have less vegetation cover as well as animal life forms. However, organisms that develop both morphological adaptations and tolerance mechanisms would survive and reproduce, hence maintain their population in the ecosystem.
Works cited
Anacker, Brian L. "The nature of serpentine endemism." American Journal of Botany 101.2 (2014): 219-224.
Cacho, N. Ivalú, and Sharon Y. Strauss. "Occupation of bare habitats, an evolutionary precursor to soil specialization in plants." Proceedings of the National Academy of Sciences 111.42 (2014): 15132-15137.
Herath, I., et al. "Immobilization and phytotoxicity reduction of heavy metals in serpentine soil using biochar." Journal of Soils and Sediments 15.1 (2015): 126-138.
Hidalgo-Triana, Noelia, Andrés Vicente Pérez Latorre, and James Hansen Thorne. "Plant functional traits and groups in a Californian serpentine chaparral." Ecological Research (2017): 1-11. Palm, E. R., and E. Van Volkenburgh. "Physiological adaptations of plants to serpentine soils." Plant ecology and evolution in harsh environments’.(Eds N Rajakaruna, RS Boyd, T. Harris) pp (2014): 129-148.
Lewis, J. F., et al. "Ophiolite-related ultramafic rocks (serpentinites) in the Caribbean region: a review of their occurrence, composition, origin, emplacement and Nilaterite soil formation." Geologica Acta: an international earth science journal 4.1-2 (2006).
Zefferman, Emily, et al. "Plant communities in harsh sites are less invaded: a summary of observations and proposed explanations." AoB Plants 7 (2015): plv056.
