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Plant Evolution & Ecology (2): snow roots
Onipchenko, V.G., Makarov, M.I., Logtestijn, R.S.P., Ivanov, V.B., Akhmetzhanova, A.A., Tekeev, D.K., Ermak, A.A., Salpagarova, F.S., Kozhevnikova, A.D. and Cornelissen, J.H.C. 2009. New nitrogen uptake strategy: specialized snow roots. Ecology Letters 12: 758-764.
Corydalis conorhiza is a flowering plant species living in the Caucasian alpine snow-bed ecosystem, which is characterized by long winters, where plant growth is limited by a short growing season (Billings and Bliss, 1959; Chapin et al., 1986; Walker et al., 1995), and N (nitrogen) and P (phosphorous) are co-limited ( Cornelissen, J.H.C., et. al. 2009: i). Snow-beds form in surface soil depressions that accumulate large amounts of snow during the winter months, and the final snowmelt does not occur until late in the growing season (Bjork, R.G. & Molau, U. 2007). In response to environmental stress, this plant has developed a unique and amazing root system that has not been described before. Usual below-ground soil roots host a kind of fungi that mainly helps to supply P (the authors think so), and unusual above-ground snow roots appear a matted network through the snow (Fig. 1). The roots grow up into the snow, but when snow melts, they will rapidly dry up and rot away.
Fig.1 Corydalis conorhiza with matted snow roots (Cornelissen, J.H.C., et. al. 2009: ii)
(Cornelissen, J.H.C., et. al. 2009: ii)
In the natural environment, the ice crust forms a strong barrier on the soil surface, so before the snow completely melts, the soil roots cannot readily access N that has accumulated in the snow. Therefore, the authors had two hypotheses: 1) snow roots take up N from snow packs; 2) this N gets transported to other plant organs.
They carried on two investigations to test the hypotheses. First, they dissected snow roots and soil roots to compare their structures; second, they used a scientific experimental method---adding a traceable N isotope (15N) in the water solution of the habitat plots to track any 15N taken up and transported.
They selected 4 treated plots in which they added labelled 15N and another 4 similar paired plots as test control groups no additions in the alpine vegetation belt on Caucasus, where the soils hold high C and N contents. During the study process, they selected another plant species from the same community as the test species. The second species does not have snow roots, so should be unable to access nutrients above the which means it cannot take the added 15N because the strong soil barrier formed by ice crust stops 15N from entering the soil, but has a similar growing environment, life form, and developmental timing, so that it was expected for could offer, according to the authors, a reasonable comparison about N uptake and transportation by snow roots and by soil roots.
In terms of cells, snow roots have clearly different characteristics compared with soil root structure. Snow roots are 1) much thinner, with; 2) no separate cortex (for water and food storage) from waterproof epidermis (outside skin); and 3) thicker and more corky endodermis to regulate water movement (inner structure regulating water coming in and out). (Fig. 2)
As opposed to soil roots, snow roots themselves, stems and leaves were greatly enriched with 15N. The second test species did not have labelled 15N anywhere.
Resource allocation strategy
The structure of snow roots shows a reduced protective function---thinner and no clear differentiation in epidermal part, but an increased ability to transport nutrients---thickened cell walls of the endodermis---to xylem, which is the tissue that moves water (vascular tissues for materials transportation from roots to other organs). The authors also found that the total length of snow roots are is much longer than soil roots both of this species itself and of other 99 other species. They and believed that this length helps to take up and transport nutrient. In addition, the highest amount of traceable 15N was found in leaves, where photosynthesis occurs, during the proceswhich N helps to synthesize food production. Plus, snow roots rapidly wither after snow. The series of observation and experimental results point to strong contributions for nutrient uptake and transport alone in snow roots with minimal investments (Cornelissen, J.H.C. et. al: ˅) in structure architecture, showing a strategic investment in which is a better resource allocation design.
Because of limited N availability, such roots with changed shape function to take N accumulated in the snow, which is almost competition excluded because only snow roots can behave in this way. This is to physiologically adapt to specific environment.
Like all other plant species, C. conorhiza has established its unique a niche in a specific habitat. (Niche: mainly a term describing a relationship of a species with its living and non-living surroundings.) The study involved in three of the four niche components proposed by Grubb (1977). First, the species tolerates a range of conditions, including limited takes resources, in this from the habitat for growth and reproduction---habitat niche; second, it changed its morphology (refers to external shapes of leaves, stems, or roots, etc.) by upward growing-up roots to adapt to its surroundings---life-form niche; third, snow roots behave differently in different time intervals, trying to seize opportunities of the moment to grow and reproduce---phenological niche. For example, when snow covers, snow roots function to take up N, but when snow melts, they wither. The paper did not discuss the fourth niche---regeneration niche. The establishment of a species in a habitat is substantially based on the capacity of propagule dispersal and seed germination, as well as juvenile phase.
Conclusions and problems
The authors confirmed that snow roots take up and transport N from snow for growth and reproduction. From this study, it is impossible to tell whether that the nitrogen uptake occurs from this now, or from the during final melt-water out, i.e. Snow roots take up N from liquid water instead of snow, thus problems aroused. When do snow roots develop and grow in snow beds and do they take up nutrients from liquid water of melting snow? How much N do they take up during the relatively stable snow-bed period and how much during final snow-melt?
Limitations and weaknesses
The paper is built on the general scientific method. The authors first observed an unusual natural phenomenon, and then they gave proposed a hypothesis. Over the course of scientific experiments (field and lab works) and data collected, they showed and testedthe predictions, i.e. (what will happen in the future if hypotheses are correct). Finally, they decided that the evidence strongly supported pronounced that their hypothesis were strongly supported. In the last part, they raised some questions that their study could not answer, and left readers some thoughts. Generally, the paper showed inductive reasoning to develop a hypothesis, and a deductive reasoningto test its prediction.
However, along the reasoning process, there exists several serious flaws.
Firstly, the authors did not rule out the possibility that soil roots can be responsible for sufficient N requirement for a very short growing season. Some studies showed that in alpine snow-bed ecosystem, all plant species tested were capable of taking up N while still snow was still present? (xilbroughet. al., C.J., Welker, J.M and Bowman, W.D. 2000.), which suggests that soil roots may be able to take up enough N in the tolerance range of the limited factor. (Fig. 3). Perhaps, whereas snow roots function in other ways. Maybe they serve as thermal insulators to protect dormant seeds until growing season. Furthermore, treated snow roots in the field experiment were from high-N-contented soil, which indicates that soil roots are have readily access to N.
Secondly, the authors used ambiguous words to make their conclusions somewhat problematic. One, they believed that soil roots host fungi, presumably for efficient P uptake in late growing season (Cornelissen, J.H.C., et. al. 2009: ii), which could be reasonable understood on the opposite way---soil roots host fungi, presumably for efficient N uptake. Two, the authors pronounced that this is unequivocal evidence that they had taken up substantial ammonium nitrogen from the snow pack, providing strong, direct support for our their first hypothesis. ( Cornelissen, J.H.C., et. al. 2009:˅), but they raised some questions that directly challenge their conclusions. Plus, "this nitrogen has been presumably translocated from the snow roots..." (Cornelissen, J.H.C., et. al. 2009:˅)
Thirdly, although the second test species are similar with C. conorhiza in some key life aspects, the authors did not give substantial evidence to prove that comparisons between the two species are convincing.
In addition, very long roots do not fall into "minimum investment" strategy. Both extended tissues and long transportation vessels cost much more energy to maintain the self-growth. ¾but isn’t there a tradeoff? It takes SOME investment to capture the nitrogen, at least they made the roots simple and relatively cheap.
The authors left some thoughts about the evolutionary adaptation to readers. First, will snow roots be advantageous or disadvantageous if conditions change? For example, climate warming is altering mountain snow habitat. Second, are there other species that have this trait? Perhaps nutrient uptake through snow roots is a more widespread environment-adaptation strategy in other similar habitats, but has not been observed yet. Third, did this trait evolve once in a related group of species, or repeated multiple times in evolutionary history?
The snow root is a spectacular example of the whole wonderful plant kingdom, and an amazing episode of the long mysterious evolutionary history. It verifies the adaptation of physical fitness upon natural selection from a point, and raises new questions about evolution in response to new environmental changes.
Normally, plant roots grow down into the ground, but a plant species, Corydalis conorhiza, in a nitrogen-limited mountain snow-bed habitat, has developed snow roots, which that grow up out of the soil covered by persisting snow. Upon observing this unusual phenomenon, the authors wondered if the snow roots function to take up nitrogen from snow and transport this nitrogen to other plant organs in a short growing season. They carried out two investigations, comparing the structure of dissected snow roots and regular soil roots, and adding a traceable nitrogen isotope into the snow of the natural habitat to trace the movement of the nitrogen isotope in the plant body. They found that the inner structure of snow roots was clearly different from regular soil roots, and that treated plants showed high amount of traceable nitrogen isotope in stems and leaves, but not regular roots, and not in plants with no snow roots. They concluded that snow roots take up nitrogen from snow and transport it to other organs to adapt the specific environment, but wither as soon as the ground thaws enough for the below-ground roots to function.
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