subject: Plant survival and functions during winter under temperate conditions [print this page] Plant survival and function during winter under temperate conditions. By Khalil Jibran Wani and Ghulam Mohyuddin Wani 4 BISMILLAH COLONY,PO SANTNAGAR,KASHMIR,190005,India Introduction One wonders why plants in winter go in dormancy and do not shoot or fruit .The upper ground portions of the plants seem to have dried as most of the leaves are shed and generally budding or grafting is not performed in winter. On the other hand rooting or planting of trees is not done in summer but preferred in winter. This question and other related geo-physiological processes are reviewed in this paper. The active layer What is it? It is the top layer of soil that thaws during the summer and freezes again during the autumn. Normally the soil is active during summer and water flows freely in and around the root zone of the plants. It is absorbed and sweeps up through the stem and utilized for photosynthesis and other life functions of the plant. When the soil freezes, it does not allow this absorption of the moisture in the desired quantity which results into shedding of leaves and drying of shoots. The leaves fall off but the shoot do have life saving nutritive stores which help it to be alive during the dormancy of winter. The resumption of summer or change in the soil temperature makes root zone active again to have abundant quantity of the water from soil for shoot revival and new leaf formation. The fruiting and other functioned reoccur. SOIL TEPERATURE. The temperature in the lower levels of the soil will remain more stable than that at the surface, where the influence of the atmospheric temperature is greatest. It is clear that a surface of soil below the ground remains unaffected by the atmospheric temperature,. Thus in plants with deep roots winch come down below the active layer which freezes continue to function even dung under lower or freezing temperatures. FROST FRONT If the winter temperature is below the freezing point of water a frost front will form in the soil. This "frost front" is the boundary between frozen and unfrozen soil and with the coming of spring and summer, the soil is thawed. This process starts from top to bottom if the heating during summer exceeds the cooling during winter, the soil will be completely thawed during the summer and there will be no permafrost. This occurs when the mean annual temperature is above 0C (32F), but also occurs when the mean annual temperature is slightly below 0C on sites exposed to the sun. PERMAFROST What is it? When there is not sufficient heat to thaw the frozen soil completely, permafrost forms. The active layer in this environment consists of the top layers of soil which thaws during the summer, whilst the inactive layer refers to the soil below which is frozen year-round because the heat fails to penetrate. Liquid water cannot flow below the active layer, with the result that permafrost environments tend to be very poorly drained and boggy. Thaw depth The thickness of the active layer, known as the thaw front is determined by how far the frost front is forced to retreat due to heating during the summer. Thus, the primary determinant of active layer thickness is the maximum temperature attained during the summer. If it is only a little above 0C, the active layer can be very thin (only 10 cm on Ellesmere Island), whilst if it is quite warm, it is much thicker (about 2.5 m at Yakutsk), and if the permafrost is discontinuous and soil begins thawing earlier, it can be still thicker (5 meters at Yellowknife). The parent material of the soil is also important. The active layers in soils made of sandy or gravelly parent materials can be up to five times deeper than those made from loam- or clay-rich parent material. This is because the coarser material allows for much greater conductivity of heat down into the soil. This is important because roots of plants cannot penetrate beyond the active layer and are restricted by its thickness. Thus, in a continuous permafrost environment plants must have shallow roots, which restrict tree growth to specialised species such as Larix. In areas of discontinuous permafrost, most conifers are able to grow easily. Because of variation in summer temperatures from year to year, the amount of heating of the active layer also varies - hence the depth of thaw is not constant. Soil formation in the active layer Cryoturbation is the dominant force operating in the active layer, and tends to make it generally uniform in composition throughout. However, variation in the composition of soils due to differences in parent rock is very marked in permafrost regions due to the low rate of weathering in the very cold climate. The slow rate of decomposition of organic material means Gelisols (permafrost soils) are very important as a sink for carbon dioxide. This carbon dioxide and other greenhouse gases (chiefly methane) forms from the very slow decomposition of the excess organic matter that remains in most Gelisols and is mixed down into the pereletok layer during relatively hot summers and below that layer during warmer periods about 5000 to 6000 years ago. This storage of carbon means thawing of permafrost may accelerate global warming - some suggest the difference could become very significant especially if the carbon has been stored since before recent glacial maxima.
Plant survival and functions during winter under temperate conditions
