The product of a long evolutionary history, plants are adapted to the climate in a way that humans are not. Humans fashion their material culture to suit the climate, or, when climate worsens, they migrate to a more hospitable locale. Plants, rooted to the ground, must adapt or die. Migration is not an option, though seed dispersal is a kind of intergenerational migration. Through dispersal, seeds travel roughly one kilometer per year, a rate too slow to adapt a plant to rapid climate change. As the climate deteriorates, some plant species die out, to be replaced by hardier species. In Neolithic and Bronze Age Denmark, for example, the climate was warmer and wetter than it is today.
The climate was warm and wet enough to sustain the growth of oak trees. In the Iron Age, however, temperatures dropped to current figures. Unable to cope with the decline in temperature and moisture, oak trees died out and were replaced by grasses and heather. In New Hampshire, the tree Pinus strobus was for centuries the most numerous tree in the region's deciduous forests. Pinus came through the fire of 1665 unscathed, but was unable to cope with climatic catastrophe. In 1921, a tornado, and in 1938 a hurricane, swept through the forests, felling large trees of several species. The catastrophes wiped out Pinus, ending centuries of its dominance, and leaving other species to reconstitute the forests.
These sudden changes in climate and flora are dramatic and easy to quantify. No less dramatic has been the effect of hydrofluorocarbons on climate and plants. Humans have released large quantities of hydrofluorocarbons into the atmosphere, where they have thinned the ozone layer. Consequently UV-B radiation, a type of ultraviolet light, penetrates the ozone layer in greater amounts than in the past. UV-B radiation damages half of all plant species. Damaged plants grow small leaves and short shoots and photo-synthesize at a slow rate. These effects are magnified by the fact that several crops are among the plants sensitive to UV-B radiation. One study concluded that a 25 percent reduction in the ozone layer would halve soybean yields.
The tropics support lush vegetation, with more than 80 in. of rain per year, temperatures nearly uniform and warm year-round, and abundant sunshine. Trees have thin bark, for they don't need insulation against the cold or protection against water loss. Trees grow in layers, with those in the innermost layers able to survive without exposure to direct sunlight. Because sunlight does not penetrate to the rainforest floor, little vegetation grows along the ground. At higher latitudes north and south of the tropics, rainfall diminishes and temperatures vary year round. These climates have seasons, with vigorous plant growth in spring and summer and dormancy in autumn and winter.
To cope with a diminution in rain, plants in temperate climates have evolved small leaves to minimize the loss of water through transpiration. In areas that have a dry season, trees evolved the shedding of leaves to stop transpiration and, in other species of trees, the growth of needles rather than leaves to minimize transpiration. With their needles, the evergreens and conifers are adapted to short summers because they can carry out photosynthesis as soon as temperatures warm, whereas deciduous trees must regrow their leaves before they can photosynthesize. Evergreens and conifers grow where winter is cold enough to freeze the ground. Once the ground freezes, roots cannot absorb water, making winter a period of drought and favoring trees that can minimize transpiration in response to frost-induced drought.
In contrast to the rainforest, sunlight penetrates to the forest floor in temperate forests, permitting the growth of plants, often grasses, in abundance along the ground. As the dry season lengthens and rainfall diminishes still further, forest gives way to grassland. Grasses need less water than trees. Trees grow alongside grasses on the African savanna, but few trees grow on the Russian steppe. There, grasses are the dominant flora. At high latitudes, temperatures fall below 40 degrees F (4 degrees C) for six to nine months per year. Summers are brief, with temperatures above 50 degrees F (10 degrees C). Rainfall ranges between 1040 in. (25-102 cm.) per year, with between 15-24 in. (38-61 cm.) typical. This climate favors the growth of coniferous forests. In addition to their needles, conifers have thick bark to retard water loss and to protect against the cold. Temperature separates coniferous forest from tundra. Where summer temperatures exceed
50 degrees F (10 degrees C), conifers predominate, but wherever summer temperatures fall below 50 degrees F (10 degrees C), tundra results. Grasses and sedges are the tundra flora. During the 50 or 60 days of summer, the sun melts a thin strip of ground. Free from the grip of ice and benefiting from the nearly continuous sunlight of summers in high latitudes, plants grow vigorously and then are dormant for the long, bitter winter.
effects of climate change on plants
The increase in temperatures that is the likely outcome of the greenhouse effect will affect plants. By one estimate, a two or three degree F increase in temperature will raise crop yields in the temperate zone, though an increase above 3 degrees F (1.7 degrees C) will decrease yields. Any temperature increase will likely reduce yields in the tropics, where crops are already at their maximum heat tolerance.
The climate of the future is sure to affect plants. Despite predictions to the contrary, the increase in carbon dioxide will likely increase temperatures. One study suggests that a doubling of carbon dioxide in the atmosphere will triple the growth rate of plants and trees. Forests will grow more densely and will extend their range to higher latitudes. Plants will grow more vigorously on marginal land. Another study indicates that a doubling of carbon dioxide will shift temperate forests 310 to 621 m. (500 to 1,000 km.) north in the northern hemisphere, and south in the southern hemisphere. The concentration of carbon dioxide is likely to double by 2080, but trees are not likely to migrate so far so fast. Global warming therefore endangers temperate forests. As the climate warms, trees will advance north and south, taking over ground that had been tundra. As temperatures rise, dead plants will decay more rapidly, liberating still more carbon dioxide into the atmosphere.
The cutting down of forests will surely harm the plants that survive. The amount of rainfall will decrease as forests are cut down. With fewer forests, the rate of transpiration will diminish. Whereas forests absorb sunlight, bare ground reflects sunlight back into space. A 15 percent decline in rainfall would replace the forests of South America with grassland. A 30 percent decrease in rainfall would replace the forests of Zaire with grasses. A 70 percent decrease in rainfall would make the Amazon basin a desert.
The climate of the future may imperil many plant species, but as a kingdom, plants are resilient. They have survived the Ice Ages and the predation of herbivores in warm climates. Humans are fortunate that plants are so adaptable, for with their agriculture, humans are dependent on plants. Life would not exist without the diversity of plants. The most numerous form of terrestrial life during the Carboniferous period, plants occupy every biome. Even in deserts, their seeds lie dormant, awaiting the infrequent rains. Plants have adapted to every climate, from the tropics to frigid tundra. Even bodies of water are home to plants. The survival of plants depends on their ability to adapt to the climate of the future. The survival of the rest of the biota depends on the success of plants.
sEE ALsO: Climate; Cretaceous Era; Greenhouse Effect; Greenhouse Gases.
BIBLIOGRAPHY. Institute for Biospheric Research, The Greening of Plant Earth: the Effects of Carbon Dioxide on the Biosphere (Western Fuels Association, 1991); D.C. Money, Climate, Soils and Vegetation (University Tutorial Press, 1965); J.I.L. Morison and M.D. Morecroft, eds., Plant Growth and Climate Change (Blackwell Publishers, 2006); Jelte Rozema, Rien Aerts, and Hans Cornelis-sen, eds., Plants and Climate Change (Springer, 2006); F.I. Woodward, Climate and Plant Distribution (Cambridge University Press, 1987).
Christopher Cumo Independent Scholar
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