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Hershey, D. R. 2002. Plants rarely thermoregulate. American Biology Teacher 64: 413. Plants rarely thermoregulateDeGolier's (2002) winter field trip idea is worthwhile, but contains some errors. In the Plant Kingdom, thermoregulation is applied only to flowers of a few species, such as skunk cabbage (Symplocarpus foetidus)(Knudson 1974, Seymour 1997, Seymour & Blaylock 1999). DeGolier (2002) does not mention skunk cabbage but it is probably the only plant where thermoregulation protects against cold damage and just cold damage to the inflorescence at that.
DeGolier, T. (2002). Cold war: Flora's undercover agents, a campus winter field trip to illustrate that plants do indeed thermoregulate. The American Biology Teacher, 64, 45-51. Knutson, R.M. (1974). Heat production and temperature regulation in Eastern skunk cabbage. Science, 186, 746-747. Seymour, R.S. (1997). Plants that warm themselves. Scientific American, 276(3), 104-109. Seymour, R.S. & Blaylock, A.J. (1999). Switching off the heater: Influence of ambient temperature on thermoregulation by Eastern skunk cabbage, Symplocarpus foetidus. Journal of Experimental Botany, 50, 1525-1532. David R. Hershey
The above letter was heavily censored by the American Biology Teacher editor, who inexplicably did not want to publish it at all and alert the readers of the many errors. Below is the original version:
DeGolier's (2002) winter field trips to examine plant adaptations to survive winter seem very worthwhile. However, the term thermoregulation is misapplied because plants rarely thermoregulate as warm-blooded animals do. In the Plant Kingdom, the term thermoregulation is applied only to flowers of a few species (Seymour 1997) such as lotus (Nelumbo nucifera), split leaf philodendron (Philodendron selloum) and skunk cabbage (Symplocarpus foetidus). Flowers in these species can raise their temperature well above ambient as warm-blooded animals do. Perhaps the only plant to thermoregulate to protect its flowers from cold injury is skunk cabbage, native throughout the Northeastern U.S. west to Iowa, south to Georgia and north into southern Canada. It can raise the internal temperature of its inflorescence 15 to 35 C above ambient air temperatures of -15 to 15 C and can maintain thermoregulation for two weeks or more when it blooms in February and March (Knutson 1974, Seymour and Blaylock 1999). Skunk cabbage often melts the snow covering it. Its inflorescence tissue is not frost resistant (Knutson 1974) so thermoregulation does seem to prevent cold damage. Other functions of flower thermoregulation are to vaporize floral scents and to attract insect pollinators. DeGolier (2002) contains several incorrect or incomplete facts as follows: - The statement that retaining normally deciduous leaves over winter would be a huge energy cost to the plant is very hypothetical and does not seem to be a major reason for leaf abscission. Leaves of deciduous species are programmed to senesce in the fall. Even if they did not senesce, deciduous leaves would soon be killed by the cold temperatures. If the deciduous leaves did somehow survive during winter, their metabolic level would be lower than normal because of the lower temperatures. They would also likely to be able to photosynthesize to some extent to at least partially meet their winter energy needs. The fact that many needleleaf and broadleaf trees are evergreen indicates that overwintering leaves are not a huge energy cost. The mean net primary production (grams per square meter per year) of temperate forests is actually slightly higher for evergreen than for deciduous forests (Salisbury and Ross 1985). It may seem counterintuitive that evergreen forests are dominant farther north than deciduous forests. Ecologists have correlated the dominance of evergreens in certain areas of both cold and warm climates with the low mineral nutrient availability in those areas (Aerts 1995). Evergreen leaves make more efficient use of mineral nutrients than deciduous leaves. In very cold biomes, such as the far north taiga or boreal coniferous forest, mineral nutrient availability is low because of the low temperatures. Therefore, deciduous forests typically dominate in areas with higher mineral nutrient availability, which are south of the taiga. With no mineral nutrient limitation, deciduous trees can make new leaves every year and not have the disadvantage of lower photosynthetic rates of evergreen leaves that are old or winter-damaged. The deciduous habit is thought to have evolved in warm climates as an advantage where there is seasonal drought (Axelrod 1966). A cold winter might also be considered a dry season. Another possible advantage of dropping leaves in cold winter climates is that there is far less surface area to accumulate loads of ice or snow sufficient to break branches (Lemon 1961). Ice storms often cause tremendous damage to deciduous trees even without their leaves. In the U.S., deciduous forests in the Northeast and Midwest correspond to areas where winter ice storms are common. - Tulips and daffodils do not die back in winter. They die back in summer. Tulip and daffodil leaves often begin to emerge aboveground during winter in USDA Cold Hardiness Zone 6 and possibly farther north if the soil is not frozen solid. Their leaves are cold hardy and survive subfreezing temperatures. - Not all ferns will "frost off" in most areas of the U.S. Some native ferns are cold hardy evergreens including the Christmas fern (Polystrichum acrostichoides), which is hardy to USDA hardiness zone 4. USDA hardiness zone 4 includes the southern half of Minnesota. - Certain trees, such as some oaks (Quercus spp.) and some beeches (Fagus), often retain some dead leaves during winter. These leaves are termed marcescent, defined as withering without falling off. There seems to be no evidence that marcescent leaves provide additional cold protection to buds. Marcescent leaves have been considered a juvenile characteristic (Leopold and Kriedemann 1975). Marcescent leaves are found mainly in the lower, more juvenile, parts of larger trees. Hormone levels are probably responsible because gibberellic acid applied to mature plants can cause them to revert to juvenile characteristics, hormones trigger abscission, and hormones can delay leaf senescence (Taiz and Zeiger 1991). Some juvenile characteristics, such as thorns and spiny needles, discourage herbivory. It has also been hypothesized that marcescent leaves are a defense against large herbivores, such as deer (Svendsen 2001). However, even in trees that normally drop all their leaves in the fall, leaf abscission is sometimes prevented by cold temperatures that kill the leaves before the abscission layer has completely formed. - Marcescent leaves do not remain "on oak buds". The oak bud is not subpetiolar or under the petiole. An example of a true subpetiolar bud is sycamore (Plantanus spp.). There seems to be a common misconception that buds on marcescent trees are subpetiolar and that bud growth in the spring simply pushs marcescent leaves off the tree. However, Hoshaw and Guard (1949) found that in pin oak (Quercus palustris) the petiole base remained alive during winter even though the rest of the leaf was dead. In West Lafayette, IN, they found anatomical changes in the petiole base starting in late February that resulted in rapid dropping of marcescent leaves at the end of March. Thus, the abscission zone completes development in late winter or early spring rather than in the fall. - There was not a clear answer for the question "Does the bark of a tree have any insulating value?" Evidence that bark is often a poor insulator in many deciduous trees is the widespread occurence of damage termed cup shakes, frost canker and frost cracks (Harris 1983, Pirone et al. 1988). These all occur when branches or trunks are rapidly thawed or frozen. Cup shakes is a separation of wood along an annual ring which occurs when a frozen trunk is quickly warmed causing the outer layers to thaw first, expand, and separate from the inner, still-frozen wood. Cup shakes are not visible externally but weaken the tree and lower lumber quality. Frost cankers and frost cracks occur when a warmed trunk or branch freezes quickly, as when the sun suddenly goes behind an opaque object. In frost canker, the cambium or bark is injured or killed. Frost cracks occur when the outer trunk layer freezes and therefore shrinks faster than the inner layers. This results in a long longitudinal crack in the bark and wood that often extends all the way to the center of the trunk. A loud noise often accompanies the formation of a frost crack. The frost cracks reopen in subsequent winters when the temperature is low enough, so they cannot heal shut. To prevent frost canker and frost cracks, young tree trunks are often wrapped with burlap or painted white to prevent them from being thawed by the sun. - Not all conifers are evergreen. Larches (Larix spp.), bald cypress (Taxodium distichum), dawn redwood (Metasequoia glyptostroboides), and golden larch (Pseudolarix kaempferi) are deciduous, needleleaf conifer trees. The deciduous larches have similar aboveground production rates as sympatric evergreen conifers and because they are deciduous, they can survive in far northern areas too harsh for evergreens (Gower and Richards 1990). Larches seem to be an exception to the generalization that evergreens are favored in areas with low mineral nutrient availability. - Many coniferous needles do not have stomata just on the underside. Pine needles have stomata on all surfaces (Mauseth 1995). Some pine species are even round in cross section. - Is there any evidence that V-shaped needles drain water away so "stomata don't drown"? The surface tension of water, the waxy cuticle, and the air that would likely be trapped by surface water in the depression above the sunken stomata would all seem to make it difficult for water to enter sunken stomata, particularly on the underside of the leaf. Given that leaves evaporate large quantities of water, temporary waterlogging of needles seems unlikely to be a serious problem. Darwin and Acton (1895) had a teaching experiment to demonstrate waterlogging of broad leaves, but not conifer needles. They reported that when a frozen ivy leaf was thawed underwater it became waterlogged because the intercellular ice melted, was absorbed back into the cells and pulled water into the leaf through the stomata. Darwin and Acton's (1895) experiment seems a bit extreme because it is unlikely that evergreen needles would be completely submerged in water as they thaw. However, conifer needles that prevent freezing by supercooling would have no intercellular ice (Taiz and Zeiger 1991) so that particular waterlogging mechanism would not work unless temperatures were too low for supercooling. Some additional items worth mentioning on a winter field trip include the following: - Any examination of plant cold hardiness would benefit from a discussion of the USDA Plant Hardiness Zone Map (Cathey 1990) or the Arnold Arboretum Plant Hardiness Zone Map (Wyman 1990). The USDA Plant Cold Hardiness Zone Map is widely available on the internet, in horticulture and gardening books and in plant catalogs. - Another important aspect of plant cold hardiness is that roots are typically less cold hardy than the shoots. People often learn this lesson the hard way when cold hardy plants grown in aboveground containers die because the roots are killed by low temperatures. - Flower buds are typically less hardy than vegetative buds. This leads to substantial economic losses nearly every year when cold temperatures kill peach or other fruit crop flower buds in some orchards. Some spring flowering shrubs and trees, such as forsythia (Forsythia spp.) and flowering dogwood (Cornus florida), may have their flower buds killed or damaged in harsh winters. David R. Hershey
Aerts, R. (1995). The advantages of being evergreen. Trends in Ecology and Evolution, 10, 402-406. Axelrod, D.I. (1966). Origin of deciduous and evergreen habits in temperate forests. Evolution, 20, 1-15. Cathey, H.M. (1990). USDA Plant Hardiness Zone Map. Washington, DC: USDA Agricultural Research Service Misc. Publication 1475. Darwin, F. and Acton, E.H. (1895). Practical Physiology of Plants. London: Cambridge University Press. DeGolier, T. (2002). Cold war: Flora's undercover agents, a campus winter field trip to illustrate that plants do indeed thermoregulate. American Biology Teacher, 64, 45-51. Gower, S.T and Richards, J.H. (1990). Larches: Deciduous conifers in an evergreen world. BioScience, 40, 818-826. Harris, R.W. (1983). Arboriculture: Care of Trees, Shrubs and Vines. Englewood Cliffs, NJ: Prentice-Hall. Hoshaw, R.W. and Guard, A.T. (1949). Abscission of marcescent leaves of Quercus palustris and Q. coccinea. Botanical Gazette, 110, 587-593. Knutson, R.M. (1974). Heat production and temperature regulation in Eastern skunk cabbage. Science, 186, 746-747. Lemon, P.C. (1961). Forest ecology of ice storms. Bulletin of the Torrey Botanical Club, 88, 21-29. Leopold, A.C. and Kriedemann, P.E. (1975). Plant Growth and Development. New York: McGraw-Hill. Mauseth, J.D. (1995). Botany: An Introduction to Plant Biology. Philadelphia: Saunders College. Pirone, P.P., Hertman, J.R., Sall, M.A. and Pirone, T.P. (1988). Tree Maintenance. New York: Oxford University Press. Salisbury, F.B. and Ross, C.W. (1985). Plant Physiology. Belmont, CA: Wadsworth. Seymour, R.S. (1997). Plants that warm themselves. Scientific American, 276(3), 104-109. Seymour, R.S. and Blaylock, A.J. (1999). Switching off the heater: Influence of ambient temperature on themoregulation by Eastern skunk cabbage, Symplocarpus foetidus. Journal of Experimental Botany, 50, 1525-1532. Svendsen, C.R. (2001). Effects of marcescent leaves on winter browsing by large herbivores in northern temperate deciduous forests. Alces, 37(2) 475-482. Taiz, L. and Zeiger, E. (1991). Plant Physiology. New York: Benjamin/Cummings. Wyman, D. (1990). Trees for American Gardens. New York: Macmillan.
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