Trees and other plants are continually buffeted by the elements, and have thus adapted their physiology over many thousands of years to suit the environment in which they naturally will grow. Below, we look at some of the adaptations plants may go through in order to safeguard themselves against the harsh winters that afflict many parts of the world.
Bud break / growth cessation
The phenological timing of spring bud burst and autumn growth cessation will tie in closely with when the period of colder weather ends and begins, respectively – the length of the night is the principal means of determining phenology (Hurme et al., 1997; Leinonen & Hanninen, 2002). Such processes for temperate and boreal tree species ensure that they are in an improved position to cope with the colder weather. For instance, the timing of bud burst in spring is crucial for the avoidance of spring frost damage, which may reduce growth, or in extreme cases kill the specimen (Arora et al., 2003). However, since spring frost damage can not be totally prevented by the adaptation that is late bud burst, it is likely that the adaptation is a compromise between the likelihood of the occurrence of damaging frosts and a sufficiently long growing season, in addition to a species’ ability to cope with cold weather periods before and following bud break (Calmé et al., 1994). Further, deciduous trees will shed their leaves so to reduce the rate of water loss during the winter, and to safeguard their foliage from potentially severe frost damage.
A thick rhytidome (outer bark) can help to protect trees from frost damage during the winter, by acting as a ‘buffer’ zone that shields the phloem, cambium and xylem from significant cold damage. This is particularly necessary with conifers, as frost-related injuries correlate highly with bark thickness. The thicker bark can retain heat in the cambium zone, thus keeping bark temperature above freezing even during winter in many periods (Gurskaya & Shiyatov, 2006). Species such as Sequoia giganteum and Pseudotsuga menziesii have very thick rhytidomes – up to 60cm for the sequoia (Dujesiefken & Liese, 2015).
As cell temperature approaches freezing point, accumulations of sugars, organic compounds and amino acids within cells will artificially lower the cell’s freezing point, and allows mild cold tolerance up to around -1°C to -2°C (Morin et al., 2007; Thomas, 2000). As temperature drops further, cells can essentially ‘supercool’ themselves so their temperature can drop to around -40°C but the water inside will not freeze. In this state, the cells act as if they are a solid when in fact they are still liquid. This is partly achieved by proteins within the cell changing in structure, acting as an anti-freeze protein (AFP) of sorts. The plasma membrane will also become ‘gel-like’, changing its state from a liquid during the warmer temperatures (George et al., 1974; Karban, 2015; Thomas, 2000).
Assuming both of the above adaptations don’t work as it’s that inhospitably cold, cells will shift water from within their structure and store it extracellularly, allowing the water to then freeze outside of the cells, and tolerating the temporary cellular desiccation during these absurdly cold conditions. Such a tactic can give trees tolerance up to around -196°C. As the water outside the cells freeze, the small amount of heat released also helps to keep the cells themselves slightly warmer (Ashworth & Abeles, 1984; Thomas, 2000).
Research has however found that plants will ‘learn’ how to adapt to the cold, so trees exposed to very cold conditions annually are more readily able to undertake such cellular changes (Karban, 2015). For example, one study saw naive rye that was killed at -5°C, whilst ‘hardened’ rye that had been subject to yearly cold periods could withstand -30°C temperatures (Thomashow, 1999).
Xylem embolism protection
Physiological studies of the responses of temperate woody plants to winter xylem embolism, which involves the freezing of xylem water, suggest that such plants minimize the impact of overwinter embolism by replacing previously-embolised vessels with new and functional vessels every year, and / or by re-filling embolised vessels by generating positive xylem pressures – at least, positive xylem pressure is found in some species, such as Acer spp., Juglans spp., and Betula spp. – to re-assimilate gas bubbles that appeared during freezing (Sakr et al., 2003; Tyree, 1983; Zhu et al., 2000).
Internal regulation of temperature
Certain plants are even able to adapt – via their mitochondria – cellular respiration rates, sometimes increasing respiration rate to the equivalent of that of a hummingbird in flight (Nagy et al., 1972). This increase in respiration raises internal temperature, thereby reducing risk of cells freezing (Karban, 2015) when temperatures fall during colder periods (Knutson, 1974; Nagy et al., 1972).
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Ashworth, E. & Abeles, F. (1984) Freezing behavior of water in small pores and the possible role in the freezing of plant tissues. Plant Physiology. 76 (1). p201-204.
Calmé, S., Bigras, F., Margolis, H., & Hébert, C. (1994) Frost tolerance and bud dormancy of container-grown yellow birch, red oak and sugar maple seedlings. Tree Physiology. 14 (12). p1313-1325.
Dujesiefken, D. & Liese, W. (2015) The CODIT Principle: Implications for Best Practices. USA: International Society of Arboriculture.
George, M., Burke, M., & Weiser, C. (1974) Supercooling in overwintering azalea flower buds. Plant Physiology. 54 (1). p29-35.
Gurskaya, M. & Shiyatov, S. (2006) Distribution of frost injuries in the wood of conifers. Russian Journal of Ecology. 37 (1). p7-12.
Hurme, P., Repo, T., Savolainen, O., & Pääkkönen, T. (1997) Climatic adaptation of bud set and frost hardiness in Scots pine (Pinus sylvestris). Canadian Journal of Forest Research. 27 (5). p716-723.
Karban, R. (2015) Plant Sensing & Communication. USA: University of Chicago Press.
Knutson, R. (1974) Heat production and temperature regulation in eastern skunk cabbage. Science. 186 (4165). p746-747.
Leinonen, I. & Hanninen, H. (2002) Adaptation of the timing of bud burst of Norway spruce to temperate and boreal climates. Silva Fennica. 36 (3). p695-701.
Morin, X., Améglio, T., Ahas, R., Kurz-Besson, C., Lanta, V., Lebourgeois, F., Miglietta, F., & Chuine, I. (2007) Variation in cold hardiness and carbohydrate concentration from dormancy induction to bud burst among provenances of three European oak species. Tree Physiology. 27 (6). p817-825.
Nagy, K., Odell, D., & Seymour, R. (1972) Temperature regulation by the inflorescence of Philodendron. Science. 178 (4066). p1195-1197.
Sakr, S., Alves, G., Morillon, R., Maurel, K., Decourteix, M., Guilliot, A., Fleurat-Lessard, P., Julien, J., & Chrispeels, M. (2003) Plasma membrane aquaporins are involved in winter embolism recovery in walnut tree. Plant Physiology. 133 (2). p630-641.
Thomas, P. (2000) Trees: Their Natural History. UK: Cambridge University Press.
Thomashow, M. (1999) Plant cold acclimation: freezing tolerance genes and regulatory mechanisms. Annual Review of Plant Biology. 50 (1). p571-599.
Tyree, M. (1983) Maple sap uptake, exudation, and pressure changes correlated with freezing exotherms and thawing endotherms. Plant Physiology. 73 (2). p277-285.
Zhu, X., Cox, R., & Arp, P. (2000) Effects of xylem cavitation and freezing injury on dieback of yellow birch (Betula alleghaniensis) in relation to a simulated winter thaw. Tree Physiology. 20 (8). p541-547.
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