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Keywords:

  • bud break;
  • climate change;
  • heat transfer;
  • Nusselt number;
  • tree line

The temperatures that leaves, buds and stems attain is generally different from air temperature. The difference is often critical for survival, especially in extreme environments. For example, the leaves of some desert species avoid lethal temperatures by transpirational cooling (Althawadi & Grace, 1986). By contrast, leaves of some dwarf plants in mountains and in polar regions can grow only because they become significantly warmer than the cold air above them, thus allowing normal cell division and expansion to occur (Wilson et al., 1987). Only a small fraction of all species generate significant heat by respiration; generally, the only means of immediate and direct thermal regulation is by adjustment of transpiration rates and leaf display. Indirectly, temperature regulation of plant organs is also achieved through evolutionary adjustment of size, shape and plant form. It is possible to calculate leaf temperature by making assumptions about the rates of heat and water vapor transfer, as was first done many years ago (Gates & Papian, 1971; Grace, 1977). However, the most difficult part of the heat balance equation to calculate is convection. Engineers have measured convection from various objects in wind tunnels and developed empirical equations for calculating heat transfer from inert subjects such as flat plates, spheres and cylinders (Pitcairn & Grace, 1985), but plants and their organs are exceedingly complex in shape and air flows in wind tunnels are often not turbulent as they are in the natural environment; consequently, these empirical relationships from engineering texts may not always apply (Grace et al., 1980; Dixon & Grace, 1983; Chen et al., 1988). Moreover, plant structures vary in shape: they do not always resemble plates, spheres and cylinders and this poses a problem that requires different approaches altogether, based upon computational fluid dynamics rather than experiments in wind tunnels (Roth-Nebelsick, 2001). It is possible that plants in cold places have novel solutions to keeping warm that have so far eluded engineers, through evolution of size, shape and ornamentation, and therefore careful measurements of heat transfer from complex botanical structures like those used by Michaletz & Johnson in this issue of New Phytologist (pp. 87–98) are always welcome. These authors used a wind tunnel to vary the wind speed; and determined the heat transfer rate by measuring the warming rates of conifer buds that had been chilled before being mounted in the wind tunnel. Conifers are among the most difficult subjects for this kind of experimentation as the structures are complex and vary as the buds are surrounded by needles and thus there may be a shelter effect.

‘If we are to properly estimate the effect of climate change on the onset of growth in trees, and on the rate of plant growth it is not enough to assume that physiology is driven by air temperature.’

Convection from buds is less than expected

  1. Top of page
  2. Convection from buds is less than expected
  3. Larger scale perspective
  4. Wind speed changes bud temperature by a crucial few degrees
  5. References

The authors show that there is a marked difference between the rates of heat transfer between cylinders and cylindrical plant parts. The heat transfer from the buds was found to be markedly less than expected from empirical relationships found in the literature.

Buds are ‘dry’ structures, having few or no stomata and therefore not exhibiting transpirational cooling. What this means is that the buds would be expected to be warmer than the air when the sun is shining, as long as the orientation of the bud is such that it intercepts solar radiation (i.e. it is not much shaded by the needles). Conversely, at night, when the radiant energy balance is negative (long-wave energy streaming from the bud to the sky) the buds may be expected to be colder than the atmosphere. The effect might have been even greater had it been possible to explore the lower range of wind speeds that characterize free convection and mixed (free and forced) convection.

Larger scale perspective

  1. Top of page
  2. Convection from buds is less than expected
  3. Larger scale perspective
  4. Wind speed changes bud temperature by a crucial few degrees
  5. References

The authors have not yet taken the study to its logical conclusion which would be to evaluate the entire energy balance of the structures and compute the diurnal and seasonal trends of temperatures that might be expected from the heat transfer data they have obtained. This would be well-worth doing, as it is often assumed that the canopy of conifer trees and forests is at the same temperature as that of the air. The statement is based mostly on the idea that these forests are aerodynamically rough and therefore the air temperature in the canopy is well-coupled to the air in the atmosphere above through convection; and secondly on the idea that needle-leaves have a thin boundary layer and therefore they are especially well-coupled to the temperature of the air in the canopy (McNaughton & Jarvis, 1983). Of course, this may be true for needles but not for bulky organs like stems, cones and buds, which heat up significantly in bright sunshine, as has been shown in pine trees growing near the tree line in Scotland by inserting extremely fine thermocouples into the buds of pine trees in the natural environment (Wilson et al., 1987; Grace et al., 1989). For mature pines the bud temperatures were 4°C warmer than the air by day, and even warmer in the dwarf krummholz pines near the tree line. It has also been shown that the needles of conifers heat up significantly because of the clumping effect, which tends to retard heat transfer (Martin et al., 1999). A full study of these processes does need to take into account the structure and height of the vegetation, as the convective heat transfer is a strong function of vegetation height, as has been demonstrated many times and discussed by McNaughton & Jarvis (1983).

These larger-scale considerations will also be important when the results are used by fire ecologists to estimate burning rates. Of course, the temperatures in that case are much higher, and natural convection will dominate forced convection under such conditions; nevertheless, much of the theory is essentially the same.

Wind speed changes bud temperature by a crucial few degrees

  1. Top of page
  2. Convection from buds is less than expected
  3. Larger scale perspective
  4. Wind speed changes bud temperature by a crucial few degrees
  5. References

The reason this work ought to be developed is that models purporting to investigate the impacts of climate change invariably assume that ‘canopy temperature’ is equal to air temperature when clearly the plant organs in the canopy can be both warmer and cooler than the air. How large is this effect? Using the equations from Michaletz & Johnson (2006) we can see that in spring sunshine (assume net radiation is 200 W m−2) the temperatures attained in buds are substantially warmer than they would be in cylinders of the same size, and are highly dependent on wind speed (Fig. 1). Plant processes of bud break, and leaf expansion are very temperature-sensitive as many studies demonstrate (Juntilla, 1986; Murray et al., 1989). If we are to properly estimate the effect of climate change on the onset of growth in trees, and on the rate of plant growth it is not enough to assume that physiology is driven by air temperature. Plant temperature is a strong function of absorbed radiation, wind speed and wetness, and may be profoundly influenced by shape factors that provide shelter or stimulate turbulence.

image

Figure 1. The effect of wind speed on the temperature of the buds of Picea glauca, estimated from the relationships given in Michaletz & Johnson (2006). Assumptions made were: net radiation is 200 W m−2 and the system is dry (i.e. no evapotranspiration). Symbols are: triangles, cylinder; filled circles, buds alone; open circles, combined data from buds and shoots.

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References

  1. Top of page
  2. Convection from buds is less than expected
  3. Larger scale perspective
  4. Wind speed changes bud temperature by a crucial few degrees
  5. References
  • Althawadi AB, Grace J. 1986. Water use by the desert cucurbit Citrullus colocynthis (L.) Schrad. Oecologia 70: 475480.
  • Chen JM, Ibbetson A, Milford JR. 1988. Boundary-layer resistances of artificial leaves in turbulent air. I. Leaves parallel to the mean flow. Boundary-Layer Meteorology 45: 137156.
  • Dixon M, Grace J. 1983. Natural convection from leaves at realistic Grashof numbers. Plant, Cell & Environment 6: 665670.
  • Gates DM, Papian LE. 1971. Atlas of Energy Budgets of Plant Leaves. London, NY: Academic Press.
  • Grace J. 1977. Plant Response to Wind. London: Academic Press.
  • Grace J, Allen S, Wilson C. 1989. Climate and meristem temperatures of plant communities near the tree-line. Oecologia 79: 198204.
  • Grace J, Fasehun FE, Dixon M. 1980. Boundary layer conductance of the leaves of some tropical timber trees. Plant, Cell & Environment 3: 443450.
  • Juntilla O. 1986. Effects of temperature on shoot growth in northern provenances of Pinus sylvestris L. Tree Physiology 1: 185192.
  • Martin TA, Hinkley TM, Meinzer FC, Sprugel DG. 1999. Boundary layer conductance, leaf temperature and transpiration of Abies amabilis branches. Tree Physiology 19: 435443.
  • McNaughton KG, Jarvis PG. 1983. Predicting effects of vegetation changes on transpiration and evaporation. In: Water Deficits and Plant Growth, Vol. VII (ed. TTKozlowski). San Diego: Academic Press, pp. 1–47.
  • Michaletz ST, Johnson EA 2006. Foliage influences forced convection heat transfer in conifer branches and buds. New Phytologist 170, 8798.
  • Murray MB, Cannell MGR, Smith RI. 1989. Date of budburst of fifteen tree species in Britain following climatic warming. Journal of Applied Ecology 26: 693700.
  • Pitcairn CER, Grace J. 1985. Convective heat transfer from leaves, In: Grace J (ed.) The Effects of Shelter on the Physiology of Plants and Animals. Lisse: Swets & Zeitlinger, pp. 115.
  • Roth-Nebelsick A. 2001. Heat transfer of rhyniophytic plant axes. Review of Palaeobotany and Palynology 116: 109122.
  • Wilson C, Grace J, Allen S, Slack F. 1987. Temperature and stature, a study of temperatures in montane vegetation. Functional Ecology 1: 405414.