evaporative stresses, which reduce the supply rate of carbon dioxide flowing into leaves. The products of reactions can suppress rates of further reaction if they accumulate in cells, which can be a problem at high temperatures. Rates of photosynthesis do not increase with a simple exponential trend like rates of respiration (Figure 2.3). The temperature effect on rates of photosynthesis might have a humped shape, reflecting the balance between carbon gains and losses in leaves, as photosynthesis declines in response to rising photorespiration (the diversion of energy to producing water rather than sugar).
The temperature of an object, such as a tree leaf, represents the thermal energy contained within the object. Thermal energy is the kinetic energy of molecules; the molecules of nitrogen in a volume of air move at velocities of about 450 to 500 m per second. A molecule of nitrogen might move about 100 nm before colliding with another air molecule, which translates into billions of collisions for each molecule each second. Molecules move faster as temperatures rise, leading to more collisions and exchanges of energy that we measure as temperature. Molecules are packed more densely in liquids and solids, leading to higher rate of collisions (and transfers of heat).
FIGURE 2.4 Rates of chemical processes increase with increasing temperature, as reflected in the rising curve for respiration of trees in a rainforest in Costa Rica
(Source: Based on Cavaleri et al. 2010).
The rate of total (gross) photosynthesis by fully illuminated leaves in a rainforest in Panama also rises with temperature to an optimum near 30 °C, and then declines
(Source: Based on Slot and Winter 2017).
The two graphs present the same data, with the upper graph focusing primarily on how each process responds to temperature, while the lower graph uses a single Y axis to give show how much larger photosynthesis is relative to respiration (across the temperature gradient).
Temperature is the Balance Point Between Energy Gains and Losses
A leaf increases in thermal energy when exposed to radiant energy from the sun, and when exposed to hot air. Absorption of solar radiation raises leaf thermal energy, raising leaf temperature. The temperature of the leaf continues to rise until the gain of energy is offset by energy losses. Leaves can lose energy by exposure to cooler air, by evaporating water, and by emitting (shining) radiant energy.
FIGURE 2.5 On an afternoon when air temperature was 25 °C, the temperatures of leaves of beech and spruce were sustained at higher levels as a result of heating by sunlight. The temperatures of leaves covered a range for each species, because of varying angles of leaf exposure to the sun's rays, and the variable levels of shade in the crowns. Beech leaves were hotter (most were about 30 °C) than spruce leaves (most were about 27 °C), indicating beech leaves had higher energy gain (absorbing more light), or lower energy loss (lower transpiration, or poorer coupling with the cooler air).
Source: Data from Leuzinger and Körner (2007).
Objects such as tree leaves and bird feathers, lose energy to the air by conduction and convection if the air is cooler than the object (Figure 2.5). The rate of energy transfer into the air depends on the gradient in temperatures, and how well the object is “coupled” to the air. Large surfaces, such as beech leaves, have stable layers of air molecules comprising a thick boundary layer, slowing energy transfer from the leaf into the air at large. Small surfaces have thinner boundary layers allowing faster transfer of thermal energy; the boundary layer for the spruce needles was only one‐fourth that of the beech leaves, explaining a large portion of the difference in temperatures between species. The coupling of surfaces to the atmosphere is improved by winds, as the thickness of the boundary layer declines with increasing wind. A wind of a few meters per second (a few kilometers per hour) can lower leaf temperatures by a few degrees, and increase the delivery of CO2 to stomata along with loss of water to the air.
The evaporation of water requires large amounts of energy, about 2.4 MJ/l evaporated. This energy flow is referred to as “latent heat,” because the temperature of the water molecules remains unchanged as the phase changes from liquid to gas. The amount of energy removed from an object such as a leaf depends of course on the rate of evaporation, and rates of evaporation depend on the water status of the plant, the dryness of the air, and the presence of wind to reduce the boundary layer. Evaporation from a leaf exposed to dry air without any wind may lower leaf temperature by about a degree; the addition of a light wind can increase evaporation enough to cool leaves by 3–5 °C.
All Objects Shine; Hot Objects Shine Brightly
Radiation from the sun can be felt by holding out a hand in bright sun; the hand absorbs the solar radiation and warms up. We can see the hand illuminated by sunlight, because some of the light reflects from the hand and reaches our eyes. Another radiation story is also occurring, invisible to our eyes. The hand is also “shining” like the sun, emitting radiation to the environment around it. The cooler temperature of the hand means the wavelengths of radiation emitted are much longer than sunlight (a long wavelength is the same as a low frequency). The emission of radiation by the hand removes energy, and would lead to cooling of the hand unless another source of energy kept the hand warm.
The radiant energy emitted by the sun is called “shortwave” radiation, because of the short wavelengths (between about 400 and 700 nm for the visible portion of the solar spectrum). Objects at temperatures commonly encountered in forests “shine” or emit radiation at longer wavelengths, on the order of 10 μm. Longwave radiation may be sensed by skin as warmth, even though the weak radiation cannot be detected by our eyes.
The differences in the wavelength of light that shines (or is “emitted”) from an object are associated with very large differences in the amount of energy transfer. A very hot object emits shorter wavelengths of light, along with much higher amounts of energy (Figure 2.6). An object at room temperature (about 20 °C) shines out about 420 W of energy for each m2 of surface area. If the same object warmed to 37 °C (typical human body temperature), the energy transfer would rise to 520 W m−2. The loss of this emitted energy cools the objects, which in turn lowers the rate of further losses of energy. The energy loss may be counteracted by any energy being added to the surface from the environment (in the form of radiation, or hot air), and temperatures stabilize when energy gains from the environment match the energy losses.
FIGURE 2.6 All objects emit radiation to the environment, and hotter objects emit more energy (for a given surface area) than cooler objects. If the temperature of a soil surface increased from 20 to 40 °C, the emission of energy from the soil into the air
