Cooling
For most leaves overheating is a far greater risk than freezing. Exposed to the light and with no form of cooling a leaf's temperature can rise by over 1°C/second (Vogel 2009).
Leaves can maintain a temperature cooler than ambient by evaporative cooling and by reduction of heat absorbance.
Reduction of absorbance seems to be more important as leaves do not necessarily transpire more in hotter environments (Nicotra
et al. 2008). Reduction of absorbance can be
generated by reflection of light by hairs or waxes, or by changing the leaf angle so the leaf is not so exposed to the sun. In the dry woodland of Western Australia species with
more upright leaves are found in higher light (Bragg and Westoby 2002).
Another way of preventing the leaf from heating up is by reducing the surface area, opting for more, smaller leaves or a highly dissected leaf. This works by increasing heat dissipation due to a smaller boundary layer. Models show that heat is lost more easily from irregularly shaped
leaves, and that entire leaves are very sensitive to orientation but lobed leaves much less so (Vogel 1970). Producing lobed leaves rather than entire ones would therefore remove the constraint on leaf orientation, requiring less investment in support structures.
Correlative studies show that compound leaved species are found in environments with high light, and less commonly in the shade (Stowe and Brown 1981). Intraspecific studies have also seen correlations seen between dissected
leaves and sunnier habitats (Andersson and Shaw 1994), and warmer climates (Gurevitch 1988, 1992a, 1992b). Viola septemloba
is a perennial violet of the south east US which produces cordate and lobed leav
es on one plant. During winter an average of 15% of the leaves are lobed, at midsummer the proportion rises to 73%. Lobed leaves average 2.2°C cooler than cordate (Winn 1999). Variation in dissection within plant also has a measurable effect on cooling.
Sassafras albidium produces fewer lobed leaves in the shade and in the middle of branches and more lobed leaves in open environments and at the proximal and distal ends of branches. The lobed leaves cool faster and are more photosynthetically active (De Soyza and Kincaid 1991). It has even been suggested that the leaf dissection produced by
herbivore activity could be an advantage to plants in the height of summer (Vogel 2009).
Dissected leaves are associated with high photosynthetic activity in Pelargonium,
Ranunculus repens and in cotton (Lynn and Waldren 2002; Stiller et al
. 2004; Nicottra et al. 2008). However, in none of these cases was it clear that this affect was due to cooling and thinner boundary layers as the Pelargonium measurements were taken in a wind tunnel to remove boundary layers,
R. repens is an aquatic plant, and in cotton leaf dissection was strongly related to variation in water use efficiency which was thought
to have a greater effect on the results. Gurevitch (1988, 1992) showed that the more dissected ecotype of Achillea
mille-folium maintained a lower temperature than the less dissected form but had lower photosynthetic rates. The difference in photosynthetic rates is seen at both the warm
temperatures native to the dissected form and at the cooler temperatures native to the less dissected form, and the less dissected form had higher photosynthetic rates at the higher
temperatures. This suggests that leaf dissection is not always an adaptation to cool leaves to a temperature at which photosynthesis is most productive.