Transpiration and the Ascent

228 7. Transpiration
Leaf area seems to be the best basis for calculating rates of transpiration and
photosynthesis because the receipt of energy is more closely related to the area
than to the weight of leaves. However, as mentioned earlier, Nobel (in Turner
and Kramer, 1980) pointed out that leaf thickness and the amount of mesophyll
tissue can affect the rates of transpiration and photosynthesis per unit of surface
area. Use of the leaf area introduces the necessity of measuring or estimating it.
If leaves can be separated sufficiently, their area can be measured on a commercialleaf
area machine or the area usually can be estimated from a formula relating
area to length or width (Wiersma and Bailey, 1975). Sometimes the relationship
between area and weight of a representative sample can be obtained
and used to estimate the area of a large mass of leaves, such as all of those on a
tree, from their weight. Johnson (1984) and Svenson and Davies (1992) discussed
methods for measurement of surface area of pine needles, and leaf area
measurement is discussed in Lassoie and Hinckley (1991, pp. 466-475).
Transpiration from stems usually is disregarded, but Gracanin (1963) reported
that the rate of water loss from the stems of several herbaceous species
constitutes a significant fraction of the total water loss. Water loss from large
herbaceous stems probably deserves further study. According to Huber (1956),
transpiration from the bark of trees is negligible in comparison to leaf transpiration.
However, enough water is lost from branches of leafless deciduous trees
during sunny winter days to reduce the pressure in the xylem sap and stop
maple sap flow by early afternoon.
Rates 01 Transpiration. Despite various errors inherent in the measurement
of transpiration, some useful information on daily and seasonal differences and
differences among species have been obtained. An example of differences in diurnal
rate between a C3 and a CAM species are shown in Fig. 7.11, and the
seasonal course of transpiration of an evergreen and a deciduous species is
shown in Fig. 7.12. The evergreen transpiration rate was lower than that of the
deciduous tree in the summer, but was higher in the winter because it retained
leaves. Table 7.8 shows transpiration rates, measured gravimetrically on potted
plants, of a number of woody species measured at different locations. It is somewhat
reassuring to find the transpiration rates of well-watered yellow poplar
(Liriodendron tulipifera) seedlings measured in August at Columbus, Ohio, and
Durham, North Carolina, so similar (10.11 and 11.78 g dm- 2day-l). Also, a
comparison of rates of transpiration of six species of European trees measured
by two methods, made by four investigators, all indicated that spruce had the
lowest rate and birch the highest (Kramer and Kozlowski, 1960, p. 299).
Roberts (1983) found that transpiration rates for trees reported from various
places in Europe were quite similar, suggesting that the rate of transpiration of
European forests is relatively stable and lower than would be expected from
potential rates of evaporation.