Transpiration canopy

Subsurface runoff. When precipitation hits the land surface, the vast majority does not go directly into the network of streams and rivers in fact, it may be cycled several times before ever reaching a river and the ocean. Instead, most precipitation that is not intercepted by the vegetation canopy and re-evaporated infiltrates into the soil, where it may reside as soil moisture, percolate down to ground-water, or be transpired by plants. 

Sellers, P. J., Berry, J. A., Collatz, G. ]., Field, C. B. and Hall, F. G. (1992). Canopy reflectance, photosynthesis, and transpiration. III. A reanalysis using improved leaf models and a new canopy integration scheme. Remote Sens. Environ. 42,187-216. 

Mature phreatophyte trees (poplar, willow, cottonwood, aspen, ash, alder, eucalyptus, mesquite, bald cypress, birch, and river cedar) typically can transpire 3700 to 6167 m3 (3 to 5 acre-ft) of water per year. This is equivalent to about 2 to 3.8m3 (600 to 1000 gal) of water per tree per year for a mature species planted at a density of 600 trees per hectare (1500 trees per acre). Transpiration rates in the first two years would be somewhat less, about 0.75 m3 per tree per year (200 gal per tree per year), and hardwood trees would transpire about half the water of a phreatophyte. Two meters of water per year is a practical maximum for transpiration in a system with complete canopy coverage (a theoretical maximum would be 4 m/yr based on the solar energy supplied at latitude 40°N on a clear day). 

California trees, citrus tree trunk and canopy growth, leaf nitrogen level, fruit yield, and fruit quality were decreased by competition from annual weeds and bermudagrass (Jordan, 1981). Suzuki (1981) reported that in Japan, weeds in summer absorb and transpire large amounts of water from the soil and compete with citrus trees. Moisture and nitrogen levels in the soil decreased particularly where large crabgrass and tufted knotweed were present (Ito and Ukei, 1981). 

During the daytime, a transpiring and photosynthesizing plant community as a whole can have a net vertical flux density of CO2 (/coz) downward toward it and a net vertical flux density of water vapor (71W) upward away from it into the turbulent air above the canopy. These flux densities are expressed per unit area of the ground or, equivalently, per unit area of the (horizontal) plant canopy. Each of the flux densities depends on the appropriate gradient. The vertical flux density of water vapor, for example, depends on the rate of change of water vapor concentration in the turbulent air, c, with respect to distance, z, above the vegetation ... 

The flux density of water vapor just above the canopy, which includes transpiration from the leaves plus evaporation from the soil, is often termed evapotranspiration. For fairly dense vegetation and a moist soil, evapotrans-piration is appreciable, usually amounting to 60 to 90% of the flux density of water vapor from an exposed water surface (such as a lake) at the ambient air temperature. The daily evapotranspiration from a forest is often equivalent to a layer of water 3 to 5 mm thick, which averages 2 to 3 mmol m-2 s-1 over a day. At noon on a sunny day with a moderate wind, Jw above a plant canopy can be 7 mmol m-2 s-1. Using Equation 9.4 (Ac v = 7wr ) and our value for of 30 s m-1, we note that over the first 30 m of the turbulent air... 

Under normal conditions of water availability, values for LAI in pastures of B. brizantha have been measured above 4.0. However with the establishment of a water deficit in the soil, these values decrease to below two or even lower in pastures of P. maximum (Roberts et al. 1996). A similar situation is found in abandoned pastures in eastern Amazonia, where a reduction of approximately 68% of green tissue has been observed in the dry season, while in an adjacent area of primary forest this reduction was only 16% (Nepstad et al. 1994). Primary forests, which have deep root systems and little seasonal variation in LAI, maintain stable subcanopy microclimatic conditions and transpirational flux, even during the dry season. Because of an evergreen forest canopy, the return of the rainy season has less impact on the microclimate near the soil in the forest than in the pastures, and the deep soil water stores are also more efficiently recharged in the forest. 

C., Vygodskaya, N. N., and Ziegler, W. (2000). Canopy transpiration in a chronosequence of central Siberian pine forests. Global Change Biol. 6, 25-37. 

P, phenological type (e, evergreen, r, raingreen, s, summergreen) minimum canopy conductance (mm/s) Eni.tx maximum daily transpiration rate (mm/day) percent of roots in the top 30 cm of soil L, leaf longevity (months) optratio, the maximum allowed C/C, ratio kk, the Beer s law extinction coefficient minimum monthly temperature for C3 photosynthesis 7 curve, modifier to the curve response of photosynthesis to temperature Rfact, modifier to the curve response of maintenance respiration to temperature Alloc, modifier to the minimum allocation Fire, the soil moisture percent threshold at which a fire day is counted. 

Sellers, P.J., 1985. Canopy reflectance, photosynthesis and transpiration. Int. J. Remote Sens. 6, 1335-1372. 

The already mentioned feedback between the water and carbon cycle (see also Fig. 2.31) is not only given by the essential role of water in sustaining all forms of life. Observational evidence indicates that transpiration rates of plants are high at the same time as CO2 fixing by the plants, and hence CO2 flux from the atmosphere to the plant canopy is large. When an environment is humid, plants grow more rapidly, draw more CO2 from the atmosphere, and release more water to the atmosphere. 

Water Stress Effects on Canopy Photosynthesis, Temperature, Transpiration and Shedding of Leaves and Fruit in Cotton 733... 

Direct measurement of transpiration is difficult for all but relatively small plants that can be grown in weighing lysimeters or enclosed in chambers in which the flux of water can be ealculated from the humidity increase in the enclosure. Common approaches used to quantify transpiration in the field include precipitation minus runoff on gauged watersheds, energy balance equations (Allen et al., 1998), eddy covariance (water vapor gradients above and below canopy) (Baldocchi, 2003), hydrologic models, and sap flow measurements (Vose et al., 2003). 

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