During selection of woody ornamentals for market, it is common in smaller nurseries to selectively choose the largest and fullest plants. Selection of the largest plants is considered beneficial in several ways. It promotes good customer relations. Larger plants in containers tend to require more irrigation, so their removal is thought to reduce irrigation frequency, or at least spot hand-watering. Furthermore, reduced plant density is thought to be associated with reduced levels of shoot diseases. However, at some undetermined reductions in plant density, irrigation requirements for sections that have been selectively harvested have been noted to be higher than in similar, adjacent unharvested blocks (R.C. Beeson Sr., Co-owner, Beeson's Rhododendron Nursery, personal communication).
Rose (1984) proposed a theoretical model for transpiration from isolated forest trees. In brief, canopy transpiration of isolated trees was partitioned into two fractions. One was termed vertical transpiration. This was defined as movement of water vapor to the air above the canopy from upper leaves responding to solar radiation, wind, and vapor pressure deficit (VPD). In biophysical terms, this would be the vertical flux density of water vapor moving by eddy diffusion, similar to uniform swaths of grass or dense-canopy agronomic crops (Nobel, 1999). The other fraction was termed horizontal transpiration, driven principally by VPDs caused by eddy movement horizontally through the shaded lower canopy (advection; Nobel, 1999). Rose's model predicted that as forest canopy cover approached 100%, i.e., complete canopy closure, air movement through the lower canopy would decline. At 100% canopy closure, where there is complete overlap among tree canopies, most if not all tree transpiration was predicted to be restricted to a thin, upper layer of a tree canopy; thus, total tree transpiration would be less than that of an isolated tree (Rose, 1984). No actual measurements accompanied this theoretical model.
In 1996, a study was completed that quantified water use during production of three representative woody shrub species of marketable size for an entire year (Beeson, unpublished data). Decades-long below-normal rainfall was prompting legislation limiting irrigation in both nurseries and landscapes. The only representative data were the research nearing completion where plants were grown in closed canopies blocks. Knowing differences in water use existed between closed canopies and isolated plants, experimental validation and defining the theoretical limits of the model proposed by Rose appeared to be an avenue to extend the results of the concluding project to landscape situations and nurseries searching for ways to conserve water. It also appeared to be a way to explain observations noted decades earlier in rhododendron nursery production. The objectives of this research were to verify theoretical reductions in plant evapotranspiration with increases in canopy closure and to begin defining the depth of the upper layer that constituted vertical transpiration. In 2003, objectives were to verify results recorded in 1997 and to test whether results were dependent on plant heights common in woody shrub nurseries.
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