Lindsey and Bassuk (1991) data. 
Michael Knee, Daniel K. Struve, Michael H. Bridgewater, and Joseph W. Phillips, Department of Horticulture and Crop Science, The Ohio State University.
The temperature of the medium for containers on gravel was not as high as expected, and there was not much difference from the temperature of containers in mulch. However, these temperatures were recorded for the bulk medium, and they may have been higher on the sun-exposed side of containers on gravel. The occurrence of local heating is consistent with the greater evaporation from containers of medium on gravel, by comparison with containers in mulch. Local heating may also be the reason why root growth was lower in containers on gravel, and this effect was most noticeable for large plants whose roots were approaching the edge of the container.
Shoot growth (height and dry weight) was adversely affected by growing on gravel only in R. pseudoacacia, and leaf area seemed to be unaffected for any species.
The higher water use by the larger trees (R.pseudoacacia and C. canadensis) in mulch than on gravel is consistent with their larger root systems and less temperature stress on the root system. Higher water use could also be associated with higher rates of photosynthesis since open stomata would permit influx of carbon dioxide as well as efflux of water vapor.
The only consistent result of this hypothetical enhancement of photosynthesis seems to have been an increase in root mass. This could include food reserves for future growth, but the limited data for growth after planting out do not show any consistent effect of production method on plant performance.
The rate of transpiration per unit of leaf area decreases as trees grow, so comparisons between species are most easily made at similar leaf areas. Lindsey and Bassuk (1991) showed a relationship between transpiration and leaf area based on averages for four tree species (Figure 6). G. triacanthos and R.pseudoacacia fit this relationship quite well, but G. dioicus and C. canadensis had lower rates of transpiration than predicted by the relationship (Figure 6).
|
Figure 6. Ratio of average transpiration (from regression of
water loss on leaf area) to potential evapotranspiration (Eo) in
relation to average leaf area for trees in containers, compared
to published data for other tree species. Cc Cercis canadensis,
Gd Gymnocladus dioicus, Gt Gleditsia triacanthos, Rp Robinia
pseudoacacia, Lindsey and Bassuk (1991) data. |
The transpiration rate of a species helps to predict its water requirement under production conditions when water is not limiting. The transpiration rate may not be an indicator of water requirement in the landscape or ability to withstand drought. Levitt et al. (1995) showed that the drought-tolerant species, Prosopis alba, had a higher transpiration rate than the less tolerant Quercus virginiana under well-watered conditions.
In conclusion, root growth can be inhibited in containers exposed to the sun, especially when the root system grows to the edge of the container. It is not clear whether this has any lasting effect on growth after transplanting to the landscape. Honeylocust (Gleditsia triacanthos) and black locust (Robinia pseudoacacia) have similar water requirements to other shade trees studied by Lindsey and Bassuk (1991), but Kentucky coffee tree (Gymnocladus dioicus) and redbud (Cercis canadensis) had lower water requirements.