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| We investigated water use and potential drought avoidance of Norway maple (Acer platanoides L.) and green ash (Fraxinus pennsylvanica Marsh) seedlings grown in protective plastic shelters. Gravimetric tree water use and reference evapotranspiration for fescue turf (ETo) were monitored for 1to 3 days during the growing season. Water use of trees was 8-14% of Eto in shelters vs. 29-40% for trees not in shelters. Trunk diameter was affected more than whole-tree water relations by lack of irrigation, suggesting that the non-irrigated trees were subjected to only mild water stress. Shelters did not improve drought avoidance, as water potentials were generally more negative and trunk diameter increment was lower for non-irrigated trees in shel-ters. Maples in shelters were affected more adversely by lack of water than were ash. Higher temperatures in shelters also may have reduced trunk growth. Air temperatures were 13 oC warmer than ambient in non-irrigated shelters, but only 5 C warmer in irrigated shelters. Tree shelters can reduce transpiration rates by over half, but benefits from reduced water loss may be offset by negative effects of higher air temperatures. Shelters reduced cold hardiness of both species, but maple was affected more than ash.
Tree shelters, developed initially to reduce herbivory on newly planted tree seedlings, increase tree height (Burger et al., 1992; Potter, 1988; Svihra et al., 1993) because greenhouse-like conditions raise temperature and humidity. In addition, shelters block air movement over the foliage, thereby increasing boundary-layer resistance to water movement and reducing over-all transpiration rates (Kjelgren, 1994). Reducing transpiration decreases soil-water depletion, potentially improving drought avoidance. In large-scale plantings of small trees, mortality is of-ten high during the first year after planting (Cleveland and Kjelgren, 1994), particularly if trees are not irrigated (Nambiar and Sands, 1991). Tree shelters may help in establishing small trees in large-scale plantings and landscapes when routine irrigation is not possible. In addition, the ability to predict water use in shelters can permit strategic timing of irrigation to conserve water and labor. The objectives of this study were to quantify water use of trees in shelters relative to reference evapotranspiration, and determine if shelters increase water-stress avoidance in bare-root whips during first-year establishment. Supproting pdf: Establishment of Norway Maple and Green Ash in Treeshelters I: Shelters Reduce Growth, Water Use, and Hardiness, but not Drought Avoidance |
