ESTABLISHMENT IN TREESHELTERS .1. SHELTERS REDUCE GROWTH, WATER-USE, AND HARDINESS, BUT NOT DROUGHT AVOIDANCE

We investigated water use and potential drought avoidance of Norway ma ple (Acer platanoides L.) and green ash (Fraxinus pennsylvanica Marsh) seedlings grown in protective plastic shelters. Gravimetric tree wate r use and reference evapotranspiration for fescue turf(ETo) were monit ored for 1 to 3 days during the growing season. Water use of trees was 8% to 14% of ETo in shelters vs. 29% to 40% for trees not in shelters . Trunk diameter was affected more than whole-tree water relations by lack of irrigation, suggesting that the nonirrigated trees were subjec ted to only mild water stress. Shelters did not improve drought avoida nce, as water potentials were generally more negative and trunk diamet er increment was lower for nonirrigated trees in shelters. Maples in s helters were affected more adversely by lack of water than were ash. H igher temperatures in shelters also may have reduced trunk growth. Air temperatures were 13 degrees C warmer than ambient in nonirrigated sh elters, but only 5 degrees C warmer in irrigated shelters. Tree shelte rs can reduce transpiration rates by over half, but benefits from redu ced water loss may be offset by negative effects of higher air tempera tures. Shelters reduced cold hardiness of both species, but maple was affected more than ash.