ESTABLISHMENT IN TREESHELTERS .2. EFFECT OF SHELTER COLOR ON GAS-EXCHANGE AND HARDINESS

We investigated the microclimate, gas exchange, and growth of field-gr own Norway maple (Acer platanoides L.) and green ash (Fraxinus pennsyl vanica Marsh) trees nonsheltered, and in brown and white shelters. She lter microclimate-air temperature (T-a), vapor pressure deficit (VPD), and radiation-and tree leaf area, growth in diameter, stomatal conduc tance (g(s)), and photosynthesis were measured during the first growin g season after bare-root transplanting. Bark temperature in midwinter were also measured. Treeshelter microclimate was greenhouse-like compa red to ambient conditions, as shortwave radiation was lower, and midda y T-a and relative humidity were higher. Although trees in shelters ha d greater shoot elongation and higher g(s) than trees grown without sh elters, photosynthesis was not different. White shelters allowed 25% m ore shortwave radiation penetration and increased T-a by 2 to 4 degree s C and VPD by 0.5-1 kPa over brown shelters. However, tree growth and gas exchange generally were not affected by shelter color. Winter inj ury was increased for trees in shelters and varied with species and sh elter color. Both species exhibited shoot dieback in shelters the spri ng following a winter where bark temperatures varied 40 to 50 degrees C diurnally. More new growth died on maple, particularly in white shel ters where several trees were killed. These data suggest that supraopt imal summer and winter temperatures may reduce vigor and interfere wit h cold tolerance of some species grown in shelters.