Effects of Soil Matric Potential On Fraser Fir (Abies fraseri) Growth and Water Stress Under Drip Irrigation In Michigan

Fraser fir (Abies fraseri) is a major species for Christmas tree production in the eastern and Midwestern United States of America. However, the species is commonly raised out of its natural range in situations where environmental conditions are not suitable for optimal growth. As a result, supplemental irrigation is commonly provided to meet the physiological needs and enhance growth. This study investigated growth and crop water stress in response to irrigation based on soil matric potential. The experiments were conducted at two research locations in Michigan, where automated irrigation system triggered by soil matric potential levels (45 KPa, 35 KPa, 25 KPa, 15 KPa) and a non-irrigated controls were setup in the spring of 2006. Tree height and caliper diameter growth were measured and correlated with soil matric potential levels to determine the optimal irrigation thresholds for A. fraseri. The effect of irrigation on tree water stress was evaluated based on crop water stress index, calculated from difference of canopy and air temperatures measured with infra red thermometers. The stem water potential was also measured using a pressure chamber. Initiating irrigation based on target soil matric potential values of 15KPa and 25 KPa significantly improved height and basal area growth of A. fraseri in smaller size classes.  Soil matric potential of 15 KPa also reduced the CWSI, maintaining it within the range of 0.2-0.6 known as suitable for most agronomic crops. Measurements of the stem water potential were more inconsistent across irrigation treatments for all size classes.