Simulated Aspen Understory Microclimate Effects on Alfalfa Growth

Understory plant growth in agroforestry systems is strongly influenced both positively and negatively by the microclimate created by the overstory. Growth chambers were used to isolate and compare the effects of photosynthetically active radiation (PAR) and humidity (and resultant leaf/atmosphere vapor pressure differences) on the early growth and development of alfalfa (Medicago sativa L.). Leaf temperature corresponded closely to air temperature and did not vary between humidity treatments. Predictably, PAR had a strong effect on alfalfa growth with significant effects on the size and mass of all yield components, as well as relative growth rates. Humidity effects were less pronounced, although still significantly increased total alfalfa leaf area (24-30%), stem mass (17-42%), shoot mass (13 -33%), and height (23-24%). Increased leaf area was attributed to 21 to 25% greater leaf numbers rather than area per leaf. Unlike the effects of PAR, elevated humidity did not increase the stem mass/length ratio, indicating the increased stem mass was due primarily to stem lengthening, and not thickening or increased tissue density. In general, humidity had small, positive effects on alfalfa growth, but was strongly expressed only within variables positively affected by PAR. This is consistent with the theory that the primary benefit of humidity is through enhanced photosynthetic gas exchange. Results suggest elevated humidity under aspen (Populus tremuloides Michx.) appears to compensate for some lost growth potential in the understory, but cannot fully counteract the reduced photosynthetic potential of alfalfa at the two PAR levels tested.