| Developing agroforestry systems in Canada requires an understanding of the ecological processes that influence production. Complementary experiments in northern aspen (Populus tremuloides Michx.) agroforests identified mixes of competition and facilitation, with above- and below-ground effects. Facilitated soil moisture and nitrogen increases, and aspen damage reduction were observed in mixtures of aspen seedlings with alfalfa ( Medicago sativa L.) and marsh reedgrass (Calamagrostis canadensis (Michx.) Beauv.). However, these effects were often masked by competition for light, water and nitrogen that reduced most aspects of aspen seedling growth. Canopy removal and root trenching in aspen stands in Aspen Parkland and Lower Boreal ecosystems revealed understory microclimates favourable for cool-season plant growth. Aspen cover reduced soil water loss during drought, frosts, air temperatures (T), and increased relative humidity (RE). Trees had less effect on averages than in moderating extremes. Aspen also competitively reduced photosynthetically active radiation (PAR) and soil water. With the exception of PAR, there were few season-long microclimate differences between partial and full canopies. Understory production and composition responded strongly to canopy removal with less consistent results from trenching. When trenching was significant, it generally coincided with increased PAR resulting from canopy removal. This interaction was particularly strong among grasses. Production under a partial Parkland canopy was comparable to that with complete canopy removal and demonstrated balanced competition and facilitation. Experiments on alfalfa quantified specific physiological and production responses in an aspen understory. In growth chambers, RH had a small, positive effect on alfalfa growth, particularly when in conjunction with greater PAR. In the field, alfalfa leaf-to-atmosphere vapour pressure difference (D) decreased with aspen cover. Alfalfa leaf T and D were strongly coupled to air T, with RH having a smaller effect. Direct PAR received also influenced leaf T, with shaded leaf T less than or equal to air T, and illuminated leaf T greater than air T. These results collectively support general theories on the importance of facilitation: facilitation mitigated intermittent extremes, but was otherwise masked by persistent competition. Balancing facilitation and competition in agroforestry design is possible through further elucidation of the appropriate level of aspen cover. |
