Facilitation of seedling establishment: Reduction in irradiance enhances winter growth of Eucalyptus pauciflora

We determined whether plants benefit from a reduction in irradiance when growth is limited by low temperatures. Growth of snow gum (Eucalyptus pauciflora Sieb. ex Spreng.) seedlings was studied in response to two irradiance regimes (100% and 50% sunlight) under field conditions during autumn and winter. Seedlings were planted on north (exposed) and south (sheltered) sides of vertical screens transmitting 50% incident sunlight. This planting arrangement allowed assessment of affects of excess irradiance on growth of seedlings subject to common temperature minima. Over winter, sheltered seedlings were less photoinhibited, had higher photosynthetic CO₂ assimilation rates, lost less leaf area, and maintained a higher leaf-area ratio than exposed seedlings. These differences were consistent with greater growth of sheltered than exposed seedlings by the end of winter. These results show that shading may contribute to facilitation of regeneration by nurse plants in frost-prone environments.