Effect of reduced light during autumn and winter on snow gum seedling establishment

Abstract

The present study tested whether eucalypt seedlings 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 for five consecutive seasons in a treeless pasture at 1000 m elevation where frost is common. Seedlings were planted at the beginning of autumn on north and south sides of vertical panels transmitting 50 % incident sunlight. This planting arrangement allowed assessment of excess irradiance on growth of seedlings subject to similar minimum temperatures. The panels remained in place for the first two seasons of the study and at the beginning of spring were removed to determine under common conditions the influence of different irradiance regimes during autumn and winter. Over winter, shaded seedlings were less photoinhibited, had higher photosynthetic CO2 assimilation rates, lost less leaf area and maintained higher leaf area ratio than unshaded seedlings. These differences were consistent with greater growth in shaded seedlings by the end of winter. The benefit of reduced irradiance during winter however did not translate into continued superior growth after the shade shelters were removed. During spring, seedlings which were previously shaded suffered more frost damage and had reduced photosynthetic rates so that overall, seedlings that were formerly shaded accumulated less biomass than seedlings exposed to full sunlight for the entire experiment. These results show that in the short term, shading at the coldest time of the year may explain why nurse plants can improve regeneration. However in the longer term if seedlings are subsequently exposed to full sunlight, the prior benefit of reduced light may not continue. Finally, recommendations are provided for tree planting in cold regions. Show more