Interactive Effects of Defoliation and Climate Change on Compensatory Growth of Silver Birch Seedlings

Atmospheric warming increases the abundance of insect herbivores and intensifies the risk of defoliation, especially in high latitude forests. At the same time, the effects of increasing temperature and CO2 on plant responses to foliage damage are poorly understood. We examined if previous-year defoliation, varying between 0 and 75% of total leaf area, and different combinations of elevated temperature, CO2 and nutrient availability alter the growth of two-year old silver birch (Betula pendula Roth) seedlings. We measured the greatest height growth in seedlings that were fertilized and defoliated twice at the level of 50% of total leaf area, and subjected to elevated temperature with ambient CO2. The lowest growth was recorded in unfertilized seedlings that were defoliated twice at the level of 25% of total leaf area, and grew under ambient temperature with ambient CO2. The total biomass increased in all seedlings that were fertilized or grew under elevated temperature. The root: shoot ratios were low in defoliated seedlings, or seedlings subjected to fertilization or temperature elevation. Our conclusion is that ability of birches to compensate height growth is highly dependent upon the magnitude and frequency of defoliation on the limits of temperature and nutrient availability. These responses imply that folivory does not necessarily lead to reduced net productivity of trees under changing climate

Conclusions

Since the late 1980s, researchers in Europe have reported that trees seem to be growing faster. The results of the pan-European project ‘Growth Trends in European Forests’ (Spiecker et al., 1996) indeed showed that the rate of forest growth has increased on many sites in Europe. The causes for this increase have now been investigated in the Recognition project (‘Relationships between Recent Changes of Growth and Nutrition of Norway Spruce, Scots Pine and European Beech Forests in Europe). The results indicate that increased nitrogen nutrition, thought to be mainly driven by the interaction of elevated atmospheric nitrogen deposition and natural recovery of the sites from former devastative land-use practices, contributes most to the observed increases in forest growth. Climatic changes and CO2 fertilization have not been the key factors for forest site productivity changes during the 20th century, but will, according to model predictions, be of increasing importance in the future