Human activities have caused global warming, increase in surface ozone concentration, and increase in nitrogen deposition to soil. Global warming is expected to increase insect herbivore outbreaks and enable the herbivore spread across the northern forests. Thus, global warming, increased ozone concentration, increased nitrogen availability in soil, and herbivore damage are all important factors that will affect the forests of Finland in the future. Scots pine is the most abundant tree species in Finnish forests. Warming and nitrogen deposition to soil may increase the growth of the Scots pine forests. Ozone is toxic to Scots pine and may cause decreased photosynthesis, and seedling growth. Feeding by insect herbivores may also decrease seedling growth.
This thesis assesses the effects of warming, nitrogen addition, ozone and shoot herbivory on Scots pine seedlings based on two field experiments. In the first experiment, Scots pine seedlings were exposed to warming, high rate of nitrogen addition to soil, increased ozone concentration and needle feeding by larvae of web-spinning pine sawfly. In the second experiment, Scots pine seedlings were exposed to air warming, moderate rate of nitrogen addition to soil, and bark feeding by large pine weevil. The seedlings were exposed to above mentioned treatments in single as well as combined exposure for three growing seasons. The effects of experimental treatments were reported from shoot and root growth, root structure, colonization of roots by mycorrhizal fungi, and emission of volatile organic compounds from belowground part of the seedlings.
Warming and nitrogen addition stimulated seedling growth in both experiments. Growth stimulation in combined exposure to warming and moderate nitrogen addition was lower than growth stimulation in single exposure regimes. High rate of nitrogen addition in the first experiment caused a preference of shoot growth over root growth. High rate of nitrogen addition and herbivore feeding decreased mycorrhizal colonization of roots. The emission of volatile organic compounds from belowground generally decreased due to all the treatments. All the treatments showed interactive effects throughout the two experiments whereby the effect of one treatment was modified in combined treatment.
Based on the results we can conclude that the future projections may overestimate the growth of Scots pine trees if based on single exposure of warming or nitrogen addition experiments. The general decrease in emissions of volatile organic compounds from belowground due to altered growth conditions in future may have implications on root growth, mycorrhizal colonization, and interactions of roots with other soil organisms e.g. herbivores living in soil. The results also suggest that warming, nitrogen addition, ozone and herbivore feeding can make the root system less efficient in absorbing nutrients which may be detrimental to Scots pine forests.
The doctoral dissertation of Master of Science Muhammad Usman Rasheed, entitled Rhizosphere processes of Scots pine seedlings under growth conditions of future climate and shoot herbivory will be examined at the Faculty of Science and Forestry on the 1st of December online. The opponent in the public examination will be Professor Håkan Wallander of the University of Lund, and the custos will be Associate Professor James D. Blande of the University of Eastern Finland. The public examination will be held in English.