The Role of Temperature in the Phenology of Temperate and Boreal Tree Species

Abstract

Dissertation AbstractIn the current era of climate change, the phenology of trees, i.e., the timing of seasonal life-cycle events, is evolving. Due to warming springs, leaf bud break has been observed to occur earlier worldwide, even though this trend has slowed in recent years. Because the phenology of trees can be important in determining range limits, annual net primary production, and interactions with other species, it is essential to understand how climate change might impact phenological timing in the future. This dissertation describes several laboratory experiments with the goal of elucidating the effect of some of the main cues, i.e., cold winter and warm spring temperatures, on the spring phenology of boreal and temperate tree species. With the help of others, I collected dormant twigs of temperate and boreal tree species at Cedar Creek, MN, and exposed them to different experimental conditions. In collaboration with the Technical University of Munich, Bavaria, I was also able to replicate some experiments in Germany to understand the different chilling and forcing needs of temperate and boreal species of the same genus, but different region. Dr. Sam Fahrner Ward (back then at the Entomology Department of the University of MN) and I additionally worked with tamarack seedlings and larch casebearer larvae to better understand if changes in the phenological synchrony of these interacting species could be responsible for recent larch casebearer outbreaks in MN. I found that for both Cedar Creek and Germany, prolonged chilling throughout the winter reduced the need for forcing/the time to bud break for the large majority of species, and that there were significant differences between species within the U.S. and Germany, and across continents. If chilling were reduced in the winter due to climate change, species with small chilling requirements could be disproportionally advantage by warmer springs, However, I also found that out of 14 species from Germany and the U.S., 7 species significantly reacted to chilling temperature and that 6 out of these 7 species broke bud faster when exposed to warmer chilling of up to 4.5ºC. In regions with very cold winter temperatures, such as MN, climate change induced winter warming could initially benefit species that prefer warmer chilling, while for species that prefer colder chilling, warmer winters could delay bud break if chilling requirements are not fully met. I also found that warmer chilling and forcing increased phenological synchrony between larch casebearer and tamarack, but only up to 27ºC of forcing, after which larval activation slowed down. Additionally, in growth chambers of 21ºC and warmer, casebearer larvae were unable to reach adulthood. Warmer winters and springs might increase synchrony of both species, which could benefit larch casebearer due to the availability of younger, more nutritious needles with reduced defense chemicals. However, in very warm springs, survivorship to adulthood might be drastically reduced.