Impact of elevated temperature and [CO2] on spring phenology and photosynthetic recovery of boreal Norway spruce

In recent decades, several methods have been devised to study the effects of elevated atmospheric CO2 concentration, [CO2], and temperature on tree and plant physiological responses. This has resulted in the development of a range of systems to expose tree species to elevated [CO2] and/or temperature and it has become obvious that there is a strong need for multifactor experiments, which at the tree and stand level can become both a technical and financial challenge. Most available information on the effects of elevated [CO2] and increased temperature on trees comes from single factor experiments on seedlings or saplings and the results from these studies may not be directly applicable to mature trees or forest stands. This thesis is based on data gathered from mature Norway spruce trees growing in whole-tree chambers in northern Sweden, where both temperature and [CO2] were controlled according to climate conditions predicted to prevail in the year 2100. As spring is such an important season for boreal tree species, it was this period in which I was most interested. Spring phenology (bud burst and shoot growth) and the timing and extent of photosynthetic recovery was studied in relation to elevated temperature and [CO2] over a three-year period. From the analysis presented herein, phenological development of buds was enhanced with elevated temperature and new shoots emerged 10 to 20 days earlier than in ambient temperature. The duration of the shoot growth period was not affected by [CO2] or temperature and while shoot length varied between treatments, neither temperature nor [CO2] had any significant effect. These differences could not be explained by an accumulation of temperature sum (Tsum) nor could ecophysiological simulation models predict these events. Although the models were not able to predict the regulation of the timing of bud burst, they did provide a clear indication that rest completion in Norway spruce trees growing in northern Sweden takes place near the spring equinox, meaning that high air temperatures are not physiologically effective until late in the spring. Elevated temperature also enhanced the recovery of the photosynthetic apparatus, as seen in variable chlorophyll fluorescence, apparent quantum yield, and light saturated photosynthesis, but elevated [CO2] had no effect during the spring period. While measurements of chlorophyll fluorescence were a reasonable indicator of photosynthetic capacity, they cannot be used as a proxy for photosynthesis measurements since net photosynthesis is so variable and responsive to temperature and [CO2]. Rising air temperatures will lead to earlier spring photosynthetic recovery of boreal Norway spruce forests and this will ultimately affect the length of the growing period. The commencement and rate of spring photosynthetic recovery are two of the most important factors governing the potential annual carbon sequestration by boreal forests.