Photosystem Stress of Understory Balsam Fir (Abies balsamea) in Ontario, Canada as Measured by Chlorophyll Fluorescence

INTRODUCTIONFor a number of years there has been substantial interest in the effects of climate change on the environment and ecosystems. Since plants are both abundant and vital components of terrestrial ecosystems, they are often chosen as subjects for studies about responses to climate change. Eco tones, or vegetation transition zones, are ideal study sites for these types of experiments, as they are efficient models due to their sensitivity to any change, and are easy to monitor for future climate change (Neilson 1993; Noble 1993; Goldblum and Rigg 2002). Pollen evidence from the region clearly shows that over the past 8,000 years, forests and tree species in North America were very responsive to climate fluctuations at the regional scale. MacDonald et al. (1998), Rizzo and Wiken (1992), and Smith et al. (1992), remarked that there is a potential for northern treelines to shift northward hundreds of kilometers in response to the predicted increase in temperature due to climate change caused by increasing levels of atmospheric greenhouse gases. As a result, the speed and magnitude of climate change could be enhanced on a global level, because if forests shift to higher latitudes, the planetary albedo will decrease and the position of the summer arctic front will change as well (MacDonald et al. 1998; Bonan et al. 1992). Balsam fir, being an evergreen species, provides an absorptive surface throughout the year, which is especially important in the winter. Therefore, if balsam fir and other evergreen species are abundant, winter albedo will be decreased. Decreasing planetary albedo contributes to an increase in regional surface temperatures when compared to the surrounding tundra, which in turn heats the overlaying troposphere, creating a thermal grathent (MacDonald et al. 1998; Hare and Ritchie 1972; Pielke and Vidale 1995). The thermal grathent would then generate into a synoptic barometric grathent, or front, that would be powerful enough to partially secure the Arctic Front position in the summer (MacDonald et al. 1998; Pielke and Vidale 1995). The possible northward shift of the summer Arctic Front could ultimately change the summer-storm patterns in the high and midlatitudes (MacDonald et al. 1998). Therefore, it is worth considering what might happen to the deciduous/boreal forest ecotone as a result of human-induced climate change, since dramatic warming has been projected to occur, in this area, over the next 100-200 years (Goldblum and Rigg 2002). If one species is currently stressed it may be an indication that the species may struggle under future climate conditions and potentially create opportunities for competing species. In that regard, balsam fir is an important understory species that may directly affect sugar maple and other deciduous species ability to migrate. In order to more accurately determine the various effects future climate change will have on individual species, one must first know their current health status and must therefore ascertain the plants' current physiological stress levels. Assessing the efficiency of plant photosys terns, as measured by chlorophyll fluorescence, is one common technique used to obtain this information.PHOTOSYSTEM STRESSIn almost all photosynthetic eukaryotes, light is required for life. However, according to Lichtenthaler and Bur kart (1999), when plants are exposed to irradiance levels above the normal light saturation point of photosynthesis, they experience high-light stress. These above normal light levels can instigate multiple responses within the plant. Increased production of destructive reactive oxygen species, made as a by-product of photosynthesis, is one of the damaging responses that occur when there is excessive light (Muller et al. 2001). In extreme cases, this ? hoto- oxidative impairment can lead to pigment bleaching and ultimately the death of the affected plant (Muller et al. 2001). To offset the damage caused by light stress, multiple internal mitigating responses are initiated. …