Alpine Treeline Stability: Mechanisms of Conifer Tree Seedling Establishment

Wake Forest University, Winston Salem NC

Investigators

Abstract

If current projections of climate warming are accurate, future global change will include the disappearance of a significant portion of the Earth's biodiversity during the 21st century, e.g. alpine tundra ecosystems in temperate and subtropical latitudes may be replaced by the subalpine forest below. Despite these predictions, mechanistic explanations for the upper elevational limits of subalpine forests across the globe have been pondered for over a century, and are still controversial today. The stability of this alpine treeline will be evaluated based on the ecophysiology of tree seedling establishment away from the forest edge and into the subalpine-alpine ecotone by the two dominant conifers, Abies lasiocarpa (subalpine fir) and Picea engelmannii (Engelmann spruce). Paradoxically, sunlight exposure during the day and leaf warming can enhance photosynthesis and growth in most plant species, while prior exposure to a clear, cold night sky (longwave thermal radiation sink) leads to lower minimum leaf temperatures, a major limiting factor at high elevation. In combination, low leaf temperatures at night followed by high sunlight the following morning can result in potentially severe low temperature photoinhibition of photosynthesis (LTP) and substantial reductions in carbon assimilation. It is hypothesized that this decrease in carbon assimilation is due primarily to excessive sky exposure and LTP, resulting in inadequate carbon gain to support the root growth necessary to survive rapidly drying soils following spring runoff. In addition, the timing and degree of mycorrhizal infection of fine roots may also play a vital role in preventing lethal water stress. Three primary venues for adaptation are proposed for seedling establishment under such a multiple stress regime: (i) microsite facilitation, (ii) seedling architecture, and (iii) photosynthetic physiology. The relative effectiveness of each of these adaptive venues for seedling establishment will be evaluated quantitatively from field measurements on photosynthesis, growth, and survival. If the hypotheses linking greater sky exposure to lower minimums in leaf temperature, greater LTP, less carbon gain and root growth, and lower seedling survival (desiccation death) are correct, conifer tree seedling establishment in this treeline ecotone could flourish under current scenarios of global warming and, thus, lead to the encroachment of subalpine forest and the potential loss of alpine tundra ecosystems.