| The mechanisms governing the occurrence and stability of timberlines and treelines have been debated for over a century and most studies have focused on correlations with mean temperatures. However, both maintenance (at the current altitude) and upward migration of treeline are dependent on new seedling establishment. Seedling establishment at high altitudes is enhanced by facilitative interactions with other plants, inanimate objects and even topography. Facilitation often results in increased carbon gain, which leads to greater root growth, mycorrhizal infection and survival. Also, plant species inhabiting high-altitude communities commonly employ self-facilitative mechanisms using leaf and plant architecture.; In the set of studies presented here, desiccation was predicted to be higher at high altitudes, but only when there was a low (wet) lapse rate. This desiccation could have particularly strong effects on young seedlings with shallow root systems. Also, photosynthetic carbon gain in young seedlings of Abies lasiocarpa and Picea engelmannii was found to be approximately 20-30% lesser at a high-altitude treeline than in the lower-elevation forest. This reduction in carbon gain was associated with colder nighttime temperatures and increased structural shading at the higher elevation site. Long-term drought (∼ 5 years) also resulted in reduced carbon gain in Abies lasiocarpa seedlings and was primarily due to reductions in stomatal conductance.; Emergence and survival of current-year Abies fraseri seedlings was found to be highest under closed and open canopies, respectively. Seedlings in open canopy sites experienced greater sunlight, had higher photosynthesis (at the same light value), and were predicted to fix 3-5 times more carbon than closed canopy seedlings. Cloud immersion resulted in more constant levels of sunlight that were nearer the species' photosynthetic maximums, resulting in greater photosynthesis in shaded microsites and less exposure to high sunlight and possible photoinhibition of photosynthesis.; Fluorescence was used as a measure of light absorption in three high-altitude species with basic leaf types: laminar leaf with palisade (Rhododendron catawbiense), needle with palisade (Abies fraseri ), and needle without palisade (Picea rubens). Adaxial illumination resulted in evenly distributed fluorescence across the mesophyll for leaves with a palisade cell layer, as well as for the cylindrical needles without palisade, when compared to abaxial illumination. This study supports the hypotheses that light propagation is associated with asymmetric mesophyll structure and that the cylindrical leaf form, such as found in many conifers, may be a structural solution to excessive sunlight that replaces the differentiated mesophyll found in most laminar-leaved species. |
