| The balance of carbon sources and sinks is a critical determinant of plant growth, and is a principal topic in the field of plant physiological ecology. Climatic stress, such as excessively high or low temperatures, can disrupt the equilibrium between photosynthesis, respiration, and biosynthesis, and reduce the rate of biomass accumulation (growth). Tree growth in the alpine-treeline ecotone was the study system where I examined how cool temperatures negatively affect one or more components of tree carbon balance, leading to severely impaired rates of growth. Traditional hypotheses suggest that reductions in photosynthesis would be the limiting factor to tree growth. In contrast, the importance of photosynthesis has been questioned because carbon sinks (respiration and biosynthesis) may be more limited by cool temperatures. I tested hypotheses of carbon balance in seedlings because of their ecological importance to tree population dynamics, and I expected photosynthetic limitations. I determined which components of carbon balance changed most with elevation, how the changes varied among a treeline (Abies lasiocarpa) and nontreeline (Pseudotsuga menziesii) species, and whether accumulations of intermediate pools of carbon (nonstructural carbohydrates, NSCs) indicate source-sink balance. Variation in carbon balance among elevations and species were driven by differences in respiration rates, leaf growth, and needle to non-needle ratios. The reduction in growth with elevation was primarily attributable to (1) a reduction in leaf area (carbon sink), which led to (2) reduced CO 2 uptake (carbon source), which had (3) a feedback on less growth (source-sink feedback). Approximately half the variation in NSCs was related to photosynthesis:respiration, although NSCs were also linked to long-term physiological processes. Seedlings exhibited needle-level traits favoring a growth strategy for rapid carbon gain, which may have alleviated some of their carbon limitations. However, A. lasiocarpa exhibited traits favoring stress tolerance, which may be necessary to cope with high-elevation climate, but came at a cost to carbon gain. Nonetheless, greater needle:non-needle of A. lasiocarpa at the whole-plant level offset reduced carbon gain at the needle-level. My studies illustrate the complex interplay occurring among components of carbon balance in seedlings at timberline, exemplifying co-limitations and challenging traditional concepts of single limiting factors. |
