Nitrogen and carbon storage in alpine plants

Synopsis Alpine plants offer unique opportunities to study the processes and economics of nutrient storage. The short alpine growing season forces rapid completion of plant growth cycles, which in turn causes competition between vegetative and reproductive growth sinks during the early part of the growing season. Mobilization of stored nitrogen and carbon reserves facilitates competing sinks and permits successful completion of reproduction before the onset of winter stress. We discuss the theoretical framework for assessing the costs and benefits of nutrient storage in alpine plants in order to lay the foundation for interpretation of observations. A principal point that has emerged from past theoretical treatments is the distinction between reserve storage, defined as storage that occurs with a cost to growth, and resource accumulation, defined as storage that occurs when resource supply exceeds demand, and thus when there is no cost to growth. We then discuss two case studies, one already published and one not yet published, pertaining to the storage and utilization of nitrogen and carbon compounds in alpine plants from Niwot Ridge, Colorado. In the first case, we tested the hypothesis that the seasonal accumulation of amino acids in the rhizome of N-fertilized plants of Bistorta bistortoides provides an advantage to the plant by not imposing a cost to growth at the time of accumulation, but providing a benefit to growth when the accumulated N is remobilized. We show that, as predicted, there is no cost during N accumulation but, not as predicted, there is no benefit to future growth. In the presence of N accumulation, reliance on stored N for growth increases, but reliance on current-season, soil-derived N decreases; thus the utilization of available N in this species is a ‘zero sum’ process. Inherent meristematic constraints to growth cause negative feedback that limits the utilization of accumulated N and precludes long-term advantages to this form of storage. In the second case study, we discuss new results showing high concentrations of cyclic polyol (cyclitol) compounds in the leaves of many alpine species dominant in the dry fellfield habitat. In Artemisia scopulorum, cyclitols were induced as the growing season progressed, and reached highest concentrations during the dry, late-summer months. Leaf cyclitol concentrations were high in all four species of the Caryophyllaceae that we examined and appeared to be constitutive components of the leaf carbohydrate pool as concentrations were high through the entire growing season. We observed correlations among seedling abundance, seeding survivorship and the presence of high leaf cyclitol concentrations. We propose that the primary function of cyclitols in the leaves of alpine, fellfield herbs is to promote drought tolerance through osmotic protection, and enhance fitness by improving seedling survival. We considered the possibility that cyclitols also function as carbon storage compounds that are remobilized at the end of the growing season and used to support growth the following year. Our observations do not support this hypothesis in the Caryophyllaceae because the requirement for high constitutive concentrations year-after-year prevents long-term advantages of storage and remobilization. However, in A. scopulorum, remobilization of cyclitols following the end of the growing season may provide storage substrates that can be used for growth the following season. From our analysis we conclude that it is difficult to use current theory that is embedded in the economic concept of costs and benefits to interpret observed dynamics in nitrogen and carbon allocation. Future theoretical developments that move away from an abstract foundation embedded in cost-benefit tradeoffs and toward phenotypic integration of source-sink relationships will improve our ability to merge observations and theory.