The location of arctic and alpine treelines, while partly historic, is primarily a reflection of conditions where environmental variables limit tree function, particularly material accumulation. In order to be able to predict treeline movements in response to global changes in climate and atmospheric CO2 concentrations (and the consequences of these movements in turn on global change), it is essential to understand the most important environmental factors and plant processes presently limiting tree species performance at treelines.
White spruce is the dominant treeline tree species in Alaska. Its growth, reproductive effort, and reproductive success is lowered at the treeline as compared to the adjacent forest. In spite of dramatically reduced soil nitrogen mineralization at the treeline as compared to the forest below in the Chugach Mountains, south-central Alaska, nitrogen fertilizer application stimulates white spruce growth only to a small degree, and the effect is no greater at the treeline than inside the forest. Thus it appears that white spruce is not differentially nitrogen limited at the treeline. Rather the treeline white spruce appears to be carbon limited based on current findings of less growth, greater premature needle mortality, reduced mycorrhizal infection levels, and increased root respiration. The PIs for this project predict increased root mortality at the treeline, which they suggest exacerbate the carbon drain by premature needle loss. They expect that increased wind, low soil temperatures, and greater soil movement cause the increase in observed and predicted tissue turnover. In this project they will evaluate an alternate material limitation reducing growth, i.e. nutrient limitation, possibly a phosphorus deficiency at the Chugach treeline and phosphorus and/or a nitrogen deficiency elsewhere. Supporting a nutrient limitation are trends of increased needle retention along an elevational gradient within the forest, high nutrient concentrations in treeline needles and fine roots, and an elevationally increasing soil nitrogen limitation in treeline mountain birch in Scandinavia.
This project will test the nutrient limitation hypothesis by studying geographically distinct arctic and alpine white spruce treelines and adjacent forest sites in Alaska. The proposed sites have been selected to represent climatic and geographical extremes. At each location the project will study three treeline sites and three forest sites at which vegetation composition, past tree growth, and accumulated needle loss will be measured. At each site seedlings will be planted with and without shelter and with and without fertilizer application. Tree and seedling growth, needle and fine root turnover, tree seed production, seed germination, and seedling establishment will be monitored at each site. Climatic and soil nutrient conditions will also be monitored at each site. The microclimatic study will indicate the relative importance of different climatic parameters on tissue loss and tissue growth.
In order to evaluate whether rising atmospheric CO2 concentration will allow the white spruce seedlings to compensate for tissue loss they suffer at the treeline, they will be exposed to different atmospheric CO2 levels (ambient and 1.5 times) and different degree of needle and/or root pruning (intact and 30% removal). By including two nutrient levels (treeline soil with or without fertilizer) to the treatments, it will be possible to further refine the carbon versus nutrient assessment. Shoot and root growth will be measured as well as photosynthesis and water relations. Soils and tissues of trees and seedlings from the field and the growth chamber will be sampled for chemical analysis (roots for mycorrhizal infection levels also) in order to test the hypothetical growth reduction mechanisms.
Together, the field and laboratory studies will allow an evaluation of the importance of premature tissue turnover and material limitation on tree performance. That knowledge, in conjunction with an understanding of the relationship between tree species growth and reproductive effort and success, will lay the foundation for a realistic prediction of treeline response to global change.
