January 8, 2014 Dr. Jessie Young, Tree Water Use Dynamics of Interior Alaska boreal forest systems International Arctic Research Center, University of Alaska Fairbanks

Tree Water Use Dynamics of Interior Alaska boreal forest systems

Dr. Jessie Young, International Arctic Research Center, University of Alaska Fairbanks

The ecohydrology of boreal forest ecosystems of Interior Alaska is not well understood largely because of challenges posed by the presence of discontinuous permafrost. Near-surface permafrost results in storage-dominated systems with cold, poorly drained soils, and slow growing, low statured coniferous trees (Picea mariana) or CDE's. The transition to permafrost-free areas can occur over a few meters and is accompanied by a vegetation community dominated by large deciduous trees (Populus sp. and Betula sp.) or DDE's. Typically, areas with permafrost are on north facing slopes and valley bottoms, and areas without permafrost are south facing. In Alaska's boreal forest, the permafrost is very warm and vulnerable to the effects of climate change. Once permafrost begins to thaw, the vegetation community shifts from coniferous to deciduous dominated. Streamflow in watersheds with a larger permafrost distribution tends to be higher and more responsive to precipitation events than in watersheds with low permafrost distribution. In fact, precipitation events in the low permafrost areas do not infiltrate past the rooting zone of the deciduous trees (~5-40 cm). This suggests that the deciduous trees may remove water from the system via uptake and transpiration.

We focus on how vegetation water use affects boreal forest hydrology in areas of discontinuous permafrost. Specifically, we ask: what are the patterns of vegetation water use in areas with and without permafrost? This study focuses on the CDE and DDE systems. Our research sites are established on low and high locations on each aspect (south facing DDE, north facing CDE) to capture the variability associated with the different hillside drainage properties. At each of the four sites during the growing season, we measured various aspects of plant water use dynamics, including water flux, water content, water sources, depth of water uptake in the soil, and water stress. We use a Bayesian framework to analyze the data. We found that, compared to the coniferous trees, the deciduous trees have higher transpiration rates, lower water stress, higher water content, and use rain-derived water during the summer and snowmelt water prior to leaf out. The amount of water taken up and fluxed by deciduous trees is greater at the high site than the low site. The deciduous trees take up nearly 40% of snowmelt water prior to leaf-out. The low water use rates of the coniferous trees suggests that they play a very small role in the boreal water cycle, resulting in more water remaining in the watershed, which eventually moves into the stream. Conversely, the very high water use of the deciduous trees suggests they have a big effect on the boreal water cycle because they remove water from the system and transpire it to the atmosphere. This suggests that if the climate warms as expected, there may be a profound shift in the boreal forest ecohydrology. This transpiration model is integrated into a storage-based hydrologic model to better understand the relationships between vegetation, permafrost, water and climate in the boreal forest ecosystem.