Methods: Precipitation exclusion treatments

The snow exclusion treatments will be applied to a set of three replicate upland birch/aspen sites (BNZ LTER UP2 sites) and a set of three replicate floodplain balsam poplar/white spruce sites (BNZ LTER FP3 sites). In addition, a series of complete precipitation (snow and rain) exclusion treatments will be installed on the floodplain. This last set of treatments will provide data to evaluate the importance of ground water levels, determined by glacial river levels, to forest ecosystems on the floodplains of interior Alaska.

One snow exclusion treatment will be placed in the replicate plots of the UP2 and FP3 sites. Treatment plot size will be 15 m X 15 m. This corresponds to an area 33% larger than the rainfall exclusion experiments used in the BNZ LTER experiments. Near the end of September, a plastic moisture barrier will be placed on the forest floor of each site for the snowfall exclusion experiment. The primary purpose of the plastic sheeting will be to minimize moisture inputs from late fall rain events and to minimize inputs due to spring snowmelt. Accumulated snowfall will be removed during the spring of each year using a snow blower. By removing the snowpack in the spring, changes to the soil temperature regime, controlled by the insulating nature of a snow pack will be kept to a minimum. The plastic sheeting will be removed after snow melt to allow natural rainfall to recharge the soil moisture in each site. The snowfall removal experiments will be installed or removed at the same time that the rainfall exclusion treatments are removed or applied, respectively. This will allow for a dovetailed treatment application for both the summer and winter precipitation exclusion experiments.

One additional treatment will be applied to the floodplain (FP3) site. This treatment will eliminate the input of precipitation (both snowmelt and rainfall) to the balsam poplar/white spruce stands. This treatment will examine the significance of soil moisture recharge linked to river level dynamics on the floodplain. This treatment will be installed in the same manner as the summer rainfall and the winter precipitation exclusion experiments. For the summer rainfall exclusion a cover will be constructed under the canopy but above the soil surface to drain rainfall from the site. The cover will be sloped, and plexiglass sheeting will be placed on the permanent framing structure. During the fall, the elevated cover will be removed and plastic sheeting will be applied on the forest floor. The plastic sheeting will minimize soil moisture recharge due to rain events that can occur in late September. During the spring before snowmelt has started, a snow blower will be used to remove most of the snow from the plastic sheeting. Plastic sheeting that is destroyed during the snow removal will be replaced. The plastic sheeting will remain in place till the summer rainfall covers are installed for the rainfall exclusion treatment.

A vertical moisture barrier will be installed in the soil around each plot to prevent subsurface water flow into, and root growth out of, the experimental areas at both the floodplain and upland sites. A trench one meter deep will be dug around each plot and a visqueen plastic sheet will be installed to prevent the sideways movement of water into and out of the plot. We will then limit moisture dynamics to vertical movement (rainfall and snowmelt infiltration and river water levels on the floodplain) of water in the plots.