Methods for the artificial communities experiment: Vegetation Manipulation

 

Experimental design:

Artificial communities of pure spruce or mixed alder and spruce were established in 1989 at each of three replicate early successional sites (FP1A, FP1B, and FP1C) on the Tanana River floodplain. At each site four pure spruce plots were established and two mixed plots were established. Each plot was 20m X 20m. Spruce seedlings were grown in a greenhouse for 6 months from local seed, then planted at a spacing of 1m in all plots. In the mixed plots alder seedlings were planted between spruce seedlings. Dead seedlings were replaced throughout the first two years of the study and naturally occurring alder were periodically weeded from pure spruce plots until 1995. Bank erosion destroyed plots at FP1A and this site was eliminated from the study after 1992.

 

Measurements:

Spruce height and condition were measured in 1992 and repeated in 1995 and 2002. Alder heights were measured in 1992 and 1995. In 2002 we harvested quadrates in each plot to estimate total plant and soil N and C accumulation and above ground biomass.

 

Aboveground biomass:

Four points were randomly selected within each plot for quadrate sampling of the entire above ground biomass (excluding planted alder). At each point we sampled the aboveground biomass of shrubs (if present), herbs, and litter. The shrub layer was sampled with a 1 m2 quadrate and the herb and litter layers were usually sampled with a 0.25 m2 quadrate.

Each layer was sorted into different components. The shrub layer was sorted into standing dead and live material. In most cases the live material was sorted as poplar or willow and further sorted into old growth and new growth branches and leaves. The herb layer was divided into standing dead, grasses, equisetum, and forbs. The standing dead and grasses were not sorted any further. We sorted forbs and in some cases were able to further identify them by genus or species. We also sorted the Equisetum by species in most cases. Litter was sorted into woody debris, non-woody debris, and green debris.

In 2003 we estimated the number of alder stems and their basal diameter and sampled 18 alders to generate allometric equations relating basal stem diameters to biomass components. Alder biomass will be estimated by using these allometric equations and the estimates of the number of alder stems and basal diameter.   

A sub-sample of these plant samples will be analyzed for total C and N with a Costech 4010 elemental combustion system (Costech analytical technologies, Valencia Ca.).

 

Soils:

We took soil cores to a depth of 1m from the center of each quadrate. The first 20 cm were sampled with a soil corer with a diameter of 5.2 cm. The depth from 20 to 80 cm was sampled with a narrower soil core with a diameter of 1.7 cm. The entire soil core was divided into 3 segments based on depth: 0-5cm, 5-20cm, and 20-100cm. Each segment was divided into mineral soil, organic soil, mixed organic and mineral soil, and buried organic soil. Buried organic soils were organic soils found beneath mineral soil because of silt deposition. All buried organic layers were composited into a single sample in each segment. Soils will be analyzed for total C and N with a Costech 4010 elemental combustion system (Costech analytical technologies, Valencia Ca.).

We recorded the wet weight of each sample and calculated the dry weight using the wet to dry ratio. The wet to dry ratio was determined in the lab after weighing, oven drying, and re-weighing a sub-sample of each soil. Bulk densities (g dwt soil/cm3) were calculated for each sample from the soil volume and dry weight.

Potential net mineralization rates were determined on the separated soil components from the top 20 cm of the each core. On 3 August, 2002, two 10 g samples (fresh weight) of each soil were placed into separate 150ml specimen cups. No adjustments for water holding capacity was made. Nitrogen was immediately extracted form one sample of each pair. The remaining sample was placed into a 1 L mason jar, covered with a polyethylene bag (to maintain moisture content but allow for gas exchange), and placed on a lab shelf to incubate at room temperature. Nitrogen was extracted from these remaining samples following 38 days of incubation.

Nitrogen was extracted with 1 M KCl. 75 ml of KCl was placed into each specimen cup. These were capped and shaken for one hour on a shaker table at approximately 250 rpm. After shaking, the samples were extracted under a vacuum through a glass fiber filter.  Solution NH4+-N and NO2/NO3-N of each extract was determined colorimetrically on a modified technicon autoanalyzer (Tarrytown, New York, USA).

Potential net nitrogen mineralization rates (µg N/g dwt soil/day) were calculated as the final extractable ammonium and nitrate concentrations minus the initial ammonium and nitrate concentrations. Potential net nitrification rates (µg N/g dwt soil/day) were calculated as the final extractable nitrate concentrations minus the initial nitrate concentrations.

 

Method details and data sets can be found at /Data_catalog_detail.cfm?dataset_id=72 and /data_catalog_detail.cfm?dataset_id=72&show=data