Methods: Herbivore exclosures
1. Unmanipulated herbivore exclosures
The original BNZ LTER herbivore exclosures measured 20m x 50m. We will continue to monitor vegetation changes during succession in the presence and absence of mammal herbivory using leaf litter traps inside and outside each exclosure (n=7). The litter traps measure 2 m2. They are emptied after leaf fall in mid October and the litter is sorted by species and dried. Litter fall (g m-2) and litter nitrogen by species concentration will be measured every 2 years along with soil carbon and nitrogen stocks to a depth of 20 cm.
2. Manipulated herbivore exclosures (planted with white spruce)
We have constructed 12 new herbivore exclosures with outside funding. These are located in early successional shrub communities at different terrace ages. Each exclosure has been planted with 96 white spruce seedlings with adjacent control plots. These exclosure will be subject to environmental monitoring and experimental measurements as described below.
Field methods
Microclimate:
In order to curtail cost we will not continuously monitor a full range of climate variables at each of the 12 exclosures and their respective control plots. Rather we plan to install six mini weather stations (HOBO microstations, Onset Computer Corporation, MA, USA) across the range of terrace ages from the oldest to the youngest. The variables measured continuously in the spruce seedling canopy at these stations include air temperature, relative humidity, and photosynthetically active radiation (PAR), as well as soil moisture.Soil and leaf temperatures will be measured with separate temperature loggers (HOBO8) and appropriate sensors (HA-6-IT).Summer precipitation will be obtained from the Bonanza Creek LTER site weather station located in the middle of our river transect (/).
Ecophysiology of planted seedlings:
Photosynthesis will be measured in the field on 10 trees of each treatment at the same sites as the weather stations, using a LICOR 6500 infra red gas analyzer fitted with a cuvette to accommodate conifer branches (Doran et al. 2002). Measurements will be made in the field at three times over the growing season during June, July, and August.
Since we are prevented from large-scale destructive sampling we will rely on several proximate indices of seedling physiological status and resource use. They include stable isotope analyses of foliar del13C, del18O, and del15N for respectively, water stress, water use (river water or precipitation), and nitrogen availability. We have successfully used these indices previously under both natural (18O, 15N) and experimental conditions (13C). We will randomly assign 20 seedlings at each exclosure for foliage sampling. Water stress and nitrogen availability will be measured once during the growing season (June), as previous studies have shown non-significant seasonal variation in del13C and del15N of foliage. By contrast, use of water sources will be measured three times over the growing season: June (prior to rise in river level and summer rains), July (after annual rise in river level), and August (after onset of normally heavy rains).
Growth
Growth measurements of seedlings (height, basal diameter, and branch/whorl counts) will be made in August of each year on all seedlings (n=96) at each exclosure and control plot (n=12).
Demography
Seed germination and survival of all planted seedlings will be measured during June at each plot for each year of the study (n=1152). We will utilize long-term (1950-present) observations of white spruce seed production at the Bonanza Creek LTER site (/) to model the periodicity of seed production. Several studies near our study sites have looked at the episodic nature of seed production and dispersal dynamics and will serve as a knowledge base for developing seed production and dispersal algorithms. Likewise, we will utilize the literature to further our understanding of seed germination and survival.
Soil chemistry
Chemical composition of surface soils in each exclosure and control plot will be measured at the onset and termination of the study, including: salinity, pH, cation concentrations (magnesium, calcium, sodium and ammonium), and anion concentrations (chloride, nitrate, sulphate, and bicarbonate), dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and free amino acids. Eight soils cores 5 cm diameter,10 cm deep, will be removed from each exclosure and adjacent control plot (n=192).
Soil chemical analyses will be performed at the Forest Soils Laboratory, School of Natural Resources and Agricultural Sciences, University of Alaska. Salinity and pH will be determined on saturation pastes using conventional soils methods. Cations and anions will be extracted by vaccum filtration and analyzed on a Beckman atomic absorption analyzer. DOC and DON will be obtained by high-speed centrifugation and analyzed on a Shimadzu TOC-500 carbon analyzer fitted with an Antek nitric oxide detector. Soil free amino acids will be determined by fluorescence spectroscopy on a FL600 microplate multidetection reader (Bio-Tek, Wisconsin, USA). Isotope analysis of foliage samples will be performed on a TCEA Thermo-Finnigan mass spectrometer at the Alaska Stable Isotope Facility, Water and Environmental Resource Center, University of Alaska Fairbanks.