Canopy leaf samples are collected annually from 5 sites arranged along an elevation gradient within or adjacent to Watershed 6, a reference watershed also known as the Bear Brook watershed. Four sites are in northern hardwoods forest: Low (551 m), Mid (622 m), Upper (726 m), and High (788 m). The fifth site is in the spruce-fir zone (784 m). Samples are collected in the four hardwood sites from sugar maple (Acer saccharum), American beech (Fagus grandifolia), and yellow birch (Betula alleghaniensis). White ash (Fraxinus americana) foliage is collected at the Low site beginning in 2018. Balsam fir (Abies balsamea) and red spruce (Picea rubens) foliage are collected at the spruce-fir site. Beginning in 2013, needles are separated into two sub-samples, current year growth and growth from previous growing seasons and analyzed separately. Current and older needles were not separated prior to 2013. Paper birch (Betula papyrifera) foliage was collected at the spruce-fir site beginning in 1999 but has not been sampled since 2017 due to increasing mortality of that species within the HBEF.
Approximately 30 sun leaves from the upper canopy were collected for each species and site in late July or August. Samples were obtained from one tree per species per site from 1992 through 2017 and from three trees per species per site beginning in 2018. Sampled trees are not consistent from year to year and leaves from different trees are not pooled. The canopy leaves were collected by climbing the trees or shooting branches and collecting them from the ground after they fell. Nitrile gloves are worn while handling leaf material. Each sample consists of leaf material only (laminae and petioles); woody tissue (twig) was removed from the samples. Petioles were removed in 2018.
Data are not available in 2009, 2012, 2014, and 2016.
These data were gathered as part of the Hubbard Brook Ecosystem Study (HBES). The HBES is a collaborative effort at the Hubbard Brook Experimental Forest, which is operated and maintained by the USDA Forest Service, Northern Research Station.
LABORATORY PROCEDURES
Following collection, leaf samples are brought into a lab and oven dried at 60 degrees Celsius until thoroughly dry (48 hours). Samples are then ground in a Wiley Mini-Mill with 20 mesh sieve (2012 through present) and stored in glass vials or 4 oz. jars. Samples are dried again in jars with caps loosened for 24 hours at 60 degrees Celsius before chemical analysis. From 1992 through 2003, samples were ground in an electric coffee grinder and dried with the cap loosened at 80 degrees Celsius for 24 hours prior to analysis.
Samples are analyzed for C and N content at the USDA Forest Service Forest Sciences Laboratory in Durham NH with a Thermo Flash EA 1112. Samples were analyzed with Carlo Erba Instruments 1500 series elemental analyzer at Boston University Stable Isotope Laboratory from 1992 through 2000, at the Center for Stable Isotope Biogeochemistry at University of California, Berkeley from 2001 through 2008, at the Goodale lab at Cornell University in Ithaca NY from 2010 through 2011.
Isotopic analyses are performed at the Center for Stable Isotope Biogeochemistry at University of California, Berkeley (CSIB) with a Finnigan MAT DeltaPlus XL isotope ratio mass spectrometer in continuous flow (Thermo Scientific, Bremen, Germany). Delta 15N values represent the per mil difference between the isotopic composition of the sample and that of atmospheric dinitrogen. Stable isotope ratios were measured at the Boston University Stable Isotope Laboratory from 1992 through 2000 using a Finnigan Delta-S mass spectrometer.
From 1992 through 2003, samples were analyzed for K, P, Ca, and Mg at Yale University. For determination of organic content and chemical analysis, a 0.3 g sample of the oven dry ground material was placed in a pre-weighed crucible and ashed overnight at 450-500 degrees C. The ashed material was cooled and weighed. The ash was eluted with 10 ml of 6N HNO3 and heated for a few minutes (to a simmer) on a hot plate. The hot extract solution was poured filtered (Whatmann # 41 ashless) in a funnel and into a 50 ml volumetric flask. The crucible and the material on the filter paper were rinsed several times with distilled deionized water. Following these several rinsings, the volume in the flask was brought up to 50 ml with the distilled deionized water. The 50 ml of extract solution was then placed in 60 ml plastic bottles for storage until chemical analysis. The elements Ca, Mg, K, P, and Mn were determined for each sample using an ICP spectrometer.
From 2012 through present, samples are analyzed for P, K, Ca, and Mg using microwave digest and an ICP Spectrometer at the USDA Forest Service Forest Sciences Laboratory in Durham NH. 0.1 g of oven dried ground material weighed to the nearest 0.001 g, is placed into a Teflon digest tube along with 9.75 mL Nitric acid. The sample is digested in a CEM microwave to 200 C and held at that temperature for 20 minutes (similar to EPA method 3052). The sample is then diluted to 50 mL and analyzed with an ICP spectrometer. Samples were analyzed for P, K, Ca, and Mg using very similar methods at the University of Michigan from 2004 through 2008