Contact Info:
Ellen Denny
USDA Forest Service
271 Mast Road
Durham, NH 03824
phone: 603-868-7683
email: ellen.denny@aya.yale.edu
Tom Siccama
School of Forestry and Environmental Studies
Yale University
Greeley Memorial Laboratory
370 Prospect Street
New Haven, CT 06511
phone: 203-432-5140
email: tgs3@pantheon.yale.edu
Introduction
Since 1967 we have conducted isolated collections of wood fern (Dryopteris spinulosa) fronds to determine plant nutrient concentrations. Wood fern is the most abundant herbaceous species at the Hubbard Brook Experimental Forest, comprising 40 to 50% of the biomass of this stratum. (Lycopodium lucidulum, an evergreen clubmoss, makes up another 40 to 50%, but only 10% of its biomass consists of new growth each year.) Most of our early efforts at Hubbard Brook were focused on nutrient cycling through the tree species. However, occasional samples of wood fern were collected and analyzed to get a rough idea of how the herbaceous layer contributed to nutrient cycling in the ecosystem. In 1985 we started a more consistent annual collection of wood fern from an area just west of the Watershed 6 weir. The annual collection of wood fern fronds was initiated on Watershed 1 in 1996. These quick and simple collections were not part of a larger study, but were conducted simply to monitor gross long-term patterns in plant chemistry. As we discovered in the summer of 2000, it is important to maintain these types of long-term datasets to get an idea of the magnitude of annual variability.
Wood fern
Dryopteris spinulosa
Methods of collection and analysis
Wood fern samples have been collected by walking over the entire area of interest and grabbing single fronds (no roots are included) from individual plants here and there. Thus, we have called them "grab" samples. All grab samples included in the graph below are from the lower reaches of the gauged watersheds on the south-facing slope of the Hubbard Brook valley. A single frond was taken from each fern plant to minimize disturbance to the plant.
A consistent annual collection was initiated in 1985 just to the west of W6 from the weir west and uphill to the first foot bridge on the trail and over to the east bank of Bear Brook--an area of roughly two hectares. Collection is routinely conducted during the second week of July, and the samples are replicated in the sense that 5 to 10 separate bags are collected, each including fronds representative of the whole area.
A collection on W1 began in 1996 when the watershed was prepared for detailed study by surveying in a 25 x 25 m plot system. Ferns are collected from four separate vegetation zones, although the only samples included in the graph below are from the low hardwood zone--all plots below the first rain gauge clearing. Grab samples from W1 are also taken during the second week of July, and 2 to 3 replicate bags are collected for each zone.
Each bag of grab sample is dried at 80o C, ground in a Wiley mill to pass a 2 mm screen, and ashed at 500o C. The ash is dissolved in a nitric acid solution (50 ml final volume), and analyzed on an ICP spectrometer for several elements including calcium, a plant macronutrient.
Long-term trend
In the early 1990s we saw a sudden drop in the calcium concentration of wood fern fronds (see graph below). We interpreted this as part of a general decline of calcium in the Hubbard Brook ecosystem, a decline which has been observed in the study of several other components of the ecosystem. The observation of this general calcium decline fueled interest in a calcium addition experiment on W1 which, after years of preparation, was finally initiated in October 1999. Over time we suspect calcium will increase in most components of the ecosystem, but have expected that the herbaceous plant species will be among the first to show an increase. Therefore, we were not surprised to find that the calcium concentration in the low elevation W1 ferns increased by 34% from July 1999 to July 2000 and stayed there, thus returning to pre-1990 levels. But we discovered a strange twist in the summer of 2000. . .
Routine analysis of the grab samples from west of W6 revealed that they, too, had increased almost as dramatically in calcium concentration from 1999 to 2000. At first we thought we must have contaminated the samples. Pressed for time, the ferns west of W6 had been collected on a rainy day, which is something we generally try to avoid. Handling wet samples increases the potential for contamination. Back at the lab in New Haven, CT we collected samples of a common urban herb (purple loosestrife) both on a rainy day and on a dry day. Analysis showed only a very slightly higher calcium concentration in the wet samples as compared to the dry samples. Still not believing our results, we returned to the collection site west of W6 in mid-August to conduct a second collection on a sunny day. Chemical analysis of the August collection revealed the calcium concentration to be very similar to that in the July collection--slightly higher, in fact.
Over a ten year period, we observed the calcium in fern fronds to drop, and then return to "original" levels. We thought that perhaps the return to higher calcium concentrations was part of a delayed response to increased light from the canopy gaps introduced after the 1998 ice storm. Or perhaps the oscillating July calcium concentrations are part of a natural cycle in wood fern populations related to the rate or timing of reproduction or some other physiological process. We did notice many more sporangia in the bottom of the collection bags in 2000 than we had in previous years. However, subsequent collections west of W6 yielded calcium concentrations back down to levels similar to those in the 1990s. We still can not explain why we found such high calcium concentrations in 2000. Perhaps it was simply an unexplained anomaly or perhaps we will see this again. At any rate, we plan to continue our long-term collections of wood fern, and we hope that several more years will shed more light on the issue of whether the pattern we observed in the ferns west of W6 is a unique event or a recurring cycle.
