Ecological Archives E081-032-A2

Jake F. Weltzin, John Pastor, Calvin Harth, Scott D. Bridgham, Karen Updegraff, and Carmen T. Chapin.  2000.  Response of bog and fen plant communities to warming and water-table manipulations.  Ecology 81:3464-3478.  

Appendix B. Methods for estimation of ANPP in mesocosm plots.

Graminoids and forbs

Annual ANPP (g/m²) of graminoids and forbs was estimated based on mean shoot weights or canopy/biomass relationships of plants destructively sampled from bog and fen source sites between 1994 and 1996. In addition, mean shoot weights for relatively common species were determined from a subset of the mesocosm plots in 1997. For all graminoid and forb taxa, non-destructive counts and/or measurements of plants in the plots or subplots were conducted between June and August of each year, depending on the phenology of each species.

In particular, with the exception of two species, we determined reproductive and/or vegetative shoot mass of most forbs and tiller mass of most graminoids present in the mesocosms (Table A1). We calculated annual ANPP for each taxa based on the product of tiller mass data and the number of shoots or tillers within each plot or its subplots, as appropriate.

Because Sarracenia purpurea has leaves of greatly variable mass, we estimated its production based on relationships between leaf biomass and leaf length and mid-sagittal leaf orifice diameter. In particular, in 1994 we measured and then destructively harvested 15 leaves from S. purpurea plants at the fen source site, and used this information to develop the following relationship: ln(mass of leaf in g) = -10.39 + ln(length in mm)1.65 + ln(mid-sagittal orifice diameter in mm)0.63; R² = 0.92, P < 0.0001). Because S. purpurea has relatively large flowers, we estimated production of flower mass based on a mean flower weight of 0.925 g determined for 20 flowers collected from the fen source site in 1996.

Tussocks of Eriophorum spissum were relatively large and clumped, so we estimated its production based on relationships between tussock biomass and tussock circumference, median height and visual estimates of percentage live (i.e., green) cover. In particular, in 1996 we collected aerial portions of twenty tussocks from the bog source site, and developed the following relationship: ln(tussock mass in g) = -7.15 + ln(tussock circumference in cm)1.20 + ln(tiller median height in cm)1.31 + ln(% green)0.44; R² = 0.91, P < 0.0001).

To determine whether our water-table setting and infrared loading treatments affected values of tiller mass used to predict graminoid ANPP, in 1997 we destructively sampled 10-15 vegetative tillers (as subsamples) per plot for one species common to all bog plots (Eriophorum spissum) and three species common to all fen plots (Carex lasiocarpa, undifferentiated Rhyncospora spp., Scheuchzeria palustris). We used a fixed-effects ANOVA to determine main and interactive effects of water-table setting and infrared loading on tiller mass.

Tiller mass (0.025 g) did not differ between treatments for E. spissum (Table A2). Although the ANPP algorithm of this species is based on tussock circumference, height, and percentage live cover, a consistent tiller mass across treatments suggests that canopy morphology of this species is relatively insensitive to the treatments applied. Accordingly, we used the least-squares multiple regression equation described above for data from all years and treatments.

Similarly, tiller mass (0.063 g) of undifferentiated Rhynchospora spp. (= R. alba + R. fusca) in 1997 did not differ between treatments (Table A2). However, because this value was roughly half that of the mean mass of 40 Rhynchospora spp. tillers (0.125 g) collected from the fen source site in 1994, we incremented tiller mass of Rhynchospora spp. linearly between 1994 and 1997, and applied these values to all treatments; ANPP for 1994 through 1997 was calculated as the product of tiller mass and tiller count data for each year.

In contrast, tiller masses of S. palustris and C. lasiocarpa were affected by infrared loading, and water-table and infrared loading, respectively (Table A2). Therefore, for these two species we incremented tiller mass linearly between the source site data collected in 1994 and each of the 9 treatment combinations in 1997. As for Rhynchospora spp., ANPP was calculated based on tiller mass and tiller count data for each year.

Shrubs

Annual ANPP (g/m²) of dominant shrubs was estimated based on canopy/biomass relationships determined for plants destructively harvested from the bog source site in 1994, the fen source site in 1995, and the mesocosm plots in 1995 through 1997 (Table A3).

Specifically, in August of 1994 at the bog source site, we measured the lengths of new shoots, which were then clipped, dried, and weighed, for 20 individuals of Chamaedaphne calyculata, Kalmia polifolia, Andromeda glaucophylla, Ledum groenlandicum, and Vaccinium oxycoccos. In August of 1995, we collected similar data for Vaccinium macrocarpon from the fen source site. In 1995 through 1997, we collected similar data for subsets of shrubs within the bog plots to determine whether the water-table setting and infrared loading treatments affected stem length/mass relationships over time.

We focused our investigation of potential treatment effects on data collected in 1997, with the rationale that treatments should be most divergent after four growing seasons. In particular, in 1997 we destructively sampled 10-20 individuals (as subsamples) per plot of each of two shrub species common to all bog plots: C. calyculata--representative of shrubs with erect growth forms, and V. oxycoccos--representative of shrubs with prostrate growth forms. For each species, we determined least-squares regression coefficients ($) for allometric relationships between the natural log of the length (in mm) of each new shoot segment versus the natural log of the biomass (in mg) of that shoot segment (Whitaker and Marks 1975).

We then used a fixed-effects ANOVA to determine main and interactive effects of water-table and infrared loading treatments on $ for each species. $ did not differ between treatments for C. calyculata or V. oxycoccos (Table A3).

In addition, to determine whether allometric relationships were changing over time, we compared $ for 1994 and 1997 for these two species (Zar 1996). $ did not differ between 1994 ($ ± 1 SE = 1.20 ± 0.12) and 1997 (1.21 ± 0.03) for C. calyculata (P = 0.20), but differed between 1994 (0.93 ± 0.13) and 1997 (0.99 ± 0.02) for V. oxycoccos (P = 0.01).

Therefore, for C. calyculata (and for other shrub species with similarly erect growth forms) we developed a single regression equation based on data pooled for all years (Table 3). For V. oxycoccos, we incremented $ and its intercept (") linearly between 1994 and 1997 (Table A2), and applied these year-specific regression equations to data for each year. For V. macrocarpon, we applied the shoot length/mass relationship developed for the 1995 dataset to all years (Table A3). In addition, for V. oxycoccos, we incorporated fruit mass into ANPP based on mean mass and counts of fruits.

We used the allometric relationships that we developed for C. calyculata to determine ANPP for the following minor shrub species: Salix spp., Populus spp., Betula glandulifera, Picea mariana, Larix laracina, Acer rubrum, and Vaccinium myrtilloides.

We counted and measured shrubs within mesocosm plots or subplots in August and September of each year. Number and canopy dimensions of each shrub species were usually collected from the entire 10-cm x 50-cm subplot. Because of time constraints, when C. calyculata, A. glaucophylla, or V. oxycoccus were especially abundant, we sampled them within a permanent 10-cm x 15-cm sub-subplot located at random within each subplot. Estimates of annual aboveground production were determined for each species based on previously determined canopy/biomass relationships.

Bryophytes

Bryophyte annual ANPP (g/m²) was estimated for bog plots only, and was based on shoot density and species-specific shoot mass/shoot length relationships, basal cover, and lineal shoot growth. Shoot density and mass/length relationships were developed from samples collected at the bog source site in October, 1994, when we collected three 10-cm x 10-cm x 4-cm deep samples of Polytrichum strictum, Sphagnum capillifolium, S. fuscum, S. magellanicum, and S. recurvum from different mono-specific populations. Samples were stored frozen until they were processed. For each species-specific sample, we determined shoot density (#/dm²), and shoot mass/length relationships for individual shoots by clipping distal portions of each shoot to 1 cm for Polytrichum strictum or 2 cm excluding the capitula for Sphagnum spp., drying shoot segments at 60EC, weighing them to the nearest mg, and calculating shoot mass/shoot length ratios (Table A4). Sphagnum fusca and S. capillifolium were difficult to differentiate after 1994 because of organic staining from water applications, so were lumped into Sphagnum Acutifolia section for all analyses.

Bryophyte basal cover was determined in July through August of 1995-1997 from subplots within each plot. Subplots were positioned using a removable sampling grid. The sampling grid was constructed to form a 185-cm open square, with taut line strung from side to side at right angles to form a grid of ~110 intersecting points at a spacing of 12.5 cm. The grid was leveled at about 20 cm above the median surface elevation of each plot, and was held rigidly by supports bolted to the tank. When sampling, we lowered a plumb bob from each grid point down to the bryophyte surface, where we placed a round, 3-cm-diam. quadrat. The percentage cover for each bryophyte species within the quadrat was then estimated visually. To minimize edge effects, we did not sample quadrats within 15 cm of the plot edge.

Lineal shoot growth was estimated using the cranked-wire method, wherein stainless steel wires set vertically into the bryophyte surface act as dynamic datum points for measuring bryophyte shoot extension (Clymo 1970). Because bryophyte growth rates are often related to relative elevation or microtopography (Clymo 1970), we placed one to six cranked-wires within each of three microtopographic zones within each plot based on relative elevation: high, low, and intermediate.

Microtopographic zones were defined each year based on the vertical distance between the bryophyte surface and each of the ~110 points on the removable sampling grid. Specifically, in each year, we ranked the vertical distances for all (27 plots x ~110 points per plot =) ~2970 points, and then divided this dataset into three equal-sized subsets: the ~990 points with the greatest vertical distance between the bryophyte surface and the frame were defined as representing low microtopographic zones in their respective plots. Similarly, the ~990 points with the least vertical distance represented high microtopographic zones, and the remainder of the points represented intermediate microtopographic zones. Cranked-wires were assigned to a microtopographic zone based on their relative elevation. Because we did not start using the removable sampling frame until 1995, we applied the 1995 cranked wire designations to the 1994 dataset.

Vertical height of bryophyte growth relative to the top of the cranked wire was measured monthly during the growing season in 1994 through 1997; monthly measurements were summed to obtain total lineal shoot growth within each microtopographic zone. Bryophyte annual ANPP was calculated for each species within each zone using species-specific stem mass/length relationships and shoot densities developed in 1994 (Table A4), lineal growth estimates for that zone, and species cover values within each zone. Although bryophyte ANPP was affected by microtopographic zone (unpublished data), we analyzed bryophyte ANPP on a plot-wise basis for the purpose of this paper. And, although bryophyte stem length/mass relationships within the mesocosms may have changed with time or treatment, the destructive sampling of the bryophyte community that would be required to determine these effects precluded their assessment.

Table A1. Mean shoot mass for graminoid and forb taxa used to determine annual ANPP (g/m²).

Notes: Tillers or shoots were either vegetative (V) or reproductive (R). Source and year codes: M = mesocosm plots, F = fen source site, B = bog source site; 4 = 1994, 5 = 1995, 6 = 1996, 7 = 1997.

Table A2. ANOVA P-values for mean tiller mass for graminoids, and regression coefficients ($) for shrubs, that were common to all bog or all fen mesocosm plots in 1997.

Table A3. Allometric relationships used to determine shrub annual ANPP (g/m²).

Notes: Source and year codes: M = mesocosm plots, F = fen source site, B = bog source site; 4 = 1994, 5 = 1995, 6 = 1996, 7 = 1997. The least-squares regression equation is in the form of ln(ANPP) = ln(new shoot length)$ + ".

Table A4. Bryophyte shoot density (#/dm²), total number of shoot segments used to calculate shoot mass/shoot length ratios, and mass/length ratios (mg/cm) used to determine bryophyte annual ANPP (g/m²).

Clymo, R.S. 1970. The growth of Sphagnum: methods of measurement. Journal of Ecology 58:13-49.

Whitaker, R.H. and P.L. Marks. 1975. Methods of assessing terrestrial productivity. Pages 55-118 in H. Lieth and R.H. Whitaker, editors. Primary productivity of the biosphere. Springer-Verlag, New York, New York, USA.

Zar, J.H. 1996. Biostatistical analysis. Prentice Hall. Upper Saddle River, New Jersey, USA.