We designed a CO2-controlled cuvette and stripping system to trace a (CO2)-C-14 pulse-label from photosynthetic assimilation by wetland plants tin this study Orontium aquaticum L.) to its release as (CH4)-C-14 by microbial respiration. The system maintained cuvette CO2 concentrations to within +/-5 Pa of the set-point, and it allowed continuous recovery of (CO2)-C-14 and (CH4)-C-14 for 17 d without damage to the enclosed plant. The first emissions of (CH4)-C-14 mere detected <12 h after photosynthetic assimilation of the label. The (CH4)-C-14 flux increased linearly from 0.12 Bq min(-1) at 12 h to 3.0 Bq min(-1) at 5 d, then plateaued at approximate to 2 Bq min(-1). We could not distinguish between (CH4)-C-14 produced by aceticlastic methanogenesis vs, that produced by CO2 reduction. Radiocarbon activity in the soil dissolved inorganic C pool peaked on the first day then declined slowly. We did not detect radiocarbon activity in soil solution pools of several low molecular weight organic acids (acetate, formate, lactate, and propionate), but the label was detected in the bulk dissolved organic C pool. We argue that radiocarbon will be useful for investigating the contribution of root exudates to methanogenic metabolism, but data interpretation will require separation of the relative contributions of CO2 reduction and aceticlastic methanogenesis to overall (CH4)-C-14 emissions. Processes such as CH4 oxidation and acetogenesis must also be considered in quantitative estimates of photosynthetic support of methanogenesis.

Tundra ecosystems appear to recover slowly from disturbance, but little long-term data concerning plant diversity has been available. We examined recovery of tundra vegetation in Alaska, U.S.A., 23 yr after fire and 24 yr after bulldozing. Primary productivity, depth of thaw, and vascular plant diversity were compared between disturbed and undisturbed tundra to determine whether recovery was complete. Productivity, species richness, and diversity did not differ between burned and unburned plots. Depth of thaw, however, remained greater in burned relative to unburned plots. In contrast, depth of thaw was the only characteristic that did not differ between bulldozed and control plots. Productivity and species richness were greater in bulldozed plots, but diversity was less than control plots. The differences between the two disturbances suggest that, ultimately, recovery depends more on the impact of disturbance on vegetation than changes in the abiotic environment. Vegetative propagules persisted in the soil after fire, but not bulldozing. Therefore, recolonization after fire included plants from the seed bank and vegetative propagules. Vegetation on bladed plots was dominated only by seed bank species. Thus, more than two decades after disturbance, recovery of tundra vegetation appeared to be a function of the nature of the disturbance.

We measured seasonal and canopy-level gas exchange in two stands of jack pine (Pinus banksiana Lamb.) and one stand of black spruce (Picea mariana (Mill.) B.S.P.) on relatively clear days from late May until mid-September 1994. Field measurements were made with a portable infrared gas analyzer, and laboratory measurements included photosynthetic oxygen evolution and needle chemical composition. Seasonally averaged light-saturated assimilation rates in the field were 4.0 micro mol m(-2) s(-1) in jack pine and 2.7 micro mol m(-2) s(-1) in black spruce. Rates of assimilation and transpiration were highest in midsummer. The seasonal pattern was especially pronounced for black spruce, probably because cold soil temperatures limited early season gas exchange rates in this species. Among stands, instantaneous water-use efficiency was highest in a young jack pine stand early in the season and higher in the upper canopy foliage than in the lower canopy foliage at all sites at the end of the season. Needles of young jack pine exhibited higher photosynthetic capacity, dark respiration and needle N concentrations than needles of trees at the old site. In both species, slight acclimation to shading was manifested by reductions in photosynthetic capacity in the lower canopy foliage. In both species, first-year needles had greater photosynthetic capacity than older needles but in situ rates of CO(2) assimilation in the field showed little difference among needle age classes. In both species, there was a strong correlation between assimilation and stomatal conductance, indicating that assimilation was highly stomatal limited and that environmental factors that alter conductance (e.g., VPD) have a strong influence on CO(2) and water fluxes, especially after early season thawing concludes.

Leaf‐level measurements of gas exchange, chemistry, morphology, and spectral optical properties were acquired at the five instrumented tower sites during the three 1994 growing season intensive field campaigns (IFCs) conducted near Prince Albert, Saskatchewan, as part of the Boreal Ecosystem‐Atmosphere Study (BOREAS). Stands included old and young aspen (OA, YA) associated with the hazelnut shrub, old and young jack pine (OJP, YJP) stands, and an old black spruce (OBS) stand; white spruce (at YA) and an understory herb (dogbane, at OJP) were also examined. Midsummer peak photosynthesis for aspen leaves in the field (A, light saturated) and laboratory ( A max light and CO 2 saturated) was ∼12.6 and 33–41 μmol CO 2 m −2 s −1 . Black spruce exhibited the lowest A, 3 μmol CO 2 m −2 s −1 . Jack pine and black spruce attained their highest A max (17–20 μmol CO 2 m −2 s −1 ) in late summer/early fall. Gas exchange by white spruce was significantly higher and stomatal limitation lower than for other conifers, at levels comparable to broadleaf responses. White spruce foliage had the highest chlorophyll content in fall (∼41 μg cm −2 ), followed by aspen (OA) and hazelnut (YA) in midsummer (∼31 μg cm −2 ). Specific leaf mass of aspen, hazelnut, and conifer foliage was 86, ∼47, and ∼174 g m −2 , respectively. Leaf nitrogen content of broadleaves (18–40 g N g −1 dry wt) was 2–3 times greater than conifer needles (8–12 g N g −1 ). Significantly larger needles were produced at OJP versus YJP, but needle number per age class was greater at YJP. The absorbed photosynthetically active radiation fraction ( f APAR) in June/July averaged ∼80% for broadleaves and ∼83% in conifer needles. The simple ratio (SR, near‐infrared/red ratio) calculated from foliar transmittances was more strongly related to f APAR than SR calculated from reflectances, with stronger correlation for broadleaves ( r =0.92) than for conifers ( r =0.78).