Increased Turnover but Little Change in the Carbon Balance of High-Arctic Tundra Exposed to Whole Growing Season Warming

Figures & data

FIGURE 1. (A) Time course of green cover during the season determined from pin-frame measurements. Means ± 1 SE on different days of the year (DOY), and fitted polynomial curve (2nd order) for the unheated (○, dotted line) and heated (•, solid line) treatment. (B) Time course of soil moisture during the season. Means ± 1 SE on different DOY, and fitted polynomial curve (5th order) for the unheated (○, dotted line) and heated (•, solid line) treatment. (C) Percentage cover of living vascular plants by species, mosses and bare soil, at the beginning (b) and end (e) of the irradiation period from 2 July to 29 August 1999. Plant parts were considered completely senesced if chlorophyll could no longer be detected visually. Average ± 1 SE of three heated (filled bars) and three unheated (open bars) plots

FIGURE 2. (A) Gross photosynthesis (P_gross ) of the tundra vegetation as a function of incident photosynthetically active radiation (PAR) in ambient conditions or under simulated heating, all measurements during the entire season combined. (B) Curves fitted (eq. 1) to P_gross , from unheated plots and heated plots on three characteristic days during the season; 1: Day of the year (DOY) 182 (spring), 2: DOY 224 (mid season with largest treatment difference), 3: DOY 235 (autumn). (C, D) Time course of stand-level quantum yield (α in eq. 1) and maximum gross photosynthesis (P_gross,max in eq. 1), respectively. Data by DOY and fitted polynomial curves (2nd order) for heated and unheated plots separately. (E) Daily mean gross photosynthesis plotted as a function of green cover for heated and unheated communities separately. (F) Time course of mean gross photosynthesis per unit green cover. Data by DOY and fitted polynomial curves (2nd order) for heated and unheated plots separately. Open symbols (○), dotted lines, and closed symbols (•), solid lines, for the unheated and heated treatment, respectively

FIGURE 3. (A) Soil respiration (R_soil ), from all measurement days, as a function of soil temperature (T_soil ) at a depth of 2.5 cm. Separate curves are fitted (eq. 3) for control (○, dotted line) and heated (•, solid line) plots. (B, C) Residual soil respiration (Respiration_observed –Respiration_fitted ) for control plots as a function of DOY and soil moisture. (D) Residual soil respiration for plots under simulated warming as a function of soil moisture

FIGURE 4. (A) Canopy respiration (R_canopy ) as a function of air temperature (T_air ) at 5 cm height. Measurements pooled over all measurement days of the year (DOY), and fitted curve (eq. 3) for control (○) and heated (•) plots combined. (B) Residual canopy respiration for control plots as a function of green cover (Respiration_observed –Respiration_fitted )

FIGURE 5. (A) Reconstructed time course of daily CO₂ exchange rate (CER) separated into the three components: photosynthesis (P_gross ), soil respiration (R_soil ) and vegetation respiration (R_canopy ). DOY: day of the year. (B) Reconstructed time course of daily total net CER_ecosystem . (C) Cumulated CER over the entire growing season, separated by the three components. Values in (A, B, C) are averages for the three control plots (open symbols or bars) and the three heated plots (closed symbols or bars). Positive values are CO₂ release

FIGURE 6. Daily course of net ecosystem CO₂ exchange rate (CER_ecosystem ) on DOY 189 (A), 213 (B), 231 (C), 239 (D), respectively (all sunny days), for the unheated (○) and heated (•) treatment. Reconstructed values based on separately modeled gross photosynthesis, soil respiration and canopy respiration. Positive values are CO₂ release

FIGURE 7. Instantaneous observed net ecosystem CO₂ exchange rate (CER_ecosystem ) as a function of instantaneous predicted CER_ecosystem . Pooled measurements of the whole growing season