Terhi Rasilo

Effects of Grazing on the Vegetation Structure and Carbon Dioxide Exchange of a Fennoscandian Fell Ecosystem

ABSTRACT We used the chamber method to measure growing season ecosystem carbon exchange and ecosystem respiration in Finnish alpine tundra. The average ecosystem respiration in the sites was 0.8–0.9 µmol m−2 s−1 and the daytime net ecosystem exchange (NEE) was around −0.4 to −0.5 µmol m−2 s−1. There were no detectable differences in cuvette-based net ecosystem exchange or ecosystem respiration between grazed fell areas and long term reindeer exclosure. Further analysis showed that net carbon exchange as well as ecosystem respiration were significantly correlated with the dwarf shrub cover, while the proportion of lichen cover (Cladina sp.) was not correlated with ecosystem carbon exchange. Clipping experiments showed that about half of the measured ecosystem respiration was heterotrophic. Plots that had been protected from reindeer grazing had almost two times higher above-ground plant biomass than grazed plots. The reason for this was 86% lower lichen biomass on the grazed side of the fell, while the biomass of Ericaceous dwarf shrubs did not differ even though there were changes in species composition. Surprisingly, the proportion of bare ground did not differ due to grazing pressure, but the reduction in biomass lead to a less stratified vegetation cover.

The effects of soil and air temperature on CO2 exchange and net biomass accumulation in Norway spruce, Scots pine and silver birch seedlings

Soil temperature is proposed to affect the photosynthetic rate and carbon allocation in boreal trees through sink limitation. The aim of this study was to investigate the effect of temperature on CO(2) exchange, biomass partitioning and ectomycorrhizal (ECM) fungi of boreal tree species. We measured carbon allocation, above- and below-ground CO(2) exchange and the species composition of associated ECM fungi in the rhizosphere of Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies K.) and silver birch (Betula pendula Roth) seedlings grown in soil maintained at 7-12, 12-15 and 16-22 °C. We found increased root biomass and photosynthetic rate at higher soil temperatures, but simultaneously with photosynthesis rate, higher temperature generally increased soil respiration as well as shoot, and root and rhizosphere respiration. The net CO(2) exchange and seedling biomass did not increase significantly with increasing temperature due to a concomitant increase in carbon assimilation and respiration rates. The 2-month-long growth period in different soil temperatures did not alter the ECM fungi species composition and the below-ground carbon sink strength did not seem to be directly related to ECM biomass and species composition in any of the tree species. Ectomycorrhizal species composition and number of mycorrhiza did not explain the CO(2) exchange results at different temperatures.

Transport and transformation of soil-derived CO2, CH4 and DOC sustain CO2 supersaturation in small boreal streams

Streams are typically supersaturated in carbon dioxide (CO2) and methane (CH4), and are recognized as important components of regional carbon (C) emissions in northern landscapes. Whereas there is consensus that in most of the systems the CO2 emitted by streams represents C fixed in the terrestrial ecosystem, the pathways delivering this C to streams are still not well understood. We assessed the contribution of direct soil CO2 injection versus the oxidation of soil-derived dissolved organic C (DOC) and CH4 in supporting CO2 supersaturation in boreal streams in Québec. We measured the concentrations of CO2, CH4 and DOC in 43 streams and adjacent soil waters during summer base-flow period. A mass balance approach revealed that all three pathways are significant, and that the mineralization of soil-derived DOC and CH4 accounted for most of the estimated stream CO2 emissions (average 75% and 10%, respectively), and that these estimated contributions did not change significantly between the studied low order (≤3) streams. Whereas some of these transformations take place in the channel proper, our results suggest that they mainly occur in the hyporheic zones of the streams. Our results further show that stream CH4 emissions can be fully explained by soil CH4 inputs. This study confirms that these boreal streams, and in particular their hyporheic zones, are extremely active processors of soil derived DOC and CH4, not just vents for soil produced CO2.

Concentrations and quality of DOC along the terrestrial–aquatic continuum in a boreal forested catchment

C cycling and dissolved organic C (DOC) inputs to boreal aquatic systems probably will change substantially with climate change. DOC concentrations already are increasing in surface waters. Terrestrial C is a major source of C to boreal freshwater ecosystems, but the interface between these 2 ecosystems, the riparian zone, has not been studied often. To improve our understanding of the importance of terrestrial inputs of DOC to aquatic systems from surrounding forests, we followed the changes of DOC concentration along a continuum of precipitation, throughfall, soil water, ground water, lake, and brook water in a pristine, boreal, forested headwater catchment and developed a lake C balance based on terrestrial and lacustrine C fluxes. We also examined DOC quality changes using the ratio of absorbance at 465 and 665 nm (E4/E6). DOC concentrations increased from 2.4 mg/L in precipitation to 132.3 mg/L in soil water as water passed through the terrestrial ecosystem. DOC concentrations in the riparian zone were correlated with DOC concentrations in the adjacent outflowing brook but not in the headwater lake. E4/E6 ratios indicated that the DOC in precipitation and throughfall was dominated by higher molecular weight compounds and that the DOC in soil and ground water was dominated by lower molecular weight compounds. The input of terrestrial DOC to the aquatic ecosystem was estimated to be 5 to 13 g C m−2 y−1, which is small compared with the C fluxes between atmosphere and vegetation, but can significantly decrease the net ecosystem exchange of an old-growth forest catchment. Terrestrial DOC was a major source of C in the lake, rendering it heterotrophic. The DOC export (3 g DOC m−2 y−1) made up almost 70% of total C export.