Carbon, water and nutrient dynamics in vascular plant- vs. Sphagnum-dominated bog ecosystems in southern Patagonia
Physical Geography
Final Report Abstract
In solely rain water-fed peatlands, i.e. ombrotrophic bogs, vegetation controls key processes such as the retention of C, water and nutrients. In bogs of the northern hemisphere, a shift from Sphagnum- to vascular plant-dominated vegetation is often traced back to climate change and increased nitrogen deposition, and coincides with substantially reduced capacities in carbon, water and nutrient retention. In southern Patagonia, bogs dominated by Sphagnum and cushion-building vascular plants coexist since millennia under similar environmental settings. Thus, the aim of this project was to study the effect of vascular plant invasion on carbon, water and nutrient balances of South Patagonian bog ecosystems and possible drivers of such vegetation shifts. We could prove previous findings that vascular plant-dominated cushion bogs are almost zero CH4 emission systems, as CH4 produced in the peat is almost entirely oxidized prior to being emitted. However, we could also show that the presence of cushion-forming vegetation may not always be a good a proxy for negligible CH4 emissions, as fluxes vary substantially on the microform level. Particularly, Sphagnum patches and Donatia lawns were identified as local hotspots of CH4 emissions. In addition, hot moments of CH4 emissions, i.e. discrete ebullition events, might play an important role, but could not be captured with chamber measurements. We provided the first experimental evidence that sea spray could be a major driver of distribution patterns of Sphagnumversus cushion-plant dominated bogs at the landscape scale. The photosynthetic performance of S. magellanicum was strongly reduced under sea spray addition whereas A. pumila was hardly affected. The analyses of elemental concentrations in the biomass of both species suggested that A. pumila benefit from mechanical and physiological adaptations to cope with drought stress induced by sea spray deposition making this species a stronger competitor in coastal areas. Formerly, assumptions that sea spray might be of relevance for the distribution patterns of S. magellanicum and cushion-forming vascular plants relied completely on explorative field studies. Applying landscape-scale eddy covariance measurements of CO2, water and energy fluxes in one Sphagnumdominated and one cushion bog site, we found the cushion bog to be highly productive with an average annual net CO2-C uptake 4.5 times larger than at the Sphagnum bog. Inter-annual net ecosystem exchange (NEE) variations were dominated by gross primary production (GPP) variability, while total ecosystem respiration (TER) was rather unaffected by temperature variations. Both NEE components, photosynthetic uptake as well as to ecosystem respiration were elevated at the cushion bog: TER was 80 % larger, |GPP| twice as large. Unexpectedly, experimental warming induced within a complementary field experiment led to generally lower photosynthethis levels and light-stress induced reductions of photosynthesis rates at Astelia pumila lawns. In contrast to the land-atmosphere flux measurements, we found long-term carbon accumulation rates in cushion bogs to be of similar height than in Sphagnum bogs, but apparently most recent C accumulation rates in cushion bogs decreased. This opens the question whether cushion plants might fail to accumulate carbon as the higher productivity could not outweigh the extremely high decomposition rates of the litter and in the rhizosphere on a longer time scale. Finally, we showed that image classification approaches can introduce a strong bias for estimates of landscape-scale CH4 fluxes upscaled from manual chamber measurements. Very high resolution images and a detailed speciesscale classification with a comparatively low area of the surface remaining unclassified proved to yield superior and more robust estimates for landscape-scale CH4 fluxes compared with commonly used microform-level classification approaches.
Publications
- High-Resolution Classification of South Patagonian Peat Bog Microforms Reveals Potential Gaps in Up-Scaled CH4 Fluxes by use of Unmanned Aerial System (UAS) and CIR Imagery, Remote Sens-Basel, 8, 173, 2016
Lehmann, J., Münchberger, W., Knoth, C., Blodau, C., Nieberding, F., Prinz, T., Pancotto, V., and Kleinebecker, T.
(See online at https://doi.org/10.3390/rs8030173) - Carbon dioxide and methane balances of pristine and degraded temperate peatlands - Empirical modeling of eddy covariance trace gas fluxes measured over heterogeneous terrain, Hamburger Bodenkundliche Arbeiten, 82, 139 pp., ISSN: 0724-6382 2017
Holl, D.
- Zero to moderate methane emissions in a densely rooted, pristine Patagonian bog – biogeochemical controls as revealed from isotopic evidence, Biogeosciences, 16, 541-559, 2019
Münchberger, W., Knorr, K. H., Blodau, C., Pancotto, V. A., and Kleinebecker, T.
(See online at https://doi.org/10.5194/bg-16-541-2019)
