THE USE OF LONG-TERM CLIMATE CHANGE EXPERIMENTS FOR IMPROVED INTERPRETATION OF PALEOENVIRONMENTAL RECORDS

Guido L.B. Wiesenberg, Cyrill U. Zosso, Nicholas E.O. Ofiti, Manuela Altermatt, Michael W.I. Schmidt

Department of Geography, University of Zurich, Zurich, Switzerland

Introduction

The common approach for interpretation of ecological or paleoenvironmental data relies on space for time approximations (Pickett, 1989), where, e.g., climate or elevation gradients are used as surrogates for temporal temperature gradients. However, small scale differences between sites, such as precipitation, exposition, microtopography, soil or bedrock likely contribute to differences between sites in confounding the effect of the main factors. During past decades an increasing number of well-controlled climate change experiments were established and conducted under near-natural conditions, and thus provided ideal conditions to determine climate responses that are specific to the manipulated parameters in these experiments. One classical type of such experiments are Free Air CO2 Enrichment (FACE) experiments, that were initiated to trace the impact of increasing future atmospheric CO2 concentrations on intact plant ecosystems. On top of CO2 concentration, often additional parameters like nitrogen fertilization of drought were used to trace their combined effects on plant productivity, and on carbon and nutrient cycling in the investigated ecosystems. The next generation of deep soil warming experiments, like the Blodgett Forest experiment in the Sierra Nevada (CA, USA; Hicks Pries et al., 2017) or the SPRUCE experiment in Minnesota (Spruce and Peatland Responses Under Changing Environments, MN, USA; Hanson et al., 2020), allow for tracing the effect of warming on whole plant-soil systems, which is one important factor frequently traced in paleoenvironmental research. While the Blodgett Forest experiment only traces belowground impact of one temperature level above the control temperature (+4°C), the SPRUCE experiment uses a multifactorial design and manipulates 5 temperature levels (control, +2.25°C, +4.5°C, +6.75°C, +9°C) in combination with two different CO2 concentrations (ambient and elevated), which enables tracing the impact of a combination of several factors on the ecosystem, while other environmental parameters such as water table in the soil and wind speed are controlled and remain largely unchanged. So far, studies using such experimental designs to specifically trace the impact of temperature and CO2 changes on the molecular composition of plant and soil organic matter are still scarce. The aim of our study was to determine the impact of warming and elevated CO2 concentration on the lipid composition of plants and to trace also the impact of the changing environment on organic matter composition and degradation in soils. 

Results

With increasing temperature, carbon, nitrogen, and total lipid concentrations increased in all higher plant tissues in the SPRUCE experiment, whereas Sphagnum ssp. did not follow this trend. The elevated CO2 concentration resulted in a less uniform signal, but typically led to decreased concentrations, especially at increased temperatures. Again, the mosses were excluded from this trend. These trends indicate a change in the biosynthesis of plant tissues for higher plants, whereas mosses were less affected most likely because of their habitat and biosynthesis mechanism. With increasing temperature, the concentrations of long-chain alkanes increased for all plant species, except under elevated CO2, under which alkane concentrations were lower. With increasing temperature plants strengthen their protective wax layers to prevent water loss, and with increasing CO2 concentration plants accelerate their biosynthetic cycling and seem to invest more into sugars and other compounds, but less into protective alkanes. Compositional changes of alkanes were species-specific and differed between all investigated species, highlighting the individual strategies of different species. The same applied to fatty acid concentrations and compositions, where concentrations were mostly lower under elevated CO2 concentration. However, temperature responses led to species-specific responses. At the ecosystem level, we observed non-uniform changes with temperature and CO2-concentration. Different species respond by opposing strategies to changing environmental conditions. 

Not only plant composition changed, but also soil organic matter was affected by changing environmental conditions. We traced molecular changes and transformation of organic matter within the soil profiles. In the peat sequence of the SPRUCE experiment, we found b impacts of increasing temperature and elevated CO2 concentration, specifically in the top-most 50 cm. With increasing soil depth these impacts decreased sharply, probably resulting from the anoxic conditions in the water-saturated zone, thus limiting organic matter degradation. Furthermore, the experiment lasted only a few years, which were by far not enough to equilibrate the whole ecosystem to the new equilibrium of experiment-derived organic matter incorporation and degradation. However, the active, topmost part of the sequence (0-30 cm), showed b changes of lipid compositions, reflecting the new environmental conditions. Alkane and fatty acid concentration and composition only partly reflected the new plant compositions. This is likely due to the increased fine-root biomass with increasing temperature, which results on the one hand in molecular alteration of the peat organic matter like less alkanes. But also the more active bacteria in the rhizosphere might preferentially degrade peat organic matter, eventually changing peat composition. In the oxic soil of the Blodgett Forest experiment the increased temperature resulted in lower root abundance especially in the deeper soil profile (below 60 cm), which resulted in less fresh organic matter input and degradation of the present organic matter at larger depth. 

To conclude, the environmental impacts of increasing temperature have b effects of organic matter production, incorporation and degradation in terrestrial ecosystems, which complicate the limitation to individual factors, such as only changing temperature, which is often associated with changing water availability. As a consequence, drawing general conclusions with respect to the impact of, e.g., temperature and CO2 concentration on plant molecular composition seems premature. We conclude, that paleoecological conclusions based on space-for-time approaches should be re-evaluated with the help of long-term field trials before they are transferred to paleoenvironmental records. 

References

Hanson, P.J. et al., 2020. Rapid net carbon loss from a whole‐ecosystem warmed peatland. AGU Advances 1, e2020AV000163.

Hicks Pries, C.E., Bird, J.A., Castanha, C., Hatton, P.-J., Torn, M.S., 2017. Long term decomposition: the influence of litter type and soil horizon on retention of plant carbon and nitrogen in soils. Biogeochemistry 134, 5-16.

Pickett, S.T.A., 1989. Space-for-time substitution as an alternative to long-term studies. In: Likens, G.E., Long-Term Studies in Ecology: Approaches and Alternatives. Springer, New York, pp 110-135.