Frank Hagedorn

Simple additive effects are rare: a quantitative review of plant biomass and soil process responses to combined manipulations of CO2 and temperature

In recent years, increased awareness of the potential interactions between rising atmospheric CO2 concentrations ([ CO2 ]) and temperature has illustrated the importance of multifactorial ecosystem manipulation experiments for validating Earth System models. To address the urgent need for increased understanding of responses in multifactorial experiments, this article synthesizes how ecosystem productivity and soil processes respond to combined warming and [ CO2 ] manipulation, and compares it with those obtained in single factor [ CO2 ] and temperature manipulation experiments. Across all combined elevated [ CO2 ] and warming experiments, biomass production and soil respiration were typically enhanced. Responses to the combined treatment were more similar to those in the [ CO2 ]-only treatment than to those in the warming-only treatment. In contrast to warming-only experiments, both the combined and the [ CO2 ]-only treatments elicited larger stimulation of fine root biomass than of aboveground biomass, consistently stimulated soil respiration, and decreased foliar nitrogen (N) concentration. Nonetheless, mineral N availability declined less in the combined treatment than in the [ CO2 ]-only treatment, possibly due to the warming-induced acceleration of decomposition, implying that progressive nitrogen limitation (PNL) may not occur as commonly as anticipated from single factor [ CO2 ] treatment studies. Responses of total plant biomass, especially of aboveground biomass, revealed antagonistic interactions between elevated [ CO2 ] and warming, i.e. the response to the combined treatment was usually less-than-additive. This implies that productivity projections might be overestimated when models are parameterized based on single factor responses. Our results highlight the need for more (and especially more long-term) multifactor manipulation experiments. Because single factor CO2 responses often dominated over warming responses in the combined treatments, our results also suggest that projected responses to future global warming in Earth System models should not be parameterized using single factor warming experiments.

Export of dissolved organic carbon and nitrogen from Gleysol dominated catchments – the significance of water flow paths

In this study, we estimated whether changes in hydrological pathwaysduring storms could explain the large temporal variations of dissolvedorganic carbon (DOC) and nitrogen (DON) in the runoff of threecatchments: a forest and a grassland sub-catchment of 1600m2 delineated by trenches, and a headwater catchment of 0.7km2.The average annual DOC export from the sub-catchments was 185 kg DOCha−1 y−1 for the forest, 108 kg DOCha−1 y−1 for the grassland and 84 kgDOC ha−1 y−1 for the headwatercatchment. DON was the major form of the dissolved N in soil and streamwater. DON export from all catchments was approximately 6 kg Nha−1 y−1, which corresponded to 60% ofthe total N export and to 50% of the ambient wet N deposition. DOC andDON concentrations in weekly samples of stream water were positivelycorrelated with discharge. During individual storms, concentrations andproperties of DOC and DON changed drastically. In all catchments, DOCconcentrations increased by 6 to 7 mg DOC l−1 comparedto base flow, with the largest relative increment in the headwatercatchment (+350%). Concentrations of DON, hydrolysable amino acids, andphenolics showed comparable increases, whereas the proportion ofcarbohydrates in DOC decreased at peak flow. Prediction of DOC and DONconcentrations by an end-member mixing analysis (EMMA) on the base ofinorganic water chemistry showed that changes in water flow pathslargely explained these temporal variability. According to the EMMA, thecontribution of throughfall to the runoff peaked in the initial phase ofthe storm, while water from the subsoil dominated during base flow only.EMMA indicated that the contribution of the DOC and DON-rich topsoil washighest in the later stages of the storm, which explained the highestDOC and DON concentrations as the hydrograph receded. Discrepanciesbetween observed and predicted concentrations were largest for thereactive DOC compounds such as carbohydrates and phenolics. Theyoccurred at base flow and in the initial phase of storms. This suggeststhat other mechanisms such as in-stream processes or a time-variantrelease of DOC also played an important role.

Central European hardwood trees in a high‐CO2 future: synthesis of an 8‐year forest canopy CO2 enrichment project

Rapidly increasing atmospheric CO2 is not only changing the climate system but may also affect the biosphere directly through stimulation of plant growth and ecosystem carbon and nutrient cycling. Although forest ecosystems play a critical role in the global carbon cycle, experimental information on forest responses to rising CO2 is scarce, due to the sheer size of trees. Here, we present a synthesis of the only study world-wide where a diverse set of mature broadleaved trees growing in a natural forest has been exposed to future atmospheric CO2 levels (c. 550ppm) by free-air CO2 enrichment (FACE). We show that litter production, leaf traits and radial growth across the studied hardwood species remained unaffected by elevated CO2 over 8years. CO2 enrichment reduced tree water consumption resulting in detectable soil moisture savings. Soil air CO2 and dissolved inorganic carbon both increased suggesting enhanced below-ground activity. Carbon release to the rhizosphere and/or higher soil moisture primed nitrification and nitrate leaching under elevated CO2; however, the export of dissolved organic carbon remained unaltered.Synthesis. Our findings provide no evidence for carbon-limitation in five central European hardwood trees at current ambient CO2 concentrations. The results of this long-term study challenge the idea of a universal CO2 fertilization effect on forests, as commonly assumed in climate-carbon cycle models.

The age of preferential flow paths

Preferential flow is a common phenomenon in soils, but the temporal stability of the heterogeneous flow pattern is largely unknown. We give evidence that preferential flow paths in a structured forest soil are persistent for decades. After staining preferential flow paths in a fine-loamy Dystric Cambisol with a dye tracer, we sampled the soil from the preferential flow paths and from the unstained matrix at five sampling times during 1 year. In preferential flow paths, the activities of 137Cs, 210Pb and 239,240Pu, as well as concentrations of soil organic carbon (SOC), were enriched by a factor of up to 3.5 relative to those of the matrix. The 137Cs originates mainly from the Chernobyl accident in 1986, the 210Pb from a continuous ‘natural’ atmospheric deposition and the 239,240Pu from nuclear weapon tests in the 1950s and 1960s. Since all of these radionuclides are only mobile in the soil immediately after deposition, the increased activities of radionuclides in the recent flow paths sampled during our experiments indicate that these flow paths were stable for decades. This is supported by the total enrichment of SOC in preferential flow paths ranging between 740 and 960 g C m−2, which is high in comparison to published accumulation rates of SOC in forest bulk soils from 20 to 60 g C m−2 year−1. The gradient of radionuclide activity and of SOC concentrations between the two flow regions was relatively constant during the 1-year experiment. In all of the five sampling times, concentrations of SOC were larger by 37–46% and the activities of 137Cs were larger by 83–150% in the preferential flow paths than in the matrix down to a depth of 50 cm. This means that despite the differing boundary conditions at the different sampling times, the pathways of infiltrating water were persistent with time.

Treeline advances along the Urals mountain range - driven by improved winter conditions?

High-altitude treelines are temperature-limited vegetation boundaries, but little quantitative evidence exists about the impact of climate change on treelines in untouched areas of Russia. Here, we estimated how forest-tundra ecotones have changed during the last century along the Ural mountains. In the South, North, Sub-Polar, and Polar Urals, we compared 450 historical and recent photographs and determined the ages of 11,100 trees along 16 altitudinal gradients. In these four regions, boundaries of open and closed forests (crown covers above 20% and 40%) expanded upwards by 4 to 8 m in altitude per decade. Results strongly suggest that snow was an important driver for these forest advances: (i) Winter precipitation has increased substantially throughout the Urals (~7 mm decade(-1) ), which corresponds to almost a doubling in the Polar Urals, while summer temperatures have only changed slightly (~0.05°C decade(-1) ). (ii) There was a positive correlation between canopy cover, snow height and soil temperatures, suggesting that an increasing canopy cover promotes snow accumulation and, hence, a more favorable microclimate. (iii) Tree age analysis showed that forest expansion mainly began around the year 1900 on concave wind-sheltered slopes with thick snow covers, while it started in the 1950s and 1970s on slopes with shallower snow covers. (iv) During the 20th century, dominant growth forms of trees have changed from multistemmed trees, resulting from harsh winter conditions, to single-stemmed trees. While 87%, 31%, and 93% of stems appearing before 1950 were from multistemmed trees in the South, North and Polar Urals, more than 95% of the younger trees had a single stem. Currently, there is a high density of seedlings and saplings in the forest-tundra ecotone, indicating that forest expansion is ongoing and that alpine tundra vegetation will disappear from most mountains of the South and North Urals where treeline is already close to the highest peaks.

Subordinate plant species enhance community resistance against drought in semi‐natural grasslands

According to the insurance hypothesis, more diverse plant communities are more likely to be resistant to drought. Whilst many experiments have been carried out to determine the effects of plant diversity on plant community insurance, the results are still contradictory. Here, we conducted a drought experiment where we tested whether the presence of subordinate species increases plant community insurance. In Swiss Jura grassland, we combined a removal experiment of subordinate species with a summer drought event using rainout shelters. Plant community composition was determined after the drought and based on biomass measurements; we estimated resistance, recovery and resilience of the plant community for each combination of treatments. Moreover, to assess drought impacts on water-use efficiency (WUE), we analysed carbon isotope ratios (13C values) in plant leaves of two dominants and two subordinates collected at the end of the drought period. We showed that subordinate species are more resistant to drought and increased community resistance by enhancing their above-ground biomass production during the imposed drought. These patterns were associated with decreased competitiveness of dominant species whose biomass decreased during drought. Significant increase in 13C values in plant tissue under drought indicated a better WUE for the measured species. Interestingly, the WUE was significantly higher in plots where subordinates were removed. Recovery and resilience were not affected by the summer drought, but the absence of subordinates reduced overall above-ground biomass in both watered and drought plots. Synthesis. We demonstrated that, independent of plant diversity, the presence of drought-resistant subordinate species increases plant community insurance against drought and, hence, is important for the functioning of grassland ecosystems.

Contrasting dynamics of dissolved inorganic and organic nitrogen in soil and surface waters of forested catchments with Gleysols

Abstract In this study, we investigated the dynamics of dissolved inorganic and organic nitrogen (N) in throughfall, in soil solutions of Gleysols, and in outflows from experimental sub-catchments as well as from a headwater catchment in Switzerland. Additionally, we studied the effect of increased N deposition on dissolved N by applying 3 g NH 4 NO 3 –N m −2 year −1 to soil plots and to one of the sub-catchments. Dissolved organic N (DON) was the dominant form of total dissolved N (TDN) in the soil and surface waters. The proportion of DON in TDN increased from the throughfall down to the subsoil, which indicates that the retention of DON was lower than that of inorganic N. Concentrations of DON in the subsoil were higher under reducing than under oxidising conditions. In the soil solution and in the runoff from all catchments, nitrate and DON displayed inverse seasonal patterns with concentrations of NO 3 − being highest in late winter and those of DON being maximal in summer and fall. This difference shows that during periods of increased biological activity, NO 3 − was retained in the forest ecosystem while the production of DON was stimulated. Concentrations of both NO 3 − and DON were negatively correlated with the amount of throughfall. In the outflow of the headwater catchment, however, the concentration–discharge relationship was negative for NO 3 − , but positive for DON. The reason for this difference appeared to be the larger contribution of topsoil water at high flow, which was poor in NO 3 − and rich in DON. Experimentally increasing the N deposition increased NO 3 − leaching significantly, but had no effects on DON leaching and on DON export from the catchments. In conclusion, our results show that the dynamics of dissolved organic and inorganic N are controlled by different factors.

Growth and community responses of alpine dwarf shrubs to in situ CO2 enrichment and soil warming

• Rising CO₂ concentrations and the associated global warming are expected to have large impacts on high-elevation ecosystems, yet long-term multifactor experiments in these environments are rare. • We investigated how growth of dominant dwarf shrub species (Vaccinium myrtillus, Vaccinium gaultherioides and Empetrum hermaphroditum) and community composition in the understorey of larch and pine trees responded to 9 yr of CO₂ enrichment and 3 yr of soil warming at the treeline in the Swiss Alps. • Vaccinium myrtillus was the only species that showed a clear positive effect of CO₂ on growth, with no decline over time in the annual shoot growth response. Soil warming stimulated V. myrtillus growth even more than elevated CO₂ and was accompanied by increased plant-available soil nitrogen (N) and leaf N concentrations. Growth of Vaccinium gaultherioides and E. hermaphroditum was not influenced by warming. Vascular plant species richness declined in elevated CO₂ plots with larch, while the number of moss and lichen species decreased under warming. • Ongoing environmental change could lead to less diverse plant communities and increased dominance of the particularly responsive V. myrtillus in the studied alpine treeline. These changes are the consequence of independent CO₂ and soil warming effects, a result that should facilitate predictive modelling approaches.

Soil warming and CO2 enrichment induce biomass shifts in alpine tree line vegetation

Responses of alpine tree line ecosystems to increasing atmospheric CO2 concentrations and global warming are poorly understood. We used an experiment at the Swiss tree line to investigate changes in vegetation biomass after 9 years of free air CO2 enrichment (+200 ppm; 2001-2009) and 6 years of soil warming (+4 °C; 2007-2012). The study contained two key tree line species, Larix decidua and Pinus uncinata, both approximately 40 years old, growing in heath vegetation dominated by dwarf shrubs. In 2012, we harvested and measured biomass of all trees (including root systems), above-ground understorey vegetation and fine roots. Overall, soil warming had clearer effects on plant biomass than CO2 enrichment, and there were no interactive effects between treatments. Total plant biomass increased in warmed plots containing Pinus but not in those with Larix. This response was driven by changes in tree mass (+50%), which contributed an average of 84% (5.7 kg m(-2) ) of total plant mass. Pinus coarse root mass was especially enhanced by warming (+100%), yielding an increased root mass fraction. Elevated CO2 led to an increased relative growth rate of Larix stem basal area but no change in the final biomass of either tree species. Total understorey above-ground mass was not altered by soil warming or elevated CO2 . However, Vaccinium myrtillus mass increased with both treatments, graminoid mass declined with warming, and forb and nonvascular plant (moss and lichen) mass decreased with both treatments. Fine roots showed a substantial reduction under soil warming (-40% for all roots <2 mm in diameter at 0-20 cm soil depth) but no change with CO2 enrichment. Our findings suggest that enhanced overall productivity and shifts in biomass allocation will occur at the tree line, particularly with global warming. However, individual species and functional groups will respond differently to these environmental changes, with consequences for ecosystem structure and functioning.

Nitrate Leaching From a Mountain Forest Ecosystem with Gleysols Subjected to Experimentally Increased N Deposition

Nitrate leaching was measured over seven years of nitrogen (N) addition in a pairedcatchment experiment in Alptal, central Switzerland (altitude: 1200 m, bulk N deposition: 12 kg ha−1 a−1). Two forested catchments (1500 m2 each) dominated by Picea abies) were delimited by trenches in the Gleysols. NH4NO3 was added to one of the catchments using sprinklers. During the first year, the N addition was labelled with 15N. Additionally, soil N transformations were studied in replicated plots. Pre-treatment NO 3 − -N leaching was 4 kg ha−1 a−1 from both catchments, and remained between 2.5 and 4.8 kg ha−1 a−1 in the control catchment. The first year of treatment induced an additional leaching of 3.1 kg ha−1, almost 90% of which was labelled with 15N, indicating that it did not cycle through the large N pools of the ecosystem (soil organic matter and plants). These losses partly correspond to NO 3 − from precipitation bypassing the soil due to preferential flow. During rain or snowmelt events, NO 3 − concentration peaks as the water table is rising, indicating flushing from the soil. Nitrification occurs temporarily along the water flow paths in the soil and can be the source of NO 3 − flushing. Its isotopic signature however, shows that this release mainly affects recently applied N, stored only between runoff events or up to a few weeks. At first, the ecosystem retained 90% of the added N (2/3 in the soil), but No 3 − losses increased from 10 to 30% within 7 yr, indicating that the ecosystem became progressively N saturated.