ALBERT

Notes: Based on review and original data, this synthesis investigates carbon pools and fluxes of Siberian and European forests (600 and 300 million ha, respectively). We examine the productivity of ecosystems, expressed as positive rate when the amount of carbon in the ecosystem increases, while (following micrometeorological convention) downward fluxes from the atmosphere to the vegetation (NEE = Net Ecosystem Exchange) are expressed as negative numbers. Productivity parameters are Net Primary Productivity (NPP=whole plant growth), Net Ecosystem Productivity (NEP = CO2 assimilation minus ecosystem respiration), and Net Biome Productivity (NBP = NEP minus carbon losses through disturbances bypassing respiration, e.g. by fire and logging). Based on chronosequence studies and national forestry statistics we estimate a low average NPP for boreal forests in Siberia: 123 gC m–2 y–1. This contrasts with a similar calculation for Europe which suggests a much higher average NPP of 460 gC m–2 y–1 for the forests there. Despite a smaller area, European forests have a higher total NPP than Siberia (1.2–1.6 vs. 0.6–0.9 × 1015 gC region–1 y–1). This arises as a consequence of differences in growing season length, climate and nutrition. For a chronosequence of Pinus sylvestris stands studied in central Siberia during summer, NEE was most negative in a 67-y old stand regenerating after fire (– 192 mmol m–2 d–1) which is close to NEE in a cultivated forest of Germany (– 210 mmol m–2 d–1). Considerable net ecosystem CO2-uptake was also measured in Siberia in 200- and 215-y old stands (NEE:174 and – 63 mmol m–2 d–1) while NEP of 7- and 13-y old logging areas were close to the ecosystem compensation point. Two Siberian bogs and a bog in European Russia were also significant carbon sinks (– 102 to – 104 mmol m–2 d–1). Integrated over a growing season (June to September) we measured a total growing season NEE of – 14 mol m–2 summer–1 (– 168 gC m–2 summer–1) in a 200-y Siberian pine stand and – 5 mol m–2 summer–1 (– 60 gC m–2 summer–1) in Siberian and European Russian bogs. By contrast, over the same period, a spruce forest in European Russia was a carbon source to the atmosphere of (NEE: + 7 mol m–2 summer–1 = + 84 gC m–2 summer–1). Two years after a windthrow in European Russia, with all trees being uplifted and few successional species, lost 16 mol C m–2 to the atmosphere over a 3-month in summer, compared to the cumulative NEE over a growing season in a German forest of – 15.5 mol m–2 summer–1 (– 186 gC m–2 summer–1; European flux network annual averaged – 205 gC m–2 y–1). Differences in CO2-exchange rates coincided with differences in the Bowen ratio, with logging areas partitioning most incoming radiation into sensible heat whereas bogs partitioned most into evaporation (latent heat). Effects of these different surface energy exchanges on local climate (convective storms and fires) and comparisons with the Canadian BOREAS experiment are discussed. Following a classification of disturbances and their effects on ecosystem carbon balances, fire and logging are discussed as the main processes causing carbon losses that bypass heterotrophic respiration in Siberia. Following two approaches, NBP was estimated to be only about 13–16 mmol m–2 y–1 for Siberia. It may reach 67 mmol m–2 y–1 in North America, and about 140–400 mmol m–2 y–1 in Scandinavia. We conclude that fire speeds up the carbon cycle, but that it results also in long-term carbon sequestration by charcoal formation. For at least 14 years after logging, regrowth forests remain net sources of CO2 to the atmosphere. This has important implications regarding the effects of Siberian forest management on atmospheric concentrations. For many years after logging has taken place, regrowth forests remain weaker sinks for atmospheric CO2 than are nearby old-growth forests.

Notes: The Sixth and Seventh Conference of the Parties (COP 6 and 7) at The Hague, Bonn and Marrakesh came to a final Agreement on the Kyoto Protocol, which is thus ready for ratification by the individual nations. The Agreement was only achieved by allowing countries to offset their fossil fuel emission targets (on average 95% of the 1990 emissions) by increasing biological carbon sequestration, and by trading carbon credits. Activities that would count as increasing biological carbon sequestration include afforestation and reforestation, and changes in management of agriculture and forestry. According to the Agreement reached in Marrakesh, biological carbon sequestration may reach an offset of up to 80% of the required reduction in fossil fuel emissions (4% of the 5% reduction commitment). We explain why the allowable offset rose as high during the course of the negotiations. It is highlighted that major unintended consequences may be a result of the policy as it stands in the Marrakesh Accord. Major losses of biodiversity and primary forest are expected. We present scientific concerns regarding verification, which lead to scientific doubts that the practices encouraged by the Agreement can actually increase sequestration under a full carbon accounting scheme. We explain that there is a ‘win-win’ option that would protect high carbon pools and biodiversity in an economically efficient way. But, this is not supported by the Agreement. Despite the very positive signal that most nations of the United Nations will devote major efforts towards climate protection, there remains a most urgent need to develop additional rules to avoid unintended outcomes, and to promote the ‘win-win’ options that we explain.

Notes: Eddy covariance was used to measure the net CO2 exchange (NEE) over ecosystems differing in land use (forest and agriculture) in Thuringia, Germany. Measurements were carried out at a managed, even-aged European beech stand (Fagus sylvatica, 70–150 years old), an unmanaged, uneven-aged mixed beech stand in a late stage of development (F. sylvatica, Fraxinus excelsior, Acer pseudoplantanus, and other hardwood trees, 0–250 years old), a managed young Norway spruce stand (Picea abies, 50 years old), and an agricultural field growing winter wheat in 2001, and potato in 2002. Large contrasts were found in NEE rates between the land uses of the ecosystems. The managed and unmanaged beech sites had very similar net CO2 uptake rates (∼−480 to −500 g C m−2 yr−1). Main differences in seasonal NEE patterns between the beech sites were because of a later leaf emergence and higher maximum leaf area index at the unmanaged beech site, probably as a result of the species mix at the site. In contrast, the spruce stand had a higher CO2 uptake in spring but substantially lower net CO2 uptake in summer than the beech stands. This resulted in a near neutral annual NEE (−4 g C m−2 yr−1), mainly attributable to an ecosystem respiration rate almost twice as high as that of the beech stands, despite slightly lower temperatures, because of the higher elevation. Crops in the agricultural field had high CO2 uptake rates, but growing season length was short compared with the forest ecosystems. Therefore, the agricultural land had low-to-moderate annual net CO2 uptake (−34 to −193 g C m−2), but with annual harvest taken into account it will be a source of CO2 (+97 to +386 g C m−2). The annually changing patchwork of crops will have strong consequences on the regions' seasonal and annual carbon exchange. Thus, not only land use, but also land-use history and site-specific management decisions affect the large-scale carbon balance.

Notes: Higher rates of nitrate assimilation are required to support faster growth in enhanced carbon dioxide. To investigate how this is achieved, tobacco plants were grown on high nitrate and high light in ambient and enhanced (700 μmol mol–1) carbon dioxide. Surprisingly, enhanced carbon dioxide did not increase leaf nitrate reductase (NR) activity in the middle of the photoperiod. Possible reasons for this anomalous result were investigated. (a) Measurements of biomass, nitrate, amino acids and glutamine in plants fertilized once and twice daily with 12 mol m–3 nitrate showed that enhanced carbon dioxide did not lead to a nitrate limitation in these plants. (b) Enhanced carbon dioxide modified the diurnal regulation of NR activity in source leaves. The transcript for nia declined during the light period in a similar manner in ambient and enhanced carbon dioxide. The decline of the transcript correlated with a decrease of nitrate in the leaf, and was temporarily reversed after re-irrigating with nitrate in the second part of the photoperiod. The decline of the transcript was not correlated with changes of sugars or glutamine. NR activity and protein decline in the second part of the photoperiod, and NR is inactivated in the dark in ambient carbon dioxide. The decline of NR activity was smaller and dark inactivation was partially reversed in enhanced carbon dioxide, indicating that post-transcriptional or post-translational regulation of NR has been modified. The increased activation and stability of NR in enhanced carbon dioxide was correlated with higher sugars and lower glutamine in the leaves. (c) Enhanced carbon dioxide led to increased levels of the minor amino acids in leaves. (d) Enhanced carbon dioxide led to a large decrease of glycine and a small decrease of serine in leaves of mature plants. The glycine:serine ratio decreased in source leaves of older plants and seedlings. The consequences of a lower rate of photorespiration for the levels of glutamine and the regulation of nitrogen metabolism are discussed. (e) Enhanced carbon dioxide also modified the diurnal regulation of NR in roots. The nia transcript increased after nitrate fertilization in the early and the second part of the photoperiod. The response of the transcript was not accentuated in enhanced carbon dioxide. NR activity declined slightly during the photoperiod in ambient carbon dioxide, whereas it increased 2-fold in enhanced carbon dioxide. The increase of root NR activity in enhanced carbon dioxide was preceded by a transient increase of sugars, and was followed by a decline of sugars, a faster decrease of nitrate than in ambient carbon dioxide, and an increase of nitrite in the roots. (f) To interpret the physiological significance of these changes in nitrate metabolism, they were compared with the current growth rate of the plants. (g) In 4–5-week-old plants, the current rate of growth was similar in ambient and enhanced carbon dioxide (≈ 0·4 g–1 d–1). Enhanced carbon dioxide only led to small changes of NR activity, nitrate decreased, and overall amino acids were not significantly increased. (h) Young seedlings had a high growth rate (0·5 g–1 d–1) in ambient carbon dioxide, that was increased by another 20% in enhanced carbon dioxide. Enhanced carbon dioxide led to larger increases of NR activity and NR activation, a 2–3-fold increase of glutamine, a 50% increase of glutamate, and a 2–3-fold increase in minor amino acids. It also led to a higher nitrate level. It is argued that enhanced carbon dioxide leads to a very effective stimulation of nitrate uptake, nitrate assimilation and amino acid synthesis in seedlings. This will play an important role in allowing faster growth rates in enhanced carbon dioxide at this stage.

Notes: Plants of Cirsium vulgare (Savi) Ten. were cultivated under five different nitrogen regimes in order to investigate the effects of nitrogen supply on the storage processes in a biennial species during its first year of growth.External N supply increased total biomass production without changing the relationship between ‘productive plant compartments’ (i.e. shoot plus fine roots) and ‘storage plant compartments’ (i.e. structural root dry weight, which is defined as the difference between tap root biomass and the amount of stored carbohydrates and N compounds). The amount of carbohydrates and N compounds stored per unit of structural tap root dry weight was not affected by external N availability during the season, because high rates of N supply increased the concentration of N compounds whilst decreasing the carbohydrate concentration, and low rates of N supply had the opposite effect. Mobilization of N from senescing leaves was not related to the N status of the plants. The relationship between nitrogen compounds stored in the tap root and the maximum amount of nitrogen in leaves was an increasing function with increasing nitrogen supply. We conclude that the allocation between vegetative plant growth and the growth of storage structures over a wide range of N availability seems to follow predictions from optimum allocation theory, whereas N storage responds in a rather plastic way to N availability.

Notes: The cost of nitrogen storage to current growth was examined in relation to N availability in the biennial Cirsium vulgare. Plants were grown outdoors, in sand culture, with continuous diel drip irrigation of fertilization medium containing one of five different N concentrations. Plants grown at the highest N concentration stored twice as much N in their tap roots as did plants grown at the lowest N concentration. In high-N-grown plants, the storage of N reserves occurred during the period of maximum growth, at the same time as tap-root production. At the time of maximum biomass, stored N was also at a maximum. During the period following maximum biomass, no additional storage of N occurred. This pattern was observed despite frequent late-season leaf senescence which resulted in a large pool of potentially mobile N which could have been stored at no cost to growth. In low-N-grown plants, the production of tap-root storage tissue and the filling of that tissue with stored N were staggered. Tap-root production and growth occurred during the period of maximum growth, as in the high-N-grown plants. However, filling of the storage tissue with N occurred late in the growing season, when the pool of mobile N from senescent leaves was large. The utilization of this late-season N source occurred with little or no cost to growth, and this N is labelled, according to previous definitions, as ‘accumulated’. The costs of storing N in plants of the different N treatments were calculated using two models based on different growth constraints. In one model, the cost of N storage was represented as lost growth due to allocation of N to storage, rather than to the photosynthetic shoot (i.e. growth was assumed to be limited by carbon acquisition). In the second model, the storage cost was calculated as lost growth due to allocation of N to storage, rather than to the nitrogen-acquiring fine-root system (i.e. growth was assumed to be limited by nitrogen acquisition). In both models, the total cost of N storage was predicted to decrease as N availability decreased due to smaller storage pool sizes in plants of the low-N treatments. The cost of filling the tap root with stored N as a percentage of the total storage cost was also reduced as N availability decreased due to the occurrence of late-season accumulation. By relying, at least in part, on late-season accumulation, plants grown at the lowest three levels of N availability reduced total storage costs by 15 to 22%. The results demonstrate that plants are capable of adjusting their storage patterns in response to low nitrogen availability such that the costs of storage are reduced.