Terrestrial ecosystems sequester 2.1 Pg of atmospheric carbon annually. A large amount of the terrestrial sink is realized by forests. However, considerable uncertainties remain regarding the fate of this carbon over both short and long timescales. Relevant data to address these uncertainties are being collected at many sites around the world, but syntheses of these data are still sparse. To facilitate future synthesis activities, we have assembled a comprehensive global database for forest ecosystems, which includes carbon budget variables (fluxes and stocks), ecosystem traits (e.g. leaf area index, age), as well as ancillary site information such as management regime, climate, and soil characteristics. This publicly available database can be used to quantify global, regional or biome-specific carbon budgets; to re-examine established relationships; to test emerging hypotheses about ecosystem functioning [e.g. a constant net ecosystem production (NEP) to gross primary production (GPP) ratio]; and as benchmarks for model evaluations. In this paper, we present the first analysis of this database. We discuss the climatic influences on GPP, net primary production (NPP) and NEP and present the CO2 balances for boreal, temperate, and tropical forest biomes based on micrometeorological, ecophysiological, and biometric flux and inventory estimates. Globally, GPP of forests benefited from higher temperatures and precipitation whereas NPP saturated above either a threshold of 1500 mm precipitation or a mean annual temperature of 10 degrees C. The global pattern in NEP was insensitive to climate and is hypothesized to be mainly determined by nonclimatic conditions such as successional stage, management, site history, and site disturbance. In all biomes, closing the CO2 balance required the introduction of substantial biome-specific closure terms. Nonclosure was taken as an indication that respiratory processes, advection, and non-CO2 carbon fluxes are not presently being adequately accounted for.

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CO2 balance of boreal, temperate, and tropical forests derived from a global database

Forest age, which is affected by stand-replacing ecosystem disturbances (such as forest fires, harvesting, or insects), plays a distinguishing role in determining the distribution of carbon (C) pools and fluxes in different forested ecosystems. In this synthesis, net primary productivity (NPP), net ecosystem productivity (NEP), and five pools of C (living biomass, coarse woody debris, organic soil horizons, soil, and total ecosystem) are summarized by age class for tropical, temperate, and boreal forest biomes. Estimates of variability in NPP, NEP, and C pools are provided for each biome-age class combination and the sources of variability are discussed. Aggregated biome-level estimates of NPP and NEP were higher in intermediate-aged forests (e.g., 30–120 years), while older forests (e.g., >120 years) were generally less productive. The mean NEP in the youngest forests (0–10 years) was negative (source to the atmosphere) in both boreal and temperate biomes (−0.1 and –1.9 Mg C ha−1 yr−1, respectively). Forest age is a highly significant source of variability in NEP at the biome scale; for example, mean temperate forest NEP was −1.9, 4.5, 2.4, 1.9 and 1.7 Mg C ha−1 yr−1 across five age classes (0–10, 11–30, 31–70, 71–120, 121–200 years, respectively). In general, median NPP and NEP are strongly correlated (R2=0.83) across all biomes and age classes, with the exception of the youngest temperate forests. Using the information gained from calculating the summary statistics for NPP and NEP, we calculated heterotrophic soil respiration (Rh) for each age class in each biome. The mean Rh was high in the youngest temperate age class (9.7 Mg C ha−1 yr−1) and declined with age, implying that forest ecosystem respiration peaks when forests are young, not old. With notable exceptions, carbon pool sizes increased with age in all biomes, including soil C. Age trends in C cycling and storage are very apparent in all three biomes and it is clear that a better understanding of how forest age and disturbance history interact will greatly improve our fundamental knowledge of the terrestrial C cycle.

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Carbon cycling and storage in world forests: biome patterns related to forest age.

METLA Project 3306: Forest Carbon Sink and Economical Costs of Kyoto Protocol prduced a database of treewise volume- and biomass equations according to diameter at breast height and/or height includes equations for common European tree species. Equations will be used when estimating biomass expansion factors for major tree species. Database was compiled as a joint project with University of Edinburgh, Finnish Forest Research Institute and partners of COST E21 Action. The attached excel-files files extended and updated versions of those published previously in the printed publication. See the figure about a sample of compiled equations (files: appendix-A.xls and appendix-BC.xls).

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Biomass and stem volume equations for tree species in Europe.

Over large areas of Europe, coniferous monocultures are being transformed into mixed forests by the re-introduction of broadleaf tree species belonging to the potential natural vegetation. One important species of interest in this changing forest policy is European beech (Fagus sylvatica). However, at present, this forest management directive has ignored potential adverse effects of global climate change on wide-spread re-introduction of beech to these areas. Average global surface temperatures have risen by approx. 0.8°C in the period between 1861 and 2005 and are expected to continue to increase until the end of this century by 1.5–5.8°C above the 1990 value. To estimate the climate change in the southern part of central Europe in future, we reviewed calculations from regional climate models. Temperature increase for the southern part of central Europe is projected to be up to 2°C within the next 40 years. In contrast, the annual precipitation will most likely remain constant over the same time period, but will experience significant changes in seasonal patterns. Rising intensities of individual precipitation events may result in increasing number and intensities of flooding events and reduced precipitation during the growing season in a higher frequency of summer droughts. Growth and competitive ability of European beech will not, necessarily, respond to increasing CO2 concentrations but may be strongly impacted by intensive drought that occurs during the growing season. Seedlings as well as adult trees may suffer from xylem embolism, restricted nutrient uptake capacity and reduced growth under limited water availability. However, it remains uncertain to what extent other environmental factors (e.g. soil properties, competitive interactions) may modify the drought response of beech, thus either enhancing susceptibility or increasing drought tolerance and resilience potential. Water-logged soils, predicted during the spring for several regions due to higher than average precipitation, could negatively impact nutrient uptake and growth of beech. Whereas other dominant species as, e.g. oak are well adapted to that environmental stress, beech is known to be sensitive to water-logging and flooding. Thus, the competitive capacity of beech might—depending on the other environmental conditions—be reduced under the expected future climate conditions. Silvicultural practices must be aware today of the potential risks which a changing climate may impose on sustainable forest development.

Potential risks for European beech (Fagus sylvatica L.) in a changing climate

Fine roots <2 mm in diameter play a key role in regulating the biogeochemical cycles of ecosystems and are important to our understanding of ecosystem responses to global climate changes. Given the sensitivity of fine roots, especially in boreal region, to climate changes, it is important to assess whether and to what extent fine roots in this region change with climates. Here, in this synthesis, a data set of 218 root studies were complied to examine fine root patterns in the boreal forest in relation to site and climatic factors. The mean fine root biomass in the boreal forest was 5.28 Mg ha−1, and the production of fine roots was 2.82 Mg ha−1 yr−1, accounting for 32% of annual net primary production of the boreal forest. Fine roots in the boreal forest on average turned over 1.07 times per year. Fine roots contained 50.9 kg ha−1 of nitrogen (N) and 3.63 kg ha−1 of phosphorous (P). In total, fine roots in the boreal forest ecosystems contain 6.1 × 107 Mg N and 4.4×106Mg P pools, respectively, about 10% ...

Fine root biomass, production, turnover rates, and nutrient contents in boreal forest ecosystems in relation to species, climate, fertility, and stand age: literature review and meta-analyses.

Growth and carbon stocks of a spruce forest chronosequence in central Europe

Mineral nutrients are a major part of all the physiological and biogeochemical processes in forest ecosystems. This is especially true for forests across Europe, which were deprived of nutrients due to intensive wood and litter use, and which experienced deposition of acids, nitrogen and sulphur over the second half of this century, resulting in significant nutrient imbalances for growth (Schulze 1989). Decreased nutrient availability can lead to a reduction of leaf size (Linder 1987), resulting in an almost instantaneous decrease in current year growth. In this way, the nutrient status of long-lived conifer needles might influence net primary production (NPP) long after a transient nutrient shortage, caused, e.g. by one dry season, has occurred. In natural forest ecosystems nutrient uptake from soil solution and nutrient release through litterfall and fine root turnover should balance each other such that the turnover time of nutrients within the system meets the requirements for stand growth (Gorham et al. 1979; Miller 1986; Attiwill and Adams 1993) and keeps the ecosystem nutrient cycle tight. Any deviation from this cycle due to anthropogenic influence (e.g. Vitousek et al. 1997) or natural disturbance (e.g. Foster et al. 1997) could alter one or more processes within the nutrient cycle with long-lasting effects on forest functioning.

M. Hein

Papers

Growth and carbon stocks of a spruce forest chronosequence in central Europe

Linking plant nutrition and ecosystem processes

Growth and carbon stocks of a spruce forest chronosequence in central Europe. For Ecol Manage (in press)

Natürliche Senken und Quellen des atmosphärischen Kohlendioxids: Stand des Wissens und Optionen des Handelns