Despite the turmoil of not-so-distant forest fires and United Airlines troubles that threw off travel schedules, some 2600 ecologists made their way to this sun-soaked canyon last month for the Ecological Society of America's 85th annual meeting. Topics ranged from ancient droughts to photosynthesis beneath snow and how trees resist insects.
Soil-surface CO2 efflux ('soil respiration') accounts for roughly two-thirds of forest ecosystem respiration, and can be divided into heterotrophic and autotrophic components. Conventionally, the latter is defined as respiration by plant roots. In Boreal forests, however, fine roots of trees are invariably covered by ectomycorrhizal fungi, which by definition are heterotrophs, but like the roots, receive sugars derived from photosynthesis. There is also a significant leaching of labile carbon compounds from the ectomycorrhizal roots. It is, therefore, more meaningful in the context of carbon balance studies to include mycorrhizal fungi and other mycorrhizosphere organisms, dependent on the direct flux of labile carbon from photosynthesis, in the autotrophic component. Hence, heterotrophic activity becomes reserved for the decomposition of more complex organic molecules in litter and other forms of soil organic matter. In reality, the complex situation is perhaps best described as a continuum from strict autotrophy to strict heterotrophy. As a result of this, and associated methodological problems, estimates of the contribution of autotrophic respiration to total soil respiration have been highly variable. Based on recent stand-scale tree girdling experiments we have estimated that autotrophic respiration in boreal forest accounts for up to 50-65% of soil respiration during the snow-free part of the year. Girdling experiments and studies of the delta(13)C of the soil CO2 efflux show that there is a lag of a few days between the carbon uptake by photosynthesis and the release by autotrophic soil respiration of the assimilated carbon. In contrast, estimates of 'bomb 14C' and other approaches have suggested that it takes years to decades between carbon uptake via photosynthesis and the bulk of soil heterotrophic activity. Temperature is normally used as a driver in models of soil processes and it is often assumed that autotrophic soil activity is more sensitive to temperature than is heterotrophic activity, but this is questionable. It is inherently difficult to make a precise separation of autotrophic and heterotrophic respiration from soils. The partitioning between these two components is highly variable in space and time, and taxonomic autotrophs and heterotrophs may perform the function of the other group to some degree. Care should be taken to disturb as little as possible the delicate plant-microbe-soil system, and this speaks for non-intrusive isotopic methods. There are, however, problems in modelling the flux of isotopes through this complex system. Girdling of tree stands is a very robust alternative approach to make the distinction between autotrophic and heterotrophic activities, but ultimately kills the trees and cannot, therefore, always be used. A further development would be to block the phloem sugar transport reversibly. We propose that thus assumption needs further critical testing.
The Boreal forest is the world's second largest forested biome occupying the circumpolar region between 50 degrees N and 70 degrees N. This heterogeneous biome stores about 25% of all terrestrial carbon. We have reviewed EC measurements of CO2 exchange between the atmosphere and Boreal forests, and assessed progress in understanding the controlling processes. We have assessed net ecosystem productivity, the net balance between net primary productivity and heterotrophic respiration, measured using the EC method, for 38 Boreal forest sites. Gross ecosystem productivity has been estimated by adding day-time EC-measured CO2 fluxes to respiration estimated from night-time relationships between respiration and temperature. Maximum midday values of gross ecosystem productivity vary from 33 pmol m(-2) s(-1) for aspen to 6 micromol m(-2) s(-1) for larch stands. Long-term EC flux measurements, ongoing at nine Boreal sites, have shown the strong impact of spring weather and growing season water balance on annual net ecosystem productivity. Estimation of net biome production, incorporating the effects of disturbance resulting from forest fires and logging, has progressed significantly in recent years. After disturbance, summer measurements in Boreal chronosequences suggest that it takes about 10 years before growing season carbon uptake offsets the decomposition emissions. Small-scale exchange rate measurements using chambers and manipulative experiments such as stem girdling and soil heating help to understand the processes and mechanisms playing major roles in the carbon balance of terrestrial ecosystems. Aircraft EC flux measurements, convective boundary layer carbon budgets, and (13)C/12C changes in the atmosphere play an important role in validating estimates of regional carbon exchange based on scaled up EC measurements. Atmospheric inverse models are an important approach to studying regional and global carbon balance but need further improvement to yield reliable quantitative results.
Seasonal profiles of sulphur, phosphorus, and potassium content in the wood of trees have been established for the first time. This became possible by using a novel laser ablation system coupled to HR-ICP-MS for measuring these elements in Norway spruce drill cores. This technique combines excellent spatial resolution with superior detection power, and makes it possible to measure low element concentrations even in relatively narrow annual rings. Despite its low quantity in wood, sulphur is an important macronutrient for plants and seems to display seasonal variations of its concentration, which correspond to actual theories of sulphur metabolism in plants. A similar seasonal pattern was also found for phosphorus, another crucial element in tree nutrition. This was unexpected, because it was previously assumed that the distribution of phosphorus remains constant throughout the year. Potassium, the third element measured, seems to be especially accumulated in the latewood. The profiles presented in this article suggest a seasonal variation, revealing some new aspects of Norway spruce (PICEA ABIES) metabolism.