P. G. Jarvis

Respiration as the main determinant of carbon balance in European forests

Carbon exchange between the terrestrial biosphere and the atmosphere is one of the key processes that need to be assessed in the context of the Kyoto Protocol. Several studies suggest that the terrestrial biosphere is gaining carbon, but these estimates are obtained primarily by indirect methods, and the factors that control terrestrial carbon exchange, its magnitude and primary locations, are under debate. Here we present data of net ecosystem carbon exchange, collected between 1996 and 1998 from 15 European forests, which confirm that many European forest ecosystems act as carbon sinks. The annual carbon balances range from an uptake of 6.6 tonnes of carbon per hectare per year to a release of nearly 1 t C ha -1 yr-1, with a large variability between forests. The data show a significant increase of carbon uptake with decreasing latitude, whereas the gross primary production seems to be largely independent of latitude. Our observations indicate that, in general, ecosystem respiration determines net ecosystem carbon exchange. Also, for an accurate assessment of the carbon balance in a particular forest ecosystem, remote sensing of the normalized difference vegetation index or estimates based on forest inventories may not be sufficient.

The human footprint in the carbon cycle of temperate and boreal forests

Temperate and boreal forests in the Northern Hemisphere cover an area of about 2 × 107 square kilometres and act as a substantial carbon sink (0.6–0.7 petagrams of carbon per year). Although forest expansion following agricultural abandonment is certainly responsible for an important fraction of this carbon sink activity, the additional effects on the carbon balance of established forests of increased atmospheric carbon dioxide, increasing temperatures, changes in management practices and nitrogen deposition are difficult to disentangle, despite an extensive network of measurement stations. The relevance of this measurement effort has also been questioned, because spot measurements fail to take into account the role of disturbances, either natural (fire, pests, windstorms) or anthropogenic (forest harvesting). Here we show that the temporal dynamics following stand-replacing disturbances do indeed account for a very large fraction of the overall variability in forest carbon sequestration. After the confounding effects of disturbance have been factored out, however, forest net carbon sequestration is found to be overwhelmingly driven by nitrogen deposition, largely the result of anthropogenic activities. The effect is always positive over the range of nitrogen deposition covered by currently available data sets, casting doubts on the risk of widespread ecosystem nitrogen saturation under natural conditions. The results demonstrate that mankind is ultimately controlling the carbon balance of temperate and boreal forests, either directly (through forest management) or indirectly (through nitrogen deposition).

BOREAS in 1997: Experiment overview, scientific results, and future directions

The goal of the Boreal Ecosystem-Atmosphere Study (BOREAS) is to improve our understanding of the interactions between the boreal forest biome and the atmosphere in order to clarify their roles in global change. This overview paper describes the science background and motivations for BOREAS and the experimental design and operations of the BOREAS 1994 and BOREAS 1996 field years. The findings of the 83 papers in this journal special issue are reviewed. In section 7, important scientific results of the project to date are summarized and future research directions are identified.

Description and validation of an array model — MAESTRO

Abstract An array model, MAESTRO, was developed to predict radiation absorption, photosynthesis and transpiration by the individual crowns of trees in a stand and by the stand as a whole. The fluxes of radiation are treated in the photosynthetic (PAR), near infrared (NIR) and thermal wavebands; beam and diffuse radiation are considered separately. The spatial heterogeneity of the leaf area density distribution within the tree crown has been incorporated into MAESTRO, which can be used to study the spatial distribution of the radiation regime, and of the water vapour and carbon dioxide exchanges of leaves within the tree crown, in relation to stand structure. This model has been tested by comparing the calculated hourly and daily fluxes of PAR with measurements made by quantum sensors at locations below the tree crowns using three different submodels of leaf area density distribution. Good agreement between measurements and predictions was obtained.

Seasonal variation of carbon dioxide, water vapor, and energy exchanges of a boreal black spruce forest

Measurements of the fluxes of latent heat λE, sensible heat H, and CO2 were made by eddy covariance in a boreal black spruce forest as part of the Boreal Ecosystem-Atmosphere Study (BOREAS) for 120 days through the growing season in 1994. BOREAS is a multiscale study in which satellite, airborne, stand-scale, and leaf-scale observations were made in relation to the major vegetation types [Sellers et al., 1995]. The eddy covariance system comprised a sonic anemometer mounted 27 m above the forest, a system for transferring air rapidly and coherently to a closed path, infrared gas analyzer and a computer with the Edinburgh EdiSol software. Over the measurement period, closure of the energy balance on a 24 hour basis was good: (H + λE)/(Rn - G - B - S) = 0.97. The midday Bowen ratio was typically in the range 1.0–2.5, with an average value of ∼1.9 in the first Intensive Field Campaign (IFCl) and 1.3–1.4 in IFC2 and IFC3. Daily ecosystem evapotranspiration from moss, understory, and trees followed daily net radiation. Mean half-hourly net ecosystem flux followed photosynthetic photon flux density (PPFD) closely, reaching −9 μmol m−2 s−1 in June and August. The mean respiratory efflux on nights during which the atmosphere was well mixed (u*>0.4 m s−1) reached 6 μmol m−2s−1. The PPFD-saturated biotic CO2 assimilation reached 20 μmol; m−2 s−1 and showed little response to air temperature or vapor pressure deficit (VPD). Storage of CO2 in the air column at night did not account adequately for respiration on stable nights, so nighttime efflux was modeled for periods when u*<0.4 m s−1. There was a net gain of CO2 on most of the 120 days, but on 31 days of high temperature or low PPFD there was a net carbon loss. High PPFD promoted influx of CO2 by the foliage, whereas high temperatures reduced net CO2 influx through high respiration rates by the roots and soil microorganisms, leading to lower net uptake at high PPFD. Over the 120 day period, 95 g m−2 of C were stored (an average of 0.8 g m−2 d−1), and 237 mm of water evaporated (an average of 2 mm d−1).

Effects of spatial scale on stomatal control of transpiration

Abstract In an earlier treatment, we used the concept of coupling between vegetation and the atmosphere to demonstrate how the sensitivity of transpiration to a change in stomatal conductance decreases as the spatial scale increases from leaf to region. We introduced the omega coefficient to define the degree of coupling quantitatively and showed the increasing dependence of transpiration on radiation and decreasing dependence on saturation deficit with the increase in scale. Whilst this approach was effective for this limited purpose, it was not capable of easy extension to include other variables and it does not clearly demonstrate the reasons for the changes in sensitivity of transpiration to the stomata and the changes in emphasis on environmental driving variables as the scale increases. In the present treatment, we develop the thesis that increasing scale leads to an increase in number of negative feedback paths that stabilise the system and diminish the sensitivity of transpiration to change in stomatal conductance. We show that a consequence of negative feedback at the leaf and canopy scales is that we need only crude models of stomatal response to environmental variables so long as the ratio of stomatal conductance to the boundary-layer conductances is large, but we need rather better models where this ratio is small. At the regional scale, the effects of the negative feedbacks acting through the planetary boundary layer are even stronger, so that the boundary-layer conductances are of little effect, and we have no need for complex multi-layer models over a wide range of large canopy conductances. When water stress causes stomatal closure, however, the importance of the stomatal and canopy conductances increases so that we need more reliable estimates of them, but there is still no benefit to be gained from multi-layer models. When the supply of water completely dominates transpiration and canopy conductance is very small, crude models again suffice.

Constraints to growth of boreal forests.

Understanding how the growth of trees at high latitudes in boreal forest is controlled is important for projections of global carbon sequestration and timber production in relation to climate change. Is stem growth of boreal forest trees constrained by the length of the growing season when stem cambial cells divide, or by the length of the period when resources can be captured? In both cases, the timing of the thaw in the spring is critical: neither cambial cell division nor uptake of nutrients and carbon dioxide can occur while the soil is frozen. Here we argue, on the basis of long-term observations made in northern Saskatchewan and Sweden, that the time between the spring thaw and the autumn freeze determines the amount of annual tree growth, mainly through temperature effects on carbon-dioxide uptake in spring and on nutrient availability and uptake during summer, rather than on cambial cell division.

Botany: Constraints to growth of boreal forests

Understanding how the growth of trees at high latitudes in boreal forest is controlled is important for projections of global carbon sequestration and timber production in relation to climate change. Is stem growth of boreal forest trees constrained by the length of the growing season when stem cambial cells divide, or by the length of the period when resources can be captured? In both cases, the timing of the thaw in the spring is critical: neither cambial cell division nor uptake of nutrients and carbon dioxide can occur while the soil is frozen. Here we argue, on the basis of long-term observations made in northern Saskatchewan and Sweden, that the time between the spring thaw and the autumn freeze determines the amount of annual tree growth, mainly through temperature effects on carbon-dioxide uptake in spring and on nutrient availability and uptake during summer, rather than on cambial cell division.

Remote sensing of photosynthetic-light-use efficiency of boreal forest

Using a helicopter-mounted portable spectroradiometer and continuous eddy covariance data we were able to evaluate the photochemical reflectance index (PRI) as an indicator of canopy photosynthetic light-use efficiency (LUE) in four boreal forest species during the Boreal Ecosystem Atmosphere experiment (BOREAS). PRI was calculated from narrow waveband reflectance data and correlated with LUE calculated from eddy covariance data. Significant linear correlations were found between PRI and LUE when the four species were grouped together and when divided into functional type: coniferous and deciduous. Data from the helicopter-mounted spectroradiometer were then averaged to represent data generated by the Airborne Visible Infrared Imaging Spectrometer (AVIRIS). We calculated PRI from these data and relationships with canopy LUE were investigated. The relationship between PRI and LUE was weakened for deciduous species but strengthened for the coniferous species. The robust nature of this relationship suggests that relative photosynthetic rates may be derived from remotely-sensed reflectance measurements. ©2000 Elsevier Science B.V. All rights reserved.

An improved open chamber system for measuring soil CO2 effluxes in the field

An “open” chamber for the measurement of soil CO2 efflux has been designed which utilizes an air seal to maintain chamber pressure within 0.004 Pa of atmospheric pressure, eliminating any mass flow and ensuring that atmospheric pressure fluctuations are transferred through to the soil surface. The system was used at the Boreal Ecosystem-Atmosphere Study (BOREAS) southern study area old black spruce site through the 1994 and 1996 field campaigns and was found to be robust and simple to use. Consistency was found between soil CO2 effluxes measured with this system and a closed dynamic chamber system.