A major problem in the analysis of environmental issues is a lack of combined physico-chemical and biological knowledge. Practical examples of the combination of physico-chemical and biological knowledge, and utilization of versatile up-to-date instrumentation for continuous long-term field measurements, are illustrated at the SMEAR stations (Station for Measuring Forest Ecosystem-Atmosphere Relations). The facility comprises two installations: SMEAR I at Värriö (northern Finland) and SMEAR II at Hyytiälä (southern Finland). The stations are similarly equipped and form a combined facility operating within the boreal pine forest under different climatic conditions.

SMEAR I (6746'N, 2935'E) is situated at the top of a hill (400 m above sea level) in a 40-year old, 7m tall Scotch pine (Pinus sylvestris L.) in the Arctic timberline. The station complements SMEAR II by being situated under different (more arctic) climatic conditions but representing the same boreal vegetation type. All SMEAR II activities (described below) are not available in SMEAR I; during intensive campaigns, however, many of SMEAR II's atmospheric and tree measurement facilities can be made available at SMEAR I. The different arctic conditions cause additional challenges to the arrangement of measurements.

The main emphasis at SMEAR I is on interactions between trees and atmosphere, air quality and aerosols [e.g., Ahonen et al., 1997]. Information on air quality as influenced by the Kola industrial areas (located less than 200 km away) and on the effects on photosynthesis in subarctic conditions is also obtained. Practically no sources of pollutants exist close to the station; however, regularly occurring episodes of heavy sulfur dioxide and aerosol pollution are observed when winds are northeasterly.

SMEAR II is located in a rather homogenous Scotch pine stand on a flat terrain at Hyytiälä Forestry Field Station, University of Helsinki (6151'N, 2417'E, 181 m above sea level), 220 km northwest of Helsinki. The most homogeneous fetch covers 1200 m x 600 m (60) and the homogeneous fetch in the prevailing wind direction is 250 m x 200 m. The natural managed stand was established in 1962 by sowing after the area had been treated with prescribed burning and light soil preparation. Besides Scotch pine, the stand consists of only 1% other species. The mean height of the trees is 12 m, the mean diameter at chest level is 13 cm, and the projected leaf area is 3 m2 (8 m2 in the case of total area) per unit area of soil. The zero-plane displacement is about 6 m and the roughness length is 0.8 m. The wood biomass is 45 t ha-1 and the tree density is 2500 ha-1. Nitrogen leaf content is 1.3%. The soil material is sandy and coarse silty glacial till. The region's annual average mean temperature is 3.0°C and the annual mean precipitation is 690 mm. The air quality at the site represents typical background conditions.

Figure 1. Overview of the SMEAR II station and the measurements carried out there [http://honeybee.helsinki.fi/HYYTIALA/smear].

The SMEAR II facility is designed to determine material and energy flows in the atmosphere-vegetation-soil continuum at different temporal and spatial scales (Figure 1). It can be divided into four operational blocks: i) atmospheric measurements accomplished using a 72 m mast, ii) tree measurements performed using a 15 m tower, iii) soil measurements carried out on two catchment (watershed with weir) areas, and iv) aerosol measurements from 2 m above the ground. The station includes some outstanding instrumentation [Vesala et al., 1998] for measurements of aerosol particle size distribution in the size range of 3-500 nm, vertical flux of aerosol particles of diameters down to 12 nm, exchange of trace gases on shoot-scale and soil surface at intervals of 1 minute, spatial distribution of irradiance by 800 sensors in the vicinity of a shoot, spatial distribution of irradiance with 200 sensors within the canopy, and two soil catchment areas (890 and 300 m2) for soil water, dissolved ions and organic carbon balances. SMEAR II is part of the global FLUXNET project [Kaiser, 1998], and international measurement campaigns are frequently organized at the station. FLUXNET is an investigation of the carbon cycle of terrestrial ecosystems using the eddy-covariance technique by which the net ecosystem exchange can be estimated. Figure 2 presents the daily CO2 exchange time series measured at SMEAR II [see also Rannik, 1998]. The daily sums are calculated only for days when no more than an hour of data is missing. Therefore, in addition to longer gaps, there are also gaps of shorter duration. Negative fluxes correspond to uptake of CO2 by the forest. This boreal forest acts as a sink for CO2 during the summer (May­September) and a rather significant source in winter.

Figure 2. Daily sums of carbon dioxide flux (net ecosystem exchange) for a Scotch pine forest at SMEAR II station [Rannik, unpublished data].

The objectives of the atmospheric measurements done from the mast are i) to determine the exchange rates of CO2 and H2O (latent heat), as well as deposition or emission rates of SO2, NOX and O3 at the stand scale, ii) to determine the sensible heat flux, iii) to determine the concentrations of trace gases, iv) to give information on meteorological conditions (including radiation), and v) to detect episodes of low-level clouds and conditions within them.

The objectives of the tree measurements are i) to determine the exchange rates of CO2 and H2O as well as the deposition or emission rates of SO2, NOX and O3 at the shoot scale, ii) to determine the accurate photosynthetic light response, iii) to determine the sap flow rate, and iv) to link the tree structure to functions and growth. The canopy PAR distribution measurements are aimed at i) determining light variations at small spatial and short temporal scales, and ii) supporting the scaling procedure from photosynthesis and transpiration rates of shoots to that of the canopy.

The objectives of the soil measurements are i) to analyze water, carbon and nutrient budgets at a stand level, ii) to monitor soil temperatures and determine the soil heat flux, iii) to follow bacterial, fine root and mycorrhizal activities, and iv) to follow soil/soil solution chemical equilibria.

The objectives of aerosol measurements are i) to find out the frequency of the aerosol formation events in the boundary layer, ii) to find out how aerosol size distribution will respond to different meteorological conditions, and iii) to analyze how aerosol dynamics are connected to atmospheric chemistry and biological processes.

In sum, research at SMEAR I and II is focused on continuous measurements using different kinds of data simultaneously. Different data sets support each other effectively [see, e.g., Vesala et al., 1998].

References

SMEAR homepage: http://honeybee.helsinki.fi/HYYTIALA/smear.

1. Ahonen, T., P. Aalto, U. Rannik, M. Kulmala, E.D. Nilsson, S. Palmroth, H. Ylitalo, P. Hari, Variations and vertical profiles of trace gas and aerosol concentrations and CO2 exchange in eastern Lapland, Atmospheric Environment, 31, 3351-3362, 1997.
   Kaiser, J., New network aims to take the world's CO2 pulse, Science, 281, 506-507, 1998.
2. Rannik, U., Turbulent atmosphere: Vertical fluxes above a forest and particle growth, Ph.D.Thesis, University of Helsinki, Department of Physics, Report Series in Aerosol Science, 35, 1998.
3. Vesala, T., J. Haataja, P. Aalto, N. Altimir, G. Buzorius, E. Garam, K. Hämeri, H. Ilvesniemi, V. Jokinen, P. Keronen, T. Lahti, T. Markkanen, J.M. Mäkelä, E. Nikinmaa, S. Palmroth, L. Palva, T. Pohja, J. Pumpanen, U. Rannik, Y. Siivola, H. Ylitalo, P. Hari, and M. Kulmala, Long-term field measurements of atmosphere-surface interactions in boreal forest combining forest ecology, micrometeorology, aerosol physics and atmospheric chemistry, Trends in Heat, Mass & Momentum Transfer, 4, 17-35, 1998.