Cloud cover increases the proportion of diffuse radiation reaching the Earth's surface and affects many microclimatic factors such as temperature, vapour pressure deficit and precipitation. We compared the relative efficiencies of canopy photosynthesis to diffuse and direct photosynthetic photon flux density (PPFD) for a Norway spruce forest (25-year-old, leaf area index 11 m 2 m À2 ) during two successive 7-day periods in August. The comparison was based on the response of net ecosystem exchange (NEE) of CO 2 to PPFD. NEE and stomatal conductance at the canopy level (G canopy ) was estimated from halfhourly eddy-covariance measurements of CO 2 and H 2 O fluxes. In addition, daily courses of CO 2 assimilation rate (A N ) and stomatal conductance (G s ) at shoot level were measured using a gas-exchange technique applied to branches of trees. The extent of spectral changes in incident solar radiation was assessed using a spectroradiometer.We found significantly higher NEE (up to 150%) during the cloudy periods compared with the sunny periods at corresponding PPFDs. Prevailing diffuse radiation under the cloudy days resulted in a significantly lower compensation irradiance (by ca. 50% and 70%), while apparent quantum yield was slightly higher (by ca. 7%) at canopy level and significantly higher (by ca. 530%) in sun-acclimated shoots. The main reasons for these differences appear to be (1) more favourable microclimatic conditions during cloudy periods, (2) stimulation of photochemical reactions and stomatal opening via an increase of blue/red light ratio, and (3) increased penetration of light into the canopy and thus a more equitable distribution of light between leaves.Our analyses identified the most important reason of enhanced NEE under cloudy sky conditions to be the effective penetration of diffuse radiation to lower depths of the canopy. This subsequently led to the significantly higher solar equivalent leaf area compared with the direct radiation. Most of the leaves in such dense canopy are in deep shade, with marginal or negative carbon balances during sunny days. These findings show that the energy of diffuse, compared with direct, solar radiation is used more efficiently in assimilation processes at both leaf and canopy levels.

The spectral composition of photosynthetically active radiation (PAR) during clear and overcast days was studied above the canopy (U) and at two layers of a dense Norway spruce stand [Picea abies (L.) Karst.] characterized with an average LAI = 7.3 ± 0.8 (middle layer: M) and 12.3 ± 0.7 (lower layer: L). Whereas the spectral composition of PAR incoming on the canopy surface during cloudy days (characterized by diffuse index DI > 0.7) was almost independent of the solar elevation angle, the proportion of the blue-green spectral region of PAR was significantly reduced at low elevation angles during days with prevailing direct radiation (DI < 0.3). The PAR spectrum at both M and L levels was only slightly enriched in the green spectral region (more pronounced for DI < 0.3). The penetration of diffuse radiation into the canopy resulted in a slight (approx. 5%) reduction of the blue region proportion that remained stable during the day. On the contrary, under clear sky conditions the penetration of blue and red radiation was dependent on the solar elevation in an opposite manner in comparison with the spectral composition of PAR incident on canopy, giving almost twofold proportion of the blue part of the spectrum at a low elevation angle at M layer. We suggest that the blue enhancement of the spectrum within the Norway spruce canopy during clear days is due to a specific spatial arrangement of the assimilatory apparatus of a coniferous stand. Further, the possible consequences of the observed dynamics of the PAR spectrum inside the canopy during clear days on the efficiency of PAR absorption of the needles located within the canopy are discussed.

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This study falls into site-specific studies (here in the Drahanská vrchovina Highland) focusing on the determination of allometric relationships between the stem dendrometric and biomass parameters in young Norway spruce stands. The total aboveground biomass (TB) of a sampled tree with 14.9 m in height and 15.5 cm in stem diameter at 1.3 m (DBH) averaged to 110.3 kg. The stem biomass participated on average by 54 %, branch biomass by 24 % and needle biomass by 22 % on the TB of the sampled spruce tree. TB of the tree and the biomass of individual aboveground tree organs were predicted with the highest accuracy (over 91 %) from DBH or a stem diameter at the one tenth of the tree height using allometric-power functions. The stem diameters up to 70 % of the relative tree height predicted TB accurately (over 95 %) as well. The biomass expansion factors based on the stem volume expansion to TB of the tree, as well as the biomass of each of the aboveground tree organs did not show functional dependency on DBH.

Mineral nutrition represents the uptake, transport, metabolism and utilization of nutrients by the forest stand. These processes influence all physiological functions of trees. A specific minimum amount of all nutrients is necessary for the healthy development and growth of forest trees. The uptake of nutrients is influenced not only by natural conditions but also by anthropogenic activities. During the period of 2000-2005 the mineral nutrition of mountain Norway spruce stands was studied at the study site Bílý Kříž (Moravian-Silesian Beskids Mts., Czech Republic). Research was carried out in a spruce stand that was limed in the past years (in 1983, 1985 and 1987) and in a spruce stand that was not limed in order to compare the liming effect on the mineral nutrition of spruce stands. A positive liming effect was detected in the calcium, magnesium and phosphorus nutrition because their contents in current needles were higher on the limed plots. No liming effect was determined in the nitrogen, potassium and microelement (Fe, Mn, Cu, Zn, Al) content in current needles. Sufficient nutrition of spruce stands only with calcium was recorded on all studied plots.