Fine roots (52 mm) are very dynamic and play a key role in forest ecosystem carbon and nutrient cycling and accumulation. We reviewed root biomass data of three main European tree species European beech, (Fagus sylvatica L.), Norway spruce (Picea abies L. Karst.) and Scots pine (Pinus sylvestris L.), in order to identify the differences between species, and within and between vegetation zones, and to show the relationships between root biomass and the climatic, site and stand factors. The collected literature consisted of data from 36 beech, 71 spruce and 43 pine stands. The mean fine root biomass of beech was 389 g m 72, and that of spruce and pine 297 g m 72 and 277 g m 72 , respectively. Data from pine stands supported the hypothesis that root biomass is higher in the temperate than in the boreal zone. The results indicated that the root biomass of deciduous trees is higher than that of conifers. The correlations between root biomass and site fertility characteristics seemed to be species specific. There was no correlation between soil acidity and root biomass. Beech fine root biomass decreased with stand age whereas pine root biomass increased with stand age. Fine root biomass at tree level correlated better than stand level root biomass with stand characteristics. The results showed that there exists a strong relationship between the fine root biomass and the above-ground biomass.

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Norway spruce (Picea abies (L.) Karst.) seedlings fertilized with boron (B) at three rates were grown in a 3:7 mix of forest humus and quartz sand for 18 weeks. Half of the seedlings were not watered during the last 9 days of the experiment (drought treatment). The role of B in drought resistance was assessed by comparing the effects of seedling internal B concentration on the water relations, photosynthesis, growth and nutrition of well-watered and drought-treated seedlings. At the end of the drought treatment, needle B concentrations were 7.0 mg x kg(-1) at the lowest B supply rate and 17.5 and 23.5 mg x kg(-1), respectively, at the higher supply rates. Seedlings at the lowest B supply rate had fewer root tips and mycorrhizas than seedlings at the higher B supply rates. Drought treatment had a more pronounced effect on the water relations and net photosynthetic rate of seedlings than B treatment. Although seedlings at the higher B supply rates lost water more rapidly than seedlings at the low B supply rate-leading to faster stomatal closure and decreased photosynthesis-drought did not affect their final height, whereas drought reduced height growth of seedlings at the low B supply rate.

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The effects of three boron levels on the growth dynamics and ectomycorrhizas of seedlings of Norway spruce [Picea abies (L.) Karst.] were studied in a growth room experiment. The seedlings were grown in forest humus mixed with quartz sand for 16 weeks. The B treatment was applied in the nutrient solution. Stem height, dry weight, number of root tips, mycorrhizas as well as B and N concentrations in the seedlings were monitored in sequential harvests. By the last harvest, in week 16, needle B concentrations were 6.6 mg kg -1 at the lowest B level and 17.5 mg kg -1 and 26.5 mg kg -1 at the two higher levels. Boron slightly increased the stem height and the total dry weight, but did not affect N content of the seedlings. Low internal B reduced the number of root tips and mycorrhizas as well as mycorrhizal percentage and root dry weight, which indicates the importance of B for root growth in Norway spruce seedlings. The seedlings grown with adequate internal B had more root tips than those receiving the two lower B treatments as early as week 9, when needle B concentrations at the lowest B supply were 16.0-17.3 mg kg -1 , which has previously been considered a sufficient B level. Therefore, the critical needle B concentrations should perhaps be re-examined.