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Increasing evidence suggests that forest soils in central and northern Europe as well as in North America have been significantly acidified by acid deposition during the last decades. The present investigation was undertaken to examine the effect of soil acidity on rooting patterns of 40-year-old Norway spruce trees by comparing fine and coarse roots among four stands which differed in soil acidity and Mg (and Ca) nutrition. The coarse root systems of four to five 40-year-old Norway spruce trees per stand were manually excavated. The sum of cross sectional area (CSA) at 60 cm soil depth and below of all vertical coarse roots, as a measure of vertical rooting intensity, was strongly reduced with increasing subsoil acidity of the stands. This pattern was confirmed when 5 additional acidic sites were included in the analysis. Fine root biomass in the mineral soil estimated by repeated soil coring was strongly reduced in the heavily acidified stands, but increased in the humic layer. Using ingrowth cores and a screen technique, we showed that the higher root biomass in the humic layer of the more acidic stands was a result of higher root production. Thus, reduced fine root biomass and coarse root CSA in deeper soil layers coincided with increased root growth in the humic layer. Root mineral analysis showed Ca/Al ratios decreased with decreasing base saturation in the deeper mineral soil (20–40 cm). In the top mineral soil, only minor differences were observed among stands. In general, low Ca/Al ratios coincided with low fine root biomass. Calcium/aluminum ratios determined in cortical cell walls using X-ray microanalysis showed a similar pattern as Ca/Al ratios based on analysis of whole fine roots, although the amplitude of changes among the stands was much greater. Aluminum concentrations and Ca/Al ratios in cortical cell walls were at levels found to inhibit root growth of spruce seedlings in laboratory experiments. The data support the idea that Al (or Ca/Al ratios) and acid deposition-induced Mg (and possibly Ca) deficiency are important factors influencing root growth and distribution in acidic forest soils. Changes in carbon partitioning within the root system may contribute to a reduction in deep root growth.

Forest hydrologists have hypothesised that canopy transpiration (E(c)) of European temperate forests occurs at rather similar rates in stands with different tree species and hydrologic regimes. We tested this hypothesis by synchronously measuring xylem sap flow in four mature stands of Fagus sylvatica along a precipitation gradient with the aim (1) of exploring the regional variability of annual canopy transpiration (E(c(t))) in this species, and (2) of analysing the relationship between precipitation (P) and E(c(t)). E(c(t)) rates of 216, 225, 272 and 303 mm year(-1) corresponded to precipitation averages of 520, 710, 801 and 1,040 mm year(-1) in the four stands. We explored the regional variability of E(c(t)) in Central European colline to sub-montane beech stands in two meta-analyses based on (1) existing sap flow data on beech (n=5 observations), or (2) all canopy transpiration data on beech obtained by different techniques (sap flow, micrometeorological or soil water budget approaches, n=25). With a coefficient of variation (CV) of 20%, the regional variability of E(c(t)) (213-421 mm year(-1)) was smaller than the variation in corresponding precipitation (550-1,480 mm year(-1)). The mean E(c(t)) for beech was 289 (+/-58) mm year(-1) (n=25). A humped-shaped relationship between E(c(t)) and P, with a broad transpiration maximum in the precipitation range from ca. 700 to 1,000 mm year(-1), was found which may indicate soil moisture limitation of transpiration for Pca. 700 mm year(-1), and reduced transpiration by increased cloudiness or leaf wetness for P1,000 mm year(-1). Thus, the precipitation level significantly influences canopy transpiration of humid temperate forests; however, the size of the P influence on E(c(t)) and, in part, the direction of its effect differ from forests in semi-arid or arid climates. European beech has the capacity to maintain high E(c) rates in both humid and partly dry summer climates (P550 mm year(-1)).