3.2. FRBandsamplingdepthThe FRB sampling depths were significantly smaller in the boreal forests than in the tropical or temperate ones (p<0.001) (Table 4), and there was significant positive correlation between total FRB and sampling depth in the tropical and temperate forests (r2 =0.48, p<0.001, N=73 and r2 =0.52, p<0.001, N=173, respec- tively). The FRB estimates extrapolated for the whole rooting depth using equation (1) were 526±321gm−2, 775±474gm−2 and 776 ± 518 g m−2 for the boreal, temperate and topical forests, respectively (Table 5). These values were 26–67% higher than those based on the original sampling depths, the differences being statis- tically significant (p < 0.001, Table 5).3.3. ContributionofunderstoreyvegetationrootstototalFRBThe proportion of the total FRB extrapolated for the whole sam- pling depth attributable to the understorey vegetation roots was assessed with data covering 74 temperate and 59 boreal forest stands. Tropical forests could not be included because there were no data available for the analyses. The proportion tended to be higher in the boreal forests (0.31±0.26) than in the temperate forests (0.20±0.29), and higher in the needleleaf forests (0.29±0.28, N = 101) than in the broadleaf forests (0.13 ± 0.25, N = 32), but the differences were not statistically significant (p > 0.05). There were relatively more forest stands without any understorey vegetation in the temperate (40 out of 74) and broadleaf forests (22 out of 32) than in the boreal (4 out of 59) or needleleaf forests (25 out of 101). No significant relationships (p > 0.05) were found between FRB of trees as a proportion of total FRB and tree stand basal area (m2 ha−1), tree stand density (treesha−1) or stand age (data not shown).3.4. Tree,understoreyandtotalFRBinthebiomesThe mean FRB of trees calculated using the original sampling depth did not differ between the temperate and boreal forests (p > 0.05), whereas it was significantly higher in the broadleaf than in the needleleaf forests (p=0.05) (Tables 5 and 6). These differ- ences disappeared when the mean FRB of trees by biome was calculated for the whole rooting depth (p > 0.05) (Tables 5 and 6).There were no significant differences in the mean understorey FRB between the temperate and boreal forests or between the broadleaf and needleleaf forests (Tables 5 and 6).The mean total FRB (trees + understorey) did not differ signifi- cantly between the biomes in the non-extrapolated original data (p > 0.05), but the extrapolated values for the whole rooting depth differed significantly between biomes (p<0.05) (Tables 5 and 6), being higher in the tropical and temperate forests than in the boreal forests (Tukey HSD test, p < 0.01). There were no significant differences (p > 0.05) in the mean total FRB biomass between the broadleaf forests and needleleaf forests or between the evergreen and deciduous tropical forests (Tables 5 and 6).3.5. RelationshipsbetweenFRBandstandandenvironmental characteristicsWe started by exploring the relationships between the eight tree stand and environmental variables by means of PCA ordination. Of the environmental variables, latitude, temperature and annual precipitation had the highest scores on the first ordination axis and only elevation on the second axis (Fig. 2), while among the tree stand variables stand density and basal area had the highest scores on the second axis and the needleleaf and broadleaf forests were located at opposite ends of the first axis.
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