Temperature control over root growth and root biomass in taiga forest trees

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Abstract
Root elongation of greenhouse-grown Alaskan [USA] taiga tree seedlings increased with increasing root temperature in all 6 species examined and was most temperature sensitive in warm-adapted aspen (Populus tremuloides Michx.). Root elongation was slower in fine than large roots and in black spruce (Picea mariana (Mill.) B.S.P.) was less temperature sensitive in fine than in large roots. Root elongation in the laboratory was slowest in black spruce, which has an inherently slow growth rate, and most rapid in poplar (Populus balsamifera L.) and aspen, which grow more rapidly. In contrast, field root elongation rates tended to be highest in black spruce from cold wet sites, suggesting that site factors other than soil temperature (e.g., moisture) predominated over genetic differences among species in determining field root elongation rates. The seasonal pattern of root elongation was closely correlated with soil temperature and reached maximum rates by July for all tree species (except aspen medium-sized roots). Most roots of each species were in the top 20 cm of soil. However, root growth penetrated to greater depth in warm compared with cold sites. Root biomass in a 130-yr black spruce forest (1230 g/m2) comprised only 15% of total tree biomass. Root biomass of 25-yr aspen and 60-yr poplar sites (517 and 5385 g/m2, respectively) comprised a greater proportion (57% in poplar) of total tree biomass than in spruce.