Influence of Environmental Variability on Root Dynamics in Northern Forests

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Abstract
Plant root systems are highly dynamic over various temporal and spatial scales, and are responsive to changes in environmental conditions. The objective of this review is to describe the dynamics of root structure and function in boreal and northern temperate forests and examine how edaphic and climatic characteristics and intra- and interspecific root competition impact root dynamics. Fine roots exhibit distinct seasonal trends of production and mortality. Over the extent of stand development, coarse root biomass increases until maturity, while the response of fine roots remains unclear. Roots are predominantly restricted to the upper soil layers, and spatial variability of roots in the horizontal direction decreases with decreasing root size. Root/shoot ratio decreases gradually through stand development. On nutrient-rich sites, roots are more concentrated around respective stems and root systems are more branched than on nutrient-poor sites. Plants generally root deeper under low soil moisture conditions, while roots tend to grow horizontally into rich rather than poor patches of soil. Plants adapt their biomass allocation strategies to edaphic and climatic variation according to the functional equilibrium hypothesis. Although root production is projected to increase, providing nutrients are not limiting, following elevated carbon dioxide concentrations and temperatures, how root turnover and above- and below-ground carbon allocation may change remains uncertain. Stands composed of species with different rooting characteristics may attain greater root production compared to single-species stands or mixtures of similar species from reduced exploitative competition. Alternatively, plants can produce greater root biomass with a competing plant than growing alone as a result of self root discrimination. Future research is needed to address how elevated carbon dioxide concentrations and temperatures will feedback upon soil resource availability to influence plant responses from the organism- to the ecosystem-level.