Predicting fine root production and turnover by monitoring root starch and soil temperature

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Abstract
To determine how the longevity of fine roots (those without secondary thickening) is controlled, shoots of Douglas-fir (Pseudotsugamenziesii Mirb. (Franco)) seedlings were exposed to light or maintained in darkness while roots were maintained at 10, 20, or 30 °C. Fine root maintenance respiration rates, estimated from rates of starch and sugar depletion in the seedlings maintained in darkness, ranged from 0.83 to 3.25 mg starch g dry weight−1 day−1. At 20 and 30 °C, starch deposition was curtailed and previously deposited starch was used to maintain the older roots, whether current photosynthate was entering the root system or not. On the other hand, at 10 °C starch was deposited in the roots whenever the root systems grew. Based on these results, we suggest that starch deposition in a fine root occurs only when the root is being formed and the root carbon balance is positive. Starch is subsequently respired to meet maintenance requirements exclusively. A simple means of estimating root biomass production and turnover based on root starch and soil temperature is described and compared with field estimates.