Inferred seasonal alternation in the anatomical structures determining maximum transpiration rate in upland boreal stand types

In the boreal forest the water-absorbing fraction of the root network undergoes an annual cycle of growth and dieback that is positively correlated with soil temperature. This study is a modelling exercise that uses duff temperature as a proxy for root uptake capacity to gain insight into its potential influence on the seasonal dynamics of maximum transpiration rate. Atmospheric and soil variables were measured hourly over six growing seasons in four boreal stand types. Empirical drying models predicting the relative contribution of evaporation and transpiration to the hourly change in duff moisture content were developed for periods when precipitation was not adding water and bulk flow was not removing it. The transpiration model was effective at predicting duff moisture loss in the springtime and late autumn but underestimated it in the intervening period. This divergence could be corrected by replacing the model's duff-temperature input variable with a calibrated fixed value between 15 degrees C and 24 degrees C. These findings suggest that root uptake capacity is the dynamic limiting factor to maximum transpiration rate in the spring and late autumn while outflow potential through the fixed number of stomata in the canopy assumes the role during the summer and early autumn once functional equilibrium with root uptake capacity has been established. Deterioration in this three-phase pattern in later iterations of the study was associated with drought-induced reductions in transpiration at the same sites in the preceding year and hints at shortfalls in the production of starch needed to initiate fine-root growth in the spring.