The influence of soil temperature on transpiration: a plot scale manipulation in a young Scots pine stand

Classic studies have stressed the importance of forest management for soil frost and the dynamics of soil warming during spring. But, we know very little about the potential loss of forest production due to cold soils. Liquid soil water may not be available to trees due to cold soil conditions, thus reducing transpiration and photosynthesis. We believe that these effects need to be quantified in order to keep silvicultural practices from inadvertently reducing forest growth through effects on soil temperature. In order to test this hypothesis it is important to know more about water uptake in the field by trees in cold environments. The snow cover was manipulated on plots in a young stand of Pinus sylvestris L. to create plots with different timings of soil warming. Soil profile temperature and liquid water content, as well as snow and frost depths, were measured together with tree physiological parameters such as sap flow, stomatal conductance and needle water potential. The study has confirmed the importance of the soil temperature for tree water uptake when soil warming was delayed beyond the start of growing season as defined by air temperature. The absence of frost (and hence the presence of liquid water) was not sufficient to enable water uptake. It was rather elevation of soil temperature above zero in the upper soil layers that was required for substantial water uptake, in combination with other site factors. Below ca. +8 °C, soil temperature was a factor in the restriction of transpiration, explained by a lower stomatal conductance and likely decreased root permeability. The effect of a certain soil temperature was not the same at different times due to its interaction with aboveground factors such as air temperature and day-length. The effect of low soil temperature increased with the persistence of low temperatures after the start of the growing season. The timings of soil warming induced by our study were within the spatial and inter-annual variation of soil warming in this region. Thus, we conclude that influences of forest management on soil temperature can affect the transpiration deficit during spring, with potential implications for forest productivity.

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