Electrical impedance spectroscopy (EIS) was applied to stems of Scots pine (Pinus sylvestris L,) in a provenance field trial during frost hardening to find an EIS parameter for assessing frost hardiness (FH) without a controlled freezing test. The FH of stems and needles assessed by controlled freezing tests was compared with the equivalent circuit EIS parameters of a distributed model of stems (not exposed to controlled freezing treatment) and with dry matter (DM) content of stems. Significant differences in the equivalent circuit parameters, FH and DM content were found between provenances, The relaxation time (tau (1)), describing the peak of the high frequency are of the impedance spectrum, and the intracellular resistance (r(i)) of stems increased with increasing FH, According to the linear regression, the coefficient of determination (R-2) between the FH of stems and needles with tau (1) of the stem was 0.87 and 0.89, and with r(i) of the stem 0.74 and 0.85, respectively, The relation between FH and tau (1) changed with the degree of hardiness, The highest coefficient of determination was 0.95 in September when the FH of needles, ranging from -10 degreesC to -25 degreesC, was predicted with an accuracy of +/- 2.0 degreesC, The resistance parameter r(2), describing the width of the low frequency are of the impedance spectrum, decreased prior to and during the initial hardening: significant differences were found between provenances, This indicates that r(2) was not related to frost hardening per se. It is concluded that it is possible to distinguish the hardening patterns of different provenances by tau (1) in the rapid phase of hardening without controlled freezing tests.
