Effects of frozen soil on growth and longevity of fine roots of Norway spruce

• We studied fine roots of boreal Norway spruce in changing frozen soil regimes.
• More frozen soil → minor effects on growth but longevity increased.
• Delayed soil thawing → harmful for growth but no effect on longevity.
• Longevity increased with root branching order and soil depth.

Frozen soil is predicted to change in the boreal areas with climate warming. We studied growth, longevity and mortality of fine roots at different levels of frozen soil in winter followed by a delayed soil thawing in spring in a 47-year-old stand of Picea abies (L. Karst.) in the boreal zone. The treatments, repeated over two winters, were: (i) natural insulating snow accumulation and melting (CTRL), (ii) snow removed during winter (OPEN), and (iii) as OPEN in winter but soil thaw delayed by insulation at the top of the forest floor (FROST). Short and long roots were monitored at different depths by minirhizotron imaging at one-month intervals from May to October in the 2 years during and 2 years after the treatments, to assess standing length (SSL), production volume (SPV) and mortality. A survival function estimate was calculated according to the nonparametric maximum likelihood estimate for interval censored data, and the mean and median root longevities were calculated as with a Kaplan–Meier estimate. CTRL and OPEN did not differ for SSL and SPV but they differed in FROST where compensatory growth occurred in the follow-up seasons. The mean longevity ranged from 276 to 305 days for short roots and from 425 to 464 days for long roots, being higher in OPEN than CTRL and FROST, and higher in the deeper soil layers than near the soil surface. The mean and median longevities were largely the same for short roots but the means were 80–100 days higher for long roots. We conclude that the winters with deep soil freezing are not detrimental for fine roots of Norway spruce, insofar as soil thawing will not prolong the growing season. The longer lifetime in OPEN suggests declining carbon flux into the soil following winters with deeply frozen soil.