Significance of snowpack for root-zone water and temperature cycles in subarctic lapland

Abstract

Snowmelt timing is a critical factor for tree growth in high latitudes, but threshold conditions with respect to soil moisture availability and soil temperature for the rootzone processes are not well known. We monitored snowpack thickness, air and soil temperature, and water content in the soil, sapwood, and roots of downy birch (Betula pubescens Roth.) in Finnish Lapland through 1999-2003. An extreme cold event in January 1999 (T AIR = 49°C) resulted in soil freezing (at 10-cm depth) down to T 10 = 26°C at a snow-free site, but beneath the 50-cm-thick snowpack the soil temperature was T 10 = -2.5°C. Snowmelt water was able to infiltrate partially frozen soil sequences, such that an increase in water content of the soil and birch roots occurred two to six weeks before soil temperatures rose notably above 0°C. The soil T 10 reached +0°C a week after the disappearance of snow. The increase in water content of birch trunks was coincidental with the air temperature rises notably above 0°C. The systematic interseasonal pattern of water content in the birch root-trunk system, i.e. high peaks in late winter-early spring and fall, suggests sap flow in downy birch.