Soil Temperatures near the Distribution Limit of the Mountain Birch ( Betula pubescens ssp. czerepanovii ): Implications for Seedling Nitrogen Economy and Survival

This paper (1) describes soil temperature conditions in an area close to the altitudinal and latitudinal distribution limit of the mountain birch, and (2) evaluates whether soil temperature is likely to impose a limit on nitrogen uptake and subsequent reductions in growth and survival. Soil temperature was monitored at two depths (–5 and –10 cm) during 3 yr in forested and nearby nonforested sites both at low altitude (ca. 400 m) and at forest line (ca. 650 m) in the Torneträsk area, northern Swedish Lapland (68°21′N, 18°49′E). The mean summer soil temperature was ca. +6 to +7°C and +5 to +7°C at –5 and –10 cm, respectively. Forest-line soils were on average 0.8°C warmer than valley soils. We found no statistically significant differences in soil temperature between sites colonized by mountain birch and nearby nonforested habitats. Given present soil temperatures, there is thus no reason why (mature) trees should not survive also on alpine and subalpine heaths currently not colonized by birch. However, relating these temperature recordings to experimentally determined relationships between soil temperature and seedling nitrogen (N) uptake, and between N uptake and survival, indicates that the mountain birch seedlings are unlikely to survive their first winter under soil temperature conditions normally prevailing in this area. Our analysis suggests that successful seedling establishment can be expected on disturbed sites when the growing season (15 June to 15 August) mean soil temperature is ca. 12°C or higher. In comparison, growing season soil temperature under undisturbed vegetation (at –5 cm) was on average ca 8.2°C during our 3 yr of soil temperature measurements. Air temperature during the same period was 1.1°C warmer than the average for the last 86 yr (1913–1998, 11.0 versus 9.9°C).