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 th e altitudinal and latitudinal distribution Limit of the mountain birch, and (2) evaluates whether soil temperature is likely to impose a limit on nitr ogen uptake and subsequent reductions in growth and survival. Soil temperat ure 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 li ne (ca. 650 m) in the Tornetrask area, northern Swedish Lapland (68 degrees 21'N, 18 degrees 49'E). The mean summer soil temperature was ca. +6 to +7 degreesC and +5 to +7 degreesC at -5 and -10 cm, respectively. Forest-line soils were on average 0.8 degreesC warmer than valley soils. We found no st atistically significant differences in soil temperature between sites colon ized 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. Howe ver, relating these temperature recordings to experimentally determined rel ationships between soil temperature and seedling nitrogen (N) uptake, and b etween N uptake and survival, indicates that the mountain birch seedlings a re unlikely to survive their first winter under soil temperature conditions normally prevailing in this area. Our analysis suggests that successful se edling establishment can be expected on disturbed sites when the growing se ason (15 June to 15 August) mean soil temperature is ca. 12 degreesC or hig her. In comparison, growing season soil temperature under undisturbed veget ation (at -5 cm) was on average ca 8.2 degreesC during our 3 yr of soil tem perature measurements. Air temperature during the same period was 1.1 degre esC warmer than the average for the last 86 yr (1913-1998, 11.0 versus 9.9 degreesC).