Low winter soil temperature affects summertime nutrient uptake capacity and growth rate of mountain birch seedlings in the subarctic, Swedish lapland

Effects of different winter soil temperature on summertime growth rate of individual seedlings of mountain birch (Betula pubescens ssp. czerepanovii) were evaluated in an outdoor pot experiment in subarctic Sweden. The hypothesis tested was that low winter soil temperature could decrease growing-season nutrient uptake capacity and growth rate by injury of the root system; further, the root damage should stimulate growth of replacement roots, which should be reflected by increased summertime biomass allocation to roots. Mountain birch seedlings were exposed to two soil temperatures during wintertime by manipulation of snow depth that accumulated on top of the pots. Minimum soil temperature (hourly recordings) during winter was -1.7degreesC in the "protected" treatment (deep snow cover) and -10.5degreesC in the "exposed" treatment (thin snow cover). The treatments simulated the soil temperature conditions of a local mountain birch forest and heath site, respectively. During June and July, the relative growth rate (R.) and nitrogen accumulation rate (RN) in the exposed treatment were lower compared to the protected treatment. In addition, the seedlings of the exposed treatment allocated a greater proportion of biomass to roots compared to the protected treatment. Thus, leaf growth was reduced in the exposed compared to the protected treatment. We concluded that lower winter temperature in soils can result in stronger root injury that stimulates the growth of replacement roots and reduces nutrient uptake capacity and growth rate during summer. In concert with other possible factors causing root disruption during winter, e.g., frequent freeze-thaw cycles, the mechanism identified here might be important for survival of tree seedlings in subarctic and alpine areas, especially near treelines. The implications are discussed specifically for mountain birch seedling growth and survival in treeless subarctic heath sites and above treelines in northern Europe.