Key message Juveniles and canopy trees may not exhibit similar nitrogen acquisition responses to soil temperature change caused by variation in snow cover over winter. The use of(15)N tracer is a powerful tool for tracking the effects of variation in soil frost on plant nitrogen acquisition. While the responses of juvenile trees to environmental change are often used to infer the responses of canopy trees, the(15)N enrichment responses of juveniles and mature canopy trees may not be comparable. We conducted a winter soil temperature manipulation study (snow exclusion, ambient snow or soil insulation) in a lowlandFagus sylvaticaforest.N-15 tracer was applied the following spring and the(15)N enrichments of soil, juvenile and mature canopy trees were examined in late fall. Within canopy trees and juveniles, the relative treatment effects on(15)N enrichment were consistent among all sampled tissues (roots, stem cores, leaves, buds and the current year's shoot growth). For juveniles,N-15 enrichment was highest under snow exclusion (coldest soil) and lowest under soil insulation (warmest soil), and lower(15)N enrichment occurred under ambient conditions than under snow exclusion. For canopy trees,N-15 enrichment also was highest under snow exclusion and lowest under soil insulation, but there was no difference in(15)N enrichment between ambient conditions and the snow exclusion treatment. Therefore, our results indicate that sampling of juveniles may overestimate the nitrogen acquisition responses of mature trees to winter temperature variation.

AimsIn this study, we investigated the effects of reduced snow depth on plant phenology, productivity, nitrogen (N) cycling, and N use in canopy and understory vegetation. We hypothesized that decreased snow depth would hasten the timing of leaf flushing and N uptake in understory vegetation, increasing its N competitive advantage over canopy trees.ResultsSnow removal did not directly affect the phenology of either canopy or understory vegetation. Understory vegetation took up more N in the snow removal plots than in the control plots, particularly in the mid- to late-growing season. Leaf production and N uptake in canopy trees also did not differ between the control and snow removal plots, but N resorption efficiency in the snow removal plots (57.6%) was significantly higher than those in control plots (50.0%).ConclusionsIncreased N uptake by understory plants may induce N limitation in canopy trees, which in turn may cause canopy trees to increase their N use efficiency. Such competitive advantage of understory vegetation over canopy trees against snow reduction may affect N cycling via litter quality and quantity not only just after the growing season but also in subsequent seasons.

Global climate change is altering snow depth in winter, which could significantly affect soil respiration and microbial communities. However, belowground responses are still uncertain as they depend on the thermal effects on soils, the acclimation of soil microbial communities and ecosystem type. Here, we conducted a snow manipulation experiment including 50% removal of snowpack (mean snow depth after treatment was 3.1 +/- 0.7 cm), ambient snow (mean snow depth was 6.3 +/- 0.7 cm), and 50% increase of snowpack (mean snow depth after treatment was 9.6 +/- 1.5 cm) to explore the effects of altered snow depth on winter soil respiration and microbial communities in a mid-latitude plantation forest with continental climate with dry winters. Winter soil CO2 effluxes varied from 0.09 to 0.84 mu mol m(-2)S(-1) with a mean of 0.32 +/- 0.07 mu mol m(-2)s(-1). The cumulative soil CO2 effluxes from 11 December 2014 to 21 March 2015 were 27.3 +/- 1.1, 26.5 +/- 2.1, and 29.5 +/- 1.3 g Cm-2 under reduced, ambient and added snowpack, which corresponded to 5.7 +/- 0.2%, 5.5 +/- 0.3%, and 5.8 +/- 0.1% of the annual soil CO2 effluxes, respectively. Our one-year observation results suggested that although snow reduction decreased soil temperature, microbial biomass carbon (MBC) and soil respiration did not change, suggesting microbial adaptation to cold conditions between - 4 degrees C and -1 degrees C. In contrast, snow addition increased soil temperature, MBC, and soil respiration. Microbial community structure (F/B, ratio of fungi to bacteria) was also changed and soil enzymatic (beta-glucosidase) activities increased under snow addition. However, these effects were short-lived and disappeared when soil temperature did not differ between the addition and control plots at the 14th day after treatment. These results indicated that the responses of soil microbial communities and respiratory activities to changing soil temperature were rapid and the response of soil respiration to snow addition was transient. Consequently, altered snow depth did not affect cumulative soil CO2 effluxes. Our study suggests that wintertime soil respiration rates are generally low and snow manipulation has minor effects on soil CO2 efflux, soil temperature (the determinant driver of wintertime soil CO2 efflux) and soil microbial biomass at our site.

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. (C) 2013 Elsevier B.V. All rights reserved.

Snow regimes affect biogeochemistry of boreal ecosystems and are altered by climate change. The effects on plant communities, however, are largely unexplored despite their influence on relevant processes. Here, the impact of snow cover on understory community composition and below-ground production in a boreal Picea abies forest was investigated using a long-term (8-year) snow cover manipulation experiment consisting of the treatments: snow removal, increased insulation (styrofoam pellets), and control. The snow removal treatment caused longer (118 vs. 57 days) and deeper soil frost (mean minimum temperature -5.5 vs. -2.2 degrees C) at 10 cm soil depth in comparison to control. Understory species composition was strongly altered by the snow cover manipulations; vegetation cover declined by more than 50% in the snow removal treatment. In particular, the dominant dwarf shrub Vaccinium myrtillus (-82%) and the most abundant mosses Pleurozium schreberi (-74%) and Dicranum scoparium (-60%) declined strongly. The C:N ratio in V. myrtillus leaves and plant available N in the soil indicated no altered nitrogen nutrition. Fine-root biomass in summer, however, was negatively affected by the reduced snow cover (-50%). Observed effects are attributed to direct frost damage of roots and/or shoots. Besides the obvious relevance of winter processes on plant ecology and distribution, we propose that shifts in the vegetation caused by frost damage may be an important driver of the reported alterations in biogeochemistry in response to altered snow cover. Understory plant performance clearly needs to be considered in the biogeochemistry of boreal systems in the face of climate change.

Climate change may cause a decrease in snow cover in northern latitudes. This, on the other hand, may result in more severe soil frost even in areas where it is not common at present, and may lead to increased stress on the tree canopy. We studied the effects of snow removal and consequent changes in soil frost and water content on the physiology of Norway spruce (Picea abies [L] Karst.) needles and implications on root biomass. The study was conducted at a 47-year-old Norway spruce stand in eastern Finland during the two winters of 2005/06 and 2006/07. The treatments in three replicates were: (i) natural snow accumulation and melting (CTRL), (ii) artificial snow removal during the winter (OPEN), and (id) the same as OPEN, but the ground was insulated in early spring to delay soil thawing (FROST). In spite of the deeper soil frost in the OPEN than in the CTRL treatment, soil warming in spring occurred at the same time, whereas soil warming in the FROST was delayed by 2 and 1.5 months in 2006 and 2007, respectively. The soil water content was affected by snow manipulations, being at a lower level in the OPEN and FROST than CTRL in spring and early summer. The physiological measurements of the needles (e.g. starch, carbon and nitrogen content and apoplastic electrical resistance) showed differences between soil frost treatments. The differences were mostly seen between the CTRL and FROST, but also in the case of the starch content in early spring 2007 between the CTRL and OPEN. The needle responses in the FROST were more evident after the colder winter of 2006. The physiological changes seemed to be related to the soil temperature and water content in the early growing season rather than to the wintertime soil temperature. No difference was found in the fine root (diameter <2 mm) biomass between the treatments assessed in 2007. In the future, conditions similar to the OPEN treatment may be more common than at present in areas experiencing a thick snow cover. The present experiment took place over the course of two years. It is possible that whenever thin snow cover occurs yearly, the reduced starch content during the early spring may be reflected in the tree growth itself as a result of reduced energy reserves. (C) 2011 Elsevier B.V. All rights reserved.