Effects of soil temperature on the nitrogen economy and growth of mountain birch seedlings near its presumed low temperature distribution limit

Improved soil surface nitrogen balance method for assessing nutrient use efficiency and potential environmental impacts within an alpine meadow dominated region

The soil surface nitrogen balance (SSNB) method is commonly used to assess the nutrient use efficiency (NUE) of agricultural systems and any associated potential environmental impacts. However, the nitrogen flow of wide natural grasslands and other natural areas differ from that of artificial croplands and mown grasslands. In this study, we integrated root growth and the important nutrient resorption process into the SSNB model and used the improved model to clarify the nitrogen (N) flow and balance in the Three Rivers Headwater Region (TRHR)-an area dominated by alpine meadows-from 2012-2019. In the grassland system, the N surplus (ΔN) was 0.274 g m^-2 year^-1, and root return (BLD) dominated the N input, accounting for 67% of the total input (3.924 g m^-2 year^-1). N resorption was the main internal N flow in the grassland system (1.079 g m^-2 year^-1), and 30% of grassland uptake (N_UP-grass). The ΔN of the agricultural system was 1.097 g m^-2 year^-1, which was four times that of the grassland, and chemical fertilizer was the largest input, accounting for 84% of the total input. The NUE in grassland was 93%, which suggests a risk of soil mining and degradation, while that of cropland was 76% and within an ideal range. The ΔN provides a robust measure of river N export, the TRHR was divided into three catchments, and the export coefficient was 16.14%-55.68%. The results of this study show that the improved SSNB model can be applied to a wide range of natural grasslands that have high root biomass and resorption characteristics.

Treeline Research—From the Roots of the Past to Present Time. A Review

Elevational and polar treelines have been studied for more than two centuries. The aim of the present article is to highlight in retrospect the scope of treeline research, scientific approaches and hypotheses on treeline causation, its spatial structures and temporal change. Systematic treeline research dates back to the end of the 19th century. The abundance of global, regional, and local studies has provided a complex picture of the great variety and heterogeneity of both altitudinal and polar treelines. Modern treeline research started in the 1930s, with experimental field and laboratory studies on the trees’ physiological response to the treeline environment. During the following decades, researchers’ interest increasingly focused on the altitudinal and polar treeline dynamics to climate warming since the Little Ice Age. Since the 1970s interest in treeline dynamics again increased and has considerably intensified from the 1990s to today. At the same time, remote sensing techniques and GIS application have essentially supported previous analyses of treeline spatial patterns and temporal variation. Simultaneously, the modelling of treeline has been rapidly increasing, often related to the current treeline shift and and its implications for biodiversity, and the ecosystem function and services of high-elevation forests. It appears, that many seemingly ‘new ideas’ already originated many decades ago and just confirm what has been known for a long time. Suggestions for further research are outlined.

Large elevation and small host plant differences in the arbuscular mycorrhizal communities of montane and alpine grasslands on the Tibetan Plateau

Understanding the diversity and community structure of arbuscular mycorrhizal fungi (AMF) in extreme conditions is fundamental to predict the occurrence and evolution of either symbiotic partner in alpine ecosystems. We investigated the AMF associations of three plant species at elevations ranging between 3105 and 4556 m a.s.l. on Mount Segrila on the Tibetan Plateau. Three of four locations were studied in two consecutive years. The AMF diversity and community composition in the roots of Carex pseudofoetida, Pennisetum centrasiaticum, and Fragaria moupinensis differed little. However, at high elevations, the abundance of members of Acaulosporaceae increased relative to that of Glomeraceae. Plants at lower elevation sites, where Glomeraceae predominated as root symbionts, had higher leaf nitrogen and phosphorus concentrations than plants at higher elevation sites, where Acaulosporaceae predominated. The overall phylogenetic relatedness of the AMF increased with increasing elevation. This suggests that abiotic filtering may play an important role in the structuring of symbiotic AMF communities along elevational gradients. The functional role of Acaulosporaceae whose relative abundance was found to increase with elevation in alpine environments needs to be clarified in future studies.

Active summer carbon storage for winter persistence in trees at the cold alpine treeline

The low-temperature limited alpine treeline is one of the most obvious boundaries in mountain landscapes. The question of whether resource limitation is the physiological mechanism for the formation of the alpine treeline is still waiting for conclusive evidence and answers. We therefore examined non-structural carbohydrates (NSC) and nitrogen (N) in treeline trees (TATs) and low-elevation trees (LETs) in both summer and winter in 11 alpine treeline cases ranging from subtropical monsoon to temperate continental climates across Eurasia. We found that tissue N concentration did not decrease with increasing elevation at the individual treeline level, but the mean root N concentration was lower in TATs than in LETs across treelines in summer. The TATs did not have lower tissue NSC concentrations than LETs in summer. However, the present study with multiple tree species across a large geographical scale, for the first time, revealed a common phenomenon that TATs had significantly lower NSC concentration in roots but not in the aboveground tissues than LETs in winter. Compared with LETs, TATs exhibited both a passive NSC storage in aboveground tissues in excess of carbon demand and an active starch storage in roots at the expense of growth reduction during the growing season. This starch accumulation disappeared in winter. Our results highlight some important aspects of the N and carbon physiology in relation to season in trees at their upper limits. Whether or to what extent the disadvantages of winter root NSC and summer root N level of TATs affect the growth of treeline trees and the alpine treeline formation needs to be further studied.

Climate Change, Nutrition, and Bottom-Up and Top-Down Food Web Processes

Climate change ecology has focused on climate effects on trophic interactions through the lenses of temperature effects on organismal physiology and phenological asynchronies. Trophic interactions are also affected by the nutrient content of resources, but this topic has received less attention. Using concepts from nutritional ecology, we propose a conceptual framework for understanding how climate affects food webs through top-down and bottom-up processes impacted by co-occurring environmental drivers. The framework integrates climate effects on consumer physiology and feeding behavior with effects on resource nutrient content. It illustrates how studying responses of simplified food webs to simplified climate change might produce erroneous predictions. We encourage greater integrative complexity of climate change research on trophic interactions to resolve patterns and enhance predictive capacities.

From greening to browning: Catchment vegetation development and reduced S-deposition promote organic carbon load on decadal time scales in Nordic lakes

Increased concentrations of dissolved organic carbon (DOC), often labelled “browning”, is a current trend in northern, particularly boreal, freshwaters. The browning has been attributed to the recent reduction in sulphate (S) deposition during the last 2 to 3 decades. Over the last century, climate and land use change have also caused an increasing trend in vegetation cover (“greening”), and this terrestrially fixed carbon represents another potential source for export of organic carbon to lakes and rivers. The impact of this greening on the observed browning of lakes and rivers on decadal time scales remains poorly investigated, however. Here, we explore time-series both on water chemistry and catchment vegetation cover (using NDVI as proxy) from 70 Norwegian lakes and catchments over a 30-year period. We show that the increase in terrestrial vegetation as well as temperature and runoff significantly adds to the reduced SO₄-deposition as a driver of freshwater DOC concentration. Over extended periods (centuries), climate mediated changes in vegetation cover may cause major browning of northern surface waters, with severe impact on ecosystem productivity and functioning.

Tolerance of ectomycorrhizal fungus mycelium to low temperature and freezing–thawing

Very little is known about the tolerance of ectomycorrhizal fungi to the freezing and repetitive freezing–thawing in northern coniferous forests. Isolates of Cortinarius multiformis , Russula densifolia , Suillus granulatus , and Lactarius deliciosus were exposed to a series of temperatures between +4 and –40 °C. The relative electrolyte leakage test indicated that the lethal temperature for 50% of samples was between –7.6 and –13.7 °C. Resume growth experiments showed that the 4 species of ectomycorrhizal fungi had a relatively high tolerance to the low temperatures, with L. deliciosus having the highest tolerance and C. multiformis the lowest. The repeated freezing–thawing delayed the growth of mycelium, which decreased with an increase in the number of freeze–thaw cycles.

Freezing tolerance of ectomycorrhizal fungi in pure culture

The ability to survive freezing and thawing is a key factor for the existence of life forms in large parts of the world. However, little is known about the freezing tolerance of mycorrhizal fungi and their role in the freezing tolerance of mycorrhizas. Threshold temperatures for the survival of these fungi have not been assessed experimentally. We grew isolates of Suillus luteus, Suillus variegatus, Laccaria laccata, and Hebeloma sp. in liquid culture at room temperature. Subsequently, we exposed samples to a series of temperatures between +5 degrees C and -48 degrees C. Relative electrolyte leakage (REL) and re-growth measurements were used to assess the damage. The REL test indicated that the lethal temperature for 50% of samples (LT(50)) was between -8.3 degrees C and -13.5 degrees C. However, in the re-growth experiment, all isolates resumed growth after exposure to -8 degrees C and higher temperatures. As many as 64% of L. laccata samples but only 11% in S. variegatus survived -48 degrees C. There was no growth of Hebeloma and S. luteus after exposure to -48 degrees C, but part of their samples survived -30 degrees C. The fungi tolerated lower temperatures than was expected on the basis of earlier studies on fine roots of ectomycorrhizal trees. The most likely freezing tolerance mechanism here is tolerance to apoplastic freezing and the concomitant intracellular dehydration with consequent concentrating of cryoprotectant substances in cells. Studying the properties of fungi in isolation promotes the understanding of the role of the different partners of the mycorrhizal symbiosis in the freezing tolerance.

Genetic and environmental factors affecting the allergenicity of birch (Betula pubescens ssp. czerepanovii [Orl.] Hämet-ahti) pollen

BACKGROUND

Environmental variation, such as an increase of mean temperature due to the greenhouse effect, as well as the genetic factors may affect the allergenicity of pollen and thus, the prevalence of allergies. The connection between these factors and the allergen content of pollen is poorly understood.

OBJECTIVES

To evaluate the role of environmental and genetic factors on the allergenicity of birch pollen.

METHODS

Mountain birch (Betula pubescens ssp. czerepanovii (Orl.) Hämet-Ahti) pollen was studied using SDS-PAGE and IgE-immunoblotting. Pollen samples were collected from the trees of 10 half-sib families. The study trees from each family were reared in two tree line gardens where the daily mean temperatures were different during the growing season.

RESULTS

The quantitative analysis of band intensities suggested that the responses of the major birch pollen allergen, Bet v 1, were stronger in the samples collected from the garden with higher daily mean temperature. Half-sib families and individual trees differed in their Bet v 1 content.

CONCLUSIONS

Our results show that both genetic and environmental factors have an effect on the amount of Bet v 1. This suggests that breeding for trees low in allergen content may be possible.