The influence of soil temperature and water content on belowground hydraulic conductance and leaf gas exchange in mature trees of three boreal species.

Understanding stomatal regulation is fundamental to predicting the impact of changing environmental conditions on vegetation. However, the influence of soil temperature (ST) and soil water content (SWC) on canopy conductance (gs ) through changes in belowground hydraulic conductance (kbg ) remains poorly understood, because kbg has seldom been measured in field conditions. Our aim was to (a) examine the dependence of kbg on ST and SWC, (b) examine the dependence of gs on kbg and (c) test a recent stomatal optimization model according to which gs and soil-to-leaf hydraulic conductance are strongly coupled. We estimated kbg from continuous sap flow and xylem diameter measurements in three boreal species. kbg increased strongly with increasing ST when ST was below +8°C, and typically increased with increasing SWC when ST was not limiting. gs was correlated with kbg in all three species, and modelled and measured gs were well correlated in Pinus sylvestris (a model comparison was only possible for this species). These results imply an important role for kbg in mediating linkages between the soil environment and leaf gas exchange. In particular, our finding that ST strongly influences kbg in mature trees may help us to better understand tree behaviour in cold environments.

In Planta Sporulation of Frankia spp. as a Determinant of Alder-Symbiont Interactions.

The Alnus genus forms symbiosis with the actinobacteria Frankia spp. and ectomycorrhizal fungi. Two types of Frankia lineages can be distinguished based on their ability to sporulate in planta Spore-positive (Sp+) strains are predominant on Alnus incana and Alnus viridis in highlands, while spore-negative (Sp-) strains are mainly associated with Alnus glutinosa in lowlands. Here, we investigated whether the Sp+ predominance in nodules is due to host selection of certain Frankia genotypes from soil communities or the result of the ecological history of the alder stand soil, as well as the effect of the sporulation genotype on the ectomycorrhizal (ECM) communities. Trapping experiments were conducted using A. glutinosa, A. incana, and A. viridis plantlets on 6 soils, differing in the alder species and the frequency of Sp+ nodules in the field. Higher diversity of Frankia spp. and variation in Sp+ frequencies were observed in the trapping than in the fields. Both indigenous and trapping species shape Frankia community structure in trapped nodules. Nodulation impediments were observed under several trapping conditions in Sp+ soils, supporting a narrower host range of Sp+ Frankia species. A. incana and A. viridis were able to associate equally with compatible Sp+ and Sp- strains in the greenhouse. Additionally, no host shift was observed for Alnus-specific ECM, and the sporulation genotype of Frankia spp. defined the ECM communities on the host roots. The symbiotic association is likely determined by the host range, the soil history, and the type of in plantaFrankia species. These results provide an insight into the biogeographical drivers of alder symbionts in the Holarctic region.IMPORTANCE Most Frankia-actinorhiza plant symbioses are capable of high rates of nitrogen fixation comparable to those found on legumes. Yet, our understanding of the ecology and distribution of Frankia spp. is still very limited. Several studies have focused on the distribution patterns of Frankia spp., demonstrating a combination of host and pedoclimatic parameters in their biogeography. However, very few have considered the in planta sporulation form of the strain, although it is a unique feature among all symbiotic plant-associated microbes. Compared with Sp- Frankia strains, Sp+ strains would be obligate symbionts that are highly dependent on the presence of a compatible host species and with lower efficiency in nitrogen fixation. Understanding the biogeographical drivers of Sp+ Frankia strains might help elucidate the ecological role of in planta sporulation and the extent to which this trait mediates host-partner interactions in the alder-Frankia-ECM fungal symbiosis.

Does genomic variation in a foundation species predict arthropod community structure in a riparian forest?

Understanding how genetic variation within a foundation species determines the structure of associated communities and ecosystem processes has been an emerging frontier in ecology. Previous studies in common gardens identified close links between intraspecific variation and multispecies community structure, and these findings are now being evaluated directly in the complex natural ecosystem. In this study, we examined to what extent genomic variation in a foundation tree species explains the structure of associated arthropod communities in the field, comparing with spatial, temporal and environmental factors. In a continuous mixed forest, arthropods were surveyed on 85 mature alders (Alnus hirsuta) in 2 years. Moreover, we estimated Nei's genetic distance among the alders based on 1,077 single nucleotide polymorphisms obtained from restricted-site-associated DNA sequencing of the alders' genome. In both years, we detected significant correlations between genetic distance and dissimilarity of arthropod communities. A generalized dissimilarity modelling indicated that the genetic distance of alder populations was the most important predictor to explain the variance of arthropod communities. Among arthropod functional groups, carnivores were consistently correlated with genetic distance of the foundation species in both years. Furthermore, the extent of year-to-year changes in arthropod communities was more similar between more genetically closed alder populations. This study demonstrates that the genetic similarity rule would be primarily prominent in community assembly of plant-associated arthropods under temporally and spatially variable environments in the field.

Nitrogen acquisition, net production and allometry of Alnus fruticosa at a young moraine in Koryto Glacier Valley, Kamchatka, Russian Far East.

Alders (Alnus spp.) often dominate at nutrient-poor sites by symbiotic relations with atmospheric nitrogen-fixing bacteria. However, little is known about quantitative relationships between root nodule as a nitrogen acquisition organ and leaf as a carbon acquisition organ. To examine carbon allocation, nitrogen acquisition and net production in nutrient-poor conditions, we examined allocation patterns among organs of shrub Alnus fruticosa at a young 80-year-old moraine in Kamchatka. Slopes of double-log allometric equations were significantly smaller than 1.0 for the root mass, leaf mass and root nodule mass against stem mass, and for the root nodule mass against root mass, indicating that smaller individuals invested disproportionally more biomass into resource-acquiring leaf and root tissues than to supportive tissues compared to older individuals. The slope of allometric equation of root depth against stem height was 0.542, indicating that smaller/younger individuals allocate disproportionally more biomass into root length growth than stem height growth. On the contrary, the root nodule mass isometrically scaled to leaf mass. The whole-plant nitrogen content also isometrically scaled to root nodule mass, indicating that a certain ratio of nitrogen acquisition depended on root nodules, irrespective of plant size. Although the net production per plant increased with the increase in stem mass, the slope of the double-log regression was smaller than 1.0. On the contrary, the net production per plant isometrically increased with leaf mass, root nodule mass and leaf nitrogen content per plant. Since the leaf mass isometrically scaled to root nodule mass, growth of each individual occurred at the leaves and root nodules in a coordinated manner. It is suggested that their isometric increase contributes to the increase in net production per plant for A. fruticosa in nutrient-poor conditions.

Absence of net long-term successional facilitation by alder in a boreal Alaska floodplain.

Long-term experiments provide a way to test presumed causes of successional or environmentally driven vegetation changes. Early-successional nitrogen (N)-fixing plants are widely thought to facilitate productivity and vegetation development on N-poor sites, thus accounting for observed vegetation patterns later in succession. We tested this facilitative impact on vegetation development in a 23-yr field experiment on an Interior Alaska (USA) floodplain. On three replicate early-successional silt bars, we planted late-successional white spruce (Picea glauca) seedlings in the presence and absence of planted seedlings of an early-successional N-fixing shrub, thinleaf alder (Alnus incana). Alder initially facilitated survivorship and growth of white spruce. Within six years, however, after canopy closure, alder negatively affected spruce survivorship and growth. Our three replicate sites followed different successional trajectories. One site was eliminated by erosion and supported no vegetation development during our study. The other two sites, which differed in site moisture, diverged in vegetation composition. Structural equation modeling (SEM) suggested that, in the drier of these two sites, alder inhibited spruce growth directly (presumably by competition) and indirectly through effects mediated by competition with other woody species. However, at the wetter site, alder had both positive and negative effects on spruce growth, with negative effects predominating. Snowshoe hares (Lepus americanus) in alder thickets further reduced height growth of spruce in the wetter site. We conclude that net effects of alder on white spruce, the late-successional dominant, were primarily inhibitory and indirect, with the mechanisms depending on initial site moisture. Our results highlight the importance of long-term research showing that small differences among initial replicate sites can cause divergence in successional trajectories, consistent with individualistic distributions of species and communities along environmental gradients. This divergence was detectable only decades later.

Evolution, plasticity and evolving plasticity of phenology in the tree species Alnus glutinosa.

Both traits and the plasticity of these traits are subject to evolutionary change and therefore affect the long-term persistence of populations and their role in local communities. We subjected clones from 12 different populations of Alnus glutinosa, located along a latitudinal gradient, to two different temperature treatments, to disentangle the distribution of genetic variation in timing of bud burst and bud burst plasticity within and among genotypes, populations, and regions. We calculated heritability and evolvability estimates for bud burst and bud burst plasticity and assessed the influence of divergent selection relative to neutral drift. We observed higher levels of heritability and evolvability for bud burst than for its plasticity, whereas the total phenological heritability and evolvability (i.e. combining timing of bud burst and bud burst plasticity) suggest substantial evolutionary potential with respect to phenology. Earlier bud burst was observed for the low-latitudinal populations than for the populations from higher latitudes, whereas the high-latitudinal populations did not show the expected delayed bud burst. This countergradient variation can be due to evolution towards increased phenological plasticity at higher latitudes. However, because we found little evidence for adaptive differences in phenological plasticity across the latitudinal gradient, we suggest differential frost tolerance as the most likely explanation for the observed phenological patterns in A. glutinosa.

Cascading effects of induced terrestrial plant defences on aquatic and terrestrial ecosystem function.

Herbivores induce plants to undergo diverse processes that minimize costs to the plant, such as producing defences to deter herbivory or reallocating limited resources to inaccessible portions of the plant. Yet most plant tissue is consumed by decomposers, not herbivores, and these defensive processes aimed to deter herbivores may alter plant tissue even after detachment from the plant. All consumers value nutrients, but plants also require these nutrients for primary functions and defensive processes. We experimentally simulated herbivory with and without nutrient additions on red alder (Alnus rubra), which supplies the majority of leaf litter for many rivers in western North America. Simulated herbivory induced a defence response with cascading effects: terrestrial herbivores and aquatic decomposers fed less on leaves from stressed trees. This effect was context dependent: leaves from fertilized-only trees decomposed most rapidly while leaves from fertilized trees receiving the herbivory treatment decomposed least, suggesting plants funnelled a nutritionally valuable resource into enhanced defence. One component of the defence response was a decrease in leaf nitrogen leading to elevated carbon : nitrogen. Aquatic decomposers prefer leaves naturally low in C : N and this altered nutrient profile largely explains the lower rate of aquatic decomposition. Furthermore, terrestrial soil decomposers were unaffected by either treatment but did show a preference for local and nitrogen-rich leaves. Our study illustrates the ecological implications of terrestrial herbivory and these findings demonstrate that the effects of selection caused by terrestrial herbivory in one ecosystem can indirectly shape the structure of other ecosystems through ecological fluxes across boundaries.

Testing the link between community structure and function for ectomycorrhizal fungi involved in a global tripartite symbiosis.

Alnus trees associate with ectomycorrhizal (ECM) fungi and nitrogen-fixing Frankia bacteria and, although their ECM fungal communities are uncommonly host specific and species poor, it is unclear whether the functioning of Alnus ECM fungal symbionts differs from that of other ECM hosts. We used exoenzyme root tip assays and molecular identification to test whether ECM fungi on Alnus rubra differed in their ability to access organic phosphorus (P) and nitrogen (N) when compared with ECM fungi on the non-Frankia host Pseudotsuga menziesii. At the community level, potential acid phosphatase (AP) activity of ECM fungal root tips from A. rubra was significantly higher than that from P. menziesii, whereas potential leucine aminopeptidase (LA) activity was significantly lower for A. rubra root tips at one of the two sites. At the individual species level, there was no clear relationship between ECM fungal relative root tip abundance and relative AP or LA enzyme activities on either host. Our results are consistent with the hypothesis that ECM fungal communities associated with Alnus trees have enhanced organic P acquisition abilities relative to non-Frankia ECM hosts. This shift, in combination with the chemical conditions present in Alnus forest soils, may drive the atypical structure of Alnus ECM fungal communities.

Do climate and soil influence phenotypic variability in leaf litter, microbial decomposition and shredder consumption?

We tested the hypothesis that water stress and soil nutrient availability drive leaf-litter quality for decomposers and detritivores by relating chemical and physical leaf-litter properties and decomposability of Alnus glutinosa and Quercus robur, sampled together with edaphic parameters, across wide European climatic gradients. By regressing principal components analysis of leaf traits [N, P, condensed tannins, lignin, specific leaf area (SLA)] against environmental and soil parameters, we found that: (1) In Q. robur the condensed tannin and lignin contents increased and SLA decreased with precipitation, annual range of temperature, and soil N content, whereas leaf P increased with soil P and temperature; (2) In A. glutinosa leaves N, P, and SLA decreased and condensed tannins increased with temperature, annual range of temperature, and decreasing soil P. On the other hand, leaf P and condensed tannins increased and SLA decreased with minimum annual precipitation and towards sites with low temperature. We selected contrasting leaves in terms of quality to test decomposition and invertebrate consumption. There were intraspecific differences in microbial decomposition rates (field, Q. robur) and consumption by shredders (laboratory, A. glutinosa). We conclude that decomposition rates across ecosystems could be partially governed by climate and soil properties, affecting litter quality and therefore decomposers and detritivores. Under scenarios of global warming and increased nutrients, these results suggest we can expect species-specific changes in leaf-litter properties most likely resulting in slow decomposition with increased variance in temperatures and accelerated decomposition with P increase.

Transcriptomes of Frankia sp. strain CcI3 in growth transitions.

BACKGROUND: Frankia sp. strains are actinobacteria that form N2-fixing root nodules on angiosperms. Several reference genome sequences are available enabling transcriptome studies in Frankia sp. Genomes from Frankia sp. strains differ markedly in size, a consequence proposed to be associated with a high number of indigenous transposases, more than 200 of which are found in Frankia sp. strain CcI3 used in this study. Because Frankia exhibits a high degree of cell heterogeneity as a consequence of its mycelial growth pattern, its transcriptome is likely to be quite sensitive to culture age. This study focuses on the behavior of the Frankia sp. strain CcI3 transcriptome as a function of nitrogen source and culture age. RESULTS: To study global transcription in Frankia sp. CcI3 grown under different conditions, complete transcriptomes were determined using high throughput RNA deep sequencing. Samples varied by time (five days vs. three days) and by culture conditions (NH4+ added vs. N2 fixing). Assembly of millions of reads revealed more diversity of gene expression between five-day and three-day old cultures than between three day old cultures differing in nitrogen sources. Heat map analysis organized genes into groups that were expressed or repressed under the various conditions compared to median expression values. Twenty-one SNPs common to all three transcriptome samples were detected indicating culture heterogeneity in this slow-growing organism. Significantly higher expression of transposase ORFs was found in the five-day and N2-fixing cultures, suggesting that N starvation and culture aging provide conditions for on-going genome modification. Transposases have previously been proposed to participate in the creating the large number of gene duplication or deletion in host strains. Subsequent RT-qPCR experiments confirmed predicted elevated transposase expression levels indicated by the mRNA-seq data. CONCLUSIONS: The overall pattern of gene expression in aging cultures of CcI3 suggests significant cell heterogeneity even during normal growth on ammonia. The detection of abundant transcription of nif (nitrogen fixation) genes likely reflects the presence of anaerobic, N-depleted microsites in the growing mycelium of the culture, and the presence of significantly elevated transposase transcription during starvation indicates the continuing evolution of the Frankia sp. strain CcI3 genome, even in culture, especially under stressed conditions. These studies also sound a cautionary note when comparing the transcriptomes of Frankia grown in root nodules, where cell heterogeneity would be expected to be quite high.

Physiological performance of an Alaskan shrub (Alnus fruticosa) in response to disease (Valsa melanodiscus) and water stress.

• Following the decades-long warming and drying trend in Alaska, there is mounting evidence that temperature-induced drought stress is associated with disease outbreaks in the boreal forest. Recent evidence of this trend is an outbreak of Cytospora canker disease (fungal pathogen Valsa melanodiscus (anamorph = Cytospora umbrina)) on Alnus species. • For Alnus fruticosa, we examined the effects of water stress on disease predisposition, and the effects of disease and water stress on host physiology. In two trials, we conducted a full-factorial experiment that crossed two levels of water stress with three types of inoculum (two isolates of V. melanodiscus, one control isolate). • Water stress was not required for disease predisposition. However, the effects of water stress and disease on host physiology were greatest near the peak phenological stage of the host and during hot, dry conditions. During this time, water stress and disease reduced light-saturated photosynthesis (-30%), light saturation point (-60%) and stomatal conductance (-40%). • Our results depended on the timing of water stress and disease in relation to host phenology and the environment. These factors should not be overlooked in attempts to generalize predictions about the role of temperature-induced drought stress in this pathosystem.

Volatile emissions from Alnus glutionosa induced by herbivory are quantitatively related to the extent of damage.

Plant volatile organic compounds (VOCs) elicited in response to herbivory serve as cues for parasitic and predatory insects. Knowledge about quantitative relationships between the extent of herbivore-induced damage and the quantities of VOCs released is scarce. We studied the kinetics of VOC-emissions from foliage of the deciduous tree Alnus glutinosa induced by feeding activity of larvae of the geometrid moth Cabera pusaria. Quantitative relationships between the intensity of stress and strength of plant response were determined. Intensity of biotic stress was characterized by herbivore numbers (0-8 larvae) and by the amount of leaf area eaten. The strength of plant response was characterized by monitoring (i) changes in photosynthesis, (ii) leaf ultrastructure, and (iii) plant volatiles. Net assimilation rate displayed compensatory responses in herbivore-damaged leaves compared with control leaves. This compensatory response was associated with an overall increase in chloroplast size. Feeding-induced emissions of products of the lipoxygenase pathway (LOX products; (E)-2-hexenal, (Z)-3-hexenol, 1-hexanol, and (Z)-3-hexenyl acetate) peaked at day 1 after larval feeding started, followed by an increase of emissions of ubiquitous monoterpenes peaking on days 2 and 3. The emission of the monoterpene (E)-ß-ocimene and of the nerolidol-derived homoterpene 4,8-dimethyl-nona-1,3,7-triene (DMNT) peaked on day 3. Furthermore, the emission kinetics of the sesquiterpene (E,E)-α-farnesene tended to be biphasic with peaks on days 2 and 4 after start of larval feeding. Emission rates of the induced LOX products, of (E)-ß-ocimene and (E,E)-α-farnesene were positively correlated with the number of larvae feeding. In contrast, the emission of DMNT was independent of the number of feeders. These data show quantitative relationships between the strength of herbivory and the emissions of LOX products and most of the terpenoids elicited in response to feeding. Thus, herbivory-elicited LOX products and terpenoid emissions may convey both quantitative and qualitative signals to antagonists of the herbivores. In contrast, our data suggest that the feeding-induced homoterpene DMNT conveys the information "presence of herbivores" rather than information about the quantities of herbivores to predators and parasitoids.

Flooding induced emissions of volatile signalling compounds in three tree species with differing waterlogging tolerance.

To gain insight into variations in waterlogging responsiveness, net assimilation rate, stomatal conductance, emissions of isoprene and marker compounds of anoxic metabolism ethanol and acetaldehyde, and stress marker compounds nitric oxide (NO), volatile products of lipoxygenase (LOX) pathway and methanol were studied in seedlings of temperate deciduous tree species Alnus glutinosa, Populus tremula and Quercus rubra (from highest to lowest waterlogging tolerance) throughout sustained root zone waterlogging of up to three weeks. In all species, waterlogging initially resulted in reductions in net assimilation and stomatal conductance and enhanced emissions of ethanol, acetaldehyde, NO, LOX products and methanol, followed by full or partial recovery depending on process and species. Strong negative correlations between g(s) and internal NO concentration and NO flux, valid within and across species, were observed throughout the experiment. Isoprene emission capacity was not related to waterlogging tolerance. Less waterlogging tolerant species had greater reduction and smaller acclimation capacity in foliage physiological potentials, and larger emission bursts of volatile stress marker compounds. These data collectively provide encouraging evidence that emissions of volatile organics and NO can be used as quantitative measures of stress tolerance and acclimation kinetics in temperate trees.

Host species and habitat affect nodulation by specific Frankia genotypes in two species of Alnus in interior Alaska.

Alders (Alnus spp.) are important components of northern ecosystems due to their ability to fix nitrogen (N) in symbiosis with Frankia bacteria. Availability of optimal Frankia may be a contributing factor in limiting the performance and ecological effects of Alnus, but the factors underlying distribution of Alnus-infective Frankia are not well understood. This study examined the genetic structure (nifD-K spacer RFLP haplotypes) of Frankia assemblages symbiotic with two species of Alnus (A. tenuifolia and A. viridis) in four successional habitats in interior Alaska. We used one habitat in which both hosts occurred to observe differences between host species independent of habitat, and we used replicate sites for each habitat and host to assess the consistency of symbiont structure related to both factors. We also measured leaf N content and specific N-fixation rate (SNF) of nodules ((15)N uptake) to determine whether either covaried with Frankia structure, and whether Frankia genotypes differed in SNF in situ. Frankia structure differed between sympatric hosts and among habitats, particularly for A. tenuifolia, and was largely consistent among replicate sites representing both factors. Leaf N differed between host species and among habitats for both hosts. SNF did not differ among habitats or host species, and little evidence for differences in SNF among Frankia genotypes was found, due largely to high variation in SNF. Consistency of Frankia structure among replicate sites suggests a consistent relationship between both host species and habitat among these sites. Correlations with specific environmental variables and possible underlying mechanisms are discussed.

Alnus sibirica encroachment promotes dissolved organic carbon biodegradation in a boreal peatland.

Symbiotic dinitrogen (N2)-fixing trees have been expanding to boreal peatlands, yet its influence on dissolved organic carbon (DOC) biodegradation is unclear. Here, we measured DOC, ammonium­nitrogen (NH4+-N), nitrate­nitrogen (NO3--N), dissolved inorganic nitrogen (DIN), and dissolved total nitrogen (DTN) concentrations, specific ultraviolet absorbance at 254 nm (SUVA254), and humification index in the extracts obtained from peats in the 0-10 cm, 10-20 cm, and 20-40 cm depths in the open peatlands and Alnus sibirica islands in a boreal peatland, Northeast China. Afterwards, the peat extracts were used to assess the effect of N2-fixing woody plant expansion on DOC biodegradation with a 42-day incubation experiment. The expansion of A. sibirica significantly increased NH4+-N, NO3--N, DIN, and DTN concentrations, but did not produce a significant effect on SUVA254 and humification index in the extracts in each depth. Following A. sibirica expansion, DOC biodegradation was enhanced by 24.5%, 15.4%, and 38.3% at 0-10 cm, 10-20 cm, and 20-40 cm depths, respectively. Furthermore, DOC biodegradation was significantly and negatively correlated with DOC:DIN and DOC:DTN ratios, but exhibited no significant relationship with SUVA254 and humification index. This implied that improved N availability and associated shifts in C:N stoichiometry determined the increase in DOC biodegradation following A. sibirica expansion. Our findings suggest that N2-fixing tree encroachment promotes microbial decomposition of DOC through improved N availability in boreal peatlands, which may cause organic C loss from soils in these C-enriched ecosystems.

Photosynthetic and leaf water potential responses of Alnus glutinosa saplings to stem-base inoculaton with Phytophthora alni subsp. alni.

Three-year-old Alnus glutinosa (L.) Gaertn. (alder) saplings were single or double inoculated at the stem base with Phytophthora alni subsp. alni Brasier & S.A. Kirk under natural climatic conditions. Lesion formation on the bark showed a biphasic pattern of development, with extension occurring at a moderate rate in spring, and more rapidly during late summer. However, large variability was encountered in pathogen development within the population of infected saplings, ranging from high susceptibility to almost complete resistance. Infection resulted in severe growth retardation, and death within two years of inoculation in 75% of the saplings. During disease development, rates of transpiration and CO(2) uptake were significantly reduced. Consequently, minimum leaf water potentials were less negative in infected saplings than in control saplings. Surviving saplings matched control trees in photosynthetic capacity, transpiration rate and water potential during the second year of infection. Leaf starch concentration of infected saplings was significantly higher than in control saplings, possibly indicating that the destruction of bark tissue by the pathogen impaired phloem transport from leaves to roots.

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Soil microbial residues are important components of soil stable carbon (C) pools. How N-fixing tree species influence microbial residues in soil aggregates in larch plantations is still unclear. To determine the effects of N-fixing tree species on the distribution of microbial residues in different aggregates, we compared the distribution of amino sugars, biomarker of microbial residues, among aggregates in a pure larch (Larix kaempferi) plantation and a mixed plantation of larch (Larix kaempferi) and alder (Alnus sibirica) in eastern Liaoning Province. The results showed that alder did not affect the distribution of amino sugars, but significantly increased amino sugars content in soil aggregates. The total amino sugars in different soil aggregates were enriched by 130%-170% in the mixed larch plantation compared with those in pure larch plantation. The contributions of glucosamine, galactosamine and muramic acid to the increases of total amino sugars caused by alder introduction were 66.5%-66.9%, 30.0%-30.6% and 2.5%-3.2%, respectively. Alder introduction significantly accelerated the glucosamine/muramic acid ratios in >2000 µm and <250 µm aggregates, but not in 250-2000 µm aggregates. Moreover, alder introduction increased the microbial contribution to soil organic C in all aggregates, but did not influence this contribution among aggregates, indicating that the effects of alder introduction on microbial contribution to aggregates were homogeneous.

Models of the spring phenology of boreal and temperate trees: Is there something missing?

According to prevailing theory, air temperature is the main environmental factor regulating the timing of bud burst of boreal and temperate trees. Air temperature has a dual role in this regulation. First, after the cessation of growth in autumn, prolonged exposure to chilling causes rest completion, i.e., removes the physiological growth-arresting conditions inside the bud. After rest completion, prolonged exposure to warm conditions causes ontogenetic development leading to bud burst or flowering. During the past three decades, several simulation models based on chilling and forcing have been developed and tested. In recent modeling studies of the timing of bud burst in mature trees, the simpler thermal-time models that assume forcing starts on a fixed date in the spring have outperformed the chilling-forcing models. We hypothesize that this discrepancy may be due to some element missing from the chilling-forcing models. We tested two new model formulations by introducing reversing, temperature-driven elements that precede forcing and by fitting the models to seven historical time series of data of flowering and leaf bud burst of common boreal tree species. In these tests, both of the new models were generally more accurate in predicting the timing of bud burst than a classical chilling-forcing model, but less accurate than the simple thermal-time model. We therefore conclude that besides chilling, other environmental factors are involved in the regulation of the timing of bud burst. Further work is needed to determine if the regulatory factors derive from air temperature or from some other environmental condition such as changes in light conditions, like day length or night length.

The dynamics of the Corylus, Alnus, and Betula pollen seasons in the context of climate change (SW Poland).

The changes in the main features of early spring tree or shrub pollen seasons are important due to the significant impact on the occurrence of pollen-related allergy symptoms. This study shows the results of pollen monitoring for a period of eleven years (2003-2013) using a Burkard volumetric spore trap. The main characteristics of the hazel, alder, and birch pollination season were studied in Wroclaw (SW Poland). The statistical analyses do not show a significant trend of annual total pollen count or shift in timing of the pollen season in the period of analysis. The research confirms a great impact (at the statistically significant level of 0.05) of the heat resources on pollination season (the value of the correlation coefficient ranges from -0.63 up to -0.87). Meteorological variables (e.g. sum of temperature for selected period) were compiled to 5-year running means to examine trends. Changes in the pollination period features due to climate change including both timing and intensity of pollen productivity, would have important consequences for allergy sufferers.