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.

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.

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.

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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.

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.

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.

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.

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.

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.

High resource-capture and -use efficiency, and effective antioxidant protection contribute to the invasiveness of Alnus formosana plants.

To investigate the traits contributing to the invasiveness of Alnus formosana and the mechanisms underlying its invasiveness, we compared A. formosana with its native congener (Alnus cremastogyne) under three light treatments (13%, 56%, and 100%). The consistently higher plant height, total leaf area, light-saturated photosynthetic rate (A(max)), light saturation point (LSP), light compensation point (LCP), respiration efficiency (RE), and non-photochemical quenching coefficient (NPQ) but lower root mass fraction (RMF) and specific leaf area (SLA) of the invader than of its native congener contributed to the higher RGR and total biomass of A. formosana across light regimes. The total biomass and RGR of the invader increased markedly with increased RMF, A(max), LSP, LCP, RE, stomatal conductance (G(s)) and total leaf area. Furthermore, compared with the native species, the higher plasticity index in plant height, RMF, leaf mass fraction (LMF), SMF, SLA, A(max) and dark respiration rate (R(d)) within the range of total light contributed to the higher performance of the invader. In addition, the activities of antioxidant enzymes were higher in the invader compared to the native, contributing to its invasion success under high/low light via photoprotection. With a decrease in light level, superoxide dismutase (SOD) and catalase (CAT) activities increased significantly, whereas total carotenoid (Car) and total chlorophyll (Chl) decreased; ascorbate peroxidase (APX) and glutathione reductase (GR) activities remained unchanged. These responses may help the invader to spread and invade a wide range of habitats and form dense monocultures, displacing native plant species. The results suggest that both resource capture-related traits (morphological and photosynthetic) and adaptation-related traits (antioxidant protection) contribute to the competitive advantage of the invader.

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.

Alnus peptides modify membrane porosity and induce the release of nitrogen-rich metabolites from nitrogen-fixing Frankia.

Actinorhizal plant growth in pioneer ecosystems depends on the symbiosis with the nitrogen-fixing actinobacterium Frankia cells that are housed in special root organs called nodules. Nitrogen fixation occurs in differentiated Frankia cells known as vesicles. Vesicles lack a pathway for assimilating ammonia beyond the glutamine stage and are supposed to transfer reduced nitrogen to the plant host cells. However, a mechanism for the transfer of nitrogen-fixation products to the plant cells remains elusive. Here, new elements for this metabolic exchange are described. We show that Alnus glutinosa nodules express defensin-like peptides, and one of these, Ag5, was found to target Frankia vesicles. In vitro and in vivo analyses showed that Ag5 induces drastic physiological changes in Frankia, including an increased permeability of vesicle membranes. A significant release of nitrogen-containing metabolites, mainly glutamine and glutamate, was found in N2-fixing cultures treated with Ag5. This work demonstrates that the Ag5 peptide is central for Frankia physiology in nodules and uncovers a novel cellular function for this large and widespread defensin peptide family.

Optimization of the AC-gradient method for velocity profile measurement and application to slow flow.

This work presents a spectroscopic method to measure slow flow. Within a single shot the velocity distribution is acquired. This allows distinguishing rapidly between single velocities within the sampled volume with a high sensitivity. The technique is based on signal acquisition in the presence of a periodic gradient and a train of refocussing RF pulses. The theoretical model for trapezoidal bipolar pulse shaped gradients under consideration of diffusion and the outflow effect is introduced. A phase correction technique is presented that improves the spectral accuracy. Therefore, flow phantom measurements are used to validate the new sequence and the simulation based on the theoretical model. It was demonstrated that accurate parabolic flow profiles can be acquired and flow variations below 200 µm/s can be detected. Three post-processing methods that eliminate static background signal are also presented for applications in which static background signal dominates. Finally, this technique is applied to flow measurement of a small alder tree demonstrating a typical application of in vivo plant measurements.

Short-term water stress impacts on stomatal, mesophyll and biochemical limitations to photosynthesis differ consistently among tree species from contrasting climates.

Predicting the large-scale consequences of drought in contrasting environments requires that we understand how drought effects differ among species originating from those environments. A previous meta-analysis of published experiments suggested that the effects of drought on both stomatal and non-stomatal limitations to photosynthesis may vary consistently among species from different hydroclimates. Here, we explicitly tested this hypothesis with two short-term water stress experiments on congeneric mesic and xeric species. One experiment was run in Australia using Eucalyptus species and the second was run in Spain using Quercus species as well as two more mesic species. In each experiment, plants were grown under moist conditions in a glasshouse, then deprived of water, and gas exchange was monitored. The stomatal response was analysed with a recently developed stomatal model, whose single parameter g1 represents the slope of the relationship between stomatal conductance and photosynthesis. The non-stomatal response was partitioned into effects on mesophyll conductance (gm), the maximum Rubisco activity (Vcmax) and the maximum electron transport rate (Jmax). We found consistency among the drought responses of g1, gm, Vcmax and Jmax, suggesting that drought imposes limitations on Rubisco activity and RuBP regeneration capacity concurrently with declines in stomatal and mesophyll conductance. Within each experiment, the more xeric species showed relatively high g1 under moist conditions, low drought sensitivity of g1, gm, Vcmax and Jmax, and more negative values of the critical pre-dawn water potential at which Vcmax declines most steeply, compared with the more mesic species. These results indicate adaptive interspecific differences in drought responses that allow xeric tree species to continue transpiration and photosynthesis for longer during periods without rain.

Migration patterns of subgenus Alnus in Europe since the last glacial maximum: a systematic review.

BACKGROUND/AIMS: Recently, new palaeoecological records supported by molecular analyses and palaeodistributional modelling have provided more comprehensive insights into plant behaviour during the last Quaternary cycle. We reviewed the migration history of species of subgenus Alnus during the last 50,000 years in Europe with a focus on (1) a general revision of Alnus history since the Last Glacial Maximum (LGM), (2) evidence of northern refugia of Alnus populations during the LGM and (3) the specific history of Alnus in particular European regions. METHODOLOGY: We determined changes in Alnus distribution on the basis of 811 and 68 radiocarbon-dated pollen and macrofossil sites, respectively. We compiled data from the European Pollen Database, the Czech Quaternary Palynological Database, the Eurasian Macrofossil Database and additional literature. Pollen percentage thresholds indicating expansions or retreats were used to describe patterns of past Alnus occurrence. PRINCIPAL FINDINGS: An expansion of Alnus during the Late Glacial and early Holocene periods supports the presence of alders during the LGM in southern peninsulas and northerly areas in western Europe, the foothills of the Alps, the Carpathians and northeastern Europe. After glaciers withdrew, the ice-free area of Europe was likely colonized from several regional refugia; the deglaciated area of Scandinavia was likely colonized from a single refugium in northeastern Europe. In the more northerly parts of Europe, we found a scale-dependent pattern of Alnus expansion characterised by a synchronous increase of Alnus within individual regions, though with regional differences in the times of the expansion. In southern peninsulas, the Alps and the Carpathians, by contrast, it seems that Alnus expanded differently at individual sites rather than synchronously in whole regions. CONCLUSIONS: Our synthesis supports the idea that northern LGM populations were important sources of postglacial Alnus expansion. The delayed Alnus expansion apparent in some regions was likely a result of environmental limitations.

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.

Micromonospora is a normal occupant of actinorhizal nodules.

Actinorhizal plants have been found in eight genera belonging to three orders (Fagales, Rosales and Cucurbitales). These all bear root nodules inhabited by bacteria identified as the nitrogen-fixing actinobacterium Frankia. These nodules all have a peripheral cortex with enlarged cells filled with Frankia hyphae and vesicles. Isolation in pure culture has been notoriously difficult, due in a large part to the growth of fast-growing contaminants where, it was later found, Frankia was slow-growing. Many of these contaminants, which were later found to be Micromonospora, were obtained from Casuarina and Coriaria. Our study was aimed at determining if Micromonospora were also present in other actinorhizal plants. Nodules from Alnus glutinosa, Alnus viridis, Coriaria myrtifolia, Elaeagnus x ebbingei, Hippophae rhamnoides, Myrica gale and Morella pensylvanica were tested and were all found to contain Micromonospora isolates. These were found to belong to mainly three species: Micromonospora lupini, Micromonospora coriariae and Micromonospora saelicesensis. Micromonospora isolates were found to inhibit some Frankia strains and to be innocuous to other strains.

Growth and N2 fixation in an Alnus hirsuta (Turcz.) var. sibirica stand in Japan.

To estimate the N2 fixation ability of the alder (Alnus hirsuta (Turcz.) var. sibirica), we examined the seasonal variation in nitrogenase activity of nodules using the acetylene reduction method in an 18-year-old stand naturally regenerated after disturbance by road construction in Japan. To evaluate the contribution of N2 fixation to the nitrogen (N) economy in this alder stand, we also measured the phenology of the alder, the litterfall, the decomposition rate of the leaf litter, and N accumulation in the soil. The acetylene reduction activity per unit nodule mass (ARA) under field conditions appeared after bud break, peaked the maximum in midsummer after full expansion of the leaves, and disappeared after all leaves had fallen. There was no consistent correlation between ARA and tree size (dbh). The amount of N2 fixed in this alder stand was estimated at 56.4 kg ha-1 year-1 when a theoretical molar ratio of 3 was used to convert the amount of reduced acetylene to the amount of fixed N2. This amount of N2 fixation corresponded to the 66.4 percent of N in the leaf litter produced in a year. These results suggested that N2 fixation still contributed to the large portion of N economy in this alder stand.