Search

Red alder (Alnus rubra Bong.) is an ecologically significant and important fast-growing commercial tree species native to western coastal and riparian regions of North America, having highly desirable wood, pigment, and medicinal properties. We have sequenced the genome of a rapidly growing clone. The assembly is nearly complete, containing the full complement of expected genes. This supports our objectives of identifying and studying genes and pathways involved in nitrogen-fixing symbiosis and those related to secondary metabolites that underlie red alder's many interesting defense, pigmentation, and wood quality traits. We established that this clone is most likely diploid and identified a set of SNPs that will have utility in future breeding and selection endeavors, as well as in ongoing population studies. We have added a well-characterized genome to others from the order Fagales. In particular, it improves significantly upon the only other published alder genome sequence, that of Alnus glutinosa. Our work initiated a detailed comparative analysis of members of the order Fagales and established some similarities with previous reports in this clade, suggesting a biased retention of certain gene functions in the vestiges of an ancient genome duplication when compared with more recent tandem duplications. [ABSTRACT FROM AUTHOR]

Watershed nutrient balance studies traditionally focus on annual fluxes. In areas with strongly seasonal, Mediterranean-type climate regimes, riverine nutrient export may be greater during wet seasons when hydrologic forcing overwhelms or bypasses retention mechanisms. By combining data on riverine export with spatially detailed nutrient inputs, we examine how nitrogen (N) supply, retention, and streamflow shape annual and seasonal riverine N export in Oregon's Willamette River Basin (WRB). The WRB has pronounced dry summers and wet winters, and the distribution of farmland, cities and forests create significant spatial variations in N inputs. Local data on N inputs were coupled with streamflow and chemistry to calculate fractional N export for 22 WRB sub-watersheds in the mid-2000s. For the entire WRB, 78% of the N inputs came from agricultural activities, mainly as synthetic fertilizer (69%); the next largest inputs were deposition (10%), alder fixation (5%) and point sources (5%). Crop-specific estimates of fertilizer agreed with county fertilizer sales rates at the high end of extension recommendations. Fractional riverine N export (annual riverine N export / net watershed N inputs) averaged 38% of net inputs in WRB tributaries, greater than other regions of North America. Fall and winter together accounted for 60–90% of the riverine N export across all watersheds. Summer export was small but was greatest in the watersheds that receive seasonal snowmelt. Large wet season losses, when biotic sinks are less active, result in a relatively high proportion of N inputs exported in this region with a Mediterranean climate and high runoff. [ABSTRACT FROM AUTHOR]

Symbiotic nitrogen (N)-fixing trees supply significant N inputs to forest ecosystems, leading to increased soil fertility, forest growth, and carbon storage. Rapid growth and stoichiometric constraints of N fixers also create high demands for rock-derived nutrients such as phosphorus (P), while excess fixed N can generate acidity and accelerate leaching of rock-derived nutrients such as calcium (Ca). This ability of N-fixing trees to accelerate cycles of Ca, P, and other rock-derived nutrients has fostered speculation of a direct link between N fixation and mineral weathering in terrestrial ecosystems. However, field evidence that N-fixing trees have enhanced access to rock-derived nutrients is lacking. Here we use strontium (Sr) isotopes as a tracer of nutrient sources in a mixedspecies temperate rainforest to showthat N-fixing trees accessmore rock-derived nutrients than nonfixing trees. The N-fixing tree red alder (Alnus rubra), on average, took up 8 to 18%more rock-derived Sr than five co-occurring nonfixing tree species, including two with high requirements for rock-derived nutrients. The increased access to rock-derived nutrients occurred despite spatial variation in community- wide Sr sources across the forest, and only N fixers had foliar Sr isotopes that differed significantly from soil exchangeable pools. We calculate that increased uptake of rock-derived nutrients by N-fixing alder requires a 64% increase in weathering supply of nutrients over nonfixing trees. These findings provide direct evidence that an N-fixing tree species can also accelerate nutrient inputs from rock weathering, thus increasing supplies of multiple nutrients that limit carbon uptake and storage in forest ecosystems. [ABSTRACT FROM AUTHOR]

Abstract Wood is among the most decay-resistant cellulosic materials; however, all organics eventually succumb to decay. Understanding the degradation process is important when considering how products perform, but this knowledge can also be useful when determining how climates affect the organisms involved and the rates at which degradation occurs. Above ground wood decay tends to occur more slowly than decay in direct soil contact and is more dependent on climatic differences and wood species chemistries. While above ground wood deterioration is a subject of interest, most research has concentrated on the effects of the organisms on the wood. There are surprisingly few comprehensive studies of the fungi involved in the process and most have been performed under aggressive tropical conditions. Fungal colonization of red alder and Douglas-fir samples exposed above ground was evaluated over a 24-month period. Red alder is highly susceptible to fungal attack while Douglas-fir is moderately resistant. As such, they presented two differing selective media for fungal colonization. Red alder was more heavily colonized than Douglas-fir, reflecting the differing susceptibility of the two wood species to degradation. A similar number of Ascomycete and Basidiomycete species were isolated (11 vs 8 species, respectively), but Basidiomycetes were more frequently isolated. These results were similar to those found under more tropical conditions. The results illustrate the wide range of species that colonize wood exposed above ground. Highlights • Ascomycotina were more abundant than Basidiomycotina • White rot fungi were the predominant. • Alder was more heavily colonized than Douglas-fir. • Plant pathogens were among fungi in decaying wood. [ABSTRACT FROM AUTHOR]

Riparian-zone vegetation can influence terrestrial and aquatic food webs through variation in the amount, timing, and nutritional content of leaf and other litter inputs. We investigated how riparian- forest community composition, understory density, and lateral slope shaped vertical and lateral litter inputs to 16 streams in the Oregon Coast Range. Riparian forests dominated by deciduous red alder delivered greater annual vertical litter inputs to streams (504 g m-2 y-1) than did riparian forests dominated by coniferous Douglas-fir (394 g m-2y-1). Deciduous forests also contributed greater lateral litter inputs per meter of stream bank on one side (109 g m-1y-1) than did coniferous forests (63 g m-1y-1). Total litter inputs from deciduous forests exceeded those from coniferous forests most strongly in November, coincident with an autumn peak in litter inputs. Lateral litter inputs contributed most to total inputs during winter in both forest types. Annual lateral litter movement increased with slope at deciduous sites, but only in spring/ summer months at coniferous sites. Neither experimental removal of understory vegetation nor installation of mesh fences to block downslope litter movement affected lateral litter inputs to streams, suggesting that ground litter moves <5 m downslope annually. N concentrations of several litter fractions were higher at deciduous sites and, when combined with greater litter amounts, yielded twice as much total litter N flux to streams in deciduous than coniferous sites. The presence of red alder in riparian forests along many small streams of the deeply incised and highly dendritic basins of the Oregon Coast Range enhances total fluxes and seasonality of litter delivery to both terrestrial and aquatic food webs in this region and complements the shade and large woody debris provided by large coniferous trees. [ABSTRACT FROM AUTHOR]

Symbiotic N-fixing tree species can accelerate ecosystem N dynamics through decomposition feedbacks via both direct and indirect pathways. Direct pathways include the production of readily decomposed leaf litter and increased N supply to decomposers, whereas indirect pathways include increased tissue N and altered detrital dynamics of non-fixing vegetation. To evaluate the relative importance of direct and indirect pathways, we compared 3-year decomposition and N dynamics of N-fixing red alder leaf litter (2.34% N) to both low-N (0.68% N) and high-N (1.21% N) litter of non-fixing Douglas-fir, and decomposed each litter source in four forests dominated by either red alder or Douglas-fir. We also used experimental N fertilization of decomposition plots to assess elevated N availability as a potential mechanism of N-fixer effects on litter mass loss and N dynamics. Direct effects of N-fixing red alder on decomposition occurred primarily as faster N release from red alder than Douglas-fir litter. Direct increases in N supply to decomposers via experimental N fertilization did not stimulate decomposition of either species litter. Fixed N indirectly influenced detrital dynamics by increasing Douglas-fir tissue and litter N concentrations, which accelerated litter N release without accelerating mass loss. By increasing soil N, tissue N, and the rate of N release from litter of non-fixers, we conclude that N-fixing vegetation can indirectly foster plant-soil feedbacks that contribute to the persistence of elevated N availability in terrestrial ecosystems. [ABSTRACT FROM AUTHOR]

Background and aims: General theory of forest floor dynamics indicates convergence of properties during detrital decomposition. This study examined the hypothesis that nutrient stoichiometry, i.e. the relative amounts of nutrients, converges during litter decomposition. Methods: Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco) and red alder ( Alnus rubra Bong.) foliar litters that decomposed in their respective forests were analyzed for N, P, Ca, Mg, K, S, Mn, Fe, Zn, and Cu. A novel approach quantified the stoichiometric difference as the angle between nutrient vectors. The approach was also used to synthesize data from 11 previously published studies representing a broad array of litter types and locations. Results: The stoichiometries of the Douglas-fir and red alder litters converged during the first 2 years of decomposition, but diverged in the subsequent 4 years. This temporal trajectory was explained by two competing sets of processes: stoichiometric convergence occurs when different litters decompose in the same environment, and divergence occurs when the same litter decomposes in different environments. Manganese, Fe, and Ca were important contributors to stoichiometric differences. Conclusions: Stoichiometric convergence processes often dominate over divergence processes. Consideration of multi-nutrient stoichiometries may enhance the understanding of the functions of litter, including rates of decomposition and relative rates at which nutrients are released. [ABSTRACT FROM AUTHOR]

While aboveground biomass and forest productivity can vary over abiotic gradients (e.g., temperature and moisture gradients), biotic factors such as biodiversity and tree species stand dominance can also strongly influence biomass accumulation. In this study we use a permanent plot network to assess variability in aboveground carbon (C) flux in forest tree annual aboveground biomass increment (ABI), tree aboveground net primary productivity (ANPPtree), and net soil CO2 efflux in relation to diversity of coniferous, deciduous, and a nitrogen (N)-fixing tree species (Alnus rubra). Four major findings arose: (1) overstory species richness and indices of diversity explained between one third and half of all variation in measured aboveground C flux, and diversity indices were the most robust models predicting measured aboveground C flux; (2) trends suggested decreases in annual tree biomass increment C with increasing stand dominance for four of the five most abundant tree species; (3) the presence of an N-fixing tree species (A. rubra) was not related to changes in aboveground C flux, was negatively related to soil CO2 efflux, and showed only a weak negative relationship with aboveground C pools; and (4) stands with higher overstory richness and diversity typically had higher soil CO2 efflux. Interestingly, presence of the N-fixing species was not correlated with soil inorganic N pools, and inorganic N pools were not correlated with any C flux or pool measure. We also did not detect any strong patterns between forest tree diversity and C pools, suggesting potential balancing of increased C flux both into and out of diverse forest stands. These data highlight variability in second-growth forests that may have implications for overstory community drivers of C dynamics. [ABSTRACT FROM AUTHOR]

Hood Canal, Washington, USA, is a poorly ventilated fjord-like sub-basin of Puget Sound that commonly experiences hypoxia. This study examined the influence of watershed soils, vegetation, physical features, and population density on nitrogen (N) export to Hood Canal from 43 tributaries. We also linked our watershed study to the estuary using a salinity mass balance model that calculated the relative magnitude of N loading to Hood Canal from watershed, direct precipitation, and marine sources. The overall flow-weighted total dissolved N (TDN) and particulate N input concentrations to Hood Canal were 152 and 49 μg l, respectively. Nitrate and dissolved organic N comprised 64 and 29% of TDN, respectively. The optimal regression models for TDN concentration and areal yield included a land cover term suggesting an effect of N-fixing red alder ( Alnus rubra) and a human population density term (suggesting onsite septic system (OSS) discharges). There was pronounced seasonality in stream water TDN concentrations, particularly for catchments with a high prevalence of red alder, with the lowest concentrations occurring in the summer and the highest occurring in November-December. Due to strong seasonality in TDN concentrations and in particular stream flow, over 60% of the TDN export from this watershed occurred during the 3 month period of November-January. Entrainment of marine water into the surface layer of Hood Canal accounted for ≈98% of N loading to the euphotic zone of this estuary, and in a worst case scenario OSS N inputs contribute ≈0.5% of total N loading. Domestic wastewater discharges and red alders appear to be a very important N source for many streams, but a minor nutrient source for the estuary as a whole. [ABSTRACT FROM AUTHOR]

This study examined the effect of host species identity on the structure of Alnus-associated Frankia bacterial assemblages in the Pacific Northwest, United States, using two approaches. First, Frankia in nodules were sampled from six stands of Alnus rubra or Alnus viridis. Second, a bioassay was conducted where A. rubra and A. viridis seedlings were grown in different soils collected from these two hosts. Frankia genotypes were characterized with nifH sequences and bacterial assemblages were compared using taxon- and divergence-based analyses. Strong host associations were evident in the field; the dominant Frankia genotypes showed significant associations with either A. rubra or A. viridis, and there were host-associated groupings at the assemblage level as well. In the bioassay, host associations among Frankia genotypes were evident but less pronounced, reflecting an interaction between host species and other factors. Although nodule abundance varied among bioassay treatments, seedling dry mass was not strongly correlated with either nodule quantity or soil chemistry. Collectively, our observation and experimental results indicate that host identity is a major factor influencing the genotype composition and abundance of Alnus-associated Frankia assemblages. [ABSTRACT FROM AUTHOR]

Cross-boundary flows of energy and nutrients link biodiversity and functioning in adjacent ecosystems. The composition of forest tree species can affect the structure and functioning of stream ecosystems due to physical and chemical attributes, as well as changes in terrestrial resource subsidies. We examined how variation in riparian canopy composition (coniferous, deciduous, mixed) affects adjacent trophic levels (invertebrate and microbial consumers) and decomposition of organic matter in small, coastal rainforest streams in southwestern British Columbia. Breakdown rates of higher-quality red alder (Alnus rubra) litter were faster in streams with a greater percentage of deciduous than coniferous riparian canopy, whereas breakdown rates of lower-quality western hemlock (Tsuga heterophylIa) litter were independent of riparian forest composition. When invertebrates were excluded using fine mesh, breakdown rates of both litter species were an order of magnitude less and were not significantly affected by riparian forest composition. Stream invertebrate and microbial communities were similar among riparian forest composition, with most variation attributed to leaf litter species. Invertebrate taxa richness and shredder biomass were higher in A. rubra litter; however, taxa evenness was greatest for T. heerophylla litter and both litter species in coniferous streams. Microbial community diversity (determined from terminal restriction fragment length polymorphisms) was unaffected by riparian forest or litter species. Fungal allele richness was higher than bacterial allele richness, and microbial communities associated with lower-quality T. heterophylla litter had higher diversity (allele uniqueness and richness) than those associated with higher-quality A. rubra litter. Percent variation in breakdown rates was mostly attributed to riparian forest composition in the presence of invertebrates and microbes; however, stream consumer biodiversity at adjacent trophic levels did not explain these patterns. Riparian and stream ecosystems and their biotic communities are linked through exchange and decomposition of detrital resources, and we provide evidence that riparian forest composition affects stream ecosystem catabolism despite similarities in microbial and invertebrate communities. [ABSTRACT FROM AUTHOR]

To quantify the genetic diversity of Frankia bacteria associated with Alnus rubra in natural settings and to examine the relative importance of site age, management, and geographic location in structuring Frankia assemblages in A. rubra forests, root nodules from four A. rubra sites in the Pacific Northwest, USA were sampled. Frankia genetic diversity at each site was compared using sequence-based analyses of a 606 bp fragment of the nifH gene. At a 3% sequence similarity cutoff, a total of 5 Frankia genotypes were identified from 317 successfully sequenced nodules. Sites varied in the total number of genotypes present, but were typically dominated by only one or two genotypes. Phylogenetic analyses showed that all of the A. rubra-Frankia genotypes grouped with other Alnus-infective Frankia. Analysis of similarity (ANOSIM) and chi-square analyses indicated that Frankia assemblages were more strongly influenced by site age/management than geographic location. This study demonstrates that the Frankia assemblages in A. rubra forests have low genotype diversity, but that genotype abundance can differ significantly in forests of different age/management history. [ABSTRACT FROM AUTHOR]

Abstract: River flow regulation by dams for hydroelectric generation has been a common practice worldwide for centuries, with diverse ecological impacts. We studied upstream and downstream reaches of three British Columbia Hydro reservoirs on the Allouette, Coquitlam, and Cheakamus Rivers to determine if riparian plant communities were affected by the dams. We determined cover of all plants in consecutive 1m2 quadrats along five to six 14m-long belt transects in each 300m upstream and downstream sections of these rivers. We encountered 166 plant species in a total of 448 quadrats. Differences in plant communities (abundance, species richness, diversity) amongst rivers were larger than that between the upstream versus downstream of the rivers. This suggests that any difference caused by reservoir management was within the natural range of variation found across the rivers. We found significant reductions in the occurrences of red alder and western red cedar downstream of reservoirs. The lack of regeneration of these two species may be attributable to the reduction in extremes of flow variation and lack of sediment transport due to the reservoirs. Coordinated flow management can help mitigate these downstream impacts. [Copyright &y& Elsevier]

Abstract  Studies on Alnus rubra Bong. (red alder) were carried out to assess its potential for use as a component of a silvopastoral system. Comparison was between two treatments: red alder at 400 stems ha−1 (silvopasture) and red alder at 2,500 stems ha−1 (forestry control). δ15N values close to zero were recorded in all red alder plant parts except for root nodules, indicating that a large proportion of N in red alder was fixed from the atmosphere. Overall, it was estimated that there was 63.45 kg N ha−1 fixed N accumulated in red alder trees, and the rate of N fixation was estimated at 30.95 kg ha−1 year−1 in the silvopasture treatment. The total amount of N that could potentially be added to the soil in the silvopasture treatment as a result of decomposition of senescent leaves, roots, and dead nodules was estimated at 40.56 kg ha−1 year−1. Of the total N added to the soil, 27.1 kg ha−1 year−1 was due to N fixation from the atmosphere. These results show that red alder has a potential to improve and maintain soil fertility in a silvopastoral system. [ABSTRACT FROM AUTHOR]

To assess the effects of timber harvesting on headwater streams in upland forests, benthic community structure was contrasted among four dominant forest management types (old growth, red alder-dominated young growth, conifer-dominated young growth, clearcut) and instream habitats (woody debris, cobble, gravel) in southeastern Alaska. Benthos in streams of previously harvested areas resulted in increased richness, densities and biomass relative to old growth types, particularly in young growth stands with a red alder-dominated riparian canopy. Woody debris and gravel habitats supported a combination of higher densities and biomass of invertebrates than cobble habitats. In addition, woody debris also supported a richer and more diverse invertebrate fauna than either cobble or gravel substrates. Maintaining both a woody debris source and a red alder component in regenerating riparian forests following timber harvesting should support greater invertebrate densities and diversity following clearcutting. [ABSTRACT FROM AUTHOR]

1 We used repeated measurements of tree growth and population-level and neighbour- hood conditions from three mixed Alnus rubral Pseudotsuga menziesii forests in the Pacific Northwest, USA to investigate why previous results regarding the importance of neighbourhood competition as a determinant of plant growth were inconsistent 2 We propose that relative dominance ot a particular species determines the importance of neighbourhood interactions, and tested whether growth performance of both species at various stand ages agrees with this relative dominance hypothesis. 3 Neighbourhood and relative dominance interaction indices were modified to accommodate two-species mixtures and were incorporated into a growth model predicting relative diameter growth rates. The corrected Akaike Information Criterion (A/Cc) was used to identify the optimum interaction measures and model forms for each species and measurement period. 4 Interaction indices reflecting the sue of a tree relative to the population were the best predictors of growth of the dominant (i e taller) species and neighbourhood interaction indices of the subordinate species 5 Performance of interaction measures as predictors of relative growth rates in our study varied in agreement with the relative dominance hypothesis for both species and on all sites. Results from other studies suggest that the hypothesis may explain growth performances on a species and individual plant level and for a variety of' life forms. 6 These findings suggest that the spatial scale of plant interactions is influenced by the sue structure of plant populations. The relative dominance hypothesis offers a framework to provide insight into the mechanism of' competition, based on the relative performance of competition indices. [ABSTRACT FROM AUTHOR]

We investigated the influence of red alder (Alnus rubra) stand density in upland, riparian forests on invertebrate and detritus transport from fishless headwater streams to downstream, salmonid habitats in southeastern Alaska. Red alder commonly regenerates after soil disturbance (such as from natural landsliding or timber harvesting), and is common along streams in varying densities, but its effect on food delivery from headwater channels to downstream salmonid habitats is not clear. Fluvial transport of invertebrates and detritus was measured at 13 sites in spring, summer and fall during two years (2000–2001). The 13 streams encompassed a riparian red alder density gradient (1–82% canopy cover or 0–53% basal area) growing amongst young-growth conifer (45-yr-old stands that regenerated after forest clearcutting). Sites with more riparian red alder exported significantly more invertebrates than did sites with little alder (mean range across 1–82% alder gradient was about 1–4 invertebrates m-3 water, and 0.1–1 mg invertebrates m-3 water, respectively). Three-quarters of the invertebrates were of aquatic origin; the remainder was of terrestrial origin. Aquatic taxa were positively related to the alder density gradient, while terrestrially-derived taxa were not. Streams with more riparian alder also exported significantly more detritus than streams with less alder (mean range across 1–82% alder gradient was 0.01–0.06 g detritus m-3 water). Based on these data, we predict that headwater streams with more riparian alder will provide more invertebrates and support more downstream fish biomass than those basins with little or no riparian alder, provided these downstream food webs fully utilize this resource subsidy. [ABSTRACT FROM AUTHOR]

Most old fields in the Queers Valley of Olympic National Park, USA, remain dominated by exotic herbs 60 y after abandonment although the fields are surrounded by temperate rain forest. However, areas of some fields have been invaded by Picea sitchensis, one of three dominant forest species (with Alnus rubra and Tsuga heterophylla). This provided an opportunity to examine local variation of factors (competition, facilitation, cervid herbivory, soils) that influence tree colonization within a set of old fields, an approach rare in previous studies. Picea sapling invasion of field edges was negatively correlated with percent cover of Agrostis gigantea and positively correlated with Anthoxanthum odoratum. Potential indicators of competition (sod thickness, thatch thickness, percent ground cover) were correlated with Agrostis cover, Picea edge invasion was also correlated with soil organic matter. In experiments, seedlings of Picea and Tsuga emerged as readily in Agrostis as in Anthoxanthum or Pteridium aquilinum, but suffered higher mortality in Agrostis. Experimental seedling establishment was low and required reduction of competing vegetation. In experiments with transplanted seedlings, cervid herbivory suppressed growth of Tsuga and Alnus, but not Picea. Growth of Picea seedlings was facilitated by Pteridium. Differential tree colonization of the fields resulted from: (1) differential competition by invaded cover types against seedling establishment, (2) selective herbivory on tree species of established seedlings and (3) facilitation by fern cover of seedlings of an unbrowsed species. [ABSTRACT FROM AUTHOR]

Variations in plant community composition across the landscape can influence nutrient retention and loss at the watershed scale. A striking example of plant species importance is the influence of N2- fixing red alder (Alnus rubra) on nutrient cycling in the forests of the Pacific Northwest. To understand the influence of red alder on watershed nutrient export, we studied the chemistry of 26 small watershed streams within the Salmon River basin of the Oregon Coast Range. Nitrate and dissolved organic nitrogen (DON) concentrations were positively related to broadleaf cover (dominated by red alder: 94% of basal area), particularly when near-coastal sites were excluded (r² = 0.65 and 0.68 for nitrate-N and DON, respectively). Nitrate and DON concentrations were more strongly related to broadleaf cover within entire watersheds than broadleaf cover within the riparian area alone, which indicates that leaching from upland alder stands plays an important role in watershed nitrogen (N) export. Nitrate dominated over DON in hydrologic export (92% of total dissolved N), and nitrate and DON concentrations were strongly correlated. Annual N export was highly variable among watersheds (2.4- 30.8 kg N ha-1 y-1), described by a multiple linear regression combining broadleaf and mixed broad- leaf-conifer cover (r² = 0.74). Base cation concentrations were positively related to nitrate concentrations, which suggests that nitrate leaching increases cation losses. Our findings provide evidence for strong control of ecosystem function by a single plant species, where leaching from N saturated red alder stands is a major control on N export from these coastal watersheds. [ABSTRACT FROM AUTHOR]

Strong seasonal increases in aquatic (stream, ground and hyporheic water) nitrate have been observed in a variety of ecosystems. In most cases, changes in hydrological and vegetative activity occur contemporaneously, making it difficult to determine whether soil leaching is being driven by increases in the availability of leachable N or is simply due to flushing of N that has accumulated over longer periods. Three studies were conducted to better determine controls on soil nitrate leaching in a near-pristine temperate floodplain ecosystem receiving large N inputs via N-fixation by red alder: 1) an artificial rainfall experiment was conducted to estimate N-leaching potential during the summer, when plant uptake is high and new inputs of organic matter are low; 2) soil solution, groundwater and surface water were sampled during a major autumn storm to document exchanges at the seasonal transition, when plant uptake is low and inputs of senescent organic matter are high; and 3) monthly samples of soil and aquatic nitrogen were collected in 1997 and 1998 to document seasonal patterns of N exchanges. Collectively, these studies demonstrate the importance of hydrologic factors in controlling N flux. Nitrate was rapidly leached from soils during actual and simulated rainstorms. Two pathways of nitrate leaching were identified. Localized flooding and direct leaching of streamside soils into surface waters contributed to high solute concentrations in peak flows. Nitrate that leached into interstitial waters was subject to various factors that could delay or reduce its delivery to surface waters. Greater residence time may increase the influence of this component of stormflow on ecosystem productivity. While soil nitrate pools were rapidly depleted during rainstorms, accumulation of soil nitrate occurred over summer dry periods. Large differences in soil and aquatic nitrate concentrations between two years with contrasting rainfall highlight the potential for inter-annual hydrologic variability to affect ecosystem nutrient cycling. [ABSTRACT FROM AUTHOR]

Presents a study that examined the variation in seed production in catkins from red alder Alnus rubra. Determination of the total number of seed produced; Analysis of the number of viable seed produced; Relationship between seed number and percent viability.

Red alder (Alnus rubra Bong.) is a nitrogen-fixing pioneer tree species of the Pacific Northwest of North America. We investigated the response of different seed sources of red alder to elevated atmospheric CO2 and to varied levels of water stress. Seeds were stratified, germinated and grown for up to 147 d under ambient (350 μ l I-1) or elevated (700 μ l I-1) CO2. There were no significant interactions of seed source latitude with either treatment, although seedlings from more northerly sources were larger. Elevated CO2 and low moisture stress resulted in larger plants with more leaf area; effects of the two factors appeared additive. Effects of both factors on biomass allocation, including root: shoot ratios, were small or nonsignificant. Elevated CO2 decreased specific nitrogenase activity and generally increased photosynthesis (A) and stomatal conductance (g). The ratio A :g, potential water use efficiency, also increased when plants were under water stress. Elevated CO2 appears to improve drought tolerance in red alder. Overall, these results indicate that red alder would benefit in total plant growth from increased ambient CO2 and could tolerate changes in precipitation. [ABSTRACT FROM AUTHOR]

1. The monoculture self-thinning concept was expanded to polycultures by de- scribing a self-thinning surface in a mixed-species size-density space. The size- density relationships of pure and mixed populations of Alnus rubra and Pseudotsuga menziesii in the Pacific Northwest. USA were investigated. 2. For polycultures of A. rubra and P. menziesu. the average size-density surface showed a near-planar region for stands consisting principally of A. rubra. Thus. A. rubra dominates stand dynamics in this region. At high proportions of P. menziesii. the surface showed a curvilinear increase towards the relative density level of pure P. menziesii stands. The curvilinear shape of the self-thinning surface cautions against using a single self-thinning line for mixed stands where the species proportion changes over time. 3. The complexity of the size-density relationship indicates the need for a more thorough exploration of the underlying population dynamics. The growth model allowed investigation of mixed-stand development by identifying stand density. relative dominance of one species, and species proportions as important factors driving stand development along the self-thinning surface. 4. Simulations of the growth model showed the need to investigate the development of younger populations, the spatial pattern of mixtures. and the effects of dominance of one species on population development. [ABSTRACT FROM AUTHOR]

The acetylene reducing activity of Alnus rubra nodules is maximal in 15 to 30% oxygen. Within the nodule, the large number and size of many of the intercellular spaces between the uninfected cells in the outer cortex facilitates the diffusion of air through the lenticels to the infected cells in the mid-cortex. Access of air to the infected cells, which are arranged in longitudinal bundles separated by one or more lines of starch-containing uninfected cells and which run at an angle away from the stele towards the nodule apex, is restricted by the reduced number and size of air spaces in this region. The cytoplasm of the infected cells shows high activity of peroxidase and o-diphenol oxidase, but the lack of correlation between the activity of these enzymes and nitrogenase activity suggests that they are not involved in the control of access of oxygen to the endophyte. It is suggested that one role of nodule peroxidase is the ignification of the infected cells associated with cellular senescence. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Presents an abstract of the study `Early senescence and reduced gas exchanged caused by ozone in older leaves of red alder,' by C. Scott Clark et al. and presented during the 1995 Annual Meeting of the Botanical Society of America.

A correction to the article "N2-Fixing Red Alder Indirectly Accelerates Ecosystem Nitrogen Cycling" that was published in a previous issue is presented.