We studied the invasion of a New Zealand mountain beech (Nothofagus solandri var. cliffortioides) forest by the exotic perennial herb, Hieracium lepidulum. We used data from 250 randomly located permanent plots (400 m2) established in 1970 that sampled 9000 ha of forest. Frequency of H. lepidulum was 11%, 43%, and 57% in 1970, 1985, and 1993, respectively. For each year of measurement, invasion patterns were related to (a) distance to the forest margin as a measure of dispersal limitation, (b) community structure, (c) stem biomass dynamics indicating disturbance history, and (d) environmental characteristics. In 1970, invaded plots had more species and lower potential solar radiation, and they were closer to the forest margin; however, invaded plots were only weakly predicted by these site variables. H. lepidulum also invaded relatively species‐rich subplots (0.75 m2) showing that community structure was also significant at a microsite scale. Using the same sets of variables, the ability to predict which plots were invaded in any year increased from 1970 to 1993. This supports our hypothesis that in early invasion stages, with dispersal limitation, an invader may occur in only a subset of suitable sites giving a weak relationship with site variables. By 1993, distance to the forest margin was no longer related to which plots were invaded, and invaded plots had more species, occurred at lower elevations on more sheltered topographic positions, and had more fertile soils than uninvaded plots. Even though site variables were not independent (e.g., plots on fertile soils tend to have more species), multiple logistic regression showed that, all else being equal, invaded plots still tended to have more species than those not invaded. Our study therefore questions the hypothesis that, all else being equal, species‐poor habitats are more prone to invasion by exotic species.

Summary1. Mast seeding is the intermittent production of large quantities of seed across a perennial plant population. Such seeding events in many plant species are initiated by climatic cues, but whether these cues act solely as triggers or also via alterations to nutrient availability is unclear. 2. Here, we examine the effect of nitrogenous fertilization on the relationship between seed production in Nothofagus solandri var. cliffortioides and two climatic cues (rainfall and temperature) at specific stages in reproductive development from 1999 to 2008. 3. Foliar nitrogen concentrations were positively correlated with rainfall among years, suggesting rainfall was affecting nitrogen availability. 4. Seedfall mass in unfertilized stands was predominantly determined by rainfall during resource priming, while seed production in fertilized stands was more affected by temperature during floral primordia development. Similarly, seedfall mass in older stands, which contain greater internal nutrient reserves, was predominantly determined by temperature. 5. Synthesis. The results of this study demonstrate that the sensitivity of seed production to climatic cues can be altered by manipulation of resource availability and therefore establish that climatic cues involved in the synchronization of mast seeding can influence reproductive effort via an effect on resource availability. These results also indicate that alterations to resource availability have the potential to alter inter-annual patterns of seed production, but further study is required to verify this finding.

No abstract

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.Abstract. We studied the invasion of a New Zealand mountain beech (Nothofagus solandri var. cliffortioides) forest by the exotic perennial herb, Hieracium lepidulum. We used data from 250 randomly located permanent plots (400 M2) established in 1970 that sampled 9000 ha of forest. Frequency of H. lepidulum was 11%, 43%, and 57% in 1970, 1985, and 1993, respectively. For each year of measurement, invasion patterns were related to (a) distance to the forest margin as a measure of dispersal limitation, (b) community structure, (c) stem biomass dynamics indicating disturbance history, and (d) environmental characteristics. In 1970, invaded plots had more species and lower potential solar radiation, and they were closer to the forest margin; however, invaded plots were only weakly predicted by these site variables. H. lepidulum also invaded relatively species-rich subplots (0.75 M2) showing that community structure was also significant at a microsite scale. Using the same sets of variables, the ability to predict which plots were invaded in any year increased from 1970 to 1993. This supports our hypothesis that in early invasion stages, with dispersal limitation, an invader may occur in only a subset of suitable sites giving a weak relationship with site variables. By 1993, distance to the forest margin was no longer related to which plots were invaded, and invaded plots had more species, occurred at lower elevations on more sheltered topographic positions, and had more fertile soils than uninvaded plots. Even though site variables were not independent (e.g., plots on fertile soils tend to have more species), multiple logistic regression showed that, all else being equal, invaded plots still tended to have more species than those not invaded. Our study therefore questions the hypothesis that, all else being equal, species-poor habitats are more prone to invasion by exotic species.

To manage biosolids in a sustainable manner, there is a need for further research in the following areas: achieving a higher degree of public understanding and acceptance for the beneficial use of biosolids, developing cost-efficient and effective thermal conversions for direct energy recovery from biosolids, advancing technology for phosphorus recovery, and selecting or breeding crops for efficient biofuel production.

Stemwood production, N pools, and N availability were determined in even-aged (10, 25, 120, and >150-year-old) stands of a monospecific mountain beech (Nothofagus solandri var. cliffortioides (Hook. f.) Poole) forest in New Zealand recovering from catastrophic canopy disturbance brought about by windthrow. Nitrogen was redistributed among stemwood biomass, coarse woody debris (CWD), the forest floor, and mineral soil following disturbance. The quantity of N in stemwood biomass increased from less than 1 kg/ha in seedling stands (10 years old) to ca. 500 kg/ha in pole stands (120 years old), but decreased in mature stands (>150 years old). In contrast, the quantity of N stored in CWD declined rapidly with stand development. Although the mass of N stored in the forest floor was greatest in the pole stands and least in the mature stands, N availability in the forest floor did not vary greatly with stand development. The mass of N in the mineral soil (0–100 mm depth) was also similar for all stands. Foliar N concentrations, net N mineralization, and mineralizable N in the mineral soil (0–100 mm depth) showed similar patterns with stage of stand development, and indicated that N availability was greater in sapling (25 years old) and mature stands than in seedling and pole stands. We conclude that declining productivity in older stands is associated more with reductions in cation availability, especially calcium, than N availability.

No abstract

Microbial denitrification plays a key role in determining the availability of soil nitrogen (N) to plants. However, factors influencing the structure and function of denitrifier communities in the rhizosphere remain unclear. Waterlogging can result in root anoxia and increased denitrification, leading to significant N loss from soil and potential nitrous oxide (N(2)O) emissions. This study investigated denitrifier gene abundance, community structure and activity in the rhizosphere of wheat in response to anoxia and N limitation. Denitrifier community structure in the rhizosphere differed from that in bulk soil, and denitrifier gene copy numbers (nirS, nirK, nosZ) and potential denitrification activity were greater in the rhizosphere. Anoxia and N limitation, and in particular a combination of both, reduced the magnitude of this effect on gene abundance (in particular nirS) and activity, with N limitation having greater impact than waterlogging in rhizosphere soil, in contrast to bulk soil where the impact of waterlogging was greater. Increased N supply to anoxic plants improved plant health and increased rhizosphere soil pH, which resulted in enhanced reduction of N(2)O. Both anoxia and N limitation significantly influenced the structure and function of denitrifier communities in the rhizosphere, with reduced root-derived carbon postulated to play an important role.