Variability in Leaf Area and Stemwood Increment Along a 300-year Lodgepole Pine Chronosequence

Large disturbances such as the 1988 Yellowstone fires produce considerable spatial heterogeneity in ecosystem processes across landscapes, in part by affecting vegetation structure. However, the persistence of this heterogeneity with time since disturbance, and thus the role of large disturbances in shaping the heterogeneity of ecosystem processes over large spatial and temporal scales, remains unclear. Such an inquiry requires that variability as well as mean conditions of forest structure and growth be examined if changes are to be projected for heterogeneous postdisturbance landscapes. We studied a chronosequence of unburned, mature lodgepole pine stands (stand ages ranging from 50 to 300 or more years) to examine the variability in stand density, leaf-area index (LAI), and stem growth [basal area increment (BAI), a surrogate for aboveground net primary productivity (ANPP)] with stand age, the relationships between these factors, and how these factors were related to stand and site characteristics. Variation in LAI and BAI was explained primarily by differences in stand density and age (r²=0.51 for both LAI and BAI), and both LAI and BAI were most variable in the youngest age class [coefficient of variation (CV), 38% and 41% for LAI and BAI]. The relationship between LAI or BAI and stand density was significantly weaker (r² < 0.20) at stand ages characterized by canopy closure (50–175 years), suggesting that stand structure and production are closely linked. Thus, the spatial variability of stand production, which is initially very high following large fires in this landscape, is detectable for over a century before successional changes in forest structure greatly affect the initial postdisturbance landscape pattern of stand production. Given the recent focus on spatial heterogeneity of ecosystem processes across large landscapes, projecting changes in postdisturbance patterns of stand production has very strong significance for ecosystem science.     

Water-level fluctuations in North American prairie wetlands

Oscillatory water-level fluctuations are reversible changes in water levels around a long-term mean. Long-term water-level studies in wetlands in the prairie pothole region of North America and proxy data (e.g., tree rings) for water levels in this region indicate that oscillatory water-level fluctuations have occurred for thousands of years. Because there has been no standard set of terms to describe oscillatory water-level fluctuations, some terminology is proposed that is based on previous work on riverine wetlands. Changes in prairie wetland vegetation caused by oscillating water levels are called wet–dry cycles. Field studies indicate that two kinds of vegetation change are common during wet–dry cycles, fluctuations and successions. Fluctuations are changes in the relative abundance of species between the wet and dry phases of the cycles. They occur whenever the range of water levels during a cycle is small (ca. 50 cm), as in seasonal wetlands. Succesions are changes in species composition. They occur wherever the range of water levels is large (ca. 1.5–2.0 m), as in semi-permanent wetlands. During successions, high water levels during the wet phase can typically eliminate most emergent species and low or no water during the dry phase allows emergent species to become re-established from seed and terrestrial annuals to dominate the vegetation. Experimental studies at the ecosystem- and species-level have confirmed observations made during field studies of semi-permanent wetlands, e.g., that water depth tolerance is the primary determinant of distribution of emergent species. Both qualitative and quantitative assembly-rule models of wet–dry cycles have been developed. When adequate data are available, the latest quantitative models can accurately predict changes in composition and distribution of emergent vegetation in semi-permanent wetlands during all or parts of a wet–dry cycle.     

Foliar and fungal <Superscript>15</Superscript> N:<Superscript>14</Superscript> N ratios reflect development of mycorrhizae and nitrogen supply during primary succession: testing analytical models

Nitrogen isotopes (¹⁵N/¹⁴N ratios, expressed as δ¹⁵N values) are useful markers of the mycorrhizal role in plant nitrogen supply because discrimination against ¹⁵N during creation of transfer compounds within mycorrhizal fungi decreases the ¹⁵N/¹⁴N in plants (low δ¹⁵N) and increases the ¹⁵N/¹⁴N of the fungi (high δ¹⁵N). Analytical models of ¹⁵N distribution would be helpful in interpreting δ¹⁵N patterns in fungi and plants. To compare different analytical models, we measured nitrogen isotope patterns in soils, saprotrophic fungi, ectomycorrhizal fungi, and plants with different mycorrhizal habits on a glacier foreland exposed during the last 100 years of glacial retreat and on adjacent non-glaciated terrain. Since plants during early primary succession may have only limited access to propagules of mycorrhizal fungi, we hypothesized that mycorrhizal plants would initially be similar to nonmycorrhizal plants in δ¹⁵N and then decrease, if mycorrhizal colonization were an important factor influencing plant δ¹⁵N. As hypothesized, plants with different mycorrhizal habits initially showed similar δ¹⁵N values (−4 to −6‰ relative to the standard of atmospheric N₂ at 0‰), corresponding to low mycorrhizal colonization in all plant species and an absence of ectomycorrhizal sporocarps. In later successional stages where ectomycorrhizal sporocarps were present, most ectomycorrhizal and ericoid mycorrhizal plants declined by 5–6‰ in δ¹⁵N, suggesting transfer of ¹⁵N-depleted N from fungi to plants. The values recorded (−8 to −11‰) are among the lowest yet observed in vascular plants. In contrast, the δ¹⁵N of nonmycorrhizal plants and arbuscular mycorrhizal plants declined only slightly or not at all. On the forefront, most ectomycorrhizal and saprotrophic fungi were similar in δ¹⁵N (−1 to −3‰), but the host-specific ectomycorrhizal fungus Cortinarius tenebricus had values of up to 7‰. Plants, fungi and soil were at least 4‰ higher in δ¹⁵N from the mature site than in recently exposed sites. On both the forefront and the mature site, host-specific ectomycorrhizal fungi had higher δ¹⁵N values than ectomycorrhizal fungi with a broad host range. From these isotopic patterns, we conclude:(1) large enrichments in ¹⁵N of many ectomycorrhizal fungi relative to co-occurring ectomycorrhizal plants are best explained by treating the plant-fungal-soil system as a closed system with a discrimination against ¹⁵N of 8–10‰ during transfer from fungi to plants, (2) based on models of ¹⁵N mass balance, ericoid and ectomycorrhizal fungi retain up to two-thirds of the N in the plant-mycorrhizal system under the N-limited conditions at forefront sites, (3) sporocarps are probably enriched in ¹⁵N by an additional 3‰ relative to available nitrogen, and (4) host-specific ectomycorrhizal fungi may transfer more N to plant hosts than non-host-specific ectomycorrhizal fungi. Our study confirms that nitrogen isotopes are a powerful tool for probing nitrogen dynamics between mycorrhizal fungi and associated plants.     

Influence of habitat,litter type,and soil invertebrates on leaf-litter decomposition in a fragmented Amazonian landscape

Amazonian forest fragments and second-growth forests often differ substantially from undisturbed forests in their microclimate, plant-species composition, and soil fauna. To determine if these changes could affect litter decomposition, we quantified the mass loss of two contrasting leaf-litter mixtures, in the presence or absence of soil macroinvertebrates, and in three forest habitats. Leaf-litter decomposition rates in second-growth forests (>10 years old) and in fragment edges (<100 m from the edge) did not differ from that in the forest interior (>250 m from the edges of primary forests). In all three habitats, experimental exclusion of soil invertebrates resulted in slower decomposition rates. Faunal-exclosure effects were stronger for litter of the primary forest, composed mostly of leaves of old-growth trees, than for litter of second-growth forests, which was dominated by leaves of successional species. The latter had a significantly lower initial concentration of N, higher C:N and lignin:N ratios, and decomposed at a slower rate than did litter from forest interiors. Our results indicate that land-cover changes in Amazonia affect decomposition mainly through changes in plant species composition, which in turn affect litter quality. Similar effects may occur on fragment edges, particularly on very disturbed edges, where successional trees become dominant. The drier microclimatic conditions in fragment edges and second-growth forests (>10 years old) did not appear to inhibit decomposition. Finally, although soil invertebrates play a key role in leaf-litter decomposition, we found no evidence that differences in the abundance, species richness, or species composition of invertebrates between disturbed and undisturbed forests significantly altered decomposition rates.     

Yeast population dynamics of industrial fuel-ethanol fermentation process assessed by PCR-fingerprinting

Yeast populations used in industrial production of fuel-ethanol may vary according to the plant process conditions and to the environmental stresses imposed on yeast cells. Therefore, yeast strains isolated from a particular industrial process may be adapted to such conditions and should be used as the starter strain instead of less adapted commercial strains. This work reports the use of a PCR-fingerprinting method based on microsatellite primer (GTG)₅ to characterize the yeast population dynamics during the fermentation period in six distilleries. The results show that indigenous fermenting strains present in the crude substrate can be more adapted to the industrial process than commercial strains. We also identified new strains that dominated the yeast population and were more commonly present either in molasses or sugar cane fermenting distilleries. Those strains were proposed to be used as starters in those industrial processes. This is the first report on the use of molecular markers to discriminate Saccharomyces cerevisiae strains from the fuel-ethanol producing process.     

Effect of solar near ultraviolet radiation on growth of Chlorella cells

Cells of Chlorella regularis (Artari) Oltmanns (S-50) were grown under solar radiation in Tokyo, using a newly constructed outdoor culture system. The maximum specific growth rate (log₂ unit h⁻¹) was about 0.3, which was a little lower than the highest value reported using artificial light in the laboratory. The near ultraviolet light, consisting mostly of UV-A, inhibited the growth from 10 to 40% in summer mornings; the inhibition was not significant in the afternoon. The percentage inhibition was correlated with the total dose of the ultraviolet radiation.     

The effect of different zooplankton grazing patterns resulting from diel vertical migration on phytoplankton growth and composition: a laboratory experiment

Diel vertical migration (DVM) of herbivorous zooplankton is a widespread behavioural phenomenon in freshwater ecosystems. So far only little attention has been paid to the impact of DVM on the phytoplankton community in the epilimnion. Some theoretical models predict that algal population growth in the epilimnion should depend on the herbivores migration and grazing patterns: even if migrating zooplankton consume the same total amount of algae per day in the epilimnion as non-migrating zooplankton, nocturnal grazing should result in enhanced algal growth and favour algal species with high intrinsic growth rates over species with lower intrinsic growth rates. To test these hypotheses we performed experiments in which several algal species were confronted with different feeding regimes of Daphnia. In the experiments algal growth did not only depend on the absolute time of grazing but was comparatively higher when grazing took place only during the night, even when the grazing pressure was the same. Furthermore, algal species with higher intrinsic growth rates had higher advantages when being grazed upon only discontinuously during the night than algal species with a smaller intrinsic growth rate. The grazing pattern itself was an important factor for relative algal performance.     

When is a cell not a cell? A theory relating coenocytic structure to the unusual electrophysiology of Ventricaria ventricosa (Valonia ventricosa)

Summary. Ventricaria ventricosa and its relatives have intrigued cell biologists and electrophysiologists for over a hundred years. Historically, electrophysiologists have regarded V. ventricosa as a large single plant cell with unusual characteristics including a small and positive vacuole-to-outside membrane potential difference. However, V. ventricosa has a coenocytic construction, with an alveolate cytoplasm interpenetrated by a complex vacuole containing sulphated polysaccharides. We present a theory relating the coenocytic structure to the unusual electrophysiology of V. ventricosa. The alveolate cytoplasm of V. ventricosa consists of a collective of uninucleate cytoplasmic domains interconnected by fine cytoplasmic strands containing microtubules. The cytoplasm is capable of disassociating into single cytoplasmic domains or aggregations of domains that can regenerate new coenocytes. The cytoplasmic domains are enclosed by outer (apical) and inner (basolateral) faces of a communal membrane with polarised K⁺-transporting functions, stabilised by microtubules and resembling a tissue such as a polarised epithelium. There is evidence for membrane trafficking through endocytosis and exocytosis and so plasmalemma and tonoplast do not have fixed identities. Intra- and extracellular polysaccharide mucilage has effects on electrophysiology through reducing the activity of water and through ion exchange. The vacuole-to-outside potential difference, at which the cell membrane conductance is maximal, reverses its sign from positive under hypertonic conditions to negative under hypotonic conditions. The marked mirror symmetry of the characteristics of current as a function of voltage and conductance as a function of voltage is interpreted as a feature of the communal membrane with polarised K⁺ transport. The complex inhomogeneous structure of the cytoplasm places in doubt previous measurements of cytoplasm-to-outside potential difference.Correspondence and reprints: UNESCO Centre for Membrane Science and Technology, Department of Biophysics, School of Physics, University of New South Wales, Sydney, NSW 2052, Australia.     

Spatial structure of genetic variation and primary succession in the pioneer tree species Antirhea borbonica on La Réunion

In habitats where colonization and extinction are recurrent, the distribution of gene frequencies among patches of suitable habitat may reflect the age structure of different populations. In this study, we quantify population genetic structure for a pioneer tree species, Antirhea borbonica, in a chrono-sequence of primary succession on the lava flows of the Piton de La Fournaise volcano (La Réunion). Using microsatellite loci and amplified fragment length polymorphism (AFLP) markers, we quantified genetic variation within and among populations for early- and late-succession populations in a landscape where extinction and recolonization are recurrent (the ‘Grand Brûlé’) and for late-succession populations in a more stable landscape. This study produced three main results. First, we detected no evidence that founder events increase genetic differentiation among colonizing populations; F_ST values among early- and among late-succession populations were similar. Second, we found no evidence for isolation by distance; genetic distance was not correlated with spatial distance within and among populations. Third, F_IS values are consistently high in all populations, despite the fact that A. borbonica populations are functionally close to dioecy and thus expected to have an outcrossing mating system. Multiple colonization events from different sources may limit differentiation among young populations and spatial isolation may enhance differentiation among late-succession populations. Ecological processes acting during colonization may create the conditions for spatial aggregation within pioneer populations, and thus contribute to the high F_IS values.     

Understanding the stoichiometric limitation of herbivore growth: the importance of feeding and assimilation flexibilities

Ecological stoichiometry suggests that herbivore growth is limited by phosphorus when this element in the diet is < 8.6 μg P mg C^?1 (C : P atomic ratio > 300). However, in nature, it is not necessarily related to the relative phosphorus content in diets. This may be the result of complex feeding and assimilation responses to diets. We examined these possibilities using herbivorous plankton fed mono‐specific and mixed algae varying in phosphorus content of 1.6 to 8.1 μg P mg C^?1. The herbivores showed a 10‐fold growth rate difference among the diets. Growth rates related poorly with phosphorus content in the diets (r² = 0.07), better with P ingestion rate (r² = 0.41) and best with phosphorus assimilation rate (r² = 0.69). Inclusion of assimilation rates for carbon and fatty acids increased 7% of the explained growth variance. These results indicate that the feeding and assimilation flexibilities play pivotal roles in acquiring a deficient element and in regulating growth rate.