The role of land plants,phosphorus weathering and fire in the rise and regulation of atmospheric oxygen

The evolution of vascular plants and their spread across the land surface, beginning ～420 Ma, progressively increased the rate of weathering of phosphorus from rocks. This phosphorus supply promoted terrestrial and marine productivity and the burial of organic carbon, which has been the major source of O₂ over geological timescales. Hence, it is predicted that the rise of plants led to an increase in the O₂ content of the atmosphere from ～12 vol %, 570–400 Ma to its present level of ～21 vol % by ～340 Ma. Previous modelling studies suggest that O₂ then rose to ～35 vol % ～300 Ma. Such high concentrations are difficult to reconcile with the known persistence of forests, because rising O₂ increases the frequency and intensity of vegetation fires, tending to decrease biomass and cause ecological shifts toward faster regenerating ecosystems. Rising O₂ also directly inhibits C3 photosynthetic carbon assimilation and increases the production of toxic reactive oxygen species in cells. These effects suppress plant‐induced phosphorus weathering and hence organic carbon burial, providing a sensitive negative feedback on O₂. A revised model predicts that this mechanism could have regulated atmospheric O₂ within the range 15–25 vol % for the last 350 million years.     

Early life stage toxicity of extracts from the African fish poison plants Tephrosia vogelii Hook. f. and Asystasia vogeliana Benth. on zebrafish embryos

Embryos of Danio rerio are highly susceptible to extracts of the plants Tephrosia vogelii and Asystasia vogeliana . The concentration of the dried extracts at which 50% of the embryos were affected (EC₅₀) after 24 h exposure were 320 and 572 μg l⁻¹, respectively; corresponding 50% mortality (LC₅₀) values after 48 h exposure were 493 and 869 μg l⁻¹. Results indicate that the use of these ichthyotoxic plants might have a severe impact on the survival of fish larvae in the field.     

Exorhopala ruficeps (Balanophoraceae): morphology and transfer to Helosis

Exorhopala ruficeps is a rare parasitic plant of the family Balanophoraceae, endemic to the north-western part of Peninsular Malaysia. It was originally described in Rhopalocnemis and later placed in a new monotypic genus on the basis of the exogenous origin of its inflorescences. A new collection of material enabled a detailed morphological study, the results of which are presented here. Inter alia, it is documented that the inflorescences originate endogenously as in all other Balanophoraceae. However, no volva is formed and the tuber-tissue covering the emerging inflorescence crumbles into pieces. The inflorescence and flower characters established here are neither in full accordance with Rhopalocnemis nor do they justify a genus of its own, but instead place the species in Helosis , which thus becomes a genus of transpacific distribution. The new combination is Helosis ruficeps (Ridl.) R. K. Eberwein.  © 2004 The Linnean Society of London, Botanical Journal of the Linnean Society , 2004, 146 , 513–517.     

Restoration Efforts for Plant and Bird Communities in Tallgrass Prairies Using Prescribed Burning and Mowing

Recent losses and fragmentation of tallgrass prairie habitat to agriculture and urban development have led to corresponding declines in diversity and abundance of plants and birds associated with such habitat. Mowing and burning are alternative management strategies for restoring and rejuvenating prairies in fragmented landscapes, but their specific, comparative effects are the subjects of ongoing evaluation. We compared the responses of plant and bird communities on four sets of mowed, burned, and untreated sites of small (3–10 ha), fragmented tallgrass prairies at the DeSoto National Wildlife Refuge (DNWR), Iowa, U.S.A., during May–July in 1998 and 1999. Species richness and diversity of plants, resident grassland birds, and communities of birds associated with grassland edges (edge species) were independent of treatment. Although not affecting species richness and diversity in plant communities, mowed sites ranked lower in total plant coverage and total forb coverage than burned sites or untreated sites. In contrast, untreated sites had more coverage by shrubs, suggesting that mowing and burning did retard shrub encroachment. Overall, abundance and diversity of plants and birds were generally insensitive to management strategies. Small, fragmented sites of rare habitat may not respond in the short term to management treatments and may not be capable of supporting highly diverse communities, no matter how intensively manipulated. It is more probable that diversity of native prairie communities can be enhanced and restored only through long‐term efforts, acquisition of large land units capable of supporting stable populations, and deliberate reintroduction of species of high conservation value.     

Colonization by alpine native plants of a stabilized road verge on the Bogong High Plains, Victoria

Summary Although many native species are now used in the revegetation of soil disturbances in Australian alpine areas, exotic species were, until recently, the only components of seed mixes. The use of exotic species and fertilizer was justified by their availability and low cost, and the prediction that native species would replace the exotic sward, presumably once soil nutrient levels dropped to those found in native vegetation. There was no apparent regard for the invasive capacities of the exotic species used. An investigation of a road verge on the Bogong High Plains, revegetated with a mixture of exotic species in the late 1950s, has shown that colonization by native plants can be very slow. Although many native species had colonized by 1993, exotic species still provided 50% of the cover. Agrostis capillaris accounted for most of this. The major native colonizers were Carex spp., Colobanthus affinis , Euchiton spp., Leptinella filicula , Poa hiemata , Ranunculus victoriensis and Scleranthus biflorus . Poa hiemata provided by far the most cover of the native colonizers. There have been considerable increases in the cover and number of native species on the verge since 1989. Cattle grazing and trampling are likely to have limited colonization of native plants prior to the removal of livestock in 1991. Recommendations are made on approaches to future revegetation in the alpine area.     

Evaluation of methods of screening strawberry cultivars for resistance to crown rot caused by Phytophthora cactorum

Four methods were evaluated in measuring resistance of strawberry cultivars to crown rot caused by Phytophthora cactorum. Meristem propagated plants grown in vitro were inoculated with mycelial discs. Four to five days after inoculation, it was possible to distinguish between cultivars with large differences in susceptibility to the disease. Ten days later, all plants were totally necrotic making it impossible to distinguish between cultivars. When detached leaves were inoculated by inoculating a plug of mycelium into the petiole, disease symptoms developed more slowly in resistant cultivars, but leaf age greatly affected the rate of symptom development. When plug plants (not cold stored) were lightly wounded in the rhizome with a scalpel and inoculated with either zoospores or mycelium, differences in disease development between cultivars were mainly as would be expected from previous information on susceptibility, but both age and size of plants influenced the rate of disease development. Unwounded, inoculated plants did not develop symptoms. When cold‐stored plug plants were either unwounded or lightly wounded with a scalpel in the rhizome and inoculated with zoospores, the relative rates of disease development consistently reflected the susceptibility to crown rot. At the time of final assessment, disease was much more severe in wounded plants, but the relative susceptibility of cultivars was not affected by the wounding.     

Phylogenetic evidence for the evolution of ecological specialization in Timema walking‐sticks

We used phylogenetic and ecological information to study the evolution of host‐plant specialization and colour polymorphism in the genus Timema, which comprises 14 species of walking‐sticks that are subject to strong selection for cryptic coloration on their host‐plants. Phylogenetic analysis indicated that this genus consists of three main lineages. Two of the lineages include highly generalized basal species and relatively specialized distal species, and one of the lineages comprises four specialized species. We tested for phylogenetic conservatism in the traits studied via randomizing host‐plant use, and the four basic Timema colour patterns, across the tips of the phylogeny, and determining if the observed number of inferred changes was significantly low compared to the distribution of numbers of inferred changes expected under the null model. This analysis showed that (1) host‐plant use has evolved nonrandomly, such that more closely related species tend to use similar sets of hosts and (2) colour pattern evolution exhibits considerable lability. Inference of ancestral states using maximum parsimony, under four models for the relative ease of gain and loss of plant hosts or colour morphs, showed that (1) for all models with gains of host‐plants even marginally more difficult than losses, and for most optimizations with gains and losses equally difficult, the ancestral Timema were generalized, feeding on the chaparral plants Ceanothus and Adenostoma and possibly other taxa, and (2) for all models with gains of colour morphs more difficult than losses, the ancestral Timema were polymorphic for colour pattern. Generation of null distributions of inferred ancestral states showed that the maximum‐parsimony inference of host‐plant generalization was most robust for the most speciose of the three main Timema lineages. Ancestral states were also inferred using maximum likelihood, after recoding host‐plant use and colour polymorphism as dichotomous characters. Likelihood analyses provided some support for inference of generalization in host‐plant use at ancestral nodes of the two lineages exhibiting mixtures of generalists and specialists, although levels of uncertainty were high. By contrast, likelihood analysis did not estimate ancestral colour morph patterns with any confidence, due to inferred rates of change that were high with respect to speciation rates. Information from biogeography, floristic history and the timing of diversification of the genus are compatible with patterns of inferred ancestral host‐plant use. Diversification in the genus Timema appears to engender three main processes: (1) increased specialization via loss of host‐plants, (2) retention of the same, single, host‐plant and (3) shifts to novel hosts to which lineages were ‘preadapted’ in colour pattern. Our evidence suggests that the radiation of this genus has involved multiple evolutionary transitions from individual‐level specialization (multiple‐niche polymorphism) to population‐level and species‐level specialization. Ecological studies of Timema suggest that such transitions are driven by diversifying selection for crypsis. This paper provides the first phylogeny‐based evidence for the macroevolutionary importance of predation by generalist natural enemies in the evolution of specialization.     

Water use of two Mojave Desert shrubs under elevated CO2

Plant responses to elevated atmospheric CO₂ have been characterized generally by stomatal closure and enhanced growth rates. These responses are being increasingly incorporated into global climate models that quantify interactions between the biosphere and atmosphere, altering climate predictions from simpler physically based models. However, current information on CO₂ responses has been gathered primarily from studies of crop and temperate forest species. In order to apply responses of vegetation to global predictions, CO₂ responses in other commonly occurring biomes must be studied. A Free Air CO₂ Enrichment (FACE) study is currently underway to examine plant responses to high CO₂ in a natural, undisturbed Mojave Desert ecosystem in Nevada, USA. Here we present findings from this study, and its companion glasshouse experiment, demonstrating that field‐grown Ephedra nevadensis and glasshouse‐grown Larrea tridentata responded to high CO₂ with reductions in the ratio of transpirational surface area to sapwood area (LSR) of 33% and 60%, respectively. Thus, leaf‐specific hydraulic conductivity increased and stomatal conductance remained constant or was increased under elevated CO₂. Field‐grown Larrea did not show a reduced LSR under high CO₂, and stomatal conductance was reduced in the high CO₂ treatment, although the effect was apparent only under conditions of unusually high soil moisture. Both findings suggest that the common paradigm of 20–50% reductions in stomatal conductance under high CO₂ may not be applicable to arid ecosystems under most conditions.     

Small Canopy Gaps Influence Plant Distributions in the Rain Forest Understory1

This study tested three hypotheses regarding how plants respond to the spatial heterogeneity in light availability in the rain forest understory: (1) understory plants occur preferentially in the lighter parts of the understory; (2) under–story palms are more shade tolerant than other understory plants; (3) rain forest plants differ in their ontogenetic response to understory light conditions. The study was carried out in old–growth rain forest in the Yasuní National Park, Amazonian Ecuador. The hypotheses were tested by comparing the distributions of 20 plant species (1454 individuals) over microsites with differing degrees of exposure to canopy gaps to the background distribution of these microsites in the forest. The gap exposure of a given microsite was described by an index based on the number and size of gaps in the canopy to which the site was exposed. Two plant height classes were studied: 0.80–2.49 and 2.50–5 m. The first and third hypotheses were accepted, while the second hypothesis was rejected. The results for the individual species corresponded well with what is known from earlier studies about the ecology of these species or close relatives, suggesting that the patterns observed can be generalized for Neotropical rain forests. Notably, the most abundant species in the study represent several different life history strategies. Thus, abundance in the rain forest understory can be achieved by several different strategies. This suggests that niche differentiation in terms of response to small changes in understory light conditions may be an important factor in the maintenance of the high local plant species richness of tropical rain forests.     

PHYLOGENETIC RELATIONSHIPS AMONG STREPTOPHYTES AS INFERRED FROM CHLOROPLAST SMALL AND LARGE SUBUNIT rRNA GENE SEQUENCES1

To gain insights into the phylogeny of charophytes and into their relationships with other green algae and bryophytes, we analyzed the chloroplast small and large subunit rRNA sequences of charophytes belonging to five orders (Charales, Coleochaetales, Desmidiales, Klebsormidiales, and Zygnematales), of chlorophytes from the four remaining classes of green algae, and of bryophytes representing the three classes reported in this group of land plants. We also probed the gene organization and intron content of the chloroplast rDNA operon in charophytes and bryophytes. The organization of this operon proved to be highly conserved, except in members of the Desmidiales and Zygnematales. Homologous group II introns were identified in the trnA(UGC) gene of all charophyte groups examined and in the trnI(GAU) gene of charophytes from all orders except the Desmidiales and Zygnematales. Phylogenetic analyses of concatenated rDNA sequences consistently placed the prasinophyte Mesostigma viride Lauterborn at the base of the Streptophyta and Chlorophyta, although alternative topologies positioning Mesostigma within the Streptophyta could not be rejected. A sister group relationship was unambiguously established between Chaetosphaeridium globosum (Nordstedt) Klebahn and members of the Coleochaetales. The Charales, Coleochaetales, Desmidiales, and Zygnematales were found to be monophyletic, and a sister group relationship was observed for the Desmidiales and Zygnematales. Although our analyses failed to resolve the branching order of the Coleochaetales, Charales, Desmidiales/Zygnematales, and bryophytes, they revealed that the problematic charophyte taxon Entransia fimbriata Hughes strongly clusters with Klebsormidium flaccidum (Kützing) Silva, Mattox et Blackwell to form a basal lineage relative to the other charophyte orders examined.