The development of early terrestrial ecosystems

Summary

In this review of terrestrialization by plants and animals in the early Phanerozoic, the classical idea of a major mid-Palaeozoic event is discarded in favour of gradual colonization over a long time period. Four phases of colonization of the land by plants are recognized but their limits are often difficult to define. The first, of microbial mats comprising prokaryotes and later photosynthesizing protists (algae) but with no direct fossil evidence, extends from the Precambrian and may persist in environments unsuitable for colonization by higher plants and animals today. The second, based on microfossils (spores and cuticles) possibly from plants of bryophyte aspect (if not affinity) started in the Ordovician (c. 460 Ma ago) and ended in the Lower Devonian, but was overlapped by the third phase beginning early in the Silurian (c. 430 Ma). This consisted of small plants of axial organization with terminal sporangia probably allied to the tracheophytes. The advent of taller vascular plants of varied organization around the Silurian — Devonian boundary (c. 420–400 Ma) signalled the final pioneering phase — that of major adaptative radiations on land, culminating in the appearance of extant groups, in changes in reproductive strategy and in the development of complex vegetation structure. The animal record is sparser than that of the plants, but suggests an early land fauna in the mid-Palaeozoic which differed from later terrestrial assemblages in lacking herbivores, with the first direct fossil evidence for land animals in the late Silurian.     

Amplectosporangium—A New Genus of Plant from the Lower Devonian of Sichuan,China

A new genus and species Amplectosporangium jiangyouense, is described from the Pingyipu Formation (Siegenian) of the Lower Devonian in Jiangyou district, Sichuan, China. The plant consists of naked dichotomizing erect axes with terminal fertile branch-systems. Sporangia are oval-shaped each with a short stalk and are arranged along the inner side of dichotomized axes in single row. By comparison with other known Devonian plants the genus is considered to represent a new morphological type of early land plants. Based on the theory of the telomic origin of the ovular integument, the plant may represent an early stage in the evolution of integument.     

A new genus of Early Devonian plants with novel strobilar structures and vegetative appendages from the Posongchong Formation of Yunnan,China

A new plant, Dibracophyton acrovatum gen. et sp. nov., is described from the Lower Devonian (Pragian) Posongchong Formation of Wenshan District, southeastern Yunnan, China. The plant has creeping axes from which arise vegetative and fertile axes. The vegetative axes helically bear lateral dichotomous appendages with curved or round tips. The fertile axes possess terminal strobili with numerous fertile units arranged in irregular helices. Each fertile unit consists of a stalked long-elliptical sporangium, with dehiscence into two equal valves, and two discrete long-ovate bracts covering sporangium from above–below directions. A new genus and species is thus established based on these characters and is temporarily regarded as incertae sedis of Tracheophyta, although it perhaps closes to the barinophytes in affinity. Detailed comparisons with other plants having a similar architecture, i.e., sporangia closely associated with modified vegetative structures, are made. The discovery of D. acrovatum further enriches the composition of the Posongchong flora and demonstrates great morphological disparity of the Early Devonian vascular plants.     

抱囊蕨属—四川早泥盆世一新属

本文描述了四川早泥盆世陆地植物的一个新属种——江油抱囊蕨(Amplectosporangiumjiangyouense)。植物体茎轴裸露,等或不等二歧式分枝多次,在其远端区密集的二歧式分枝构成了一个椭圆状或梨状轮廓的能育顶枝系。孢子囊椭圆形,具短柄,呈单列侧生于能育枝的内侧。根据新植物显示的形态特征,除与一些早期陆地维管植物作了比较外,还初步探讨了它与种子植物珠被演化间的关系。     

Biomass Allocation in the Dioecious Tropical Palm Chamaedorea tepejilote and Its Life History Consequences

Abstract The patterns of resource allocation are described for a dioecious tropical palm, Chamaedorea tepejilote. Resource allocation was measured by harvesting fifteen plants of C. tepejilote. The relative allocation of biomass in the stem increased with the size of the plant; that in the leaves decreased and that in the other structures remained roughly constant. Female plants showed a greater total reproductive effort, though male plants produced more inflorescences during the flowering season. Both male and female plants allocated more resources to prop root than to hypogeal roots. The annual productivity of reproductive and vegetative parts of C. tepejilote was estimated using allometric relationships for different plant structures and from demographic data obtained from the field. Annually, female plants allocated significantly more resources to leaves than male plants. Yearly productivity of inflorescences was higher for male plants, while female plants had greater total reproductive productivity (inflorescences and fruits). Correlation analysis showed an increase in reproductive effort with plant size, and an inverse relationship between fecundity and probability of survival, fecundity and residual reproductive value, and reproductive effort and life expectancy; these relationships suggested a cost in reproduction. Additionally, mature plants with different growth rates exhibited differences in fecundity: tall plants (>2.5m height) that grew more than 40 cm in height in four years had lower values of fecundity in comparison to plants of slower growth. These data were discussed in the context of the implications in the life history of a dioecious tropical plant.     

Resource partitioning to male and female flowers of Spinacia oleracea L. in relation to whole-plant monocarpic senescence

Male plants of spinach (Spinacea oleracea L.) senesce following flowering. It has been suggested that nutrient drain by male flowers is insufficient to trigger senescence. The partitioning of radiolabelled photosynthate between vegetative and reproductive tissue was compared in male (staminate) versus female (pistillate) plants. After the start of flowering staminate plants senesce 3 weeks earlier than pistillate plants. Soon after the start of flowering, staminate plants allocated several times as much photosynthate to flowering structures as did pistillate plants. The buds of staminate flowers with developing pollen had the greatest draw of photosynthate. When the staminate plants begin to show senescence 68% of fixed C was allocated to the staminate reproductive structures. In the pistillate plants, export to the developing fruits and young flowers remained near 10% until mid-reproductive development, when it increased to 40%, declining to 27% as the plants started to senesce. These differences were also present on a sink-mass corrected basis. Flowers on staminate spinach plants develop faster than pistillate flowers and have a greater draw of photosynthate than do pistillate flowers and fruits, although for a shorter period. Pistillate plants also produce more leaf area within the inflorescence to sustain the developing fruits. The (14)C in the staminate flowers declined due to respiration, especially during pollen maturation; no such loss occurred in pistillate reproductive structures. The partitioning to the reproductive structures correlates with the greater production of floral versus vegetative tissue in staminate plants and their more rapid senescence. As at senescence the leaves still had adequate carbohydrate, the resources are clearly phloem-transported compounds other than carbohydrates. The extent of the resource redistribution to reproductive structures and away from the development of new vegetative sinks, starting very early in the reproductive phase, is sufficient to account for the triggering of senescence in the rest of the plant.     

Maternal effects in progeny of field pansy (Viola arvensis) subjected to different herbicide and nitrogen rates

Observations were made on the effects of reduced rates of herbicide and nitrogen on naturally occurring populations of Viola aruensis. Progeny arising from these plants were grown in a uniform environment and monitored through to maturity. The size and number of reproductive structures produced by the maternal plants were positively correlated with the maternal plant weight. Herbicide dose affected the plant height of offspring and the effects were more pronounced in those from plants which had received 160 kg N ha^-1than 40 kg N ha^-1. Although the maternal effects on offspring size diminished with time, the number of reproductive structures in the offspring was significantly correlated with plant height during early development. Increased nitrogen availability to the maternal plant in the absence of herbicide may therefore increase the overall productivity of the subsequent generation.     

Teruelia diezii gen. et sp. nov.: an early polysporangiophyte from the Lower Devonian of the Iberian Peninsula

A new basal land plant, Teruelia diezii gen. et sp. nov., is described from the shallow‐water marine deposits of the Lower Devonian (Lochkovian–Pragian) Nogueras Formation of the Iberian Peninsula (north Gondwana palaeocontinent). Teruelia is preserved as a compression fossil and consists of isotomously branched, robust stems terminated in large, fusiform, twisted sporangia. This morphology suggests that Teruelia is very probably equivalent to Aglaophyton, a permineralized early polysporangiophyte known up to now only from the Lower Devonian (early Pragian to ?earliest Emsian) Rhynie Chert in Scotland (Laurussia palaeocontinent), which represents an early terrestrial hot‐spring ecosystem. Accepted phylogenies identify Aglaophyton as sister to vascular plants. Our phylogeny‐based results identify the Aglaophyton/Teruelia biological entity (i.e. Aglaophyton anatomical characters plus Teruelia external morphology) as the most direct vascular plant precursor. It shows that at least one Rhynie Chert type plant had a much wider distribution than previously known and suggests that Aglaophyton was not restricted to hydrothermal environments, unlike other Rhynie Chert plants.     

New insights into early land ecosystems: a glimpse of a lilliputian world

Small, relatively uncompressed, very fragmentary plant remains (mesofossils) are described from a Silurian (P ídolí) and a Lower Devonian (Lochkovian) locality in the Welsh Borderland. Excellent cellular preservation provides characters leading to the demonstration of diversity in plants of simple gross morphology and allows deliberations on functional anatomy (e.g. of stomata), and reproductive biology (including development and dehiscence of sporangia). A survey of in-situ spores is presented, and preliminary comparisons made with dispersed spore assemblages especially in relation to reconstruction of vegetation on local and regional scales. The earliest body fossils of unequivocal terrestrial arthropods isolated from the same locality as the P ídolí plants suggest that the decomposer/microherbivore/predator soil and litter communities found in the Lower and Middle Devonian extend back at least into the Silurian. Evidence for plant—animal interaction in the Lower Devonian comes from spore-dominated coprolites believed to have been produced by litter-feeding myriapods.     

Stachyophyton gen. Nov. Discovers from Lower Devonian of Yunnan and Its Significance

This paper describes a new plant—Stachyophyton yunnanense gen. et sp. nov., from Posongchong Formation (Siegenian) of the Lower Devonian in Wenshan district, Yunnan, China. This plant shows some similar organization in fertile region and sterile parts in comparison with other Devonian genera (Protolepidodendron, Colpodexylon, Leclercqia, Enigmophyton, Krithodeophyton). It may be that the plant represents an intermediate stage in evolution between Enigmophyton and Krithodeophyton, and shows perhaps a certain affinity to lycopods. According to the salient features of Stachyophyton, it can not be, at present, placed in any group known in vascular plant. It can be assigned to definite position of taxon basing on much fossils record and further studies.     

Terrestrial-marine teleconnections in the Devonian: links between the evolution of land plants,weathering processes,and marine anoxic events

The Devonian Period was characterized by major changes in both the terrestrial biosphere, e.g. the evolution of trees and seed plants and the appearance of multi-storied forests, and in the marine biosphere, e.g. an extended biotic crisis that decimated tropical marine benthos, especially the stromatoporoid-tabulate coral reef community. Teleconnections between these terrestrial and marine events are poorly understood, but a key may lie in the role of soils as a geochemical interface between the lithosphere and atmosphere/hydrosphere, and the role of land plants in mediating weathering processes at this interface. The effectiveness of terrestrial floras in weathering was significantly enhanced as a consequence of increases in the size and geographic extent of vascular land plants during the Devonian. In this regard, the most important palaeobotanical innovations were (1) arborescence (tree stature), which increased maximum depths of root penetration and rhizoturbation, and (2) the seed habit, which freed land plants from reproductive dependence on moist lowland habitats and allowed colonization of drier upland and primary successional areas. These developments resulted in a transient intensification of pedogenesis (soil formation) and to large increases in the thickness and areal extent of soils. Enhanced chemical weathering may have led to increased riverine nutrient fluxes that promoted development of eutrophic conditions in epicontinental seaways, resulting in algal blooms, widespread bottomwater anoxia, and high sedimentary organic carbon fluxes. Long-term effects included drawdown of atmospheric pCO₂ and global cooling, leading to a brief Late Devonian glaciation, which set the stage for icehouse conditions during the Permo-Carboniferous. This model provides a framework for understanding links between early land plant evolution and coeval marine anoxic and biotic events, but further testing of Devonian terrestrial-marine teleconnections is needed.     

An anatomically preserved arborescent lycopsid, Sublepidodendron songziense (Sublepidodendraceae), from the Late Devonian of Hubei, China

Sublepidodendron is a common megafossil plant in the Late Devonian of China, but historically the generic delimitation based on leaf bases masked its true systematic position. A reinvestigation of S. songziense from the Late Devonian Hsiehchingssu Formation, Hubei, China, provides new insights into its internal anatomy and reproductive morphology. This arborescent lycopsid is characterized by small, vertically elongated leaf bases arranged in spirals, presence of false leaf scars, possibly bearing separate cones, and association with a stigmarian rhizomorph. The potential for organic connections of these detached organ genera has been noted for other Sublepidodendron species. The anatomy of S. songziense axes from two levels reveals that the thinner axis may bear an ectophloic siphonostele with a filamentous pith and an outer cortex. The thicker axis has a siphonostele with a branch gap, two-zoned pith with secondary thickenings, multiseriate rays across secondary xylem, a thick periderm, and primary and secondary tracheid walls characterized by "Williamson's striations." Similarities to synapomorphies of Diaphorodendraceae and Lepidodendraceae suggest that S. songziense bears a closer affinity to Lepidodendrales rather than Protolepidodendrales, as formerly thought. Widespread occurrence of Sublepidodendron implies that phylogenetically advanced arborescent lycopsids must have diverged by the Late Devonian.     

Palaeoecology and palaeophytogeography of the rhynie chert plants: evidence from integrated analysis of in situ and dispersed spores

The Rhynie cherts yield exceptionally preserved early land plants, and provide a unique insight into the nature of Lower Devonian vegetation. Hitherto they have been poorly age constrained, and the palaeoecology and palaeophytogeography of the flora are poorly understood. Well-preserved dispersed-spore assemblages have been recovered from a number of borehole cores through the stratigraphical sequence of the Rhynie outlier. They are all similar and belong with the polygonalis-emsiensis (PE) spore zone, indicating an Early Devonian age (Early (but not earliest) Pragian to earliest Emsian). Comparisons with PE spore-zone assemblages from elsewhere suggest that the flora of the Rhynie drainage basin was slightly impoverished, with certain plant taxa that occurred at other locations not represented. This probably reflects differences between the flora of an inland intermontaine basin (Rhynie) and that of the lowland flood-plains. In situ spores have been characterized for all seven Rhynie chert plants. Analysis of the distribution of these spore types in the Rhynie sequence, in addition to those of coeval deposits from elsewhere, enables interpretation of the palaeoecology and palaeophytogeography of the Rhynie chert plants. It is concluded that at least some of the plants were not highly specialized or adapted to the peculiar hot-springs environment in which they are preserved. Rather, they were components of a diverse and widespread flora, but were the only elements able to tolerate the inhospitable hot-springs environment (i.e. they were preadapted).     

The importance of floral and circum-floral nectar to ants inhabiting dry tropical lowlands

This paper tries to explain why ants visit the flowers and other reproductive parts of plants inhabiting dry tropical lowlands (in this example, coastal Yucatan, Mexico), and the factors that promote these interactions. The plants adapted to the dry Yucatan coast have structural characteristics to cope with low moisture, the same structures serve also as herbivore deterrents. Thus, during the dry season, some of the most attractive rood sources are the rich tissues of the reproductive structures. The nectar produced by those structures may play an important role, not only in insect nutrition, but in their water balance as well. It is proposed that owing 10 rood shortage during the dry season in coastal Yucatan, ants will rely on the nectar produced on the flowers and other reproductive structures as their main liquid energy source. Twenty-three plant specks arc reported as being visited by ants in coastal Yucatan. Two of these species were studied in more detail: Avicennia germinans , where ants take floral nectar and pollination is not effected, so nectar thievery is taking place; Schomburgkia tibicinis , where ants take circum-floral nectar, and this activity benefits the plant (fewer damaged infloreseences and more matured fruits).     

Reproductive effort in cotton grass tussock tundra

Eriophorum vaginatum ssp. spissum is a dominant plant species of undisturbed cotton grass tussock tundra in Alaska. It also quickly invades and dominates recently disturbed sites. The hypothesis tested in this research was that the success of E. vaginatum on disturbed sites might be achieved through a higher allocation of biomass to reproductive structures relative to other tundra species. Reproductive allocation of tundra plants in general also was compared with plants of the temperate zone. The results indicate that E. vaginatum is about average among the common tundra species in terms of total reproductive allocation, allocation to seeds, and the proportion of total reproductive allocation that is accounted for by viable seeds. Tundra species, on a relative basis, allocate less biomass to all reproductive structures than temperate species but not necessarily less biomass to the output of viable seeds.     

Correlative controls of senescence and plant death in Arabidopsis thaliana (Brassicaceae)

Like most monocarpic plants, longevity of Arabidopsis thaliana plants is controlled by the reproductive structures; however, they appear to work differently from most dicots studied. Neither male- and female-sterility mutations (ms1-1 and bell1, respectively) nor surgical removal of the stems with inflorescences (bolts) at various stages significantly increased the longevity of individual rosette leaves, yet the mutants and treated plants lived 20-50 d longer, measured by the death of the last rosette and/or the last cauline leaf. A series of growth mutations (clv2-4, clv3-2, det3, vam1 enh, and dark green) also increased plant longevity by 20-30 d but did not delay the overall development of the plants. The mutations prolonged plant life through the production of new leaves and stems with inflorescences (bolts) rather than by extending leaf longevity. In growing stems, the newly-formed leaves may induce senescence in the older leaves; however, removal of the younger leaves did not significantly increase the life of the older leaves on the compressed stems of Arabidopsis. Since plants that produce more bolts also live longer, the reproductive load (dry weight) of the bolts did not seem to drive leaf or whole plant senescence here. The developing reproductive structures caused the death of the plant by preventing regeneration of leaves and bolts, which are green and presumably photosynthetic. They also exerted a correlative control (repression) on the development of additional reproductive structures.     

REPRODUCTIVE PLASTICITY IN THE KELP ALARIA NANA (PHAEOPHYCEAE)1

Unlike terrestrial plants, reproductive control in algae has not been attributed to biotic factors or internal chemical signals. However, experimental removal of reproductive structures (sporophylls) from the kelp Alaria nana Shrader resulted in the development of functional reproducing structures on the vegetative frond in 45% (10/22) of the experimental plants but not on the control plants where sporophylls were left intact (n = 35). In naturally occurring Alaria nana, this compensatory reproductive response occurred at a low incidence and was restricted primarily to larger individuals in the population. Thus, compensation and developmental plasticity, mediated In plant size, may play an important role in how algae respond to reproductive tissue loss.     

From over to undercompensation: Variable responses to herbivory during ontogeny of a Neotropical monocarpic plant

Empirical and theoretical work has suggested that plants can change their compensatory responses to herbivory as they develop. However, such a relationship is likely to be more complex than previously thought since the amount and type of damage a plant receives can also change as the plant develops. Here, we determined the survival, growth, and reproductive output of plants (Actinocephalus polyanthus) from different ontogenetic stages that received variable levels of natural or simulated herbivore damage. Juvenile plants and non‐reproductive adults in which leaves were damaged showed full vegetative compensation, whereas pre‐reproductive plants were not able to replace the lost leaves. However, these same pre‐reproductive plants produced more inflorescences and thus more seeds and seedlings than control plants. In contrast, damage to vegetative and/or reproductive structures during the reproductive phase resulted in a negative effect on seed and seedling production. Herbivory effects on plant survival, growth, and reproduction during the vegetative and pre‐reproductive phases were independent of the amount of damage. However, during reproduction, the magnitude of these effects was strongly influenced by the amount of damage and the reproductive stage of the plant at the time of the damage. In short, our results demonstrate that the survival, growth, and reproductive responses to herbivory of A. polyanthus can be dependent on the timing and/or intensity of damage. The reproductive response of A. polyanthus to our simulated herbivory treatments during the pre‐reproductive phase represents an example of overcompensation. Furthermore, it indicates that vegetative regrowth is not necessarily a driving factor for tolerance.     

Density-dependent reproductive and vegetative allocation in the aquatic plant Pistia stratiotes (Araceae)

Pistia stratiotes is an aquatic macrophyte that grows in temporary-ponds in the southern Pantanal, Brazil. It reproduces both sexually and asexually and is usually observed forming dense mats on the water surface, a condition favored by the plant's vegetative reproduction coupled with an ability for rapid growth. In this study we examined the effect of densely crowded conditions on the production of reproductive and vegetative structures. In addition, we verified whether there is a trade-off between clonal growth and investment in sexual reproductive structures, and whether there is an allocation pattern with plant size. Individual plant biomass and the number of the rosettes producing sexual reproductive structures and vegetative growth structures both increased with density. Increase in plant size resulted in increased proportional allocation to sexual reproductive structures and vegetative growth structures. Allocation of biomass to reproduction did not occur at the expense of clonal growth. Thus, the density response appears as a increase of rosettes producing sexual reproductive structures and vegetative growth structures. Therefore, long leaves and stolons may be adaptive under densely crowded conditions where competition for light is intense. An important aspect in the study of trade-offs is the size-dependency of the allocation patterns .Usually, larger plants produce more biomass. Therefore, larger plants can allocate more biomass to both vegetative and sexual reproduction than smaller plants and thus show a positive correlation between both traits rather than the expected negative one.     

The dawn of symbiosis between plants and fungi

The colonization of land by plants relied on fundamental biological innovations, among which was symbiosis with fungi to enhance nutrient uptake. Here we present evidence that several species representing the earliest groups of land plants are symbiotic with fungi of the Mucoromycotina. This finding brings up the possibility that terrestrialization was facilitated by these fungi rather than, as conventionally proposed, by members of the Glomeromycota. Since the 1970s it has been assumed, largely from the observation that vascular plant fossils of the early Devonian (400 Ma) show arbuscule-like structures, that fungi of the Glomeromycota were the earliest to form mycorrhizas, and evolutionary trees have, until now, placed Glomeromycota as the oldest known lineage of endomycorrhizal fungi. Our observation that Endogone-like fungi are widely associated with the earliest branching land plants, and give way to glomeromycotan fungi in later lineages, raises the new hypothesis that members of the Mucoromycotina rather than the Glomeromycota enabled the establishment and growth of early land colonists.