Estimation of the relative importance of sexual and vegetative reproduction in the clonal woodland herb Anemone nemorosa

Anemone nemorosa is a perennial rhizomatous plant of European woodlands. The “probability of clonal identity” method estimated the relative proportion of sexual to vegetative reproduction in this species to be 4.4% from allozyme genotype distributions. This result is congruent with investigations on the germination, short-term demography, population genetics, and breeding system of this species, and supports the hypothesis that even low levels of seedling recruitment can maintain considerable intrapopulational genetic diversity. Received: 2 March 1998 / Accepted: 24 July 1998     

Interactions in the patterns of vegetative growth and reproduction in woody dioecious plants

Summary Interactions between vegetative growth and reproduction were evaluated in Peumus boldus, Lithraea caustica and Laretia acaulis, three woody dioecious species in central Chile. Phenological observations were made periodically on marked branches of male and female plants, and biomass allocation (dry weight) to vegetative and reproductive tissues was measured. The magnitude of flowering was evaluated in groups of plants in three successive seasons. The patterns of activities are species- and sex-dependent, and cycles of 2–4 years have been established. Branches that produce flowers either do not grow or grow less than branches without flowers, and males and females have differential resource allocation: male branches attain higher biomass values. Groups of plants show seasonal behavior that suggest synchrony in their reproductive activities.     

Optimal allocation to vegetative and sexual reproduction in plants: the effect of ramet density

Vegetative reproduction is a very common alternative by which plants can contribute to the next generations. There are many considerations predicting which mode of reproduction, vegetative or sexual, should be favored and numerous experimental studies to verify them. However, the results are inconsistent especially when the effect of plant density is considered. I apply here a dynamic optimization model to predict the rate of vegetative and sexual reproduction in plants as a response to changes in the local plant density. The population is assumed to occupy a heterogeneous environment consisting of patches in which growth and reproduction of plants are possible and unfavorable space between them. As the environment is globally stable, the seeds, which can disperse without restriction, exhibit a constant recruitment rate. The ramets are assumed to settle only within the patch of the mother plant. The rate of ramet production effects local density, which in turn determines ramet recruitment. The optimal strategy maximizes the expected lifetime genetic contribution, realized via both vegetative and sexual reproduction. The solutions obtained under these assumptions are dualistic. The model predicts that different approaches applied in studying the effect of ramet density should give opposite outcomes. When the comparison is between patches in natural populations, a positive relationship between relative ramet allocation and density is expected. When the density is experimentally manipulated or its effect is analyzed across different successional stages, a negative relationship should be found. The results seem to be confirmed by empirical studies.     

Photosynthetic and gas exchange characteristics of dominant woody plants on a moisture gradient in an African savanna

We determined key photosynthetic gas exchange parameters, and their temperature dependence, in dominant woody plants at four savanna sites on a moisture gradient in Botswana, southern Africa. Leaf stable carbon and nitrogen (N) isotope and morphological measures were made concurrently. Sampling of these predominantly non‐N‐fixing species took place during an exceptional rainfall season, representing near‐optimum conditions for primary production at these sites. The mean specific leaf area and leaf size were positively related to mean annual rainfall (MAR); species with larger leaves of lower density were more abundant in wetter sites. Almost all species at all sites showed high net light‐saturated photosynthetic rates (A_max?10 μmol CO₂ m^?2 s^?1) due both to high CO₂ carboxylation (V_c,max) and RubP‐regeneration capacity (J_max). These high rates were associated with high values of leaf [N]. Across all sites, the temperature response of A_max showed no clear optimum, and a gradual drop from 25°C to 35°C, without notable temperature limitation at leaf temperatures in excess of 35°C. Dark respiration rate (R_day) across all species and sites increased exponentially with increasing leaf temperature. Species sampled at selected sites revealed a negative relationship between leaf δ¹³C (stable carbon isotope ratio) and MAR, suggesting higher leaf‐level water‐use efficiency at drier sites when integrated over the life of the leaf. At wetter sites, specific leaf [N] was lower and photosynthetic nitrogen‐use efficiency increased, a pattern reflected at the ecosystem level by less ¹⁵N enrichment of leaves at these sites. Taken together, the results suggest a switch from water‐use to nitrogen‐use efficiency constraints with increasing moisture availability. These constraints impact leaf form and function significantly, and may emerge at the ecosystem level in aspects of water and N cycling.     

Relationships between leaf deciduousness and flowering traits of woody species in the Brazilian neotropical savanna

The relationships between foliage permanence and flowering throughout the year were analyzed in 92 woody species of Cerrado vegetation categorized as either deciduous (DE), semideciduous (SD) or evergreen (EV). Flowering of DE, SD and EV species was investigated via three variables, measured over the course of the year: flowering duration (FLD), calculated as the number of months in flower in each species; flowering distribution (FDI), calculated as the number of species in flower per month; and flowering peak (FPE), defined as the four consecutive months yielding the highest number of species in flower. The months with the highest numbers of species in flower were October (52 species), September (50) and August (49). These months correspond to the period of transition from the dry season to the wet season. In the majority of species studied, seasonal climatic factors were strong enough to induce fruit formation in the dry season and seed dispersal in the following wet season, when sufficient water was available to support germination and plantlet growth. However, significant differences in FLD, FDI and FPE were found among the leaf phenological groups. High FLD in EV species is likely favored by the continuous input of resources from the year-round foliage. In contrast, DE species employ reserves of carbon, water and nutrients to form new leaves and flowers on a crown free of foliage at the end of the dry season. In DE species, their low FLD may reduce the impact of flowering on reserve consumption. SD species showed an intermediate level of foliage persistence, resulting in intermediate FLD values. In addition, SD species exhibited a different pattern of flowering distribution from those of DE and EV species. Many SD species have two flowering periods per year. The first period occurs when the crowns are full of leaves, in the middle of the dry season in June, similar to EV species. The second occurs when only half of the original foliage area is present, near the peak of the dry season in September, similar to DE species. Therefore, despite a strong influence of seasonal climatic conditions on the flowering behavior of DE, SD and EV woody species of Cerrado vegetation, these leaf phenological groups differ significantly in FLD, FDI and FPE.     

The relative importance of sexual reproduction versus clonal spread in an aridland bunchgrass

Festuca idahoensis (Idaho fescue) is a perennial caespitose grass, common in semi-arid rangelands of the Intermountain West. To determine how individuals are recruited into a population, we studied two long-term monitoring plots that were established in 1937 at the Northern Great Basin Experimental Range in southeastern Oregon. The plots measured 3.05×3.05 m, and were located approximately 30 m apart. One plot was ungrazed, the other was subject to moderate levels of cattle grazing. The number of F. idahoensis plants in both plots increased ten-fold between 1937 and 1996, but whether this was due primarily to reproduction by seed or clonal fragmentation was unknown. In 1996, we mapped and sampled 160 plants of F. idahoensis. We used dominant inter-simple sequence repeat (ISSR) markers and codominant allozyme markers in order to identify genetic individuals and measure genetic diversity. Both plots were characterized by high levels of genetic and clonal diversity. When information from ISSRs, allozymes and sample location were combined, 126 genets were recognized, each consisting of one to four samples (ramets). By measuring the diameter of clones surrounding plants that were present in 1937, we estimated that clonal spread occurred at a rate of approximately 3.7 cm per decade, and thus was of secondary importance in the maintenance and increase of F. idahoensis stands. Sexual reproduction, rather than clonal fragmentation, accounted for most of the recruitment of new plants into these plots. The grazed plot had fewer ramets, genotypes, and clones than the ungrazed plot, but the ramets were significantly larger. Levels of genetic diversity did not differ in the grazed and ungrazed plots, but there was some evidence for a small, but significant level of genetic differentiation between the two. The results also indicate that F. idahoensis has the potential to be a long-lived species with some individuals persisting in excess of 60 years. This study demonstrates how long-term monitoring can be supplemented by genetic analysis to obtain detailed information on the population dynamics of plants. In the case of this community dominant species, this provides essential information for understanding succession and developing management and restoration strategies.Electronic Supplementary Material Supplementary material is available in the online version of this article at http://dx.doi.org/10.1007/s00442-003-1332-2     

Asynchronicity in root and shoot phenology in grasses and woody plants

Phenology is central to understanding vegetation response to climate change, as well as vegetation effects on plant resources, but most temporal production data is based on shoots, especially those of trees. In contrast, most production in temperate and colder regions is belowground, and is frequently dominated by grasses. We report root and shoot phenology in 7‐year old monocultures of 10 dominant species (five woody species, five grasses) in southern Canada. Woody shoot production was greatest about 8 weeks before the peak of root production, whereas grass shoot maxima preceded root maxima by 2–4 weeks. Over the growing season, woody root, and grass root and shoot production increased significantly with soil temperature. In contrast, the timing of woody shoot production was not related to soil temperature (r=0.01). The duration of root production was significantly greater than that of shoot production (grasses: 22%, woody species: 54%). Woody species produced cooler and moister soils than grasses, but growth forms did not affect seasonal patterns of soil conditions. Although woody shoots are the current benchmark for phenology studies, the other three components examined here (woody plant roots, grass shoots and roots) differed greatly in peak production time, as well as production duration. These results highlight that shoot and root phenology is not coincident, and further, that major plant growth forms differ in their timing of above‐ and belowground production. Thus, considering total plant phenology instead of only tree shoot phenology should provide a better understanding of ecosystem response to climate change.     

Leaf structure of the cerrado (Brazilian savanna) woody plants

With the aim of recognizing the commonest leaf pattern found in the woody flora of the cerrado (the Brazilian savanna) we analyzed the leaf anatomy of 30 representative species. The leaves are mostly dorsiventral and hypostomatic and covered by trichomes and a thick layer of wax and cuticle; the vascular bundles are surrounded by a sheath of fibers. The mesophyll has a developed palisade tissue, dispersed sclerified cells and idioblasts bearing crystals and phenolic compounds. We compared the results with those reported for other species (60 species) from the same biome and for the families that the studied species belong. The present study suggests that the xeromorphism observed for the cerrado leaves is related to the evolutionary history of this biome, since its first floristic elements must have faced deficient water conditions as well as the consequent soil acidity and toxicity. Therefore we may infer that the leaf anatomical pattern here observed was already present in the first elements of the cerrado and was selected to guarantee the survival of those species in the new environment. Furthermore, the xeromorphic features present in those leaves continue nowadays to help the plants protecting themselves from the different biotic and abiotic factors they are subjected to.     

Topo-edaphic, floristic and physiognomic gradients of woody plants in a semi-arid African savanna woodland

The variation in topo-edaphic conditions, woodland composition and physiognomic structure of a semi-arid savanna woodland, namely the 59 400 ha Klaserie Nature Reserve, was studied with multivariate techniques. Spatial variation in the texture and chemistry of both the A-and B-horizons was substantial. One gradient of the A-horizon was of increasing clay, silt, organic matter, pH and conductivity, while a second gradient was of increasing rockiness, slope and gravel content, associated in part with upper landscape positions. Physiognomic structure of the woodland also displayed substantial spatial variation. Variation in tree density had the greatest effect on total canopy volume, and the density of coppice and dead individuals were closely correlated. Topo-edaphic variation, after the effect of vegetation composition had been accounted for, had a direct effect on woodland structure, as clay content of soils or rockiness influenced shrub density, and catenal position together with silt-or gravel-content of soils influenced coppice density. Ordination analyses identified that the included topo-edaphic variables accounted for a significant amount of floristic variation, but also showed that important environmental variables were omitted. Floristic variation of woody elements was of the nature of a large number of relatively equivalent, independent gradients rather than of a few simple primary gradients. Ten woodland types were identified by TWINSPAN which differed in their topo-edaphic and physiognomic character. Distinctive woodland assemblages were found on rocky outcrops or alluvial areas, on deep, sandy soils or on clay-or silt-rich soils. Colophospermum mopane formed the densest woodlands. Characteristics of the B-horizon were often dissimilar to those of the A-horizon, and seemed important for woodland composition.     

The relative importance of sexual and asexual reproduction in the spread of Spartina alterniflora using a spatially explicit individual-based model

This study investigates a spatially explicit, individual-based model for simulating the spread of invasive smooth cordgrass (Spartina alterniflora) in Yancheng coastal wetlands from 1995 to 2010. The model, which considers the landscape heterogeneity and changes detected by remote sensing, also reveals the relative importance of sexual and asexual reproduction in the spread by global sensitivity analysis. The model was verified as suitable for simulating the range expansion of S. alterniflora. The results show that: (1) although seedling recruitment is low, it significantly contributes to the range expansion of S. alterniflora. Removing sexual propagation greatly reduces the expansion rate. Rapid expansion requires both sexual and asexual reproduction; (2) in the global sensitivity analysis, the most significant affecters of S. alterniflora invasion were seed dispersal distance, adult survival rate and asexual recruitment survival rate. Sexual propagation contributes much more significantly to quick range expansion than asexual reproduction, but asexual reproduction is the main source of recruitment. Invasion control strategies should target a single reproduction mode. Here, limiting the germination and dispersal of seeds is suggested as a realistic strategy for controlling and managing invasion by this species.     

Differential costs of sexual and vegetative reproduction in wild strawberry populations

Summary The CO₂ costs of producing sexual and vegetative reproductive propagules were calculated for two species of wild strawberry, Fragaria virginiana and F. vesca. Five populations on sites representing a gradient of successional regrowth near Ithaca, New York, USA, were studied for two or three years each. Field studies of phenology, biomass, demography, and environment and laboratory studies of CO₂ exchange were integrated using a computerbased model of CO₂ dynamics to estimate costs of propagules.The percentage of plants flowering and the number of flower buds produced were highest in an open, recently disturbed habitat and lowest in a forest habitat. The openhabitat plants had the greatest success in converting flower buds into ripe fruits and also produced the highest numbers of runners and runner plantlets. On the basis of total investments in structure and respiration minus any photosynthetic gain of all reproductive structures, the cost per seed was lowest in the most open habitats and highest and increasingly variable in the more closed habitats. The cost of plantlets also was lowest in the most open habitat. The differences among habitats in cost of plantlets alive after one or two growing seasons increased due to differential survivorship of plantlets, with the open habitat continuing to have the lowest cost per plantlet. Theoretical treatments of life history characteristics such as reproductive effort should recognize that costs of equivalent type and size of propagule may vary among environments.     

Size-dependent allocation to sexual and vegetative reproduction in four clonal composites

Summary Populations of Silphium speciosum, Vernonia baldwinii, Solidago canadensis and Pityopsis graminifolia were studied to determine whether biomass allocation to sexual and vegetative reproduction and the balance between them were size-dependent and whether interpopulation differences in allocation patterns could be predicted from differences in population size distributions. All four species showed strong linear relationships between inflorescence mass and vegetative mass with negative y-intercepts. As a result, sexual reproductive effort (SRE) was a monotonically increasing function of ramet size. Genet size was a poor predictor of SRE. In each species, the regression parameters of these relationships differed significantly between burned and unburned habitats indicating size-independent interpopulation differences in patterns of reproductive effort as well as sizedependent effects. Interpopulation variation in vegetative reproductive effort (VRE) was greater than variation in SRE, but neither VRE nor the pattern of partitioning of VRE among daughter rhizomes showed significant relationships to plant size.     

Effects of large herbivore browsing on the functional groups of woody plants in a southern African savanna

This study examined the effect of different large herbivore species and stocking rates in savanna ecosystems of Zimbabwe on the richness and abundances of woody plant functional groups and woody plant functional attributes. Seven fence-lines with different herbivore species and stocking rates on either side of the fence were sampled. Plots were placed on both sides of each fence at each of 18 randomly selected positions. The size and species of each woody plant was recorded for each plot. It was found that the number of species with different functional attributes of spinescence, leaf longevity, fruit type and dispersal mechanism and in the functional groups of palatability were not different on the different sides of the fence. However, there were differences in plant abundances for 26 out of the 35 tests carried out on plant abundances with different functional attributes and functional groups. It was hypothesised that the time needed to change woody plant species richness is hundreds of years in these systems, whereas the time needed to change woody plant abundances is decades.     

Spatial variation of the stable nitrogen isotope ratio of woody plants along a topoedaphic gradient in a subtropical savanna

Variation in the stable N isotope ratio (δ¹⁵N) of plants and soils often reflects the influence of environment on the N cycle. We measured leaf δ¹⁵N and N concentration ([N]) on all individuals of Prosopis glandulosa (deciduous tree legume), Condalia hookeri (evergreen shrub), and Zanthoxylum fagara (evergreen shrub) present within a belt transect 308 m long × 12 m wide in a subtropical savanna ecosystem in southern Texas, USA in April and August 2005. Soil texture, gravimetric water content (GWC), total N and δ¹⁵N were also measured along the transect. At the landscape scale, leaf δ¹⁵N was negatively related to elevation for all the three species along this topoedaphic sequence. Changes in soil δ¹⁵N, total N, and GWC appeared to contribute to this spatial pattern of leaf δ¹⁵N. In lower portions of the landscape, greater soil N availability and GWC are associated with relatively high rates of both N mineralization and nitrification. Both soil δ¹⁵N and leaf [N] were positively correlated with leaf δ¹⁵N of non-N₂ fixing plants. Leaf δ¹⁵N of P. glandulosa, an N₂-fixing legume, did not correlate with leaf [N]; the δ¹⁵N of P. glandulosa’s leaves were closer to atmospheric N₂ and significantly lower than those of C. hookeri and Z. fagara. Additionally, at smaller spatial scales, a proximity index (which reflected the density and distance of surrounding P. glandulosa trees) was negatively correlated with leaf δ¹⁵N of C. hookeri and Z. fagara, indicating the N₂-fixing P. glandulosa may be important to the N nutrition of nearby non-N₂-fixing species. Our results indicate plant ¹⁵N natural abundance can reflect the extent of N retention and help us better understand N dynamics and plant-soil interactions at ecosystem and landscape scales.     

The drivers of woody species richness and density in a Neotropical savannah

Environmental filtering prevents species without certain attributes from occurring in local communities. Traits respond differently to different abiotic factors, assembling communities with varying composition along environmental gradients. Here, we measured proxies of soil fertility, disturbance by fire, response and physiological traits to assess how these variables interact to determine woody species richness and density in a Neotropical savannah. We explicitly incorporated our assumptions about how different abiotic filters influence different subsets of traits into a statistical model using structural equation modelling, yielding a more accurate representation of the assembly process. Fire had an effect on resistance traits, whereas soil fertility influenced physiological traits. Resistance traits explained both the richness and density of plots, whereas physiological traits explained only the density. Fewer fire events led to richer and denser plots. Similarly, areas with lower cation exchange capacity assembled less dense communities. Furthermore, we showed that structural equation modelling yielded a realistic representation of the bivariate interactions of distinct environmental filters with different subsets of traits.