Vegetation Effects on Soil Organic Matter Chemistry of Aggregate Fractions in a Hawaiian Forest

We examined chemical changes from leaf tissue to soil organic matter (SOM) to determine the persistence of plant chemistry into soil aggregate fractions. We characterized a slow (Dicranopteris linearis) and fast-decomposing species (Cheirodendron trigynum) and surface (O), and subsurface (A-horizon) SOM beneath each species using pyrolysis-gas chromatography/mass spectrometry (py-GC/MS), with and without derivatization. The live tissues of Dicranopteris had greater lignin content whereas Cheirodendron had a greater lipid, N-bearing, and polysaccharide component. Despite this difference in leaf chemistry, SOM chemistry was similar between soil aggregate fractions, but different between horizons. The O-horizon contained primarily lignin and polysaccharide biomarkers whereas the A-horizon contained polysaccharide, aromatic, and N-derived compounds, indicating considerable microbial processing of plant litter. The soils beneath Cheirodendron inherited a greater lipid signal composed of cutin and suberin biomarkers whereas the soils beneath Dicranopteris contained greater aromatic biomarker content, possibly derived from plant lignins. The soils beneath both species were more similar to root polysaccharides, lipids, and lignins than aboveground tissue. This study indicates that although plant-derived OM is processed vigorously, species-specific biomarkers and compound class differences persist into these soils and that differences in plant chemical properties may influence soil development even after considerable reworking of plant litter by microorganisms.     

Size-dependent foraging efficiency,cannibalism and zooplankton community structure

Only recently has the importance of positive interactions among plant species in structuring natural communities been supported by experimental evidence. Most studies have focused on interactions between a pair of species at a single life-history stage. In this study positive interactions between a woody nitrogen-fixing shrub (Myrica pensylvanica) and two herbaceous sand dune species (Solidago sempervirens, Ammophila breviligulata) which frequently grow beneath shrub canopies are examined throughout the life cycles of the herbaceous species. Comparisons of S. sempervirens and A. breviligulata growing beneath and outside M. pensylvanica shrubs show that plants growing in association with shrubs are larger, are more likely to flower, produce greater numbers of flowers and seeds, have higher midday xylem water potentials, have higher tissue nitrogen concentrations, and have higher photosynthetic efficiencies. Measurements of environmental conditions show that areas beneath shrubs are more shaded, have lower soil temperatures, and have higher soil nitrogen levels. The results from experimental manipulations designed to test the effects of Myrica shrubs on understory species suggest that the observed differences in plant performance are strongly influenced by canopy shading and soil nutrient enrichment associated with the shrubs. The results demonstrate that M. pensylvanica facilitates growth, reproduction, and recruitment of S. sempervirens and A. breviligulata growing beneath it. This study, one of the few to examine positive interactions at different life-history stages, supports previous predictions that positive interactions may be particularly important in plant communities characterized by physiologically stressful conditions. Received: 21 July 1999 / Accepted: 18 January 2000     

Facilitation by nurse shrubs of resprouting behavior in a post-fire shrubland in northern Patagonia,Argentina

Abstract. The nurse-plant syndrome is a widely recognized example of positive (facilitative) influences of plant species on the establishment and growth of other species. Most studies of the nurse-plant syndrome have been on species that reproduce mainly from seed rather than vegetatively. In this study, we experimentally compared the influences of two species of nurse shrubs, Schinus patagonicus and Berberis buxifolia, on the survival and growth of vegetatively reproducing herbaceous and woody plants in a post-fire shrubland in northern Patagonia, Argentina. The vegetation beneath shrubs was removed by clipping and, in a paired-sample design, one half of the canopy of each shrub was removed. We determined species richness, counted number of resprouts, and measured photon flux density and soil moisture beneath cut and uncut halves of each shrubs. Abundances of resprouts were several times greater beneath the uncut vs. the cut shrubs, as was the mean number of species. Thus, shrubs have a strong facilitating influence as measured by resprout densities and the number of species. Numbers of resprouts and of species were twice as high beneath Schinus as beneath Berberis, implying important differences in the facilitative effects of the two shrubs species. Microsites beneath Schinus were characterized by lower and more heterogeneous light levels but by greater soil moisture. Even though the reproductive mode in this post-fire shrubland is overwhelmingly vegetative, facilitation by nurse shrubs is important and differentially effective for different species of nurse shrubs.     

Factors affecting seed rain beneath fleshy-fruited plants

We investigated factors affecting seed rain beneath nine fleshy-fruited fruiting plant species growing in a 1-ha plot of planted Pinus thunbergii in central Japan. We tested whether the numbers of seeds and seed species dropped by birds beneath fruiting plants were correlated with the number of fruits removed by birds from the plants. Most of fruiting plant species with high fruit removal had significantly high seed rain. Both the numbers of seeds and seed species dropped were significantly, positively correlated with the number of fruits removed across for all fruiting plant species. Therefore, fruit removal predicted the difference among heterospecific fruiting plants in seed rain. We also tested whether the number of fruits removed from fruiting plants by birds was related with fruit crop size, fruit size, and height of the plants, and the numbers of fruits and fruit species of neighboring plants near the plants. Most of fruiting plant species with high fruit crop size had significantly high fruit removal. The number of fruits removed was significantly, positively correlated with both the fruit crop size and the number of neighboring fruits across the nine fruiting plant species. However, the effect of the neighboring fruit density on fruit removal was lower remarkably than that of fruit crop size. Therefore, fruit crop size best predicted the differences among heterospecific fruiting plants in fruit removal. We suggest that fruiting plant species with high fruit crop size and high fruit removal contribute to intensive seed rain beneath them. This revised version was published online in June 2006 with corrections to the Cover Date.     

The role of plant–soil feedbacks and land-use legacies in restoration of a temperate steppe in northern China

Plant–soil feedbacks affect plant performance and plant community dynamics; however, little is known about their role in ecological restoration. Here, we studied plant–soil feedbacks in restoration of steppe vegetation after agricultural disturbance in northern China. First, we analyzed abiotic and biotic soil properties under mono-dominant plant patches in an old-field restoration site and in a ‘target’ steppe site. Second, we tested plant–soil feedbacks by growing plant species from these two sites on soils from con- and heterospecific origin. Soil properties generally did not differ between the old-field site and steppe site, but there were significant differences among mono-dominant plant patches within the sites. While soil species origin (i.e., the plant species beneath which the soil was collected) affected biomass of individual plant species in the feedback experiment, species-level plant–soil feedbacks were ‘neutral’. Soil site origin (old-field, steppe) significantly affected biomass of old-field and steppe species. For example, old-field species had higher biomass in old-field soils than in steppe soils, indicating a positive land-use legacy. However, soil site origin effects depended on the plant species beneath which the soils were collected. The predictive value of abiotic and biotic soil properties in explaining plant biomass differed between and within groups of old-field and steppe species. We conclude that the occurrence of positive land-use legacies for old-field species may retard successional replacement of old-field species by steppe species. However, high levels of idiosyncrasy in responses of old-field and steppe plant species to con- and heterospecific soils indicate interspecific variation in the extent to which soil legacies and plant–soil feedbacks control successional species replacements in Chinese steppe ecosystems.     

Positive effects of shrubs on plant species diversity do not change along a gradient in grazing pressure in an arid shrubland

Facilitative or positive interactions among species are driven mainly by the environmental amelioration or protection from grazing provided by nurse plants. Some studies have suggested that protection from grazing is inconsequential in water-limited environments because of low herbivore densities and their grazing effects. Others, however, argue that herbivores have a major effect on semi-arid plant communities, and that protection from grazing is a significant factor driving positive plant–plant interactions in such environments. We identified a gradient in grazing pressure in a semi-arid shrubland in south-eastern Australia along which we compared soil condition, incident radiation and plant composition beneath two nurse shrub species with open (shrub-free) interspaces. Our aim was to assess the degree of microclimatic amelioration provided by both shrubs, and changes in the interactions (intensity, importance and frequency) between both nurse shrubs and their understorey species, and their effects on species richness at the community level. Both the relative interaction intensity (RII) and interaction importance (I_imp) indices of plant–plant interactions were generally positive and independent of grazing pressure. Soil beneath both nurse plants had significantly greater indices of nutrient cycling and infiltration, and contained more C and N than soil in the open. Almost twice as many species occurred under the canopies of both shrubs (44 species) than in the open (23 species), and the composition of species differed significantly among microsites. Fifty-four percent of all perennial plant species occurred exclusively under shrubs. Our results suggest that environmental amelioration is a stronger driver of the facilitatory effect of shrubs on their understorey species than protection from grazing. Our conclusions are based on the fact that the substantial effect of plant–plant interactions on plant species richness was largely independent of grazing pressure. Irrespective of the underlying mechanism for this effect, our study illustrates the ecological role of shrubs as refugia for understorey plants in semi-arid environments and cautions against management practices aimed at reducing shrub populations.     

Directed dispersal and decomposition drive cyclic succession in arid subtropical thicket

The influence of animal-mediated processes on the purported cyclical succession model of vegetation dynamics in arid subtropical thicket of southern South Africa was examined. We used spatially explicit sampling methods to evaluate the influence of dispersal and decomposition on plant community structure. The harvester ant Messor capensis was the only ant species recorded harvesting Portulacaria afra fruit and the elaiosome bearing seed of the tree Gymnosporia polyacantha from experimental piles. Propagules were dispersed to nest mounds positioned beneath woody shrubs. Vertebrates dispersed fleshy-fruited tree species to regeneration sites beneath P. afra clumps. The minute seeds of succulent shrubs were secondarily dispersed from beneath shrubs to ant nest sites located in the open. The results from two experiments indicate that soil nutrients and decomposition activity are spatially localised. The rate of below-ground decomposition, and the incidence of an invertebrate detritivore assemblage, was high beneath P. afra clumps compared with exposed nutrient-enriched and grassy sites. Animal dispersal vectors are integral to the regeneration of major plant groups in arid subtropical thicket by moving seeds from competitively harsh environments to safe sites beneath nurse plants. Decomposition and nutrient release are localised beneath P. afra clumps due to the unique conditions that support a suite of detritivore taxa recorded nowhere else at the study site.     

Temporal and spatial patterns of seed dispersal in two Cistus species (Cistaceae)

Cistus species are obligate seeding, early colonizers that follow disturbance, particularly fire, in Mediterranean ecosystems. We studied seed release, seed dispersal and soil seed populations in stands of Cistus ladanifer and C. libanotis. Seed release started in mid- to late summer (C. ladanifer) or in early autumn (C libanotis), and continued for a very extended period: 8-10 months in C. ladanifer, and for a mean of 16 months in C. libanotis. The xerochastic capsules of both species released seeds by successive dehiscence of the locules. All capsules begin to dehisce simultaneously at the start of the seed release period, but in C. libanotis capsule fragmentation replaced dehiscence early in the seed release period. In plants of both species, seed shadows were characterized by a peak of density beneath the plant canopy and a very short tail of much lower densities, indicating that seeds are concentrated beneath mother plants when dispersed. Nevertheless, in late May, at the onset of the fire season, soil seed densities beneath plant canopies were low compared with densities expected from seed shadows, but were apparently high enough to allow recovery of the stands if a disturbance, such as fire, had taken place. Seed-eating Bruchidae in summer and granivorous ants during the seed release period were apparently the main causes of seed losses. Results suggest that in both Cistus species, the staggered seed release could constitute an efficient risk-reducing trait. The plant pool of seeds existing throughout most of the year could be a relevant component of Cistus seed banks.     

Role of nurse plants in Araucaria Forest expansion over grassland in south Brazil

Abstract Araucaria Forest expansion over grassland takes place under wet climate conditions and low disturbance and it is hypothesized that isolated trees established on grassland facilitate the establishment of forest woody species beneath their canopies. Forest with Araucaria angustifolia is a particular type of Brazilian Atlantic Forest and the main forest type on the highland plateau in south Brazil, often forming mosaics with natural Campos grassland. The objectives of this paper were to evaluate the role of isolated shrubs and trees as colonization sites for seedlings of Araucaria Forest woody species on grassland, to determine which species function as preferential nurse plants in the process and the importance of vertebrate diaspore dispersal on the structure of seedling communities beneath nurse plants. The study was conducted in São Francisco de Paula, Rio Grande do Sul State, where we sampled isolated shrubs and trees in natural grassland near Araucaria Forest edges. Seedlings were counted and identified, and seedling diaspore dispersal syndromes, size and colour were registered. We detected 11 woody species with a potential role in nucleating grassland colonization by forest species. Beneath the canopies of nurse plants more forest species seedlings were found compared with open field grassland and the seedlings had diaspores mostly dispersed by vertebrates. Also, more seedlings were found under the canopy of A. angustifolia than beneath other nurse plant species. We conclude that A. angustifolia trees established on grassland act as nurse plants, by attracting disperser birds that promote colonization of the site by other forest species seedlings, and that under low level of grassland disturbance, conservation of frugivorous vertebrate assemblages may increase forest expansion over natural grassland and also facilitate the regeneration of degraded forest areas.     

Facilitation by nurse plants contributes to vegetation recovery in human-disturbed desert ecosystems

Aims Facilitation by nurse plants is a common interaction in harsh environments and this positive plant–plant interaction may promote vegetation recovery in ecosystems affected by human activities. Determining the relevance of this process, however, requires assessing how nurse plants influence the establishment of other species, as well as the proportion of species in the regional species pool that would benefit from the presence of nurse plants in human-disturbed areas. Further, since vegetation recovery is a time-dependent process, the community-level consequences of facilitation are likely to vary among landscapes with different disturbance history. Thus, an integrative perspective of the relevance of nurse plants for vegetation recovery could be obtained by measuring their effects across different human-disturbed landscapes of the target region. This study focuses on these issues and uses a regional-scale approach to assess the community-level effects of a widespread nurse plant of American deserts, the creosotebush (Larrea tridentata).Methods This study was conducted in the southernmost portion of Chihuahuan Desert because most floodplain valleys of this region have been affected by human activities during the past centuries. For this study, we selected 10 floodplain valleys differing in their age (i.e. the time elapsed after human activities were ceased). At each landscape, we measured the cover of creosotebushes and the proportion of plant species positively associated with them, as well as the density of seeds in the soil beneath creosotebush canopies. All these data were regressed against the age of the landscapes. Further, to assess whether positive association patterns were due to facilitation or other processes, we conducted field experiments and measured the ecophysiological performance of plant species established beneath and outside creosotebush canopies.Important findings Most plant species from the target region were positively associated to creosotebushes, and our field experiments and ecophysiological measures indicated that these distribution patterns can be attributed to facilitative interactions. In most landscapes, the density of seeds was higher beneath creosotebushes than in the surrounding habitats, suggesting that these shrubs may also act as seed traps. The community-level effects of creosotebushes increased with landscape age and creosotebush cover, indicating that magnitude of these effects depends on the disturbance history of each site. These results highlight the relevance of performing large-scale assessments for identifying the consequences of facilitation on vegetation recovery across space and time. We then propose that this kind of large-scale approach should be taken into account in the development of conservation programs aimed at the recovery and preservation of plant biodiversity in harsh environments.     

Small Tent-Roosting Bats Promote Dispersal of Large-Seeded Plants in a Neotropical Forest

In Neotropical regions, fruit bats are among the most important components of the remaining fauna in disturbed landscapes. These relatively small-bodied bats are well-known dispersal agents for many small-seeded plant species, but are assumed to play a negligible role in the dispersal of large-seeded plants. We investigated the importance of the small tent-roosting bat Artibeus watsoni for dispersal of large seeds in the Sarapiquí Basin, Costa Rica. We registered at least 43 seed species > 8 mm beneath bat roosts, but a species accumulation curve suggests that this number would increase with further sampling. Samples collected beneath bat feeding roosts had, on average, 10 times more seeds and species than samples collected 5 m away from bat feeding roosts. This difference was generally smaller in small, disturbed forest patches. Species-specific abundance of seeds found beneath bat roosts was positively correlated with abundance of seedlings, suggesting that bat dispersal may influence seedling recruitment. Our study demonstrates a greater role of small frugivorous bats as dispersers of large seeds than previously thought, particularly in regions where populations of large-bodied seed dispersers have been reduced or extirpated by hunting.     

Contagious dispersal of seeds of synchronously fruiting species beneath invasive and native fleshy‐fruited trees

In subtropical Australia, many native and invasive plant species rely on a shared suite of frugivores, largely birds, for seed dispersal. Many native plants fruit during summer in this region, whereas most invasive plants fruit during winter, thus providing the opportunity for contagious dispersal of seeds beneath synchronously fruiting species. We sampled invasive and native seed rain beneath the canopy of a native summer‐fruiting tree Guioa semiglauca and an invasive winter‐fruiting tree Cinnamomum camphora, in three study sites over the course of a year. In July, during peak fruiting season for C. camphora and other invasive species, seed rain of invasive species was higher beneath C. camphora than G. semiglauca. This was partly due to the invasive tree Ligustrum lucidum, whose seed rain was three times higher beneath C. camphora than beneath the native tree. In February, seed rain of native species was more abundant beneath the canopy of G. semiglauca than beneath C. camphora, despite the fact that C. camphora was also fruiting at this time. This was probably due to the larger fruit crop produced by G. semiglauca at this time of year. Our study provides evidence that the presence of invasive bird‐dispersed plants may facilitate contagious seed dispersal of other invaders, and likewise native species may facilitate seed spread of other native plants.     

The Reaction of the Medium in Relation to Root Formation in Coleus

Background: As seed dispersal can vary among years and individuals, studies that focus on a single year or on a few individuals may lead to erroneous conclusions.

Aims: To study temporal and spatial intraspecific variation of seed dispersal in Scrophularia canina, a widespread species with capsule-type fruit.

Methods: Primary seed dispersal was quantified by placing traps in each cardinal direction around 10 individuals during two consecutive years. We correlated several seed shadow parameters (modal dispersal distance, kurtosis, skewness, percentiles, slope, and seed percentage beneath the plant canopy) with three plant features (maximum height, lateral spread and seed production).

Results: Scrophularia canina dispersed their seeds by boleochory, giving rise to a typical leptokurtic curve, but behaving as a barochorous species, because about 90% of seeds landed beneath the plant canopy. Temporal dispersal in S. canina included several seed waves associated with maximum wind speeds. Plant lateral spread was significantly positively correlated with seed percentiles and percentage of seeds beneath the plant canopy regardless of year. A seed production effect was only evident when both years were considered together.

Conclusions: Although time-consuming, investigation of the dispersal process for more than 1 year provides more realistic information on seed dispersal. Lateral spread is the main plant feature determining seed shadow.     

Vegetation,microclimate and soils associated with the latest-lying snowpatches in Australia

Background : The Snowy Mountains contain Australia's longest-lasting snowpatches. Because of climate change, their longevity has declined, with the loss of some specialist vegetation in the underlying snowbeds.

Aims: To characterise the current status of the vegetation associated with the longest-lasting snowpatches in Australia and its association with abiotic factors.

Methods: We assessed plant composition, soil depth, moisture and nutrients and subsurface temperatures in five zones of increasing vegetation height and cover in snowbeds.

Results: The zone beneath the middle of snowpatches was characterised by little vegetation cover and lower species richness, later emergence from snow, skeletal soils, and lower mean soil temperatures than zones further downslope where soils increased in depth and nutrient levels. Vegetation beneath these snowpatches no longer occurs in distinct communities. Plants have not simply migrated upslope, instead, areas that have deep soil that used to have snowpatch specialist species are being colonised upslope by grasses and downslope by tall alpine herbfield species that prefer bare ground.

Conclusions: Reduced longevity of Australia's longest-lasting snowpatches has led to the loss of distinct snowpatch plant communities. With limited soils beneath the centre of current snowpatches, and a lack of other suitable sites there is no location for these plant communities to migrate to.     

Effects of plant species and soil history on root morphology,arbuscular mycorrhizal colonization of roots,and biomass in four tallgrass prairie species

Although dominant C₄ grasses in tallgrass prairie are highly mycotrophic, for many non-dominant species neither extent of mycorrhizal colonization nor root morphology effects on plant–soil feedback interactions are known. In a laboratory study conducted from November 2013 to February 2014 at Governors State University (University Park, IL), we grew plants of a dominant C₄ grass (Andropogon gerardii) and three non-dominant forbs (Symphyotrichum laeve var. laeve, Symphyotrichum novae-angliae and Parthenium integrifolium) individually in soil collected in the field beneath a conspecific, collected beneath another study species, or in sterilized soil. The study addressed the following questions: (1) Is extent of mycorrhizal colonization of roots related to root structure? (2) How does soil history interact with plant root traits to influence plant–soil feedback? (3) How might plant–soil feedback patterns influence competitive interactions among study species? We found that proportion arbuscular mycorrhizal fungi (AMF) colonization decreased with increasing specific root length. Soil history had a stronger influence than plant species on total biomass of plants, with all species having highest total biomass when grown in Andropogon soil. Consequently, net, or heterospecific, feedback did not differ among pairwise species combinations, and was not different from 0. While these results suggest that no study species should have a competitive advantage in the field, Andropogon might still have an advantage through mechanisms such as competition for light. Future work in the field and including less mycotrophic species is needed to better understand AMF effects on competitive interactions.

     

Spatial Variability of Soil Properties in the Shortgrass Steppe: The Relative Importance of Topography, Grazing, Microsite, and Plant Species in Controlling Spatial Patterns

We conducted a study to evaluate the relative importance of topography, grazing, the location of individual plants (microsite), and plant species in controlling the spatial variability of soil organic matter in shortgrass steppe ecosystems. We found that the largest spatial variation occurs in concert with topography and with microsite-scale heterogeneity, with relatively little spatial variability due to grazing or to plant species. Total soil C and N, coarse and fine particulate organic matter C and N, and potentially mineralizable C were significantly affected by topography, with higher levels in toeslope positions than in midslopes or summits. Soils beneath individual plants (Bouteloua gracilis and Opuntia polyacantha) were elevated by 2–3 cm relative to surrounding soils. All pools of soil organic matter were significantly higher in the raised hummocks directly beneath plants than in the soil surface of interspaces or this layer under plants. High levels of mineral material in the hummocks suggest that erosion is an important process in their formation, perhaps in addition to biotic accumulation of litter beneath individual plants. Over 50 y of heavy grazing by cattle did not have a significant effect on most of the soil organic matter pools we studied. This result was consistent with our hypothesis that this system, with its strong dominance of belowground organic matter, is minimally influenced by aboveground herbivory. In addition, soils beneath two of the important plant species of the shortgrass steppe, B. gracilis and O. polyacantha, differed little from one another. The processes that create spatial variability in shortgrass steppe ecosystems do not affect all soil organic matter pools equally. Topographic variability, developing over pedogenic time scales (centuries to thousands of years), has the largest effect on the most stable pools of soil organic matter. The influence of microsite is most evident in the pools of organic matter that turn over at time scales that approximate the life span of individual plants (years to decades and centuries).     

Seedling emergence and survival in contrasting soil microsites in Patagonian Monte shrubland

Abstract. The potential of two perennial species (Larrea divaricata and Stipa tenuis) to colonize different soil microsites was analyzed in the Patagonian Monte shrubland. We hypothesize that the short-lived grass S. tenuis is more able to colonize the soil of microsites beneath vegetation patches where N-fertility is higher than those in bare soil, while the long-lived shrub L. divaricata colonizes different soil microsites irrespective of their N fertility. A greenhouse experiment was carried out to evaluate the emergence and survival of both species in different soil microsites at different water, inorganic N and seed densities. In all cases soil microsites were seed limited since the addition of viable seeds increased seedling emergence. Both species showed, however, different abilities to emerge and survive in different soil microsites. Microsites of bare soil were more favourable for seedling emergence and survival of L. divaricata than those beneath vegetation patches, independent of their water status. This ability of L. divaricata can not be explained on the basis of increased water or N availability, but probably because of lower salt content of bare soil. The addition of inorganic N reduced the survival of L. divaricata in both microsites but increased individual plant performance. The emergence and survival of S. tenuis was not different in both types of soil microsites but the addition of inorganic N increased seedling emergence and plant biomass. According to these results, emergence and plant performance of S. tenuis may be promoted during humid years by increased concentration of inorganic N. Since N mineralization occurs at a higher rate in soil microsites beneath vegetation patches than in those of bare soil, higher plant performance and probably establishment of S. tenuis is to be expected. These results are consistent with an existing conceptual model of plant dynamics under various grazing intensities in the Patagonian Monte shrubland based on previous field observations.     

Environmental factors influencing decomposition rates in two Antarctic moss communities

Summary Rates of disappearance of dead material of Polytrichum alpestre and Chorisodontium aciphyllum from a moss turf community and of Drepanocladus uncinatus, Calliergon sarmentosum and Cephaloziella varians from a moss carpet community, measured using litter bags over 2 years, were 1.5% year^-1. Decomposition potential, estimated using loss in tensile strength of cotton strips inserted into the different bryophytes on the two sites, was also low. Ranking the five plant species in order of decomposition potential, from highest to lowest, gave D. uncinatus, C. aciphyllum, C. sarmentosum, P. alpestre and C. varians. The time taken for the tensile strength of the cotton strips at depths of 1–3 and 4–6 cm beneath the surface to decline by 50% varied from 1–2 years under the first two species to 3–4 years beneath the last two species. The main causes of these slow rates were low temperatures, short active season and low pH. Differences in decomposition between species, sites and with depth were related to temperature, nutrient status, water content and anaerobic conditions. Variation in anaerobic conditions beneath D. uncinatus, C. sarmentosum and C. varians in the moss carpet resulted in wide variation of decomposition rate beneath these species and with depth beneath C. varians. The peat in the moss turf was aerobic and experienced higher temperatures, but the average decomposition rate was no higher than in the moss carpet, because the peat was of a poorer quality and had a lower pH.     

Pine savanna overstorey influences on ground‐cover biodiversity

Question: Does the overstorey of pine savannas influence plant species biodiversity in the ground cover? Location: Camp Whispering Pines (30°41’N; 90°29’W), eastern Louisiana (USA). Methods: We used ecologically sensitive restoration logging to remove patches of Pinus palustris (longleaf pine) in a second‐growth loess plain Pinus palustris savanna managed using frequent lightning season fires. Five years later, we measured numbers of vascular plant species and transmitted light in replicated 100‐m² plots. Treatments involved three different overstorey conditions: no overstorey for 5 years, no overstorey for several decades, and overstorey pines present for decades. Results: Both recent and long‐term openings contained, on average, about 100 vascular plant species per 100 m², 20% more than in similar‐sized areas beneath overstorey trees. Responses varied with life form; more herbaceous species occurred in recent and older overstorey openings than beneath overstorey trees. Total numbers of all species and of less abundant forb species were positively and linearly related to light transmitted to ground level. Those species responding to openings in the overstorey and positively associated with increased transmitted light levels were monocarpic and shortlived perennial forb and grass species with a seed bank in the soil. In addition, community structure, as reflected in species composition and abundances, appeared to vary with canopy condition. Conclusions: Restoration involving ecologically sensitive removal of patches of overstorey pines in frequently burned pine savannas should benefit the ground cover and increase plant species biodiversity as a result of increased abundance of seed bank species.     

Soil fertility increases with plant species diversity in a long-term biodiversity experiment

Most explanations for the positive effect of plant species diversity on productivity have focused on the efficiency of resource use, implicitly assuming that resource supply is constant. To test this assumption, we grew seedlings of Echinacea purpurea in soil collected beneath 10-year-old, experimental plant communities containing one, two, four, eight, or 16 native grassland species. The results of this greenhouse bioassay challenge the assumption of constant resource supply; we found that bioassay seedlings grown in soil collected from experimental communities containing 16 plant species produced 70% more biomass than seedlings grown in soil collected beneath monocultures. This increase was likely attributable to greater soil N availability, which had increased in higher diversity communities over the 10-year-duration of the experiment. In a distinction akin to the selection/complementarity partition commonly made in studies of diversity and productivity, we further determined whether the additive effects of functional groups or the interactive effects of functional groups explained the increase in fertility with diversity. The increase in bioassay seedling biomass with diversity was largely explained by a concomitant increase in N-fixer, C4 grass, forb, and C3 grass biomass with diversity, suggesting that the additive effects of these four functional groups at higher diversity contributed to enhance N availability and retention. Nevertheless, diversity still explained a significant amount of the residual variation in bioassay seedling biomass after functional group biomass was included in a multiple regression, suggesting that interactions also increased fertility in diverse communities. Our results suggest a mechanism, the fertility effect, by which increased plant species diversity may increase community productivity over time by increasing the supply of nutrients via both greater inputs and greater retention.