Links between the detritivore and the herbivore system: effects of earthworms and Collembola on plant growth and aphid development

Effects of Collembola (Heteromurus nitidus and Onychiurus scotarius) and earthworms (Aporrectodea caliginosa and Octolasion tyrtaeum) on the growth of two plant species from different functional groups (Poa annua and Trifolium repens), and on the development of aphids (Myzus persicae) were investigated in a laboratory experiment lasting 20 weeks. Using soil from a fallow site which had been set aside for about 15 years, we expected that nitrogen would be of limited supply to plants and hypothesized that the soil animals studied, particularly earthworms, would increase nutrient availability to plants and thereby also modify aphid reproduction and development. Plant growth was modified strongly by the presence of soil animals. Earthworms caused a more than twofold increase in shoot and root mass of P. annua but increased that of T. repens by only 18% and 6%, respectively. However, earthworms neither affected plant shoot/root ratio nor the nitrogen concentration in plant tissue. In contrast, the presence of Collembola caused a reduction in plant biomass particularly that of P. annua roots, but plant tissue nitrogen concentration was increased, although only slightly. Aphid reproduction on T. repens was lowered in the presence of Collembola on average by 45% but on P. annua increased by a factor of about 3. It is concluded that Collembola decrease aphid reproduction on more palatable host plants like T. repens but increase that on less palatable ones like P. annua. Earthworm presence also affected aphid reproduction but the effect was less consistent than that of Collembola. In the presence of earthworms, aphid reproduction was in one experimental period increased by some 70%. Earthworms also modified the numbers of Collembola and their vertical distribution in experimental chambers. Exploitation of deeper soil layers by H. nitidus was increased but, generally, O. scotarius numbers were reduced whereas those of H. nitidus increased in earthworm treatments. The presence of Collembola also influenced earthworm body mass during the experiment. In general it declined, but in the presence of Collembola loss of body mass of A. caliginosa was more pronounced. We conclude that inhibiting effects between Collembola and earthworms resulted from the use of a common resource, litter material rich in nitrogen. This is supported by the higher C/N ratio of the litter material in the presence of earthworms and Collembola by the end of the experiment. Effects of soil invertebrates like Collembola and earthworms on plant performance and aphid development are assumed to be modified by complex direct and indirect interactions among soil animal groups. Received: 6 July 1998 / Accepted: 1 March 1999     

Earthworm effects on plant growth do not necessarily decrease with soil fertility

Earthworms are known to generally increase plant growth. However, because plant-earthworm interactions are potentially mediated by soil characteristics the response of plants to earthworms should depend on the soil type. In a greenhouse microcosm experiment, the responsiveness of plants (Veronica persica, Trifolium dubium and Poa annua) to two earthworm species (in combination or not) belonging to different functional groups (Aporrectodea. caliginosa an endogeic species, Lumbricus terrestris an anecic species) was measured in term of biomass accumulation. This responsiveness was compared in two soils (nutrient rich and nutrient poor) and two mineral fertilization treatments (with and without). The main significant effects on plant growth were due to the anecic earthworm species. L. terrestris increased the shoot biomass and the total biomass of T. dubium only in the rich soil. It increased also the total biomass of P. annua without mineral fertilization but had the opposite effect with fertilization. Mineral fertilization, in the presence of L. terrestris, also reduced the total biomass of V. persica. L. terrestris did not only affect plant growth. In P. annua and V. persica A. caliginosa and L. terrestris also affected the shoot/root ratio and this effect depended on soil type. Finally, few significant interactions were found between the anecic and the endogeic earthworms and these interactions did not depend on the soil type. A general idea would be that earthworms mostly increase plant growth through the enhancement of mineralization and that earthworm effects should decrease in nutrient-rich soils or with mineral fertilization. However, our results show that this view does not hold and that other mechanisms are influential.     

Effects of earthworms and organic litter distribution on plant performance and aphid reproduction

Human management practices and large detritivores such as earthworms incorporate plant litter into the soil, thereby forming a heterogeneous soil environment from which plant roots extract nutrients. In a greenhouse experiment we investigated effects of earthworms and spatial distribution of ¹⁵N-labelled grass litter on plants of different functional groups [Lolium perenne (grass), Plantago lanceolata (forb), Trifolium repens (legume)]. Earthworms enhanced shoot and root growth in L. perenne and P. lanceolata and N uptake from organic litter and soil in all plant species. Litter concentrated in a patch (compared with litter mixed homogeneously into the soil) increased shoot biomass and ¹⁵N uptake from the litter in L. perenne and enhanced root proliferation in P. lanceolata when earthworms were present. Growth of clover (T. repens) was rather independent of the presence of earthworms and organic litter distribution: nevertheless, clover took up more nitrogen in the presence of earthworms and exploited more ¹⁵N from the added litter than the other plant species. The magnitude of the effects of earthworms and organic litter distribution differed between the plant species, indicating different responses of plants with contrasting root morphology. Aphid (Myzus persicae) reproduction was reduced on P. lanceolata in the presence of earthworms. We suggest that earthworm activity may indirectly alter plant chemistry and hence defence mechanisms against herbivores.     

Do endogeic earthworms change plant competition? A microcosm study

Plants compete for limited resources. Although nutrient availability for plants is affected by resource distribution and soil organisms, surprisingly few studies investigate their combined effects on plant growth and competition. Effects of endogeic earthworms (Aporrectodea jassyensis), root-knot nematodes (Meloidogyne incognita) and the spatial distribution of ¹⁵N labelled grass litter on the competition between a grass (Lolium perenne), a forb (Plantago lanceolata) and a legume (Trifolium repens) were investigated in the greenhouse. Earthworms promoted N uptake and growth of L. perenne. Contrastingly, shoot biomass and N uptake of T. repens decreased in the presence of earthworms. P. lanceolata was not affected by the earthworms. We suggest that earthworms enhanced the competitive ability of L. perenne against T. repens. Nematodes increased the proportion of litter N in each of the plant species. Litter distribution (homogeneous vs. patch) did not affect the biomass of any plant species. However, P. lanceolata took up more ¹⁵N, when the litter was homogeneously mixed into the soil. The results suggest that endogeic earthworms may affect plant competition by promoting individual plant species. More studies including decomposers are necessary to understand their role in determining plant community structure.     

Earthworm invasion into previously earthworm-free temperate and boreal forests

Earthworms are keystone detritivores that can influence primary producers by changing seedbed conditions, soil characteristics, flow of water, nutrients and carbon, and plant–herbivore interactions. The invasion of European earthworms into previously earthworm-free temperate and boreal forests of North America dominated by Acer, Quercus, Betula, Pinus and Populus has provided ample opportunity to observe how earthworms engineer ecosystems. Impacts vary with soil parent material, land use history, and assemblage of invading earthworm species. Earthworms reduce the thickness of organic layers, increase the bulk density of soils and incorporate litter and humus materials into deeper horizons of the soil profile, thereby affecting the whole soil food web and the above ground plant community. Mixing of organic and mineral materials turns mor into mull humus which significantly changes the distribution and community composition of the soil microflora and seedbed conditions for vascular plants. In some forests earthworm invasion leads to reduced availability and increased leaching of N and P in soil horizons where most fine roots are concentrated. Earthworms can contribute to a forest decline syndrome, and forest herbs in the genera Aralia, Botrychium, Osmorhiza, Trillium, Uvularia, and Viola are reduced in abundance during earthworm invasion. The degree of plant recovery after invasion varies greatly among sites and depends on complex interactions with soil processes and herbivores. These changes are likely to alter competitive relationships among plant species, possibly facilitating invasion of exotic plant species such as Rhamnus cathartica into North American forests, leading to as yet unknown changes in successional trajectory.     

Introduced plants on Kilimanjaro: tourism and its impact

Kilimanjaro, a world heritage site and an icon of global change, not only suffers from climatic alterations but also is undergoing a drastic socio-economic upheaval. A strong increase of tourism enhances the risk of introducing alien plant species in particular in the upper zones of Kilimanjaro. One such species is Poa annua L., a cosmopolitan weed of European origin on roadsides and pastures. The aim of this study is to document its distribution, the speed of its propagation and risks for the indigenous vegetation of Kilimanjaro, and to compare the findings with other introduced species on this mountain. Based on a complete survey of the vegetation of Kilimanjaro with about 1,500 vegetation plots, plant communities invaded by Poa annua are determined. As with most of the other neophytes on Kilimanjaro, Poa annua invades only anthropogenic vegetation but not undisturbed natural vegetation. Similar to the situation in middle Europe, this neophyte is on Kilimanjaro a constituent of the vegetation of trampled ground, occurring between about 1,600 and 4,000 m asl along climbing routes or their vicinity. On a newly opened climbing route a rapid invasion (5.6 km in 3 months) was observed, which makes it likely that Poa annua spread on Kilimanjaro during the last 30 years in parallel to the increase of the climbing tourism. Although Poa annua is still in the stage of propagation, an invasion of natural vegetation types seems to be unlikely.     

Silicon enhances the tolerance of <Emphasis Type="Italic">Poa annua</Emphasis> to cadmium by inhibiting its absorption and oxidative stress

Silicon (Si) could enhance plant tolerance to heavy metals; however, the mechanism of Si-mediated alleviation of cadmium (Cd) toxicity in Poa annua was not clear. In this study, we found that 100 μM Cd significantly inhibited the growth of Poa annua seedlings. Furthermore, Cd enhanced the H₂O₂ and malondialdehyde content. The activities of superoxide dismutase and ascorbate peroxidase were enhanced, but the catalase and peroxidase activities were reduced by Cd treatment. Cd also altered the activity and expression of glucose-6-phosphate dehydrogenase (G6PDH) in Poa annua roots. Application of Na₃PO₄, an inhibitor of G6PDH, decreased the activity of G6PDH, the expression of G6PDH, and increased the Cd toxicity, suggesting that G6PDH is involved in the regulation of oxidative stress induced by Cd. Application of 1 mM Si alleviated the inhibition of Cd on the growth of Poa annua seedlings. Si application not only led to reduced oxidative injuries but also decreased the accumulation of Cd in Poa annua seedlings under Cd stress. Furthermore, Si decreased the activity of G6PDH and the expression of G6PDH under Cd stress, which demonstrated that Si attenuates the Cd toxicity in Poa annua probably through decreasing the expression of G6PDH under Cd stress. When G6PDH was inhibited, the alleviation impact of Si on Cd stress was abolished. Taken together, these results demonstrated that the Cd tolerance in Poa annua enhanced by Si is mainly due to the decrease of Cd uptake in roots and lowering the oxidative stress induced by Cd.     

Effects of grazing on plant communities and successional processes vary along an aridity gradient at a northern temperate grassland

The intermountain grasslands of North America reach their most northern geographic extent in interior British Columbia’s Cariboo-Chilcotin region. Here, this study examined the long-term effects of livestock grazing exclusion and reductions in grazing severity on plant community characteristics including plant and litter cover, species richness and abundance of leading species of 33 grassland sites across a broad aridity and soil property gradient. Across the aridity gradient, grazing reduced species richness, plant cover, and litter cover. However, the effects of grazing on dominant species varied across the gradient. In more arid grasslands, historical grazing substantially reduced cover of late-seral native bunchgrass Psuedoroegnaria spicata, and repeated measurements indicate that very long time periods are necessary for successional processes associated with recovery of native bunchgrasses. At the cool-wet end of the aridity gradient, successional processes are more rapid but dominated by exotic species Poa pratensis and Tragopogon pratensis. Recent (past 20 years) light grazing and rest-rotation have favored Poa pratensis at the expense of native needlegrasses (Achnatherum spp. and Hesperostipa spp.). We suggest that absence of a dominant large-stature native bunchgrass for mesic grasslands was a key factor in the invasion and dominance of Poa pratensis.

     

Primary succession on glacier forelands in the subantarctic Kerguelen Islands

Abstract. Primary succession was studied on recently de-glaciated areas in front of the Ampère Glacier, Kerguelen Islands (49° 30′S, 69° 30′E). Vegetation, colonization processes and soil development were investigated on seven sites on the outwash plain over a distance of 5 km and representing a > 200-yr old chronosequence. Seven species are involved in the succession, showing four patterns of change: (1)‘pioneer’ (Poa kerguelensis, Poa annua, Colobanthus kerguelensis and Cerastium fontanum); (2)‘intermediate’ (Festuca contracta); (3)‘late colonizer’ (Azorella selago); and (4)‘fluctuating’ (Agrostis magellanica). Two of the pioneers are introduced species (Poa annua and Cerastium). The total plant cover increases significantly with the age of the sites but never exceeds 4.2 %. The plant succession pattern observed in this study appears to be related to soil development. Root architecture and fine particle dynamics in the soil may explain the behaviour of the different species along the chronosequence. The pioneer species have a very shallow root system whereas the intermediate and late colonizers are deeper and longer rooted. The dominance of Azorella selago (Apiaceae) in the oldest community may be attributed to its unique root system in relation to the vertical distribution of silt cappings in the soil.     

The potential effect of high atmospheric CO2 on soil fungi–invertebrate interactions

Litter mixtures of four meadow plant species, Cardamine hirsuta, Poa annua, Senecio vulgaris, and Spergula arvensis, were produced from laboratory model terrestrial ecosystems maintained at either ambient or enriched (ambient + 200 µmol mol^?1) CO₂ concentrations. The effect of litter source on the oviposition attractivity of fungi to the sciarid fly Lycoriella ingenua was tested for seven fungal species (Absidia glauca, Cladosporium cladosporioides, C. herbarum, Fusarium oxysporum, Penicillium arenicola, P. chrysogenum, and P. janthinellum). For all species, except F. oxysporum, oviposition attractivity increased when the fungi were growing on litter derived from CO₂‐enriched environments. The relative increase of the oviposition attractiveness of fungi growing on CO₂‐enriched litter differed substantially and resulted in a shift in sciarid fly oviposition preference. For example, when P. chrysogenum and C. herbarum grew on ambient litter, P. chrysogenum was more attractive; the opposite was true for mycelia growing on enriched litter. The effect of litter source on the suitability of four fungal species for larval development was also tested. In two species of fungi (A. glauca and C. herbarum) suitability was significantly higher if growing on CO₂‐enriched litter. With P. chrysogenum the opposite was true. The consequences of these rarely considered CO₂‐induced trophic interactions on ecosystem processes such as nutrient feedback cycles between plants and soil decomposition are considered.     

Biosynthesis of plant phenolic compounds in elevated atmospheric CO2

Experiments were carried out to determine the effects of elevated atmospheric carbon dioxide (CO₂) on phenolic biosynthesis in four plant species growing over three generations for nine months in a model plant community. Results were compared to those obtained when the same species were grown individually in pots in the same soils and controlled environment. In the model herbaceous plant community, only two of the four species showed any increase in biomass under elevated CO₂, but this occurred only in the first generation for Spergula arvensis and in the second generation for Poa annua. Thus, the effects of CO₂ on plant biomass and carbon and nitrogen content were species‐ and generation‐specific. The activity of the principle phenolic biosynthetic enzyme, phenylalanine ammonia lyase (PAL), increased under elevated CO₂ in Senecio vulgaris only in Generation 1, but increased in three of the four plant species in Generation 2. There were no changes in the total phenolic content of the plants, except for P. annua in Generation 1. Lignin content decreased under elevated CO₂ in Cardamine hirsuta in Generation 1, but increased in Generation 2, whilst the lignin content of P. annua showed no change, decreased, then increased in response to elevated CO₂ over the three generations. When the species were grown alone in pots, elevated CO₂ increased PAL activity in plants grown in soil taken from the Ecotron community after nine months of plant growth, but not in plants grown in the soil used at the start of the experiment (‘initial' soil). In P. annua, phenolic biosynthesis decreased under elevated CO₂ in initial soil, and in both P. annua and S. vulgaris there was a significant interaction between effects of soil type and CO₂ level on PAL activity. In this study, plant chemical composition altered more in response to environmental factors such as soil type than in response to carbon supply. Results were species‐specific and changed markedly between generations.     

The effects of trophic structure and soil fertility on the assembly of plant communities: a microcosm experiment

Using closed, ventilated outdoor microcosms, plant communities were allowed to assemble from a pool of 48 herbaceous species comprising a wide range of plant functional types. The experiment involved factorial manipulation of soil fertility, invertebrate herbivores (slugs, grass aphids) and their predators (ground beetles, ladybirds). In the absence of herbivores the vegetation on soils of high, moderate and low fertility was dominated by plant species which under natural field conditions are restricted to fertile soils. At high fertility, the rate of competitive exclusion was rapid compared with communities persisting at lower fertility which remained species‐rich with high densities of individuals.
The effects of herbivory were profound and attributable mainly to the slug Deroceras reticulatum. At moderate and low soil fertility the main effect of herbivores was to change the ranking of plant species; palatable grasses (e.g. Poa annua, Poa trivialis and Lolium perenne) were suppressed and plant species of slower growth rate (e.g. Festuca rubra, Anthoxanthum odoratum, Festuca ovina) were promoted. More subtle impacts of herbivory altered the size structure of plant populations suggesting the preference, by slugs, for seedlings and small (suppressed) plants. Although ladybirds and their larvae persisted in low numbers in some microcosms, carnivory was more strongly in evidence (lower densities of D. reticulatum and reduced consumption of lettuce discs) in relation to the activities of the ground beetle Pterostichus melanarius. Only in the low fertility treatment, however, was this potential translated into effects on vegetation; for several plant species the frequency of large individuals increased in the presence of P. melanarius.
We conclude that outdoor microcosms provide a valuable ecological tool and, in particular, allow investigation of (a) the role of herbivores in promoting slow‐growing plant species in the vegetation of infertile habitats and (b) the protection of palatable plants against herbivores by carnivores.     

Experimental Evaluation of Herbivory on Live Plant Seedlings by the Earthworm Lumbricus terrestris L. in the Presence and Absence of Soil Surface Litter

Background

Recent studies suggested that the earthworm Lumbricus terrestris might act as a seedling predator by ingesting emerging seedlings, and individuals were observed damaging fresh leaves of various plant species in the field. To evaluate the significance of herbivore behavior of L. terrestris for plant and earthworm performance we exposed 23- to 33-days-old seedlings of six plant species to earthworms in two microcosm experiments. Plants belonged to the three functional groups grasses, non-leguminous herbs, and legumes. Leaf damage, leaf mortality, the number of leaves as well as mortality and growth of seedlings were followed over a period of up to 26 days. In a subset of replicates 0.1 g of soil surface litter of each of the six plant species was provided and consumption was estimated regularly to determine potential feeding preferences of earthworms.

Results

There was no difference in seedling growth, the number of live seedlings and dead leaves between treatments with or without worms. Fresh leaves were damaged eight times during the experiment, most likely by L. terrestris, with two direct observations of earthworms tearing off leaf parts. Another nine leaves were partly pulled into earthworm burrows. Lumbricus terrestris preferred to consume legume litter over litter of the other plant functional groups. Earthworms that consumed litter lost less weight than individuals that were provided with soil and live plants only, indicating that live plants are not a suitable substitute for litter in earthworm nutrition.

Conclusion

Our results demonstrate that L. terrestris damages live plants; however, this behavior occurs only rarely. Pulling live plants into earthworm burrows might induce microbial decomposition of leaves to make them suitable for later consumption. Herbivory on plants beyond the initial seedling stage may only play a minor role in earthworm nutrition and has limited potential to influence plant growth.     

Exotic Ecosystem Engineers Change the Emergence of Plants from the Seed Bank of a Deciduous Forest

The anthropogenic spread of exotic ecosystem engineers profoundly impacts native ecosystems. Exotic earthworms were shown to alter plant community composition of the understory of deciduous forests previously devoid of earthworms. We investigated the effect of two exotic earthworm species (Lumbricus terrestris L. and Octolasion tyrtaeum Savigny) belonging to different ecological groups (anecic and endogeic) on the emergence of plants from the seed bank of a northern North American deciduous forest using the seedling emergence method. We hypothesized that (1) exotic earthworms change the seedling emergence from the plant seed bank, (2) L. terrestris increases the emergence of plant seedlings of the deeper soil layer but decreases that of the upper soil layer due to plant seed burial, and (3) O. tyrtaeum decreases plant seedling emergence due the damage of plant seeds. Indeed, exotic earthworms altered the emergence of plant seedlings from the seed bank and the functional composition of the established plant seedlings. Surprisingly, although L. terrestris only marginally affected seedling emergence, O. tyrtaeum changed the emergence of native plant species from the seed bank considerably. In particular, the number of emerging grass and herb seedlings were increased in the presence of O. tyrtaeum in both soil layers. Moreover, the impacts of earthworms depended on the identity of plant functional groups; herb species benefited, whereas legumes suffered from the presence of exotic earthworms. The results highlight the strong effect of invasive belowground ecosystem engineers on aboveground ecosystem characteristics and suggest fundamental changes of ecosystems by human-spread earthworm species.     

Tree leaf litter composition and nonnative earthworms influence plant invasion in experimental forest floor mesocosms

Dominant tree species influence community and ecosystem components through the quantity and quality of their litter. Effects of litter may be modified by activity of ecosystem engineers such as earthworms. We examined the interacting effects of forest litter type and earthworm presence on invasibility of plants into forest floor environments using a greenhouse mesocosm experiment. We crossed five litter treatments mimicking historic and predicted changes in dominant tree composition with a treatment of either the absence or presence of nonnative earthworms. We measured mass loss of each litter type and growth of a model nonnative plant species (Festuca arundinacea, fescue) sown into each mesocosm. Mass loss was greater for litter of tree species characterized by lower C:N ratios. Earthworms enhanced litter mass loss, but only for species with lower C:N, leading to a significant litter × earthworm interaction. Fescue biomass was significantly greater in treatments with litter of low C:N and greater mass loss, suggesting that rapid decomposition of forest litter may be more favorable to understory plant invasions. Earthworms were expected to enhance invasion by increasing mass loss and removing the physical barrier of litter. However, earthworms typically reduced invasion success but not under invasive tree litter where the presence of earthworms facilitated invasion success compared to other litter treatments where earthworms were present. We conclude that past and predicted future shifts in dominant tree species may influence forest understory invasibility. The presence of nonnative earthworms may either suppress of facilitate invasibility depending on the species of dominant overstory tree species and the litter layers they produce.     

Plant and Litter Influences on Earthworm Abundance and Community Structure in a Tropical Wet Forest1

Plant communities differ in species composition and litter input. To examine the influence of plant species on the abundance and community structure of soil fauna, we sampled earthworms in areas close to and away from the bases of Dacryodes excelsa and Heliconia caribaea, two distinct plant communities within a tropical wet forest in Puerto Rico. We also carried out a litter manipulation experiment to examine the short–term responses of earthworms to litter removal and litter addition treatments. We found that: (1) the density and biomass of both soil–feeding endogeic and litter-feeding anccic worms did not differ between areas close to and away from Dacryodef trees (in contrast, the density and biomass of anecic worms was higher in areas away from Heliconia plants despite the lack of differences for endogeic worms); and (2) total dry weight of earthworms tended to be higher in the litter addition treatment than in the control within the Heliconia community. Our results suggest that Heliconia caribaea has a strong negative influence on anecic earthworms and that earthworms in the Heliconia community are more sensitive to litter input than in the Dacryodes community.     

The effect of host mycorrhizal status on host plant–parasitic plant interactions

Two pot experiments were conducted to examine three-level interactions between host plants, mycorrhizal fungi and parasitic plants. In a greenhouse experiment, Poa annua plants were grown in the presence or absence of an AM fungus (either Glomus lamellosum V43a or G. mosseae BEG29) and in the presence or absence of a root hemiparasitic plant (Odontites vulgaris). In a laboratory experiment, mycorrhizal infection (Glomus claroideum BEG31) of Trifolium pratense host plants (mycorrhizal versus non-mycorrhizal) was combined with hemiparasite infection (Rhinanthus serotinus) of the host (parasitized versus non-parasitized). Infection with the two species of Glomus had no significant effect on the growth of P. annua, while hemiparasite infection caused a significant reduction in host biomass. Mycorrhizal status of P. annua hosts (i.e. presence/absence of AM fungus) affected neither the biomass nor the number of flowers produced by the attached O. vulgaris plants. Infection with G. claroideum BEG31 greatly increased the biomass of T. pratense, but hemiparasite infection had no effect. The hemiparasitic R. serotinus plants attached to mycorrhizal hosts had higher biomass and produced more flowers than plants growing with non-mycorrhizal hosts. Roots of T. pratense were colonized by the AM fungus to an extent independent of the presence or absence of the hemiparasite. Our results confirm earlier findings that the mycorrhizal status of a host plant can affect the performance of an attached root hemiparasite. However, improvement of the performance of the parasitic plant following attachment to a mycorrhizal host depends on the extent to which the AM fungi is able to enhance the growth of the host. Accepted: 23 February 2001     

Reintroduction of Wild Lupine (Lupinus perennis L.) Depends on Variation in Canopy,Vegetation, and Litter Cover

We experimentally examined the effects of canopy, vegetation, and leaf litter cover on the demography of Wild lupines (Lupinus perennis) in a central North American oak savanna spanning 9 years. We also compared the distribution of Wild lupine across the landscape to results predicted by the demographic experiments. With less canopy cover, soil temperatures were warmer and seedlings emerged earlier. Seedling survival increased 14% with each additional leaf grown. Seedling survival was four times greater in openings and partial shade than in dense shade. Seedling survival was also influenced by interactions between canopy cover and vegetation cover, between canopy cover and leaf litter, and among canopy cover, vegetation cover, and litter cover. In openings, seedlings had higher survival when vegetative cover was present, suggesting a positive shading effect on survival, but with greater canopy cover vegetative cover reduced survival. Seedling survival was greater for plants that experienced herbivory, a result that was probably related to plant size and quality rather than having been eaten. Survival of lupines to 9 years after seed planting was greatest in the partial shade, moderate in openings, and least in dense shade. Wild lupine cover across the landscape was greatest when litter cover was low and canopy cover and ground layer cover were moderate. Reduction of canopy cover by burning or cutting, and reduction of leaf litter by prescribed burning will benefit the reintroduction of Wild lupine by increasing light, reducing litter cover, and creating disturbances; however, the reduction of vegetation cover in openings may hinder lupine reintroduction.     

Frankia in the rhizosphere of nonhost plants: A comparison with root-associated N2-fixing Enterobacter,Klebsiella and Pseudomonas

Bacterial growth in the rhizosphere and resulting changes in plant growth parameters were studied in small aseptic seedlings of birch (Betula pendula and B. pubescens) and grasses (Poa pratensis and Festuca rubra). The seedlings were inoculated with three Frankia strains (Ai1a and Ag5b isolated from native Alnus root nodules and Ai17 from a root nodule induced by soil originating from a Betula pendula stand), and three associative N₂-fixing bacteria (Enterobacter agglomerans, Klebsiella pneumoniae and Pseudomonas sp., isolated from grass roots). Microscopic observations showed that all the Frankia strains were able to colonize and grow on the root surface of the plants tested without addition of an exogenous carbon source. No net growth of the associative N₂-fixers was observed in the rhizosphere, although inoculum viable counts were maintained over the experimental period. Changes in both the biomass and morphology of plant seedlings in response to bacterial inoculation were recorded, which were more dependent on the plant species than on the bacterial strain.     

Invasion by <Emphasis Type="Italic">Aegilops triuncialis</Emphasis> (Barb Goatgrass) Slows Carbon and Nutrient Cycling in a Serpentine Grassland

Invasive plant species alter plant community composition and ecosystem function. In the United States, California native grasslands have been displaced almost completely by invasive annual grasses, with serpentine grasslands being one of the few remaining refugia for California grasslands. This study examined how the invasive annual grass, Aegilops triuncialis, has altered decomposition processes in a serpentine annual grassland. Our objectives were to (1) assess howA. triuncialis alters primary productivity and litter tissue chemistry, (2) determine whether A. triuncialis litter is more recalcitrant to decomposition than native litter, and (3) evaluate whether differences in the soil microbial community in A. triuncialis-invaded and native-dominated areas result in different decomposition rates of invasive and/or native plant litter. In invaded plant patches, A. triuncialis was approximately 50% of the total plant cover, in contrast to native plant patches in which A. triuncialis was not detected and native plants comprised over 90% of the total plant cover. End-of-season aboveground biomass was 2-fold higher in A. triuncialis dominated plots compared to native plots; however, there was no significant difference in belowground biomass. Both above- and below-ground plant litter from A. triuncialis plots had significantly higher lignin:N and C:N ratios and lower total N, P, and K than litter from native plant plots. Aboveground litter from native plots decomposed more rapidly than litter from A. triuncialis plots, although there was no difference in decomposition of belowground tissues. Soil microbial community composition associated with different soil patch types had no effect on decomposition rates. These data suggest that plant invasion impacts decomposition and nutrient cycling through changes in plant community tissue chemistry and biomass production.