Effects of sedge and cottongrass tussocks on plant establishment patterns in a post-mined peatland,northern Japan

Facilitation (positive inter-specific interaction) plays an important role in promoting succession in harsh environments. To examine whether tussocks facilitate the establishment of other species, after peat mining, investigations were carried out in a formerly Sphagnum-dominated wetland (Sarobetsu mire, northern Japan). Two tussock-forming species, Carex middendorffii and Eriophorum vaginatum, have established in sparsely vegetated areas, with a dry ground surface, since peat extraction ended. The following factors were examined, in three microhabitats created by tussocks (center = raised tussock center, edge = tussock edge covered with litter, and flat = flat areas without tussocks): (1) relationships between tussock microhabitats and plant distributions, and (2) the effects of tussocks on survival, growth, flowering and seed immigration of common species. Two (1 × 10 m) plots were established, in each of three sparsely vegetated sites, in September 2005. Tussocks were mapped in each plot, and species, location, flowering, growth stage (seedling, juvenile and fertile) and size of all plants were recorded, during snow-free periods from September 2005 to September 2006. Seed traps were used to investigate seed dispersal from June to October 2006. Four native species, Drosera rotundifolia, Lobelia sessilifolia, Moliniopsis japonica, Solidago virgaurea, and an exotic species, Hypochaeris radicata, were most common. During seedling and juvenile stages, these species were distributed more densely at the tussock edge than in the flat areas, but were less common at the center. H. radicata had a higher survival rate at the edge than in the flat during the winter. The annual growth of H. radicata, L. sessilifolia and S. virgaurea was higher at the edge. Seed traps detected that D. rotundifolia seeds accumulate more at the edge. In conclusion, tussocks facilitated plant establishment in the edge microhabitat by providing litter cover, enhancing seed accumulation, germination and survival, and thus promoted revegetation. However, Sphagnum mosses have not established in the study sites, and the vegetation differs strongly from the areas where no peat mining had taken place.     

Seedling establishment of late colonizer is facilitated by seedling and overstory of early colonizer in a post-mined peatland

The seedling establishment of late colonizer may be promoted in the environments created by the early colonizer without interspecific seedling competition. To confirm the hypothesis, seed-sowing experiments were conducted in a post-mined peatland, northern Japan. Seeds on three grasses (Rhynchospora alba, Moliniopsis japonica and Phragmites australis) were sown in four vegetation sites: bareground, R. alba grassland, M. japonica grassland, and grassland mixed with three examined species. R. alba is the earliest colonizer, and M. japonica and P. australis are the later ones. Seedling emergence, survival, growth and resource allocation were monitored for two growing seasons. The seedling emergence of all the species was lowest on bareground. The emergence of R. alba and P. australis was high in R. alba and mixed grasslands, but was low in M. japonica grassland. M. japonica seedlings emerged more in vegetation with moderate plant cover and litter. R. alba seedlings yielded the highest biomass on bareground, and the biomass decreased with increasing shading. R. alba did not change resource allocation of height to above-ground biomass between vegetation types. The seedling biomass of M. japonica was affected little by shading, probably because M. japonica adjusted resource allocation with shading. In the two summers, R. alba and M. japonica seedlings survived more when the neighboring seedlings emerged more. P. australis seedlings failed in overwintering. The seedling characteristics of each species explained the species replacement, i.e., R. alba invaded bareground and facilitated the establishment of late colonizer, M. japonica, by the shelter effects of the overstory and seedling.     

Occurrence patterns of facilitation by shade along a water gradient are mediated by species traits

In disturbed habitats, shade often has facilitative effects on plants by ameliorating water and thermal stresses. Facilitation by shade tends to increase as water availability decreases. At the same time, several studies have suggested that facilitation by shade is not affected by water status or collapses under extremely dry conditions. We hypothesized that traits of beneficiary plants, specifically, the flexibility in the allocation of biomass between shoots and roots, would mediate variation in the relationship between facilitation by shade and water status. To test this hypothesis, we examined the responses of two bog species to shade under various water conditions in a post-mined peatland. The seeds of Rhynchospora alba and Moliniopsis japonica were sown under three water levels (dry: 53% peat water content, wet: 77%, and control: 71%) × two shading levels (50% shaded and unshaded). The survival, biomass, and biomass allocation between the shoots and roots of the two species were monitored for two years. Shade increased the survival and biomass of both species. However, the facilitation of R. alba by shade was independent of water level, whereas the strength of the facilitative effects on M. japonica increased as water content decreased. R. alba preferentially allocated biomass to roots under dry conditions and was highly drought tolerant. M. japonica did not alter the allocation of its biomass in response to either shade or water level and was drought intolerant. Our results suggest that flexibility in biomass allocation of beneficiary plants mediates occurrence patterns of facilitation by shade along a water gradient. The facilitation of species with inflexible biomass allocation by shade through the amelioration of water stress increases as water availability decreases, whereas the facilitation of species with flexible biomass allocation is independent of water status. Such species-specific facilitation would promote the coexistence of diverse species in a community.     

Light field heterogeneity among tussock grasses: Theoretical considerations of light harvesting and seedling establishment in tussocks and uniform tiller distributions

Although the tussock growth form of caespitose graminoids is widespread, the effect of this growth form on light interception and carbon gain of tillers has received little attention. Daily incident photosynthetic photon flux density (PFD_inc) and carbon gain in monospecific stands of tussock grasses were compared with those of a hypothetical distribution with the equivalent tiller density per total ground area, but evenly distributed rather than clumped in tussocks. This was computed for two tussock grasses Pseudoroegneria spicata (Pursh) A. Löve (bluebunch wheatgrass) and Agropyron desertorum (Fisch, ex Link) Schult. (creasted wheatgrass) at different plant densities. Daily PFD_inc and net photosynthesis (A) were greater if tillers were distributed uniformly rather than clumped in tussocks, except when the density of tussocks was so great as to approach a uniform canopy. When tussock density per ground area was low, much of the difference between tussock and uniform tiller densities in PFD_inc and A was due to shading within the tussocks; up to 50–60% of the potential carbon gain was lost in A. desertorum due to shading within tussocks. In a matrix of tussocks, the light field for establishing seedlings was very heterogeneous; potential A ranged from 7 to 96% relative to an isolated seedling. The mean of daily PFD_inc and A for seedlings in a tussock stand were nearly identical to the values in corresponding stands of uniform tiller distributions. It is hypothesized that the loss of A resulting from clumping tillers into tussocks is offset by benefits of protecting sequestered belowground resources from invasion by seedlings of competitors.     

Carex sempervirens tussocks induce spatial heterogeneity in litter decomposition, but not in soil properties, in a subalpine grassland in the Central Alps

Tussocks of graminoids can induce spatial heterogeneity in soil properties in dry areas with discontinuous vegetation cover, but little is known about the situation in areas with continuous vegetation and no study has tested whether tussocks can induce spatial heterogeneity in litter decomposition. In a subalpine grassland in the Central Alps where vegetation cover is continuous, we measured soil properties [concentration of N, C, organic matter (OM) and pH] and monitored litter decomposition traits (dry mass loss, loss of C, N, P and K) inside and outside tussocks of Carex sempervirens. Soil C, N, OM concentrations or pH inside tussocks did not differ significantly from those outside tussocks. After 1 year of decomposition, litter dry mass loss, C and K loss were significantly smaller inside than outside tussocks. The slower litter decomposition inside tussocks was likely caused by the elevated tussock base, which made environmental conditions inside tussocks much dryer than those outside in early spring when snow melts. Our results suggest that in areas with continuous vegetation cover tussocks induce spatial heterogeneity in litter decomposition but not in soil properties.     

Kobresia tibetica tussocks facilitate plant species inside them and increase diversity and reproduction

Many plant species grow inside tussocks of some graminoids, but the underlying mechanisms remain unclear. We address whether some species occur and flower mainly inside tussocks so that species diversity and sexual reproduction are higher inside than outside tussocks, and whether relieving biological and physical stress is the mechanism associated with the facilitative process. In a heavily grazed grassland on the eastern Tibetan Plateau, where both physical (due to high altitude) and biological conditions (due to heavy grazing) are extremely harsh, we investigated vegetation in paired plots inside and outside 150 Kobresia tibetica tussocks and measured tussock basal area (=plot area). We also measured temperatures at soil surface, 5 and 10 cm depth and the number of animals (yaks, sheep and horses) grazing inside and outside tussocks. Sixty-seven percent of the species occurred and 42% flowered more frequently inside than outside tussocks, but none less frequently. Inside tussocks 78% species flowered, but outside tussocks only 31% did. Consequently, number of species, number of flowering species and number of inflorescences were all markedly larger inside than outside tussocks. Differences in number of species, number of flowering species and number of inflorescences inside and outside tussocks increased with increasing tussock basal area. Soil temperatures were lower inside than outside tussocks, but grazing intensity was much larger outside tussocks. Therefore, tussocks of K. tibetica facilitated the species inside them likely by grazing prevention, but not by increasing warmth. This study provides evidence that plant species colonizing tussocks of graminoids can be facilitated by the tussock species, and facilitation by grazing prevention may be one mechanism causing the coexistence of the species inside tussocks.     

The responses of an early (<Emphasis Type="Italic">Rhynchospora alba</Emphasis>) and a late (<Emphasis Type="Italic">Molinia japonica</Emphasis>) colonizer to solar radiation in a boreal wetland after peat mining

The responses of seedlings to solar radiation, including ultraviolet (UV), were investigated for Rhynchospora alba, an early colonizer, and Molinia japonica, a late colonizer, in a mined peatland in northern Japan. The solar radiation and rainfall were, respectively, higher and lower in 2008 than in 2009 during the field surveys. The seedlings were transplanted to bare ground, and measurements were made of the biomass, the allocation of biomass to shoots and roots, the absorbance of ultraviolet A and ultraviolet B, and the concentrations of anthocyanin and chlorophyll. R. alba did not change its biomass in response to any solar radiation treatment in 2008 and decreased shoot biomass with low UV and decreased root biomass with shade in 2009. Additionally, M. japonica did not change its biomass in 2008 but decreased its root biomass with low UV in 2009. The chlorophyll concentration of R. alba did not change in 2008 or 2009, whereas the chlorophyll concentration of M. japonica increased with decreased solar radiation, including UV. The UV absorbance of R. alba decreased under shade and with high peat moisture. In contrast, the content of UV-absorbing substances remained unchanged in M. japonica. Therefore, R. alba, the early colonizer, adapted more to strong solar radiation by changing its shoot-root allometry and producing UV-absorbing substances, whereas M. japonica, the late colonizer, tended to respond more to peat moisture. These differing responses to solar radiation and peat moisture may explain the temporal patterns of species replacement from early to late colonizers.     

Facilitation of plant species richness and endangered species by a tussock grass in a moist tall grassland revealed using hierarchical Bayesian analysis

Using hierarchical Bayesian analysis, we tested the hypothesis that a perennial tussock grass, Ischaemum aristatum var. glaucum, facilitates the species diversity of vegetation and the regeneration of individual native vascular plants in Ukishima Marsh, a moist tall grassland of eastern Japan. We analyzed microscale distribution patterns of sympatric plant species in response to ground height. Both the species richness of the whole plant assemblage and the occurrence of many native vascular plants, including two endangered species, were positively correlated with ground elevation, which was directly mediated by I. aristatum var. glaucum tussocks and/or the occurrence of mosses on tussocks. Susceptibility to late spring inundation of microsites on the tussocks was significantly lower than that on bare ground. These results suggest that the facilitation by I. aristatum var. glaucum contributes to maintaining the high species richness of the marsh. Consideration of the facilitation among native vascular plants is necessary for establishing sound conservation plans of wetland vegetation.     

Influence of a dominant macrophyte, <Emphasis Type="Italic">Juncus effusus</Emphasis>, on wetland plant species richness,diversity, and community composition

Few studies have experimentally investigated the influence of competition for light on structure and composition of wetland vascular plant communities, despite the well-documented high productivity in such systems. Influences of the dominant emergent wetland plant Juncus effusus on the surrounding macrophyte community were evaluated in a shallow freshwater wetland through two consecutive growing seasons. Permanent transects were constructed along diagonals of randomly oriented 1 m² plots centered around isolated, colonizing J. effusus tussocks. Percent areal cover was measured for each species or identifiable taxon group in 400 cm² sub-plots centered 10, 20, 30, and 40 cm from the tussock edge, to evaluate species richness, diversity, and dominance indices. Observational studies during the first growing season indicated that plots having larger, more heavily shading tussocks yielded significantly lower cover and species richness in the surrounding plant community than less shaded plots. Shading by J. effusus was reduced experimentally during the second growing season by holding culms in a vertical position in half of the J. effusus-occupied study plots in order to assess directly the influence of shading by J. effusus. Manipulated plots became more diverse and species-rich and developed higher total percent areal cover. Within shaded reference plots, reductions in cover, richness, and diversity were correlated with intensity of shading; each of these measures was lowest in sub-plots centered 10 cm from the tussock, where measured light reduction was highest. Neighborhood analyses of biomass, species composition, and light reduction indicated that individual species biomass varied with distance from J. effusus tussocks and shading intensity, an indication of the potential for shifts in community composition as populations of this dominant macrophyte expand to fill a wetland area. A mathematical model is presented to illustrate shading effects of J. effusus as a population grows from isolated, colonizing tussocks to an interacting system of tussocks in an established population of this dominant macrophyte.     

Negative Plant–Soil Feedback and Positive Species Interaction in a Herbaceous Plant Community

Increasing evidence shows that facilitative interaction and negative plant–soil feedback are driving factors of plant population dynamics and community processes. We studied the intensity and the relative impact of negative feedback on clonal growth and seed germination of Scirpus holoschoenus, a ‘ring’ forming sedge dominant in grazed grassland, and the consequences for species coexistence. The structure of aboveground tussocks was described. A Lithium tracer assessed belowground distribution of functional roots. Seed rain and seedling emergence were compared for different positions in relation to Scirpus tussocks. Soil bioassays were used to compare growth on soil taken from inside and outside Scirpus tussocks of four coexisting species (Mentha acquatica, Pulicaria dysenterica, Scirpus holoschoenus and Dittrichia viscosa). We also compared plant performance of dominant plant species inside and outside Scirpus tussocks in the field. The ‘ring’ shaped tussocks of S. holoschoenus were generated by centrifugal rhizome development. Roots were functional and abundant under the tillers and extending outside the tussocks. The large roots mats that were present in the inner tussock zone were almost all dead. Seedling emergence and growth both showed a strong negative feedback of Scirpus in the inner tussock zone. Scirpus clonal development strongly reduced grass biomass. In the degenerated tussock zone, Pulicaria and Mentha mortality was lower, and biomass of individual plants and seed production were higher. This positive indirect interaction could be related to species-specific affinity to soil conditions generated by Scirpus, and interspecific competitive release in the degenerated tussock zone. We conclude that Scirpus negative feedback affects its seedling emergence and growth contributing to the development of the degenerated inner tussock zone. Moreover, this enhances species coexistence through facilitative interaction because the colonization of the inner tussock zone is highly species-specific.     

Regeneration of native vascular plants facilitated by Ischaemum aristatum var. glaucum tussocks: an experimental demonstration

Facilitation of a perennial tussock grass, Ischaemum aristatum var. glaucum, was hypothesized to be the provision of germination safe sites against inundation. We conducted field census and seed addition experiments to test this hypothesis. Facilitation effects of I. aristatum var. glaucum tussocks were obvious; in contrast to 280 seedlings/m² of ten native species observed on tussocks, seedlings hardly emerged on bare ground, even after seed addition. Although effects of moss occurrence at tussocks were not significant on the total number or species richness of emerged seedlings, significantly positive effects were observed on the seedling survival of some species, including endangered species. Conservation of facilitators will efficiently ensure the regeneration success of native vascular plants.     

Potential and constraints for grasses to cope with spatially heterogeneous radiation environments

Placing plant organs into upper canopy layers or gaps is considered advantageous for avoiding neighbours. Current research only covers the lack of selective branching in response to heterogeneous radiation in dicots. Due to some unique shade avoidance regulatory pathways and clonal structure, directional response may occur in grasses. I used Festuca rubra L., a grass with two types of branches: intravaginal tillers developed from young buds close to the primary shoot, and extravaginal tillers developed from older buds and placed on rhizomes in a certain distance from the mother tussocks. Tussocks grown from vegetativelly multiplied initial tillers were exposed to three radiation regimes in two green-house experiments: full sunlight, full shading simulating canopy and heterogeneous radiation. Interaction of the treatments with orientation of the initial tillers relative to their mother tillers, which may constrain morphology of the developing tussocks, were studied. Even though shading decreased the number of intravaginal tillers, heterogeneous radiation did not influence direction of their outgrowth. However, shading activated dormant buds at the tussock base and thus stimulated extravaginal tillering, with a tendency to develop more extravaginal tillers towards the shaded sites.     

The propagule supply, litter layers and canopy shade in the littoral community influence the establishment and growth of Myriophyllum aquaticum

The exotic macrophyte species Myriophyllum aquaticum (Vell.) Verdc. is widely dispersed, mainly through stem fragments and rhizomes, in shallow littoral zones in which uniform canopy conditions are created by the pre-existing resident wetland plant species. Therefore, interactions between these two types of asexual propagules and the resident species may occur during the early establishment and growth phases of M. aquaticum. We tested the hypothesis that the establishment and growth of M. aquaticum are affected by three factors: the number of propagules supplied, the presence of a standing biomass of Carex japonica Thunb. and the presence of thick litter layers in the littoral zone. M. aquaticum rhizomes and stem fragments were introduced into a standing biomass of C. japonica in mesocosms using two types of sediments (littoral and sand-clay) and two propagule supply levels. After 4?months, the survival rates of both the rhizomes and stem fragments were high under all of the experimental treatments. The propagule supply positively affected the survival rates and growth of M. aquaticum. The survival of the rhizomes was unaffected by the presence of either C. japonica or littoral sediments, whereas the survival of the stem fragments was reduced by the presence of C. japonica. The presence of litter layers is a primary factor facilitating the growth of M. aquaticum propagules because of the high nutrient content of the litter. In addition, the presence of the standing C. japonica biomass and newly growing Eleocharis yokoscensis (Franch.et Sav.) Tang et Wang (a small ruderal species that, unexpectedly, grew rapidly following the removal of the standing biomass of C. japonica) reduced the growth of the M. aquaticum stem fragments and rhizomes, respectively. Our findings suggest that the loss of vegetative cover resulting from intense cattle herbivory and other factors in littoral zones may accelerate the invasion of M. aquaticum. An effective approach for preventing M. aquaticum invasion is to reduce the propagule supply and prevent propagules from dispersing.     

Fire temperatures and postfire plant community dynamics in Ecuadorian grass páramo

Three aspects of the páramo vegetation's response to fires were investigated: the measurement of fire temperatures, general observations of changes in plant communities following fires, and monitoring the fate of individual plants after burning.Fire temperatures were strongly influenced by the physiognomy of the vegetation, dominated by tussocks of Calamagrostis spp. Temperatures were highest amongst the upper leaves of the tussock (sometimes >500°C). The middle levels of the tussock experienced temperatures in excess of 400°C, but in the dense leaf bases temperatures were often below 65°C. On the ground between tussocks, temperatures were variable, whereas 2 cm below ground temperatures failed to reach 65°C.Plant survival depended on the intensity of the fire and the plant's position within the tussock structure. Survival was often the result of high temperature avoidance (with buds shielded by other plant parts or buried beneath the soil surface).Post-fire Calamagrostis tiller mortality rates were high and tussock regrowth was slow. Some other species appear to maintain their populations by exploiting this recovery phase for seedling establishment on tussocks.Between tussocks, changes of occupancy at the level of the individual plants were greater after fire than in control vegetation. Most transitions were random. Those which departed from random often involved gaps and were related to post-fire mortality, regrowth from below-ground parts, colonisation or, in the case of a clonal mat-forming species, to spatial rearrangement of rosettes. Recovery was slower at higher altitude. Recovery was much slower in burned plots when the upper 2 cm of soil was removed (along with buried plant parts) compared with burned plots.Qualitative observations suggest that recovery may consist of a cyclical process, mediated by the serial dominance of several species that are physiognomically important.The frequency of fires determines the amount of fuel accumulated within grass tussocks and some plants may be unable to survive repeated burning. Chance survival of species in unburned patches of vegetation and random colonisation of gaps may be important determinants of subsequent community structure.     

Multiclonal tussocks in the grass Achnatherum splendens (Trinius) Nevskia (Poaceae)

Clonal plant species often form genetically diverse populations, even when sexual reproduction in a population is rarely observed. Here we test whether the spatially discrete clusters of plants (tussocks of graminoids) formed within populations of some clonal species can likewise be multiclonal. We sampled leaves of ramets (shoots) within 20 tussocks of the grass Achnatherum splendens in the Otindag Sandland in Inner Mongolia, China, and genotyped the ramets using standard molecular protocols. The 20 tussocks were allocated to three classes: (i) small, circular, (ii) large, circular and (iii) large, irregular. Most tussocks (80%) were multiclonal and some contained at least eight different clones. Irregularly shaped tussocks contained twice as many clones as circular tussocks; neither size nor cover within a tussock affected number of clones per tussock, and the smaller clones in a tussock showed no tendency to occur on the edge or near the center of a tussock. These patterns seem more consistent with formation of multiclonal tussocks by coalescence than by colonization. Therefore, individual tussocks, especially large, irregular ones, cannot a priori be treated as genetic individuals without assessing their genetic information in, e.g., population demography, genetics and evolution studies.     

Performance of two Picea abies (L.) Karst. stands at different stages of decline

Summary The annual replacement of tillers of Agropyron desertorum (Fisch. ex Link) Schult., a grazing-tolerant, Eurasian tussock grass, was examined in the field following cattle grazing. Heavy grazing before internode (culm) elongation seldom affected tiller replacement. Heavy grazing during or after internode elongation, which elevates apical meristems, increased overwinter mortality of fall-produced tillers and reduced the number and heights of these replacement tillers. Unexpectedly, tussocks grazed twice within the spring growing season tended to have lower overwinter tiller mortality, greater tiller replacement, and larger replacement tillers than tussocks grazed only once in late spring. These responses of twice-grazed tussocks, however, were still less than those of ungrazed tussocks or tussocks grazed moderately in early spring. The presence of ungrazed tillers on partially grazed tussoks did not increase the replacement of associated grazed tillers relative to tillers on uniformly grazed plants. This result indicates that resource sharing among tillers, if present, is short-lived or ecologically unimportant in this species. Although A. desertorum is considered grazing-tolerant, tiller replacement on heavily grazed tussocks, particularly those grazed during or after internode elongation when apical meristems were removed, was usually inadequate for tussock maintenance. These observations at the tiller (ramet) level of organization in individual tussocks (genet) may explain the often noted reduction in stand (population) longevity with consistent heavy grazing.     

Occupational strategy of runner reed (Phragmites japonica): Change of growth patterns with developmental aging

The characteristics of P. japonica stands growing along ten transect in an AFP were assessed. Especially those of the reproductive organs in terms of developmental stages were focused. Four consecutive stages were observed based on the existence of reproductive organs (flowers and stolons). Plants in the ‘stolon’ and ‘flower and stolon’ stages played a role as a colonizer by stolonization (the longest length of stolon was more than 9 m) while those in the ‘flower’ stage played a role as an occupant via forming tall and dense shoots on tussocks (more than 2 m and about 20 shoots per tussock including dead standing shoots). Consecutive developmental stages according to changes in growth patterns, particularly in the conversion of reproductive organ from asexual (stolon) to sexual (flower), was the critical strategy of P. japonica for efficient monotypic occupation. This occupational strategy of P. japonica also resulted in a sharp decrease of the plant species richness (two to three including P. japonica) growing in the same area with P. japonica in the mature (‘flower’) stage.     

Effects of tussock size and soil water content on whole plant gas exchange in Stipa tenacissima L.: Extrapolating from the leaf versus modelling crown architecture

In this study we evaluated daily whole plant transpiration and net photosynthetic rates in Stipa tenacissima L. (Poaceae) tussocks of different sizes subjected to three levels of soil moisture. The crown architecture of 12 tussocks was reconstructed with the 3D computer model Yplant taking into account the morphology and physiology of the leaves determined at different soil moisture levels. We also calculated whole plant transpiration by extrapolating leaf transpiration in different senescence conditions measured with a diffusion porometer. This extrapolated transpiration overestimated transpiration, particularly when the soil moisture level was high (>15% of volumetric soil water content). At this high level of soil moisture, large tussocks (>60 cm in diameter), which were sexually mature and had a large leaf surface area, were the most efficient with regard to daily water use efficiency (whole plant net photosynthesis/whole plant transpiration). Whole plant water use efficiency decreased with tussock size primarily because small tussocks exhibited high transpiration rates. Small tussocks were more sensitive to soil drying than large and intermediate ones, presenting a faster rate of leaf senescence as water deficit increased. Leaf acclimation to irradiance, which was significantly influenced by the degree of mutual shading among neighbouring leaves, along with the ontogeny of the tussock and its effect upon leaf senescence were found to be the main mechanisms involved in the different responses to water limitations found in whole plant gas exchange variables. Our results show that the size of each individual plant must be taken into account in processes of scaling-up of carbon gain and transpiration from leaf to stand, as this is a particularly relevant aspect in estimating water use by semiarid vegetation.     

Environmental Heterogeneity, Animal Disturbances, Microsite Characteristics, and Seedling Establishment in a Quercus havardii Community

In restoration experiments it is imperative to consider the study of mechanisms of how species are maintained and preserved in a system. This paper reports on the results of a field experiment examining the growth and survival of seedlings of Schizachyrium scoparium, a dominant perennial bunchgrass member of the Quercus havardii (sand shinnery oak) communities of semiarid western Texas, on mounds of displaced soil produced by Sylvilagus auduboni (rabbit). The central question posed is: does environmental heterogeneity created by small mammals influence seedling survival and growth? The specific questions addressed are: (1) Does seedling survival, growth, and nutrient uptake vary when grown on mounds, off-mound soils, and artificially created mounds?; (2) What is the influence of the microbial and litter components of mound soils on seedling survival?; and (3) In communities where animal disturbances create environmental heterogeneity and may impact seedling establishment, is it possible to artificially create mounds that could serve a similar function? Results show that characteristics of mound soils increase seedling survival, shoot and root biomass, root length, number of tillers, mycorrhizal infection, and nutrient uptake more in plants grown on mounds than off mounds. Both the microbial and litter components of mound soils are essential components of this effect. Artificial mounds generated from soils associated with the herbaceous community were more similar to intact rabbit mounds than artificial mounds generated from soils associated with the oaks. The results indicate that rabbits produce rich patches (both nutrient and microbial) favorable to the growth of seedlings of the dominant bunchgrass, and point to the potential importance of rabbit disturbances in shaping the dynamics of this plant community. Thus, rabbit-generated disturbances produce environmental heterogeneity in the sand shinnery oak community, similar to that produced by harvester ants in this community as shown by an earlier study. Small animal activity that results in soil displacement, and influences soil characteristics, may indirectly contribute to the persistence of certain plant species within a community. Thus, disturbances may well operate in semiarid communities to produce nutrient and microbe rich microsites which may function to maintain diversity.     

Water Table Regime Regulates Litter Decomposition in Restiad Peatlands,New Zealand

A 5-year litterbag study examined decomposition rates at four sites representing restiad peatland succession in Waikato, New Zealand. Early successional sites were dominated by Baumea rubiginosa, or Leptospermum scoparium, mid-successional by Empodisma robustum, and late successional by Sporadanthus ferrugineus. Leaf/culm materials from these species were placed on the surface, and roots of Empodisma and Sporadanthus buried at depths of 5, 25, and 55 cm to test the influence of succession on species and site decomposition rates. Typha latifolia leaves from a Canadian bog were placed at the surface and three depths to allow comparisons with northern peatlands. Litterbags were retrieved after 0.5, 1, 2, 3, 4, and 5 years, and mass remaining characterized by an exponential model k value. Surface litter k values (0.12–0.80 y^?1) decreased from early to late successional species; however, decomposition was slower at more waterlogged early successional sites. Buried litter k values (0.04–0.24 y^?1) decreased with depth and increased from early to late successional sites, with Empodisma roots having the slowest rates. Few strong relationships existed between litter quality and decomposition rates. In contrast, water table regime strongly influenced decomposition rates; k values for the “standard” Typha litter decreased exponentially as period of saturation increased, irrespective of site successional status, nutrients, or other factors. Lower water tables in the more aerated later successional sites have led to faster decomposition rates. Ongoing drainage combined with the potential impacts of climate change may increase organic matter decomposition and accelerate carbon release into the atmosphere.