Above‐ and below‐ground plant biomass response to experimental warming in northern Alaska

Question: Does experimental warming, designed to simulate future warming of the Arctic, change the biomass allocation and mycorrhizal infection of tundra plants? Location: High Arctic tundra near Barrow, Alaska, USA (71°18′N 156°40′W). Methods: Above and below ground plant biomass of all species was harvested following 3–4 yr of 1‐2°C of experimental warming. Biomass allocation and arbuscular mycorrhizal infection were also examined in the two dominant species, Salix rotundifolia and Carex aquatilis. Results: Above‐ground biomass of graminoids increased in response to warming but there was no difference in total plant biomass or the ratio of above‐ground to below‐ground biomass for the community as a whole. Carex aquatilis increased above‐ground biomass and proportionally allocated more biomass above ground in response to warming. Salix rotundifolia increased the amount of above‐ and below‐ground biomass allocated per leaf in response to warming. Mycorrhizal infection rates showed no direct response to warming, but total abundance was estimated to have likely increased in response to warming owing to increased root biomass of S. rotundifolia. Conclusions: The community as a whole was resistant to short‐term warming and showed no significant changes in above‐ or below‐ground biomass despite significant increases in above‐ground biomass of graminoids. However, the patterns of biomass allocation for C. aquatilis and S. rotundifolia did change with warming. This suggests that long‐term warming may result in changes in the above‐ground to below‐ground biomass ratio of the community.     

Differential responses to defoliation frequency in little bluestem (<Emphasis Type="Italic">Schizachyrium scoparium</Emphasis>) in tallgrass prairie: implications for herbivory tolerance and avoidance

Plant responses to herbivory are complex. In grasses, relative growth rate (RGR), seed, and vegetative reproduction, resource allocation, and architecture vary differentially and often nonlinearly with grazing intensity. High grazing tolerance may be achieved through compensatory photosynthesis and leaf growth, or through demographic mechanisms such as activation of a belowground dormant bud bank. This study assessed the relationship between grazing frequency and responses of Schizachyrium scoparium (little bluestem) in a tallgrass prairie, and examined the roles of tiller growth, reproduction, and bud (meristem) populations in its persistence under grazing. Genets were subjected to varying simulated grazing frequencies for a period of 2 years. Strong differential responses were observed among plant traits. RGR, biomass, and flowering showed strong nonlinear reductions in response to increasing clipping frequency, with no evidence of threshold effects. However, meristem density was unaffected, and plants maintained a large bud bank across all clipping treatments. Tiller natality decreased initially, but increased with >4 clippings, suggesting that declines in tiller RGR are partially offset by increasing tiller natality, and that variation in genet size is driven more by demography than by variation in individual tiller growth. Increased grazing frequency also resulted in differential activation of buds at different positions (emerging within vs. outside the subtending leaf sheath), explaining the shift to a more prostrate growth form observed in many caespitose grasses under persistent grazing. Thus, although this grass species lacks the capacity for compensatory foliage re-growth, the maintenance of a large dormant bud bank and the differential activation of buds in different positions contribute to its grazing tolerance and avoidance, respectively, and its long-term persistence in grazed grasslands.     

Does selective defoliation mediate competitive interactions in a semiarid savanna? A demographic evaluation

Abstract. Three patterns of target-neighbor plant defoliation were imposed on a late-seral, perennial, C₄-grass, Bouteloua curtipendula, in three long-term grazing regimes to determine the influence of selective defoliation on competitive interactions and species replacement in a semiarid savanna on the Edwards Plateau, Texas, USA. Short-term (3-yr) target plant defoliation did not significantly affect either tiller or plant responses in any of the three grazing regimes. Neighbor plant defoliation, either alone or in combination with target plants, produced a significant defoliation interaction with time for tiller number and basal area per plant, but not for tiller recruitment or mortality. The minimal effect of selective defoliation on the intensity of competitive interactions in this semiarid community indicates that selective grazing has a less definitive role in mediating herbivore-induced species replacement than it does in mesic grasslands and savannas. This interpretation is discussed within the context of long-term (45-yr) change in herbaceous vegetation associated with grazing in this community. Cumulative tiller recruitment in the intensively grazed regime was only 44% of that in the ungrazed regime because of greater plant mortality and fewer surviving plants that recruited tillers. Target plant mortality (50%) only occurred in the intensively grazed regime and the proportion of target plants that initiated tillers decreased by 70, 48 and 32% in the ungrazed, moderately and intensively grazed regimes, respectively, during the final two years of the investigation. The decrease in cumulative tiller recruitment in all grazing regimes was probably mediated by a drought-induced increase in median tiller age the second year of the study. However, tiller per tiller recruitment rate among plants that recruited at least one tiller remained relatively constant among grazing regimes and years. Intensive, long-term grazing has modified the population structure of this late-seral perennial grass to the extent that population responses to both herbivory and periodic drought have been altered in comparison with those of ungrazed and moderately grazed populations. Ecological consequences of a herbivore-induced transition in population structure may be to minimize the effect of selective herbivory on competitive interactions and to function as an avoidance mechanism to reduce the probability of localized population extinction in response to intensive long-term herbivory.     

Plant responses to selective grazing by bison: interactions between light,herbivory and water stress

Two abundant tallgrass prairie forb species, Ambrosia psilostachya and Vernonia baldwinii, are commonly found intact in patches where the grasses have been selectively grazed by bison. Microclimatic patterns and physiological responses of these forbs were measured in grazed and ungrazed patches. These experiments demonstrated that bison herbivory indirectly enhanced water availability and productivity of forbs growing in grazed patches. This was due primarily to the reduction in transpiring grass leaf area in grazed patches and an increase in light availability. In grazed patches, incident light at forb mid-canopy height was 53% greater than ungrazed sites at midseason and soil temperatures were always warmer (e.g., 10°C at 5 cm), perhaps enabling forbs to initiate growth earlier in the spring. Enhanced leaf xylem pressure potential and stomatal conductance in plants in grazed areas were most evident when water availability was low (i.e., late in the growing season and over short-term dry periods characteristic of the tallgrass prairie environment). Relative to individuals in ungrazed areas, end-of-season biomass of A. psilostachya was 40% greater and reproductive biomass and head number of V. baldwinii was 45% and 40% greater, respectively, in plants in grazed patches. A favorable growing environment maintained in grazed patches during periods of water limitation enhances carbon gain in forbs leading to increased biomass and potential fitness.     

VARIATION OF STOMATAL DISTRIBUTION IN CAREX AQUATILIS (CYPERACEAE)

The density and distribution of stomates in Carex aquatilis Wahl. in the Pacific Northwest were examined using epidermal peels of samples of leaves from natural populations, from greenhouse-grown transplants and from seedling families grown under controlled conditions. These were compared to stomatal distributions of populations in eastern North America. Populations of Carex aquatilis Wahl. form 2 groups based on the distribution and density of stomates. Carex aquatilis var. dives (Holm) Kükenthal is epistomatic, with adaxial stomatal densities of 28.7–48.5/0.1 mm². The C. aquatilis var. aquatilis is amphistomatic, with adaxial stomatal densities of 8.1–22.2/0.1 mm² and abaxial densities of 11.3–24.5/0.1 mm² in the Pacific Northwest. Total stomatal frequencies are similar in both groups. Stomatal distribution and densities are here shown to not vary significantly within populations and appear to be genetically determined, as shown by progeny tests and growth of seedlings under uniform and experimental conditions. Stomatal distribution in Carex aquatilis appears to be adaptive, and intraspecific variation provides a system for determining the adaptive significance of differences in stomatal patterns.     

Grazing‐induced changes in plant–soil feedback alter plant biomass allocation

Large vertebrate herbivores, as well as plant–soil feedback interactions are important drivers of plant performance, plant community composition and vegetation dynamics in terrestrial ecosystems. However, it is poorly understood whether and how large vertebrate herbivores and plant–soil feedback effects interact. Here, we study the response of grassland plant species to grazing‐induced legacy effects in the soil and we explore whether these plant responses can help us to understand long‐term vegetation dynamics in the field. In a greenhouse experiment we tested the response of four grassland plant species, Agrostis capillaris, Festuca rubra, Holcus lanatus and Rumex acetosa, to field‐conditioned soils from grazed and ungrazed grassland. We relate these responses to long‐term vegetation data from a grassland exclosure experiment in the field. In the greenhouse experiment, we found that total biomass production and biomass allocation to roots was higher in soils from grazed than from ungrazed plots. There were only few relationships between plant production in the greenhouse and the abundance of conspecifics in the field. Spatiotemporal patterns in plant community composition were more stable in grazed than ungrazed grassland plots, but were not related to plant–soil feedbacks effects and biomass allocation patterns. We conclude that grazing‐induced soil legacy effects mainly influenced plant biomass allocation patterns, but could not explain altered vegetation dynamics in grazed grasslands. Consequently, the direct effects of grazing on plant community composition (e.g. through modifying light competition or differences in grazing tolerance) appear to overrule indirect effects through changes in plant–soil feedback.     

Demographic variability in tiller populations of two perennial pampa grasses

Abstract. The general objectives of this study were: (1) to investigate the importance of internal influences in regulating the tiller dynamics in natural populations of the warm-season perennial grasses Paspalum dilatatum and Sporobolus indicus, coexisting in Argentine flooded pampa, in as much as they act independently of the underlying external environment, and (2) to evaluate the extent to which interactions between internal and external factors affect the variation in tiller dynamics within such populations. Within-population variation in seasonal development of plants and tillers with different neighbour composition was studied for an annual growth cycle. Tiller survival and tillering were significantly influenced by tiller size. Tiller age influenced tiller fate, as suggested by the additive effects of age and size of tillers. These relationships varied with season and with species. Size and age of tillers showed additive effects with their neighbouring species on the tiller fate of P. dilatatum, but the effects of age and size of S. indicus changed according their neighbourhood. Tiller survival of S. indicus during the early growth season was more size-dependent when the cold-season species Poa lanigera, was the principal neighbour. Flowering and tillering probabilities were positively related through their common positive responses to tiller size. Tiller survival and recruitment between different seasons were strongly related. Independently of neighbour composition, tiller survival was generally inversely related to tiller recruitment in previous seasons. Therefore, significant density-dependent mortality of tillers was found for both species during the early summer when tiller density was expressed by basal area units.     

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.     

Direct and indirect effects of grazing constrain shrub encroachment in semi‐arid Patagonian steppes

Question: What are the long‐term effects of grazing exclusion on the population structure and dynamics of, and interactions among, three dominant shrub species? Location: Grass‐shrub Patagonian steppe, Chubut, Argentina. Methods: Permanent plots were established in grazed paddocks and paddocks excluded from grazing in representative Patagonian rangelands. Shrub abundance, population size‐structure, short‐term (two 3‐yr periods) and long‐term (matrix models) population dynamics, and neighborhood interactions of three native and codominant shrub species (Mulinum spinosum, Senecio filaginoides and Adesmia volckmanni) were measured and analysed using different statistical approaches. Results: The total density of shrubs was 74% higher in paddocks excluded from grazing, owing mainly to increases in Mulinum (80%) and Senecio (68%) species. However, differences in size structure between ungrazed and grazed paddocks were only detected in Mulinum. Demographic rates differed between shrub species, time‐periods and grazing conditions. In particular, recruitment in the short term (especially in wet years) and population growth rate in the long term (λ) were higher in paddocks excluded from grazing only in Mulinum populations. Senecio populations showed a marginal increase in recruitment and mortality independent of the grazing condition in the wet and dry period. Grazing exclusion modified the balance of neighborhood interactions among the three shrub species. In grazing‐exclusion paddocks, there was a balance between positive and negative interspecific interactions, while in grazed paddocks there were more negative intraspecific and interspecific interactions, resulting in a net negative balance of neighborhood interactions. Conclusions: Our understanding of woody encroachment in arid rangelands can be informed through evaluation of direct and indirect effects of grazing exclusion on the abundance and demography of dominant woody species. In Patagonian arid steppes, the occurrence of woody encroachment in rangelands excluded from grazing can be explained by altered responses in plant‐animal and plant‐plant interactions among shrub species.     

Effects of livestock exclusion on density,survival and biomass of the perennial sagebrush grass Hymenachne pernambucense (Poaceae) from a temperate fluvial wetland

In Argentina, the intensification of soybean production has displaced a substantial proportion of cattle ranching to fluvial wetlands such as those in the Delta of the Paraná River. Cattle grazing affects structure and dynamics of native forage plants but there is little information on this impact in populations from fluvial wetlands. This study addresses the effect of cattle ranching on density, survival, mean life-span and aerial biomass of Hymenachne pernambucense (Poaceae), an important forage species in the region. The study was carried out monthly for one year in permanents plots subject to continuous grazing and plots excluded from grazing in the Middle Delta of the Paraná River. In plots excluded from grazing, tillers showed significantly higher population density and survival, and a two-fold increase in mean life-span, while continuous grazing decreased survival of cohorts. The largest contribution to tiller density in ungrazed and grazed populations was made by spring and summer cohorts, respectively. Total and green biomass were significantly higher in the ungrazed population, with highest differences in late spring-early summer. Cattle grazing affected the relationship between tiller density and green biomass suggesting that cattle prefer sprouts because they are more palatable and nutritious than older tissue.     

Local Adaptation and Effects of Grazing among Seedlings of Two Native California Bunchgrass Species: Implications for Restoration

Adaptation to environmental factors may influence the germination and establishment of focal species in ecological restoration. Reciprocal transplants remain one of the best methods to detect local adaptation, but long‐term studies are often not feasible. We conducted reciprocal transplants of the native California bunchgrasses Elymus glaucus and Bromus carinatus between two central California locations to seek evidence of adaptation to local environmental conditions in a single growing season. Experimental plots at one location included grazed and ungrazed sites. The combination of locations and grazing treatments allowed us to determine whether the ability to detect evidence for adaptation depended on grazing regime. In addition, we measured the direct effects of grazing on seedling growth and survival concurrent with our investigation of local adaptation. We detected a homesite advantage for seedling growth or survival in both species, but the factors contributing to adaptive differentiation were species specific. Evidence of local adaptation was detected for seedling biomass in Bromus and for survivorship in Elymus. The homesite advantage observed in both species was greatly reduced under grazed conditions and in Elymus was significant only in the ungrazed plots. Climate and soil analyses detected significant differences between locations in five soil attributes and two climate variables. In particular, differences in exchangeable magnesium indicated that one of the two transplant locations consisted of serpentine soil, which is widely known to drive adaptation in plant populations. Together, these results suggest that it is possible to investigate the scale and factors involved in local adaptation with short‐term transplant studies.     

Reproductive challenges of a rare grass,Calamagrostis porteri subsp. insperata (Swallen) C. Greene: implications for habitat restoration

Background: Habitat management for reproductively challenged rare species is a problem when there is insufficient knowledge of their autecology. This study investigated reproductive failure in the rare grass Calamagrostis porteri ssp. insperata (Swallen) C. Greene (Reed bentgrass). Does the management recommendation of high light stimulate clonal growth, flowering, and seed production? Location: Shawnee National Forest, IL, USA, and in a greenhouse and an experimental garden at Southern Illinois University, Carbondale, IL, USA. Methods: Clones obtained from the three known Illinois populations were grown in a glasshouse under experimental light and soil moisture treatments. After 3 years, plants from the high light treatment were planted outside in an experimental garden where the light treatments were maintained for two more years. In the field, vegetative and flowering tiller density, canopy cover, and associated biotic and abiotic variables including abundance of co‐occurring plant species were monitored for 5 years. The overhead tree canopy was cleared over a portion of one population. Results: In the glasshouse, plants increased in size under high light and moist soil, and there were size differences among populations. Sixty‐six per cent (20 of 30) of the genets flowered when planted outdoors under full sunlight but did not produce seed. In the field, flowering only occurred in Calamagrostis growing in the cleared area, but no seed were produced. The plants in the flowering population were smaller than plants in the other two populations. The herbaceous community associated with Calamagrostis in the open diverged from the communities remaining under the shade. Conclusions: This study highlights the difficulty of managing reproductively challenged rare species. Calamagrostis populations can be managed to enhance clonal growth, but establishment of new populations would require translocation of vegetative material as it is highly unlikely that seed can be obtained.     

Shoot and leaf demography of Carex curvula ssp. Curvula and Carex curvula ssp. rosae in the Central Alps

Abstract. Populations of Carex curvula ssp. curvula, C. curvula ssp. rosae and their intermediate form were investigated in the Central Alps over a three-year period. The closely related taxa showed a different dominance behaviour in their respective communities. This may be caused by different growth strategies and a different reproduction biology. Therefore, the main aim of the study was to compare the demography of the three taxa. Shoot and leaf turnover, flower and seed production and population half-lives were determined. Differences in growth dynamics were less pronounced between the two species of Carex curvula than between these taxa and their intermediate form which showed the highest shoot turnover, highest number of fertile inflorescences and highest number of seeds per inflorescence. Carex curvula ssp. rosae showed a slightly higher shoot and leaf production but a lower reproduction capacity than Carex curvula ssp. curvula. Recruitment of populations of the three Carex-forms was only by vegetative shoots. Flowering had a striking effect on the shoot dynamics of the species grown in the grassland sites: up to about 70 % of all dead shoots could be identified as dead flowering shoots from the previous year. The different dominance behaviour of the three taxa could not be explained by their demographical features. Interspecific interactions and the occurrence of microniches might affect the growth and reproduction processes of the taxa.     

Morphologic and allozymic variation between long-term grazed and non-grazed populations of the bunchgrass Schizachyrium scoparium var. frequens

Summary Plant populations of Schizachyrium scoparium var. frequens with a history of long-term grazing by domestic herbivores were characterized by shorter and narrower leaf blades and tillers of lower weight than plants from populations with a history of no grazing. Following four biweekly defoliation events plants from the grazed populations additionally displayed lower specific leaf weights, lower amounts of biomass removed per tiller upon defoliation and a greater number of new and regrowing tillers. Production values per plant remained similar between the two populations because of a greater number of tillers per plant in the grazed population.A large amount of allozymic polymorphism was observed in both the grazed and non-grazed populations. Results of the electrophoretic analyses indicated that a minimum of 66% of the plants sampled represented separate genotypes. No distinction could be made between grazed and non-grazed populations in terms of allozymic partitioning. The morphological variation observed between these two populations may represent a shift in the relative dominance of a series of genotypes variously adapted to herbivory.     

Growth responses of arctic graminoids following grazing by captive lesser snow geese

The effects of grazing by captive goslings of the Lesser Snow Goose on coastal vegetation at La Pérouse By. Manitoba were investigated. Swards of Carex subspathacea, Festuca rubra and Calamagrostis deschampsioides were grazed once for different periods (0–180 min) and regrowth of vegetation determined, based on measurements of standing crop, net above-ground primary production (NAPP) and forage quality (leaf nitrogen content). The amounts of foliage removed from swards of Carex subspathacea increased with the length of the grazing period, but after 44 days of regrowth there were no significant differences in above-ground biomass between control and grazed plots. While the amount of foliage removed by goslings from swards of Festuca rubra increased with the length of the grazing period (except after 150 min of grazing), the increase in biomass following defoliation was similar among treatments. Goslings removed little biomass from swards of Calamagrostis deschampsioides, even when the opportunity for grazing was 180 min. No significant differences in standing-crop or NAPP between grazed and ungrazed plots were detected by the end of summer. Grazing had no significant effect on amounts of nitrogen in leaf tissue of all species, suggesting that faecal nitrogen was not rapidly incorporated into plant biomass within the growing season. Patterns of regrowth of these species are compared to that of Puccinellia phryganodes. An increase in goose numbers in recent years has led to birds foraging on less preferred species, such as Calamagrostis deschampsiodes and Festuca rubra. Their poor nutritional quality and a lack of a rapid growth response following defoliation may explain, in part, the decline in the weight of wild goslings recorded over the last decade.     

Effects of bison grazing on Andropogon gerardii and Panicum virgatum in burned and unburned tallgrass paririe

Summary Responses to clipping and bison grazing in different environmental contexts were examined in two perennial grass species, Andropogon gerardii and Panicum virgatum, on the Konza Prairie in northeastern Kansas. Grazed tillers had lower relative growth rates (RGR) than clipped tillers following defoliation but this difference was transient and final biomass was not affected by mode of defoliation. Grazed tillers of both species had higher RGR throughout the season than ungrazed tillers, resulting in exact compensation for tissue lost to defoliation. However, A. gerardii tillers which had been grazed repeatedly the previous year (1988) had reduced relative growth rates, tiller biomass and tiller survival in 1989. This suggests that the short-term increase in aboveground relative growth rates after defoliation had a cost to future plant growth and tiller survival.In general, the two species had similar responses to defoliation but their responses were altered differentially by fire. The increase in RGR following defoliation of A. gerardii was relatively greater on unburned than burned prairie, and was influenced by topographic position. P. virgatum responses to defoliation were similar in burned and unburned prairie. Thus grazing, fire, and topographical position all interact to influence tiller growth dynamics and these two species respond differently to the fire and grazing interaction. In addition, fire may interact with grazing pattern to influence a plants' grazing history and thus its long-term performance.     

Alterations in the biomass-specific productivity of periphyton assemblages mediated by fish grazing

1. In an experimental flume, we examined the effects of a biomass reduction and alteration of taxonomic composition, because of grazing by the fish Plecoglossus altivelis, on the net biomass accumulation of periphyton. 2. Grazed and ungrazed assemblages with different biomass and taxonomic composition were first prepared in fish enclosures and exclosures, respectively. These assemblages were then set out in the flume and incubated for 2 days under grazing‐free conditions to examine (i) the relationship between biomass and biomass accumulation rate and (ii) the effect of taxonomic composition on the relationship between these two. 3. The grazed and ungrazed assemblages were dominated by upright filamentous cyanobacteria and diatoms, respectively. The rate of biomass accumulation decreased with increasing periphyton biomass in both the grazed and ungrazed assemblages, and was lower in the grazed than the ungrazed assemblages at any biomass level. 4. The results showed that the reduction in biomass and the alteration of taxonomic composition due to fish grazing have opposite effects on biomass‐specific productivity. Biomass accumulation rate increased in response to biomass reduction, although a shift in dominance from diatoms to upright filamentous cyanobacteria decreased the overall productivity of the periphyton.     

Below‐ground biomass and productivity of a grazed site and a neighbouring ungrazed exclosure in a grassland in central Argentina

Abstract We estimated the below‐ground net plant productivity (BNPP) of different biomass components in an intensively and continuously 45‐ha grazed site and in a neighbouring exclosure ungrazed for 16 years for a natural mountain grassland in central Argentina. We measured approximately twice as much dead below‐ground biomass in the grazed site as in the ungrazed site, with a strong concentration of total below‐ground biomass towards the upper 10 cm of the soil layer in both sites. The main contribution to total live biomass was accounted for by very fine (<0.5 mm) and fine roots (0.5–1.0 mm) both at the grazed (79%) and at the ungrazed (81%) sites. We measured more dead biomass for almost all root components, more live biomass of rhizomes, tap roots and bulbs, and less live biomass of thicker roots (>1 mm) in the grazed site. The seasonal variation of total live below‐ground biomass mainly reflected climate, with the growing season being limited to the warmer and wetter portion of the year, but such variation was higher in the grazed site. Using different methods of estimation of BNPP, we estimated maximum values of 1241 and 723 g m^?2 year^?1 for the grazed and ungrazed sites, respectively. We estimated that very fine root productivity was almost twice as high at the grazed site as at the ungrazed one, despite the fact that both sites had similar total live biomass, and root turnover rate was twofold at the grazed site.     

Top-down impact through a bottom-up mechanism: the effect of limpet grazing on growth,productivity and carbon allocation of Zostera marina L. (eelgrass)

The unusual appearance of a commensal eelgrass limpet [Tectura depicta (Berry)] from southern California at high density (up to 10 shoot^–1) has coincided with the catastrophic decline of a subtidal Zostera marina L. meadow in Monterey Bay, California. Some commensal limpets graze the chloroplast-rich epidermis of eelgrass leaves, but were not known to affect seagrass growth or productivity. We evaluated the effect on eelgrass productivity of grazing by limpets maintained at natural densities (8±2 shoot^–1) in a natural light mesocosm for 45 days. Growth rates, carbon reserves, root proliferation and net photosynthesis of grazed plants were 50–80% below those of ungrazed plants, but biomass-specific respiration was unaffected. The daily period of irradiance-saturated photosynthesis (H _sat) needed to maintain positive carbon balance in grazed plants approached 13.5 h, compared with 5–6 h for ungrazed plants. The amount of carbon allocated to roots of ungrazed plants was 800% higher than for grazed plants. By grazing the chlorophyll-rich epidermis, T. depicta induced carbon limitation in eelgrass growing in an other-wise light-replete environment. Continued northward movement of T. depicta, may have significant impacts on eelgrass production and population dynamics in the northeast Pacific, even thought this limpet consumes very little plant biomass. This interaction is a dramatic example of top-down control (grazing/predation) of eelgrass productivity and survival operating via a bottom-up mechanism (photosynthesis limitation).     

Neighbour influence on the tiller demography of two perennial pampa grasses

Abstract. The aim of this study was to evaluate the possibility to use neighbour species composition to explain demographic variations in seasonal tillers in perennial grasses. Two warm-season species of the Argentina flooded pampa, Paspalum dilatatum and Sporobolus indicus, were used as models. Tiller structure and dynamics of target plants and the identity of their nearest neighbours were analyzed within a natural grassland community for an entire annual growth cycle. Canonical Correlation Analysis of tillering behaviour of target plants and neighbour species composition showed significant relationships for summer neighbourhood data. Community components affecting tiller demography were identified, and their spatial variation was described as different gradients of neighbourhood composition (NCG). NCG varied between target species, determining different spatial variation in neighbouring canopy dynamics and tiller dynamics. The tiller structure and dynamics of P. dilatatum plants were significantly correlated with the seasonal pattern of their neighbouring canopy density (NCD). Plants placed on the NCD-fluctuating extremes along NCG showed an unstable modular structure and short-lived tillers, as compared to those placed in the centre of the gradient. Relative density of the neighbouring canopy showed little variation along the NCG of S. indicus plants, which presented a tiller structure more stable and uniform than P. dilatatum plants did. Identification of NCG may be a promising approach to understanding changes in tiller dynamics of target species in relation to spatial and temporal changes in community structure.