Plant responses to nutrient addition and predictive ability of vegetation N:P ratio in an austral fen

1. Previous studies of the N:P ratio in wetland plants have been carried out in northern hemisphere wetlands where atmospheric nitrogen deposition is higher. There is little research on foliar N:P ratio as a potential indicator of nutrient limitation in vegetation communities in southern hemisphere wetlands. This study aimed to redress this knowledge gap and answer the following questions: how well does the plant tissue nitrogen to phosphorus (N:P) ratio predict wetland plant community nutrient limitation, as indicated by vegetation standing stocks and below-ground biomass, in southern hemisphere fens? Secondly, what are the impacts of realistic upper levels of farm nutrient run-off on natural montane fen vegetation?
2. Low (35 kg ha⁻¹ year⁻¹) and high (70 kg ha⁻¹ year⁻¹) levels of nitrate-N or ammonium-N with and without P (20 kg ha⁻¹ year⁻¹) were added to 81 vegetation plots over a period of 2.75 years. Species composition, plant nutrient status, and above-ground live vegetation standing stocks were assessed after 3 years, and below-ground biomass after 2 years.
3. Plant tissue analysis suggested the community was N limited or N and P co-limited; we found greater standing stocks of vegetation in plots treated with 70 kg ha⁻¹ year⁻¹ ammonium-N, indicating N limitation. No difference between other treatments was found in above-ground standing stocks or below-ground biomass. Plant species cover increased in both high N treatments, consistent with N limitation. These changes in plant species cover were accompanied by significant decreases in species richness in both high N treatments. Native species dominated the vegetation and this was unaffected by nutrient addition (90% cover).
4. This is one of the first studies to test and find support for the N:P ratio in southern hemisphere wetlands. Observed declines in species richness after N fertilisation in an N-limited fen suggests increased N may pose risks to austral wetlands. Responses by plant communities (changes in composition, biomass) to lower levels of nutrient addition may require longer periods of fertilisation to be apparent in slow growing ecosystems.

     

Biomechanical properties and holdfast morphology of coenocytic algae (Halimedales, Chlorophyta) in Bocas del Toro, Panama

For attached marine organisms, specific biomechanical properties may result in detachment or in tissue loss, when sufficient tensile force is applied. Algae experience such forces through water movement, which may thus act to limit size, abundance, and species composition, of populations of algae.Coenocytic construction is uncommon in the algae, but it occurs relatively more frequently in green algae found in shallow subtidal sediments associated with coral reefs, e.g., at our study site of Isla Colon, Bocas del Toro, Panama. We studied the biomechanical properties of some tropical coenocytic algae (Udotea flabellum (Ellis et Solander) Lamouroux, Penicillus capitatus Lamarck, P. pyriformis A. and E.S. Gepp, and Halimeda gracilis Harvey) anchored in sediments. We compare our results with published data on other coenocytic algae, as well as with multicellular algae. Our results show that properties of sand-dwelling coenocytes, such as mean force to dislodge (4.9-12.7 N), mean force to break (6.6-22.1 N), and mean strength (1.0-7.0 MN m^− 2), are all within the range reported for temperate, multicellular, algae. In contrast, the coenocytes differed markedly from the temperate non-coenocytes in the consequences of applied tensile force: coenocytes were removed whole, while most temperate algae attached to rocks break within the thallus. Some multicellular algae can regrow from tissue left on the substratum; three of the four coenocytic species we examined had rhizoids connecting closely adjacent (0.1-0.15 m) individuals, and these rhizoids may serve to regrow a new individual. While our experiments indicated that sufficient tensile force results in dislodgment, calculations using the experimentally determined variables led us to conclude that water velocities sufficient to dislodge individuals are unlikely to occur. Since dislodgment is usually fatal for algae, the role of the holdfast is a critical one. All of the species we investigated had similar holdfast morphology, a mass of rhizoids which entrained sand, the entire unit forming a hemispherical to cylindrical mass. Despite the consistency in holdfast form, and the initial prediction that this was an optimal form for anchoring these algae, our data suggest that this is not the case.     

Plant growth, community structure, and nutrient removal in monoculture and mixed constructed wetlands

The aim of this study was to compare the growth, community structure, and nutrient removal rates between monoculture and mixed wetlands, based on the hypothesis that it depends on the plant species used in the wetlands as to whether monoculture or mixed wetland is superior in plant growth and nutrient removal. Pilot-scale monoculture and mixed constructed wetlands were studied over 4 years. The monoculture wetland had a community height similar to the mixed wetland during the early years but a significantly lower height than the mixed wetland (P < 0.05) during the last year. The mixed wetland also displayed a higher plant density than the monoculture wetland (P < 0.05). The leaf area index in the monoculture wetland was significantly higher in the first year (P < 0.05) and significantly lower in the later years (P < 0.05) than that in the mixed wetland. The monoculture wetland had a similar vertical distribution of below-ground biomass over 4 years, while the mixed wetland showed a significant change in vertical distribution of below-ground biomass in the last 2 years. The monoculture wetland had a larger (P < 0.05) above-ground biomass and a similar leaf biomass in the first year, and a smaller above-ground biomass (P < 0.05) and a smaller leaf biomass (P < 0.05) than the mixed wetland during the latter 2 years. The amount of standing dead mass was smaller in the mixed wetland than in the monoculture wetland (P < 0.05). The mixed wetland exhibited a significantly lower NH₄-N removal rate in the first year (P < 0.05), and significantly higher NH₄-N removal rate in the last year, when compared to the monoculture wetland (P < 0.05). The study indicated that species competition and stubble growth resulted in significant differences between monoculture and mixed constructed wetlands in plant growth, community structure, and nutrient removal rates.     

Above- and below-ground resource acquisition strategies determine plant species responses to nitrogen enrichment

Background and AimsKnowledge of plant resource acquisition strategies is crucial for understanding the mechanisms mediating the responses of ecosystems to external nitrogen (N) input. However, few studies have considered the joint effects of above-ground (light) and below-ground (nutrient) resource acquisition strategies in regulating plant species responses to N enrichment. Here, we quantified the effects of light and non-N nutrient acquisition capacities on species relative abundance in the case of extra N input.MethodsBased on an N-manipulation experiment in a Tibetan alpine steppe, we determined the responses of species relative abundances and light and nutrient acquisition capacities to N enrichment for two species with different resource acquisition strategies (the taller Stipa purpurea, which is colonized by arbuscular mycorrhizal fungi, and the shorter Carex stenophylloides, which has cluster roots). Structural equation models were developed to explore the relative effects of light and nutrient acquisition on species relative abundance along the N addition gradient.Key ResultsWe found that the relative abundance of taller S. purpurea increased with the improved light acquisition along the N addition gradient. In contrast, the shorter C. stenophylloides, with cluster roots, excelled in acquiring phosphorus (P) so as to elevate its leaf P concentration under N enrichment by producing large amounts of carboxylate exudates that mobilized moderately labile and recalcitrant soil P forms. The increased leaf P concentration of C. stenophylloides enhanced its light use efficiency and promoted its relative abundance even in the shade of taller competitors.ConclusionsOur findings highlight that the combined effects of above-ground (light) and below-ground (nutrient) resources rather than light alone (the prevailing perspective) determine the responses of grassland community structure to N enrichment.     

Allometric relationships of field populations of two clonal species with contrasting life histories, Cladium jamaicense and Typha domingensis

Allometric analysis was used to examine morphological relationships in field populations of two clonal plants, Cladium jamaicense and Typha domingensis, in a Florida Everglades wetland. We found that allometric relationships of individuals sampled from field populations could be adequately derived and applied to analyzing both leaf and ramet growth responses to site differences along a nutrient gradient. Overall, the allometric relationships showed a significant departure from isometry which indicates that the relationships were size-dependent. Leaf-level morphological relationships were significantly different between species and between sites along the nutrient gradient. These differences, however, were not expressed on the ramet-level. Neither species expressed a plastic allocation response to site differences along the nutrient gradient. Biomass allocation between above- and below-ground for both species indicated significant size-dependent relationships with decreasing relative allocation below-ground with increasing size. Models for predicting total plant biomass (above- and below-ground) for both C. jamaicense and T. domingensis were developed based on two non-destructive measurements that are easily obtainable in the field. The models followed the equation log (biomass) = α + β₁ × log (height) + β₂ × log (basal area), where α was species specific while β₁ and β₂ were similar for both species but significantly different according to site along the nutrient gradient. Analysis of this model showed that plant height had a relatively greater influence on biomass than basal area at all sites. This difference was greatest at the un-enriched area where plants tend to be short and thick and the least at the moderately enriched site where the relative influence of both parameters was similar.     

Nitrogen losses from perennial grass species

Nitrogen losses from plants may occur through a variety of pathways, but so far, most studies have only quantified losses of nutrients by above-ground litter production. We used ¹⁵N pulse labelling to quantify total nitrogen losses from above- and below-ground plant parts. Using this method we were able to include also pathways other than above-ground litter production. To test the hypothesis that species from nutrient-poor habitats lose less nitrogen than species from more fertile soils, six perennial grasses from habitats with a wide range of nutrient availability were investigated: Lolium perenne, Arrhenatherum elatius, Anthoxanthum odoratum, Festuca rubra, F. ovina and Molinia caerulea. The results of an experiment carried out in pots in a green-house at two fertility levels show that statistically significant losses occur through pathways other than above-ground litter production. In the low fertility treatment, most (70%) losses from L. perenne occurred by litter production, but in Ar. elatius, F. rubra, F. ovina and M. caerulea, more than 50% of labelled N losses took place by root turn-over, leaching or exudation from roots. When nutrient supply increased, the ¹⁵N losses in above-ground dead material increased in all species and in Ar. elatius, A. odoratum and F. rubra the ¹⁵N losses via other pathways decreased. Ranked according to decreasing turnover coefficient the sequence of species was: L. perenne, A. odoratum, F. rubra, F. ovina, Ar. elatius, M. caerulea. These results suggest that species adapted to sites with low availability of nutrients lose less nitrogen (including above- and below-ground losses) than species adapted to more fertile soils.     

Competitive effects of Phragmites australis on the endangered artesian spring endemic Eriocaulon carsonii

We investigated the relative importance of above- and below-ground competition by reeds (Phragmites australis (Cav.) Trin. ex Steud) on the growth rate of Eriocaulon carsonii F.Muell. subsp. carsonii, an endangered plant threatened by reeds on artesian springs in Australia. Soil-filled buckets containing E. carsonii were frequently watered to simulate artesian spring conditions and subject to three treatments: (1) no Phragmites (control), (2) Phragmites (ABG), and (3) Phragmites with shoots tied back (BG). After thirteen months, Phragmites mean below-ground biomasses had increased to c. 3 kg m⁻² and mean above-ground biomasses to c. 1 kg m⁻². After the same period, mean root biomass of E. carsonii plants was significantly lower in buckets subject to both Phragmites treatments compared with control plants, as was E. carsonii foliage area. Comparison of the two Phragmites treatments indicated that below-ground competition was the primary cause of this reduced growth in E. carsonii. The vulnerability of E. carsonii to competitive exclusion by P. australis is in part due to the highly synchronized phenologies of the two species.     

Responses of three invasive aquatic macrophytes to nutrient enrichment do not explain their observed field displacements

In some eutrophic inland waters the invasive aquatic macrophyte Elodea canadensis has been displaced by the morphologically similar species Elodea nuttallii and subsequently E. nuttallii by Lagarosiphon major. We investigated whether differences in the responses of these species and their associated epiphytic floras to five nutrient loadings in the range 30–480 μg L⁻¹ P and 0.21–3.36 mg L⁻¹ N could explain their observed field displacements. The mean relative growth rate (RGR) of E. nuttallii (RGR 0.086 d⁻¹) was significantly higher than that of either E. canadensis (RGR 0.066 d⁻¹) or L. major (RGR 0.063 d⁻¹). All three species exhibited a plastic morphological response to increasing nutrient loadings with mean root weights reduced at the highest nutrient loading compared with the lowest loading by 33, 75 and 56% for E. canandensis, E. nuttallii and L. major, respectively. Mean tissue nitrogen concentrations increased significantly with increasing nutrient loading, with concentrations in E. canadensis (1.83–2.10% dry wt.) significantly higher than either E. nuttallii (1.56–2.10% dry wt.) or L. major (1.50–1.90% dry wt.). Tissue phosphorus concentrations likewise increased with increasing nutrient loadings although this trend was not as pronounced. Epiphyte biomass per unit photosynthetic surface area (PSA) was significantly higher on E. canadensis than on either E. nuttallii or L. major, but did not increase significantly with increasing nutrient loadings. We suggest that differences in species responses to nutrient enrichment do not explain the species displacements observed in the field. E. nuttallii's higher RGR may, regardless of nutrient supply, enable this species to shade out neighbouring species and outpace the establishment of algae on its leaves.     

Uptake and resource allocation of inorganic carbon by the temperate seagrasses Posidonia and Amphibolis

Productivity measurements from carbon uptake have been suggested as good indicators of the physiological health of seagrasses. As seagrasses acquire carbon from the surrounding water, the rate of uptake often provide a good measure of the efficiency at which seagrasses meet their resource demands for growth. This rate is often used to assess the photosynthetic efficiency of the plants, a proxy for the physiological status of seagrass. This has special relevance to the Adelaide region as over 5000 ha of seagrasses have been lost from Adelaide coastal waters over the last 70 years, with much of this loss attributed to nutrient inputs from wastewater, industrial and stormwater discharges. This study used an in-situ inorganic carbon isotope-labelling and spike approach to obtain ecologically relevant estimates of seasonal variability in carbon uptake and its allocation in two species of temperate seagrass common to this coast (Amphibolis antarctica and Posidonia angustifolia). Uptake of carbon by the seagrass complex (leaves, roots, phytoplankton and epiphytes) was affected by both season and species. Carbon uptake rates of phytoplankton were generally higher than other components of the system. Uptake rates ranged from 0.01 mg C g^− 1 DW h^− 1 (summer) to 0.61 mg C g^− 1 DW h^− 1 (spring) in Posidonia and 0.02 mg C g^− 1 DW h^− 1 (summer) to 0.93 mg C g^− 1 DW h^− 1 (winter) in Amphibolis. Carbon uptake by the Amphibolis complex was higher than in the Posidonia complex. The Amphibolis complex had higher uptake rates in summer whereas the Posidonia complex was higher in spring. Fine sediments probably from a nearby dredging operation, are likely to have resulted in lower carbon uptake and a reduction in the above-ground and below-ground biomass in summer.     

Production of rhizoids by Caulerpa prolifera in culture

Studies on Caulerpa prolifera rhizoids were carried out to determine the possibility of mass culture, because the rhizoids produce a bio-adhesive. Rhizoids can be induced by cutting the base of a blade and floating it in a media or planting it in sand. Measurement of rhizoid production included determination of number, length, and the weight of attached sand grains. The growth experiments were for 1–2 weeks and fronds growth was compared to rhizoid production. Optimal conditions for rhizoid growth included low levels of nitrogen and phosphate (less than 5 and 2 μM, respectively), low irradiance (30 μmol photon m⁻² s⁻¹), moderate temperature (22–28°), continuous shaking, addition of microelements and auxin (1 ppm) and initially detached fronds followed by attachment. Under these optimal conditions maximal weekly growth reached 70–170 rhizoids per blade, 7–11 mm length and maximal attachment to sand grains. Blade growth of C. prolifera responded similarly to rhizoid production and reached a weekly growth rate of 30–130%.     

Typha productivity in a Texas pond: Implications for energy and nutrient dynamics in freshwater wetlands

Above-and below-ground biomass of Typha angustifolia L. was sampled monthly for 18 months from a small Texas pond. Maximum above-ground biomass was 2559±284 g AFDW (ash-free dry weight) m⁻² in 1983 and 2895±217 g AFDW m⁻² in 1984. Peak below-ground biomass for these 2 years was 2506±278 g AFDW m⁻² and 2314±226 g AFDW m^t-2, respectively. Stepwise multiple linear regression analyses revealed that mean above-ground biomass accrual was related to duration of growing season, cumulative precipitation, cumulative degree days and/or cumulative pan evaporation. The same was not true for below-ground biomass increases. Analysis of covariance revealed that the rates of above-ground biomass production were not significantly different (F test, p < 0.05) between the 1983 and 1984 growing seasons. Below-ground biomass turnover times for 1983 and 1984 were 2.47 and 1.21 years, respectively.     

Seasonal growth dynamics of Ruppia maritima L.s.l. and Halodule wrightii aschers. In Southern Texas and evaluation of sediment fertility status

Annual growth dynamics of Ruppia maritima L.s.l. and Halodule wrightii Aschers. at two southern Texas, U.S.A., coastal sites were compared using plant biomass to monitor production. While sparse Ruppia normally coexists with dense Halodule in southern Texas bays, these two sites contained extensive stands of dense Ruppia mixed with or adjacent to Halodule. Corresponding measurements of water-column salinity and temperature revealed that vigorous Ruppia growth correlated positively with cool spring temperatures and not with low water salinities. In contrast, Halodule growth increased only after warm summer temperatures were reached. Partitioning of biomass into above-ground and below-ground tissues is suggested as an important autecological factor in resource competition between the two species. Ruppia, with a maximum 31% below-ground biomass in mid-spring, could be out-competed by Halodule, with a minimum of 66% below-ground biomass, when sediment or other conditions become favorable for Halodule growth.Analyses of sediment interstitial water from Ruppia or Halodule grass beds showed differences in rhizosphere nutrient pools characteristic of each species. Ruppia-dominated beds contained up to 15 μM nitrate during spring, whereas Halodule beds showed only a trace (< 1 μM). Halodule-dominated beds showed consistently higher levels of sediment H₂S compared to Ruppia areas (130 μM vs. 50 μM, respectively, average autumn concentration in top 10 cm). Based on these measurable edaphic differences, growth responses to sediment fertilization with two distinct fertilizer formulations were tested: (1) Osmocote^TM, an ammonium nitrate-based inorganic mixture, and (2) Hyponex^TM an organic mixture derived from fish meal. Contrasting responses to these two sediment fertilizers were observed. Halodule showed growth stimulation in early autumn by the organic fish meal only, while Ruppia responded to both types of fertilizer in both spring and early autumn. It is postulated that Ruppia normally grows on low-nutrient sediments, whereas Halodule prefers organic-rich sediments with substantial sulfate reduction activity.     

A comparison of mineral uptake and translocation by above-ground and below-ground root systems ofSalix syringiana

Nutrient uptake and translocation by above-ground adventitious roots and below-ground roots of woodySalix syringiana saplings were studied with gamma spectrometry. Each of four radionuclides (⁷⁵Se,¹³⁸Cs,⁵⁴Mn, and⁶⁵Zn) administered to adventitious and belowground roots were detected in stems and leaves within one month. Nuclides tended to be immobilized in the leaves and branches closest to the adventitious roots that absorbed them, while nuclides absorbed from below-ground sources were distributed more evenly throughout the plant. The capacity of adventitious roots to acquire nutrients from above-ground sources suggests they function as a potential auxiliary pathway of nutrient uptake and might enhance plant nutrient status where below-ground root uptake it hindered by adverse soil conditions.     

Morphological and physiological adaptive traits of Mediterranean narrow endemic plants: The case of Centaurea gymnocarpa (Capraia Island,Italy)

A case study on Centaurea gymnocarpa Moris & De Not., a narrow endemic species, was carried out by analyzing its morphological, anatomical, and physiological traits in response to natural habitat stress factors under Mediterranean climate conditions. The results underline that the species is particularly adapted to the environment where it naturally grows. At the plant level, the above-ground/below-ground dry mass (1.73 ± 0.60) shows its investment predominately in the above-ground structure with a resulting total leaf area per plant of 1399 ± 94 cm². The senescent attached leaves at the base of the plant contribute to limit leaf transpiration by shading soil around the plant. Moreover, the dense C. gymnocarpa leaf pubescence, leaf rolling, the relatively high leaf mass area (LMA = 12.3 ± 1.3 mg cm⁻²) and leaf tissue density (LTD = 427 ± 44 mg cm⁻³) contribute to limit leaf transpiration, also postponing leaf death under dry conditions. At the physiological level, a relatively low respiration/photosynthesis ratio (R/P_N) in spring results from high R [2.26 ± 0.59 μmol (CO₂) m⁻² s⁻¹] and P_N [12.3 ± 1.5 μmol (CO₂) m⁻² s⁻¹]. The high photosynthetic nitrogen use efficiency [PNUE = 15.5 ± 0.4 μmol (CO₂) g⁻¹ (N) s⁻¹] shows the large amount of nitrogen (N) invested in the photosynthetic machinery of new leaves, associated to a high chlorophyll content (Chl = 35 ± 5 SPAD units). On the contrary, the highest R/P_N ratio (1.75 ± 0.19) in summer is due to a significant P_N decrease and increase of R in response to drought. The low PNUE [1.5 ± 0.2 μmol (CO₂) g⁻¹ (N) s⁻¹] in this season is indicative of a greater N investment in leaf cell walls which may contribute to limit transpiration. On the contrary, the low R/P_N ratio (0.05 ± 0.02) in winter is resulting from the limited enzyme activity of the respiratory apparatus [R = 0.23 ± 0.08 μmol (CO₂) m⁻² s⁻¹] while the low PNUE [3.5 ± 0.2 μmol (CO₂) g⁻¹ (N) s⁻¹] suggests that low temperatures additionally limit plant production. The experiment of the imposed water stress confirms that the C. gymnocarpa growth capability is in conformity with the severe conditions of its natural habitat, likewise as it may be the case with others narrow endemic species that have occupied niches with similar extreme conditions.     

Patterns of ramet population of Iris japonica Thunb. and their effects on herb diversity in different microsites on Jinyun Mountain, China

Spatial patterns of ramet population of Iris japonica Thunb. and their effect on species diversity in the herb layer of 3 microsites (open area of forest edge (OAFE), bamboo forest (BF) and evergreen broad-leaved forest (EBF)) on Jinyun Mountain were studied using spatial pattern, niche and diversity analyses in a combination of population and community methods. The results were as follows: (1) judged by V/m and Morisita index (Iδ), ramet population of I. japonica in 3 microsites all clumped from scale 0.5 m × 0.5 m to 2 m × 2 m; (2) the pattern scale and pattern intensity both gradually decreased on all scales, and the density of ramet population of I. japonica decreased with the increase in canopy density and the decrease in relative photon flux density (RPFD) and R/FR from OAFE to EBF. In OAFE and BF, widespread I. japonica had significantly negative influence on the dominance of original dominant species and on species diversity in the herb layer (p < 0.05), while those influences in EBF were extremely weak. The mechanisms that pattern characteristics of ramet population of I. japonica influence herb diversity in 3 microsites were different. In OAFE, strong regeneration niche (above-ground spatial and below-ground root) and trophic niche (nutrient) competition had significantly negative influence on the species diversity of rare herbs and dwarf herbs. In BF, strong regeneration niche (below-ground root) and trophic niche (above-ground for light and below-ground for nutrient) competition had negative effect on the occurrence of rare species and on the survival of other herb species. In EBF, weak niche competition had little effect on the survival of herb species. Intensity of regeneration niche and trophic niche competition between I. japonica and other herb species is the determinant to the mechanism that ramet population of I. japonica influences herb diversity.     

Competition between Lemna minuta and Lemna minor at different nutrient concentrations

We investigated the differential responses of invasive alien Lemna minuta and native Lemna minor to nutrient loading as well as the mechanism of competition between the species. The role of nutrients, species identity, species influence in determining the outcome of competition between the species was estimated using the Relative Growth Rate Difference (RGRD) model. The two species differed in their response to nutrient loading. The native L. minor responded indifferently to nutrient loading. The species Relative Growth Rate (RGR) was 0.10 d⁻¹, 0.11 d⁻¹ and 0.09 d⁻¹ in high, medium and low nutrients, respectively. On the other hand, the invasive L. minuta responded opportunistically to high nutrient availability and had an RGR of 0.13 d⁻¹, 0.10 d⁻¹ and 0.08 d⁻¹ in high, medium and low nutrients, respectively. As a result, the invasive species was dominant in high nutrient availability but lost to the native species at low nutrient availability. The invader formed approximately 60% and less than 50% of the stand final total dry biomass in high and low nutrient availability, respectively. Species RGR were reduced by both intra- and interspecific competition but intraspecific effects were stronger than interspecific effects. On the overall, the species significantly differed in their constant RGR. These differences in RGR between the species (species identity) and the differential response to nutrient loading were the main determinant of change in final biomass composition of these species in mixture. Species influence (competition) only had a small influence on the outcome of competition between the species. The observed species response to nutrient loading could be targeted in management of the invasive species. Lowering nutrients can be proposed to reduce the impact of the invasive L. minuta.     

Nitrogen and phosphorus effect on Typha domingensis Presl. rhizome growth in a matrix of Schoenoplectus americanus (Pers.) Volkart ex Schinz and Keller

In an outdoor mesocosm experiment of 80 weeks, the effect of nitrogen and phosphorus addition was tested on growth of Typha domingensis Presl. rhizomes in a matrix of Schoenoplectus americanus (Pers.) Volkart ex Schinz and Keller, under loading rates of 0.23 gm⁻² d⁻¹ of nitrogen, 0.17 gm⁻² d⁻¹ of P, both nutrient together and control conditions, to assess the potential expansion of T. domingensis in response to nutrient inputs.     

Functional morphology and microclimate of Festuca orthophylla, the dominant tall tussock grass in the Andean Altiplano

Plant growth is driven by the rate of photosynthetic uptake of carbon, the loss of carbon and by allocation of photoassimilates to certain plant compartments, which leads to particular morphologies. Performance, vitality and persistence of a plant are affected by this partitioning process and vice versa. Under harsh climatic conditions such as cold temperature and seasonal drought, perennial plants often invest more in below-ground than above-ground structures. Festuca orthophylla in the subtropical Bolivian Altiplano does not match this ‘rule’. This species produces tall, evergreen tussocks, persisting decades and dominating the semi-arid, Andean landscape over thousands of square kilometers at elevation between 3600 and 4600 m a.s.l. The shallow rooting system represents only 21% of total biomass. The tussock base (root-stocks composed of the network of branching below-ground shoots and tiller meristems) comprises 28% of the total biomass. Although located partly below the soil surface, much of this biomass compartment is functionally above-ground (the basis of shoots). With their below-ground position, tiller meristems are protected against grazing and trampling by camelids as well as, to some degree, against fire and freezing. Fifty one percent of the biomass is above-ground (live leaves and inflorescences). In terms of phytomass (including attached necromass), 75% is above-ground. On average, a tussock consists of 3200 tightly packed total tillers (56% are live). Tillers emerge regularly intravaginally (i.e. within the leaf sheath of an existing mother tiller), resulting in dense canopies with strong self-shading: eighty percent of green foliage experience less than 50% of the incident light. The most important Altiplano plant species thus has morphological traits in favour of protection and survival rather than productivity.     

Intraspecific and interspecific pair-wise seedling competition between exotic annual grasses and native perennials: plant–soil relationships

Few studies have examined plant–soil relationships in competitive arenas between exotic and native plants in the western United States. A pair-wise competitive design was used to evaluate plant–soil relationships between seedlings of the exotic annual grasses Bromus tectorum and Taeniatherium caput-medusae and the native perennial grasses Elymus elymoides and Pseudoroegneria spicata. Two soils were tested: an arid soil (argid) occupied by E. elymoides and presently invaded by B. tectorum and a high elevation, high organic matter, soil (aquept) where none of the tested species would typically occur. Plant growth proceeded for 85 days at which time above-ground biomass and tissue nutrient concentrations were quantified. Soil also was collected from the rooting zone beneath each species and analyzed for various nutrient pools. The exotic species had significantly greater above-ground biomass than the natives and grew far better in the aquept soil than the argid soil. Growth of B. tectorum, and to some degree, T. caput-medusae was suppressed in intraspecific competition and enhanced, especially in the aquept soil, when competing with the natives. Although not significant, biomass of natives strongly trended downward when competing with the exotic grasses. Overall, concentrations of tissue nutrients were minimally affected by competition, but natives tended to be more negatively affected by competition with exotics. Except for phosphorus (P), all species had significantly greater nutrient concentrations when growing in the aquept soil compared to the argid soil. In both soils, exotics had significant greater tissue concentrations of manganese (Mn), magnesium (Mg), and iron (Fe), while natives had significantly greater nitrogen (N). Species affects on soil nutrient pools occurred mostly in the aquept soil with exotic species significantly decreasing pools of available N, potentially available N, and soil-solution pools of calcium (Ca²⁺), potassium (K⁺), and magnesium (Mg²⁺) relative to natives. Overall, the data suggest that, in the seedling state, B. tectorum is a superior competitor. Moreover, when the natives compete intra- or interspecifically, particularly in the aquept soil, availability of N and other nutrients in their rooting zone is consistently greater than when they compete interspecifically with the exotic grasses. These data suggest the exotics are able to co-opt nutrients in the rooting zone of the natives and perhaps gain a competitive advantage.     

Pattern and process in above-ground and below-ground components of grassland ecosystems

Abstract. This paper describes patterns of below-ground components in grassland ecosystems. It provides estimates of the contribution of below-ground organs to the total phytomass of the community and of different species to the below-ground phytomass; it describes the distribution of above- and below- ground organs of different species and the spatial and temporal correlation between above-ground and below-ground phyto-mass – both total standing crop and net primary production. 10 Siberian grasslands (meadows and steppes) were investigated during 15 yr. Ca. 70 % of the living phytomass is located in the soil and no less than 70 % of the net primary production is allocated in below-ground organs. Phytomass distribution in the soil layer is more homogeneous than above-ground. For some species the spatial distribution within 1-m² plots of the green and below-ground phytomass is similar, for others it is quantitatively or qualitatively different. According to the dominance-diversity curve, the above-ground size hierarchy is much stronger than the below-ground one. The active growth of above- and below-ground organs of a species may occur at different times of the season and it varies from year to year. Allocation of organic substances to rhizomes and roots occurs simultaneously and with proportional intensity.