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.     

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.     

Morphological Constraints of Shoot Demography of a Clonal Plant: Extra- and Intravaginal Tillers of Festuca rubra

Abstract Festuca rubra forms tillers in two different ways: extravaginally and intravaginally. Demography of these two tiller types was observed in seventeen selected tussocks of Festuca rubra s.s. over four growing seasons. Extravaginal tillers were bigger at birth and on the average produced twice as many daughter tillers per tiller. In general, the natality and mortality of extravaginal tillers were less regular than that of intravaginal tillers. Overall tillering rate per tiller was correlated with the density of the surrounding vegetation; mortality, natality and tiller life span were not. High density of the surrounding vegetation did not result in increased formation of extravaginal tillers. The proportion of the extravaginal tillers was not correlated with the density of the F. rubra tussocks. There is no evidence for foraging by extravaginal tillers, but they do act as founders of small clusters of tillers.     

Morphological Analysis of Tillering in Agropyron spicatum and Agropyron desertorum

The caespitose grasses Agropyron spicatum and Agropyron desertorumexhibit a striking difference in tillering response followingexperimental clipping treatment, with plants of A. desertorumproducing up to 18 times more tillers. The two species are similarin many aspects of their phenology and physiology. Previousexamination of current photosynthate production and levels ofstored carbohydrates indicate only slight differences betweenthe species. The possible role of three anatomical/morphologicalconstraints in controlling tillering was examined. No evidencefor such constraints was found. A basal cluster of buds is presenton the parent tillers. The mean bud number per tiller was similarfor both species and the range (3–9) was identical. Nearlyall of the bud apical meristems appeared anatomically viablethroughout the growing season and vascular development occurredto within 250 to 490 µm of the various bud apices of bothspecies. Both normal fall tillers and summer tillers producedunder clipping treatment originated from the largest, most distalbuds of the basal cluster of buds. However, precocious, morphologicallydistinctive, second-order tillers occasionally grew out fromthe smaller, most basal buds of some elongating fall tillers. Agropyron spicatum, Agropyron desertorum, bluebunch wheatgrass, crested wheatgrass, bud, tiller, tillering ability, meristematic potential, vascular development, regrowth     

The effects of the spatial pattern of defoliation on regrowth of a tussock grass

Summary The spatial pattern of foliage removal from a tussock grass can influence regrowth through effects on daily carbon gain (CER_d). This field study examined the extent to which tussock photosynthetic responses to different defoliation patterns were associated with changes in whole-canopy attributes (e.g., foliage age structure, canopy light microclimate). During the spring growing season, 60% of the green foliage area was removed from individual Agropyron desertorum tussocks with scissors in different spatial patterns. These patterns represented extremes of defoliation patterns that might be inflicted by natural herbivores. Tussock photosynthesis (per unit foliage area) at high light (2000 mol photons m^–2 s^–1 between 400 and 700 nm; P₂₀₀₀) increased following clipping with all defoliation patterns. The increases in P₂₀₀₀ were greater when leaves were removed from low in the tussock (older leaves) than if leaves high in the canopy (younger leaves) were removed. These relative changes of P₂₀₀₀ among clipping patterns paralleled the responses of CER_d and regrowth from an earlier study. Furthermore, the changes in P₂₀₀₀ corresponded with increases in the proportion of foliage within the tussocks that was directly illuminated at midday. The greater photosynthesis of tussocks after lower-leaf removal was directly related to a higher proportion of younger foliage and a smaller fraction of foliage shaded within the tussock. In a dense canopy, such as these grass tussocks, the influence of defoliation on whole-canopy attributes may be of primary importance to whole-plant photosynthetic responses.     

Age-states,population structure,and seedling regeneration of Chionochloa pallens in Canterbury alpine grasslands,New Zealand

Abstract. Age-states, population structure, and seedling regeneration of the grass Chionochloa pallens were investigated in eight alpine grassland stands representing the range of canopy cover (ca. 5–75 %) and basal area (112 - 3889 m²/ha^-1) in the headwaters of the Avoca catchment, Canterbury, New Zealand. In open-canopy stands (< 50 % canopy cover, < 2000 m²/ha^-1 basal area) C. pallens individuals were clearly distinguishable. Rela-tionshipsbetweenbasal diameter, number of tillers, height, and % crown death suggested four putative age-states: ‘seedlings‘, ‘juvenile tussocks’, ‘mature tussocks’ and ‘senescent tussocks’. Four types of population structure were identified depending onthe proportions of seedlings and juvenile and mature tussocks. C. pallens seedlings were present in seven stands at densities ranging from 0.6 to 123.4 / m². They were most frequent on microsites protected from frost-heave by short herbaceous vegetation or litter, but with little competition from taller vegetation. Differences in seedling densities between stands partly reflected the proximity and abundance of seed sources and degree of protection from frost-heave. C. pallens population structures appear to be influenced by the frequency and magnitude of geomorphic disturbances and browsing by introduced animals.     

Coping with herbivory: Photosynthetic capacity and resource allocation in two semiarid Agropyron bunchgrasses

Summary Agropyron desertorum, a grazing-tolerant bunchgrass introduced to the western U.S. from Eurasia, and Agropyron spicatum, a grazing-sensitive bunchgrass native to North America, were examined in the field for photosynthetic capacity, growth, resource allocation, and tiller dynamics. These observations allowed identification of physiological characteristics that may contribute to grazing tolerance in semiarid environments. A uniform matrix of sagebrush, Artemisia tridentata, provided an ecologically relevant competitive environment for both bunch-grass species. Physiological activity, growth, and allocation were also followed during recovery from a severe defoliation treatment and were correlated with tiller dynamics.Potential photosynthetic carbon uptake of both species was dominated by stems and leaf sheaths during June, when maximum uptake rates occurred. For both species, water use efficiency of stems and sheaths was similar to that of leaf blades, but nitrogen investment per photosynthetic surface area was less than in blades. In addition, soluble carbohydrates in stems and sheaths of both species constituted the major labile carbon pools in control plants. Contrary to current theory, these findings suggest that culms from which leaf blades have been removed should be of considerable value to defoliated bunchgrasses, and in the case of partial defoliation could provide important supplies of organic nutrients for regrowth. These interpretations, based on total pool sizes, differ markedly from previous interpretations based on carbohydrate concentrations alone, which suggested that crowns contain large carbohydrate reserves. In this study, crowns of both species contained a minor component of the total plant carbohydrate pool.Following defoliation, A. desertorum plants rapidly reestablished a canopy with 3 to 5 times the photosynthetic surface of A. spicatum plants. This difference was primarily due to the greater number of quickly growing new tillers produced following defoliation. Agropyron spicatum produced few new tillers following defoliation despite adequate moisture, and carbohydrate pools that were equivalent to those in A. desertorum.Leaf blades of regrowing tillers had higher photosynthetic capacity than blades on unclipped plants of both species, but the relative increase, considered on a unit mass, area, or nitrogen basis, was greater for A. desertorum than for A. spicatum. Agropyron desertorum also had lower investment of nitrogen and biomass per unit area of photosynthetic tissues, more tillers and leaves per bunch, and shorter lived stems, all of which can contribute to greater tolerance of partial defoliation.Greater flexibility of resource allocation following defoliation was demonstrated by A. desertorum for both nitrogen and carbohydrates. Relatively more allocation to the shoot system and curtailed root growth in A. desertorum resulted in more rapid approach to the preclipping balance between the root and shoot systems, whereas root growth in A. spicatum continued unabated following defoliation. Nitrogen required for regrowth in both species was apparently supplied by uptake rather than reserve depletion. Carbohydrate pools in the shoot system of both species remained very low following severe defoliation and were approximately equivalent to carbon fixed in one day by photosynthesis of the whole canopy.Dedicated to Drs. Michael Evenari and Konrad Springer     

Growth and tillering patterns within tussocks of Eriophorum vaginatum

Growth patterns were investigated for Eriophorum vaginatum tussocks from disturbed and undisturbed tussock tundra at two sites in Alaska. Total basal area of tussocks decreased with increased cryoturbation but mean basal area per tussock did not. Flowering was observed in tussocks of significantly smaller size on disturbed compared to undisturbed tundra. For tussocks with < 10% cover by shrubs and moss, number of tillers per tussock was linearly related to tussock diameter in most disturbed and undisturbed sites. Exceptions occurred in an area that had been bladed with a bulldozer 7 yr before our survey where tiller number increased as the square of diameter and in an area with much frost activity where tiller number was not emulated with diameter. The ratio of daughter tillers to adult tillers decreased with diameter in disturbed tundra, whereas the trend was less pronounced in undisturbed tundra.
Microsuccession in undisturbed tussock tundra was investigated by sampling tussocks with different amounts of shrub and moss cover for number of daughter tillers per adult tiller, weight per tiller, percent nitrogen, and percent phosphorus. Small tussocks without cover by other species and large, partially covered tussocks were not significantly different by any measures, but tussocks that were almost completely buried had significantly (p < 0.05) lower values of tillering index, weight per tiller, and percent phosphorus.     

Comparative demography of co-occurring introduced and native tussock grasses: persistence and potential expansion

Summary Demographic characteristics associated with the maintenance and growth of populations, such as seed dynamics, seedling emergence, survival, and tiller dynamics were examined for two tussock grasses, the native Agropyron spicatum and the introduced Agropyron desertorum in a 30-month field study. The introduced grass was aerially sown onto a native grassland site. Seed production of the introduced grass was greater than the native grass in both above- and below-average precipitation years. Seeds of A. spicatum were dispersed when they mature, while A. desertorum retained some seeds in inflorescences, and dispersed them slowly throughout the year. This seed retention allowed some seeds of the introduced grass to escape peak periods of seed predation during the summer and allowed seeds to be deposited constantly into the seed bank. Carryover of seeds in the seed bank beyond one year occurred in the introduced grass but not in the native species. For both species, seedling emergence occurred in both autumn or spring. Survival rates for A. desertorum were higher than A. spicatum when seedlings emerged between November and March. Survival rates of cohorts emerging before November favored A. spicatum whereas survival rates did not differ between species for cohorts emerging after March. Individuals of both species emerging after April were unable to survive the summer drought. Demographic factors associated with seeds of A. desertorum seemed to favor the maintenance and spread of this introduced grass into native stands formerly dominated by A. spicatum.     

Characteristics of successful competitors: an evaluation of potential growth rate in two cold desert tussock grasses

Summary Within the first few weeks after seedling emergence, Agropyron desertorum, a more competitive tussock grass, had a much higher mean relative growth rate (RGR) than Agropyron spicatum, a very similar, but less competitive species. However, beyond the early seedling stage, the two grasses had a remarkably similar whole-plant RGR in hydroponic culture and aboveground RGR in glasshouse soil, if root temperatures were above approximately 12°C. At soil temperatures between 5 and 12°C, A. desertorum exhibited a 66% greater aboveground RGR than A. spicatum (P<0.05). Both species responded similarly to warming soil temperatures. In the field, however, tiller growth rates were generally similar. Neither species showed marked tiller elongation until a couple of weeks after snowmelt, by which time soil temperatures, at least to a depth of 10 cm, were above 12°C for a significant portion of the day. Aboveground biomass accumulation over a three-year period indicated that both grasses had similar potential growth rates whereas Artemisia tridentata ssp. vaseyana, a common neighbor planted in the same plots, had a much greater potential growth rate. The greater competitive ability of adult A. desertorum, as compared to A. spicatum, cannot be attributed to appreciable differences in potential growth rates.     

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.     

Avian predation at a Southern Rockhopper Penguin colony on Staten Island,Argentina

We studied predation risk in relation to nest location and subcolony size in Southern Rockhopper Penguins (Eudyptes chrysocome chrysocome) during the chick-rearing period. Striated Caracaras (Phalcoboenus australis), the main predator, preferentially attacked from tussock grasses which are found in the periphery of all subcolonies (peripheral tussocks) and often scattered within them (central tussocks). The greatest numbers of predation and attempted predation events were observed on nests in the periphery of the subcolony next to peripheral tussocks, and on those nests next to central tussocks. Central tussocks offer Striated Caracaras an additional “edge” area from which to prey, much in the same way as do the peripheral tussocks. Predation rate per individual was not correlated with subcolony size possibly due to the presence of central tussocks which, by creating an extra edge area, change the subcolony shape. There is a suggestion (P = 0.06) of increased probability of nest success with subcolony size.     

A numerical study of successions in an abandoned,damp calcareous meadow in S Sweden

Summary Two sites at Örup, SE skåne, Sweden, have been investigated, viz. a grazed, unimproved, tussocky pasture on a calcareous moraine clay; and an originally similar, adjacent area that was abandoned about 1960. On the grazed site the vegetation is extremely rich in species. This vegetation type was formerly widespread, but nowadays it is rare and therefore it is important to try to conserve examples of it. In total, 80 0.1 m² quadrates distributed at random within 4 homogeneous plots were investigated. The species composition and the cover, the height of the vegetation, the position on a tussock or in a depression and the amount of litter were recorded. The cover data were ordinated (PCA). The differentiation between tussocks and depressions and the effects of ceasing grazing were clearly separated. The connections between the vegetation and the position of the quadrat, grazing intensity etc. were investigated. The vegetation of the ungrazed parts had become dominated by Filipendula ulmaria and Carex disticha; on an average there were 5.5 species/0.1 m². The grazed quadrats contained three times as many species and showed much higher spatial variation, important species being Carex panicea, C. flacca, Molinia caerulea, Festuca ovina, Potentilla erecta, Centaurea jacea and Serratula tinctoria.Nomenclature of vascular plants follows Lid (1974), for names of bryophytes see Nyholm (1954–69) and Arnell (1956).This paper is mainly based on a more detailed paper in Swedish (Regnéll 1979).I thank Dr. E. van der Maarel, Prof. Nils Malmer, Fil. lic. Anders Larsson, Fil. Dr. Eva Waldemarsson-Jensén and Fil. kand. Stefan Persson for valuable discussions and encouragement.     

Grass-to-tree facilitation in an arid grazed environment (Aïr Mountains, Sahara)

The scarcity of tree regeneration is a major concern for the long-term conservation of tree cover in the Aïr-Ténéré Nature Reserve (Sahara, Niger), and the causes of this shortage are poorly understood. We raised the hypothesis that livestock pressure is a key-factor in the mortality of tree seedlings and that facilitation processes among plants may enhance tree regeneration. We tested whether the tussock grass Panicum turgidum could facilitate the regeneration of a keystone tree species, Acacia tortilis var. raddiana, under a grazing regime. We transplanted 3-month-old seedlings of Acacia to plots representing different neighbourhood conditions (in areas without vegetation, amongst intact tussocks, among cropped tussocks, and under branches), which were either fenced or exposed to grazing by large herbivores. Survival and height of 160 transplants were recorded during one year. Acacia performance was much lower in the grazed plots, indicating that large herbivores had a strong negative impact on tree regeneration. Presence of Panicum tussocks had a positive effect on the height of seedlings, likely because they improved soil properties and water availability. No aboveground positive effect on Acacia was detected though the grass did reduce atmospheric aridity. We detected an indirect positive effect of Panicum on Acacia survival in the presence of herbivores when aridity and grazing pressure reached their maximum levels. We obtained evidence that indirect facilitation may occur in arid grazed areas, although it has been predicted to mainly exist in mesic competitive environments. Additionally, our results support the stress-gradient hypothesis against assumptions that competitive effects may dominate in extremely arid environments. Since most of the positive effect on Acacia survival is indirect (biotic grazing refuge) due to the presence of herbivores, transplantation of tree seedlings amongst Panicum may be a simple and low-cost technique for the restoration of Saharan ecosystems degraded by herbivore grazing.     

Growth and carbon allocation of Agropyron desertorum following autumn defoliation

Summary Growth and carbon allocation of a cool season tussock grass, Agropyron desertorum, following defoliation of newly initiated tillers in the autumn of 1988 and 1989 were investigated. Tiller density and mortality, reproductive shoot density, root density, biomass, individual tiller weight, carbon allocation, and soil water depletion were used to evaluate the response of A. desertorum to autumn grazing. Tiller recruitment was lower in the autumn-defoliated treatment in both years compared with the control because of the cessation of tiller development following autumn defoliation. Autumn defoliation also significantly reduced the movement of ¹³C to the roots in 1988 but not in 1989. Soils were cooler and drier in 1989. Other plant growth measurements and soil water depletion rates were not different between treatments. Autumn defoliation in 1988 did not influence tiller recruitment in the following autumn. Two consecutive years of autumn defoliation did not affect tiller overwinter mortality or peak standing crop in 1990.     

不同载畜率下短花针茅分蘖数量对内源激素浓度的响应

放牧是荒漠草原最主要的利用方式之一,载畜率的变化严重影响着植物的生长发育;而内源植物激素是调节植物生长发育的开关,且植物在不同的生长发育阶段具有不同的生理要求和环境适应能力。通过测定放牧条件下短花针茅(Stipa breviflora)分蘖叶内源激素的变化,研究短花针茅分蘖生长对放牧的响应,并分析了分蘖数量受内源激素影响的机制。结果表明,(1)内源激素与载畜率之间存在显著二次相关关系,说明放牧能够显著增加内源激素的浓度(P0.05),但这种相关只存在于中、小株丛的短花针茅中。(2)其次,放牧能够在一定程度上影响短花针茅植株个体分蘖的数量(P0.05),重度放牧是增加短花针茅植株个体分蘖数量最显著的载畜率。(3)过高浓度的生长素(IAA)会抑制短花针茅的分蘖数量(P0.01)。而细胞分裂素(CTK)与短花针茅的分蘖数量之间尚未发现相关关系。     

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.     

Species interactions at the level of fine roots in the field: influence of soil nutrient heterogeneity and plant size

Interference at the level of fine roots in the field was studied by detailed examination of fine root distribution in small soil patches. To capture roots as they occur in natural three-dimensional soil space, we used a freezing and slicing technique for microscale root mapping. The location of individual roots intersecting a sliced soil core surface was digitized and the identity of shrub and grass roots was established by a chemical technique. Soil patches were created midway between the shrub, Artemisia tridentata, and one of two tussock grasses, Pseudoroegneria spicata or Agropyron desertorum. Some soil patches were enriched with nutrients and others given only deionized water (control); in addition, patches were located between plants of different size combination (large shrubs with small tussock grasses and small shrubs with large tussock grasses). The abundance of shrub and grass roots sharing soil patches and the inter-root distances of individual fine roots were measured. Total average rooting density in patches varied among these different treatment combinations by only a factor of 2, but the proportion of shrub and grass roots in the patches varied sixfold. For the shrub, the species of grass roots sharing the patches had a pronounced influence on shrub root density; shrub roots were more abundant if the patch was shared with Pseudoroegneria roots than if shared with Agropyron roots. The relative size of plants whose roots shared the soil patches also influenced the proportion of shrub and grass roots; larger plants were able to place more roots in the patches than were the smaller plants. In the nutrient-enriched patches, these influences of grass species and size combination were amplified. At the millimeter- to centimeter-scale within patches, shrub and grass roots tended to segregate, i.e., avoid each other, based on nearest-neighbor distances. At this scale, there was no indication that the species-specific interactions were the result of resource competition, since there were no obvious patterns between the proportion of shrub and grass roots of the two species combinations with microsite nutrient concentrations. Other potential mechanisms are discussed. Interference at the fine-root level, and its species-specific character, is likely an influential component of competitive success, but one that is not easily assessed.     

Does morphological plasticity of the <Emphasis Type="Italic">Phalaris arundinacea</Emphasis> canopy increase invasiveness?

Morphological plasticity could facilitate invasions of wetland plants into areas that experience increased durations of flooding and eutrophication. We explored canopy plasticity of Phalaris arundinacea, an aggressive invader of wetlands, as it differentially invaded wet prairie mesocosms under 3 flooding durations and 3 levels of nutrient addition. Phalaris grew as a sward with intermittent and early-season flooding but shifted to tussocks under constant flooding. These two growth forms differed by >20% in several canopy ratios. Clones that formed tussocks produced 45% more shoots per unit biomass (P = 0.007) and a 25% higher ratio of total shoot length to biomass (P = 0.04). Lighter-weight shoots supported 33% fewer leaves and, consequently, had 35% less leaf area per shoot height (P < 0.002). Tussocks developed a continuous mat of adventitious roots, with root mats reaching 20.9 ± 0.6 cm in diameter and 4.7 ± 0.3 cm in height over two growing seasons. While forming tussocks, Phalaris tolerated longer durations of flooding and more than doubled its aboveground biomass. Invasions occurred rapidly, with Phalaris exceeding 75% canopy cover and accounting for 66% of the total aboveground biomass under constant flooding. Early-season flooding increased the lateral spread of individual shoots. High nutrient addition produced shoots that were 27% taller and 50% heavier (P < 0.02), with 81% more leaf area (P < 0.0003) than shoots that received no nutrients. Consequently, under early-season flooding with high nutrient additions, Phalaris was primed to invade, nearly doubling its proportion of the total aboveground biomass and exceeding 50% canopy cover during year two.     

The timing and degree of root proliferation in fertile-soil microsites for three cold-desert perennials

Summary Root proliferation in nutrient-rich soil patches is an important mechanism facilitating nutrient capture by plants. Although the phenomenon of root proliferation is well documented, the specific timing of this proliferation has not been investigated. We studied the timing and degree of root proliferation for three perennial species common to the Great Basin region of North America: a shrub, Artemisia tridentata, a native tussock grass, Agropyron spicatum, and an introduced tussock grass, Agropyron desertorum. One day after we applied nutrient solution to small soil patches, the mean relative growth rate of Agropyron desertorum roots in these soil patches was two to four times greater than for roots of the same plants in soil patches reated with distilled water. Most of the increased root growth came from thin, laterally branching roots within the patches. This rapid and striking root proliferation by Agropyron desertorum occurred in response to N-P-K enrichment as well as to P or N enrichment alone. A less competitive bunchgrass, Agrophyron spicatum, showed no tendency to proliferate roots in enriched soil patches during these two-week experiments. The shrub Artemisia tridentata proliferated roots within one day of initial solution injection in the N-enrichment experiment, but root proliferation of this species was more gradual and less consistent in the N-P-K and P-enrichment experiments, respectively. The ability of Agropyron desertorum to proliferate roots rapidly may partly explain both its general competitive success and its superior ability to exploit soil nutrients compared to Agropyron spicatum in Great Basin rangelands of North America.