Increase In Shoot Density Research Articles

Overcoming ecological feedbacks in seagrass restoration

Overcoming ecological feedbacks is a major limiting factor reducing the success of many seagrass restoration projects. Negative feedbacks occur when biotic or abiotic conditions of a site are changed sufficiently after the loss of seagrass to prevent its recovery, even after the original stressors are remediated. While negative feedbacks in seagrass restoration are common, there remain limited studies of ways to reduce them and kick‐start the necessary positive feedbacks to promote recovery. We used field and laboratory experiments to investigate key ecological feedbacks in seagrass (Zostera marina) restoration by testing the role of hessian bags and seed burial in reducing seed predation and promoting plant development. We used a double‐hurdle model approach to predict “seagrass emergence success” and “seagrass growth success” across planted field plots. We found that planting seeds in hessian bags and burying them in the sediment improved the likelihood of seeds developing into mature plants. We recorded an average 13‐fold increase in shoot density for seeds planted in buried bags relative to raked furrows. This could be the combined result of reduced predation as well as bags mimicking emergent traits of mature seagrass to withstand physical impacts. We supplement these findings with laboratory evidence that hessian bags provide protection from predation by green shore crabs. Overall, we found a low and variable success rate for seed‐based restoration and indicate other feedbacks in the system beyond those we controlled. However, we show that small methodological changes can help overcome some key feedbacks and improve restoration success.

Reproductive Responses to Increased Shoot Density and Global Change Drivers in a Widespread Clonal Wetland Species, Schoenoplectus americanus

The expansion of many wetland species is a function of both clonal propagation and sexual reproduction. The production of ramets through clonal propagation enables plants to move and occupy space near parent ramets, while seeds produced by sexual reproduction enable species to disperse and colonize open or disturbed sites both near and far from parents. The balance between clonal propagation and sexual reproduction is known to vary with plant density but few studies have focused on reproductive allocation with density changes in response to global climate change. Schoenoplectus americanus is a widespread clonal wetland species in North America and a dominant species in Chesapeake Bay brackish tidal wetlands. Long-term experiments on responses of S. americanus to global change provided the opportunity to compare the two modes of propagation under different treatments. Seed production increased with increasing shoot density, supporting the hypothesis that factors causing increased clonal reproduction (e.g., higher shoot density) stimulate sexual reproduction and dispersal of genets. The increase in allocation to sexual reproduction was mainly the result of an increase in the number of ramets that flowered and not an increase in the number of seeds per reproductive shoot, or the ratio between the number of flowers produced per inflorescence and the number of flowers that developed into seeds. Seed production increased in response to increasing temperatures and decreased or did not change in response to increased CO2 or nitrogen. Results from this comparative study demonstrate that plant responses to global change treatments affect resource allocation and can alter the ability of species to produce seeds.

Evaluating the use of the branched‐chain amino acids in combination with urea on creeping bentgrass at establishment

AbstractThe branched‐chain amino acids (BCAA) leucine (L), isoleucine (IL), and valine (V) are synthesized in plants and essential to growth in most organisms. Research has shown that when foliarly applied, these compounds can be absorbed by the plant, however, plant catabolism of BCAA is not completely understood. A ratio of 4:1:1 (L:IL:V) applied at 3.4 kg‐nitrogen (N) ha−1 was identified as the optimal application ratio for increasing creeping bentgrass (Agrostis stolonifera L.) shoot density, however, material incompatibility issues occurred when BCAA were applied at 8:1:1 and 12:1:1 in previous work. The objective of this study was to determine if shoot density could be further increased by using lower BCAA ratio rates in combination with urea and GreenNCrease to equalize N at establishment. Creeping bentgrass samples grown in sand‐filled pots were subjected to N application (3.4 kg‐N ha−1 application−1) using urea, GreenNCrease, BCAA 4:1:1, 8:1:1 and 12:1:1. The BCAA were also applied as 4:1:1+N, 8:1:1+N, and 12:1:1+N, which consisted of BCAA half‐rates (1.7 kg‐N ha−1) and urea‐N (1.7 kg‐N ha−1). At the end of the study, material incompatibility previously observed with 8:1:1 and 12:1:1 was alleviated through use of the BCAA half‐rates, however, 4:1:1 and 4:1:1+N led to a greater root weight compared to urea alone. After 10‐weeks, BCAA 4:1:1 and 4:1:1+N, increased shoot density by 54% and 49%, respectively, compared to equal urea N. This very low rate of nitrogen supplied by the urea applications was not enough to elicit the typical growth response that is seen from urea at higher levels. According to these results from creeping bentgrass grown in pots, using BCAA 4:1:1, either as a standalone N application or tank mixed with urea as a portion of the total applied N, will lead to best increases in shoot density and root weight compared to equal urea N.

Utilizing Branched-chain Amino Acids for Increasing Shoot Density and Establishment Rate in Creeping Bentgrass

The branched-chain amino acids (BCAA) leucine (L), isoleucine (IL), and valine (V) are synthesized in plants and are essential to growth in most organisms. These compounds can be absorbed by the plant when foliarly applied, but plant catabolism of BCAA is not completely understood. A recent study observed that BCAA applied in a 2:1:1 or 4:1:1 ratio (L:IL:V) increased creeping bentgrass (Agrostis stolonifera) shoot density compared with applications of equal urea nitrogen (N) at 3.03 lb/acre N. The present study investigated whether those increases could translate to a quicker establishment rate of creeping bentgrass grown from seed in standard greenhouse pots. The BCAA applications were compared with equal N applications using urea and a commercially available amino acid product. All N treatments were applied at 3.03 lb/acre N, per application and applied a total of four times on a 14-day interval starting 14 days after seeding. Measurements included final shoot density counts and root and shoot weights, as well as digital image analysis of percent green cover for each greenhouse pot every 7 days. No differences were observed after 70 days in shoot weight, or percent green cover between BCAA treatments and urea; however, BCAA 2:1:1 and 4:1:1 increased shoot density 21% and 30%, respectively, compared with urea, and were equal to the commercially available amino acid product. Applications of BCAA 4:1:1 also increased creeping bentgrass rooting weight by a factor of 7 compared with urea N.

Effects of biennial cropping and controlled shoot density on yield performance and fruit quality in red raspberry (Rubus idaeus L.)

ABSTRACTEffects of annual versus biennial cropping with varying shoot densities on plant structure, berry yield and quality were studied in ‘Glen Ample’ raspberry over a period of four seasons (two cropping years). In the vegetative phase, primocane height and internode length were larger in the annual than in the biennial cropping system. These parameters as well as Botrytis infestation increased with increasing shoot density. In both cropping years, berry yields per unit area were about 20% higher in the biennial cropping system, whereas yields per shoot were not significantly different in the two systems. In both cropping systems, yields per shoot strongly declined with increasing shoot density, while yields per metre row increased slightly. Regardless of cropping system, yields per metre row did not increase with increasing shoot density beyond eight shoots per metre. The concentrations of dry matter, soluble solids, titratable acidity and ascorbic acid as well as the intensity of juice colour all declined with increasing shoot density. We conclude that under controlled shoot density conditions, there is little scope for biennial yield increases that fully compensates for the lost crops every second year. However, the system greatly facilitates berry harvest and eases plant disease pressure.

Performance of Miscanthus x giganteus (Greef et Deu) established with plastic mulch and grown from a range of rhizomes sizes and densities in a cool temperate climate

The effect of biodegradable plastic mulch, rhizome size and planting density on establishment, annual harvested biomass yield of miscanthus and nutrient content were studied at two sites in Northern Ireland. In a split plot experimental design plastic mulch treatment was applied randomly to main plots (covered or left uncovered) after planting rhizome fragments varying in size (by weight) and planting density which were assigned randomly to subplots in four replicated blocks, at each site.Mean shoot density over both sites was 7.8 and 12.6m−2 for unmulched and mulched in October of the establishment year. Averaged over all sites and planting treatments, uncovered total biomass yields were 0.47, 4.79, 7.74 and 8.12tDMha−1, for the first four annual spring harvests, respectively, mulch increasing these yields by 68, 49, 36 and 24%, respectively. Although there were significant effects of rhizome size and density, the effect of plastic declined at successive harvests for large and medium sized rhizomes and high and standard densities. The main effect of plastic mulch was to increase the shoot population density, and shoot number per plant, especially in plants from rhizomes planted at a low number per unit area. Although there were soil moisture deficits in some months, mulching was considered to have improved yields mainly by increasing shoot density due to higher soil temperatures during establishment followed by a mild winter. By the third year, plastic mulch had no significant effect on recommended planting density at all harvests. Plastic mulch had no marked effect on content of N, P or K in biomass.Covering miscanthus rhizomes with a biodegradable plastic mulch is an effective management tool to increase miscanthus biomass production in a cool temperate climate, at least in the first 3–4 years of production. This is mainly due to increase in soil temperature during establishment and the subsequent year. Mulching allows the optimum planting density of rhizomes to be reduced. Despite additional cost plastic mulch represents an economic benefit to the crop.

규산 함유 액상비료 시비에 따른 크리핑 벤트그래스의 생육과 품질 변화

본 연구는 규산 함유 액비(LFSi)의 시비에 따른 잔디의 생육과 품질의 변화를 확인하기 위해 잔디의 가시적 품질, 줄기 밀도, 뿌리 길이, 엽록소 함량, 예지물 및 양분 함량을 조사하였다. 처리구는 대조구(CF), LFSi 1,000배 처리구(SiF-1), LFSi 500배 처리구(SiF-2) 및 LFSi 250배 처리구(SiF-3)로 구분되었다. LFSi 처리 전후에 토양화학성은 차이를 나타내지 않았다. LFSi 처리구와 대조구를 비교하였을 때, 잔디 품질은 7월, 10월 및 11월 조사에서 높았으며, 엽록소 함량과 잔디 예지물은 10월 27일 조사에서 증가하였다. 줄기 밀도, 뿌리 길이, 잎조직 내 질소 함량 및 칼륨 함량은 LFSi 처리구에서 증가하였다. 잔디 잎 조직 중 규소 함량은 칼리 함량이나 줄기 밀도와 정의 상관성을 보였고, 줄기 밀도는 잔디 품질이나 엽록소 함량과 각각 정의 상관성을 나타내었다. 이 결과들을 종합해 볼 때, 크리핑 벤트그래스에서 규산 함유 액비의 시비는 잔디의 잎 조직 중 칼륨 함량이나 줄기 밀도가 증가되어 가시적 품질이 향상되는 것을 확인 할 수 있었다. Superintendents have used a silicate fertilizer to improve a resistance of turfgrass against several diseases, drought damage and wear stress. This study was conducted to evaluate the effect of liquid fertilizer containing silicate (LFSi) on changes of turfgrass quality and growth by investigating visual quality, chlorophyll content-chlorophyll a, chlorophyll b, and total chlorophyll, root length, shoot length, dry weight of clipping, and nutrient content in leaves tissue. Treatments were designed as follows; control fertilizer (CF), SiF-1 (CF + <TEX>$1ml\;m^{-2}$</TEX> LFSi), SiF-2 (CF + <TEX>$2ml\;m^{-2}$</TEX> LFSi), and SiF-3 (CF + <TEX>$4ml\;m^{-2}$</TEX> LFSi). As compared with CF, soil chemical properties, visual turfgrass quality, chlorophyll content, and dry weight of clipping of LFSi treatments were not significantly. Contrastingly, shoot density, root length, and the content of nitrogen or potassium were increased by application of LFSi. The content of Si in the tissue was positively correlated with potassium content or shoot length, and similarly shoot density positively with chlorophyll content or visual quality, respectively. These results suggested that the application of LFSi improved the turfgrass quality by increasing shoot density or K content in leaf tissue of creeping bentgrass.

Artificial seagrass leaves shield transplanted seagrass seedlings and increase their survivorship

Seagrass meadows provide important ecosystem functions and services. The progressive global deterioration of seagrass meadows requires management strategies to stop and recover seagrass losses. Seagrass restoration can be promoted using seagrass seedlings germinated in vitro, but early mortality of seedlings is a severe bottleneck for successful restoration programs. In this study, we tested a technique to promote in situ survivorship of in vitro germinated seedlings of the seagrass Cymodocea nodosa. In particular, we predicted that artificial seagrass leaves (plastic green raffia, i.e. ‘Artificial Seagrass Shields', ASSs) surrounding seagrass seedlings would increase the long-term survivorship of seedlings by decreasing herbivory-induced mortality. After 2 years, the survivorship of seagrass seedlings in plots without the ASSs was nil, whereas one third of plots containing ASSs developed small-sized seagrass patches, including a one order of magnitude exponential increase in shoot density. Herbivory intensity on seagrass leaves significantly decreased when surrounded with ASSs. This study demonstrates how ASSs facilitate the establishment of transplanted seagrass seedlings and suggests ASSs could be a cheap, easy-to-use, restoration element to promote the survivorship of seagrass seedlings by decreasing the magnitude of herbivory.

Assessing methods for restoration of eelgrass (Zostera marina L.) in a cold temperate region

More than 50% of eelgrass habitats have disappeared from the Swedish NW coast in the last 30years. Restoration is being proposed to assist recovery but little is known regarding methods suitable under Scandinavian conditions; e.g. short growing seasons and scouring by ice. In the present study we evaluated different restoration methods using shoots and seeds in a Swedish fjord and assessed if eelgrass could be successfully transplanted between sites with different depth and exposure. The study demonstrates that both shoot- and seed methods can be successfully used to restore eelgrass at this latitude. Survival and growth of unanchored single shoots, transplanted without sediment in shallow habitats (1.0–1.5m) was very high (>500% increase in shoot density after 14months). This restoration method showed 2–3.5 times higher growth rate and was 2–2.5 times faster compared with shoots anchored in the sediment and shoots transplanted in sediment cores, respectively, and is recommended for shallow habitats in Sweden. Growth within deeper habitats (3.0–4.5m) was substantially lower (40% loss to 50% increase) due to light limitations and high winter mortality. Restoration using seeds distributed from mesh-bags showed very low seedling establishment rates (approximately 1%) making this method less cost-effective than transplanting single shoots in shallow habitats. However, growth of seedlings was high and this method is recommended for deep habitats with soft sediment where shoot transplantation is difficult. Despite dramatic differences in eelgrass morphology between habitats with different depth and exposure, all shoots within a planting site had the same morphology at the end of the study, independent of origin. A baseline genetic survey supported that the observed changes in morphology of transplants were due to a plastic response, suggesting that donor populations do not have to exactly match the morphology of the plants targeted for restoration.

Effects of 3-year air warming on growth of two perennial grasses (Phragmites australis and Imperata cylindrica) in a coastal salt marsh reclaimed for agriculture

The aim of this study was to examine the potential effects of air warming on growth of two perennial grasses (a C3 species Phragmites australis (Cav.) Trin. ex Steud. and a C4 species Imperata cylindrica (Linn.) Beauv.) in a coastal salt marsh reclaimed for agriculture. For this purpose, open-top chambers (OTCs) were used with a mean air temperature elevation of approximately 1.5°C on Chongming Island located at the Yangtze Estuary. After 3 years’ treatment, a series of growth-related traits of the two species under warming conditions were compared to the same plant materials under the ambient conditions. Regarding P. australis, air warming significantly decreased leaf net photosynthetic rate by 28%, together with leaf area, aboveground biomass and relative growth rate of aboveground biomass by 22%, 28% and 36% at the shoot level, respectively; but it markedly increased specific leaf area by 12% at the shoot level, together with shoot density, leaf area index and aboveground biomass by 142%, 87% and 69% at the population level, respectively. On the other hand, for I. cylindrica, air warming did not significantly change leaf net photosynthetic rate, morphological or growth traits of aboveground part at the shoot level; but it markedly decreased shoot density, leaf area index and aboveground biomass by 49%, 45% and 47% at the population level, respectively. Air warming had no significant effect on rhizome biomass of P. australis down to a depth of 20cm; but it significantly decreased the rhizome biomass of I. cylindrica by 42%. Air warming had no significant effect on the mean soil temperature, volumetric moisture or inorganic nitrogen in the upper soil layer; but it markedly increased the mean soil porewater salinity by 119%. In an environment with elevated air temperature and warming-increased soil salinity, P. australis enhanced population-level aboveground biomass by increasing shoot density and specific leaf area despite of the decreased leaf photosynthetic rate and shoot-level growth, while the clonal propagation of I. cylindrica was suppressed by warming-increased soil salinity and inter-specific competition from P. australis. Our results suggest that P. australis may well maintain the dominance over I. cylindrica in the reclaimed salt marsh, when air temperature elevation is within 1.5°C over the next few decades.

Seagrass Proliferation Precedes Mortality during Hypo-Salinity Events: A Stress-Induced Morphometric Response

Halophytes, such as seagrasses, predominantly form habitats in coastal and estuarine areas. These habitats can be seasonally exposed to hypo-salinity events during watershed runoff exposing them to dramatic salinity shifts and osmotic shock. The manifestation of this osmotic shock on seagrass morphology and phenology was tested in three Indo-Pacific seagrass species, Halophila ovalis, Halodule uninervis and Zostera muelleri, to hypo-salinity ranging from 3 to 36 PSU at 3 PSU increments for 10 weeks. All three species had broad salinity tolerance but demonstrated a moderate hypo-salinity stress response – analogous to a stress induced morphometric response (SIMR). Shoot proliferation occurred at salinities <30 PSU, with the largest increases, up to 400% increase in shoot density, occurring at the sub-lethal salinities <15 PSU, with the specific salinity associated with peak shoot density being variable among species. Resources were not diverted away from leaf growth or shoot development to support the new shoot production. However, at sub-lethal salinities where shoots proliferated, flowering was severely reduced for H. ovalis, the only species to flower during this experiment, demonstrating a diversion of resources away from sexual reproduction to support the investment in new shoots. This SIMR response preceded mortality, which occurred at 3 PSU for H. ovalis and 6 PSU for H. uninervis, while complete mortality was not reached for Z. muelleri. This is the first study to identify a SIMR in seagrasses, being detectable due to the fine resolution of salinity treatments tested. The detection of SIMR demonstrates the need for caution in interpreting in-situ changes in shoot density as shoot proliferation could be interpreted as a healthy or positive plant response to environmental conditions, when in fact it could signal pre-mortality stress.

Fish herbivory leads to shifts in seagrass Posidonia oceanica investments in sexual reproduction

Although the dominant ecological paradigm considers herbivory to play an insignificant role in seagrass ecosystems, past herbivore densities were high enough to result in significant reduc- tion of seagrass growth. To study the long-term impact of sustained and intense herbivory on sea- grass meadows, we compared morphological, population and reproductive (flowering) parameters of Posidonia oceanica meadows inside a Marine Protected Area (MPA; where herbivore fish popula- tions are very high) with unprotected meadows. In addition, we evaluated short-term seagrass responses by manipulating herbivore access to seagrass plots with caging experiments conducted inside the MPA. The density and individual sizes of the herbivorous fish Sarpa salpa were greater in the MPA, with a biomass 10 times higher than in unprotected areas. Fish bite marks on leaves were 50% more abundant inside the MPA. Shoot surface, rhizome sugar content and flower density were 80, 20 and 70% lower in the MPA, respectively, but shoot density was 30% higher in protected meadows than in unprotected meadows. The caging (fish exclusion) experiment generally corrobo- rated these results, although the caging period was probably too short to produce changes in shoot density. P. oceanica responded to severe biomass removal by herbivores (80% of the photosynthetic biomass) and consequent reductions in carbon storage in the rhizome by reducing sexual reproduc- tion (flowering intensity) and by gradually increasing clonal growth (increasing shoot density). This plasticity suggests an evolutionary adaptive mechanism to deal with historically high herbivore num- bers and is evidence of the importance of herbivory as a controlling process in the structuring and functioning of seagrass meadows in the past.

Long-Term Effects of Mechanical Winter Pruning on Growth, Yield, and Grape Composition of Barbera Grapevines

Vine performance was tested over five years (2005–2009) on Vitis vinifera L. cv. Barbera either manually spur-pruned (HP) or mechanically hedged with light (SMP-LF) or severe (SMP-SF) hand follow-up. Although mechanical treatments retained 2- to 2.5-fold higher count nodes per vine than did HP, yield per vine (~5 kg) was almost identical between treatments due to the strong offsetting effect of reduced budbreak. Weak compensation and no compensation were seen for cluster weight and bud fruitfulness, respectively. Except for a slight reduction in anthocyanin concentration, overall grape composition was similar among treatments throughout the trial. As minor differences in vine vigor and capacity were found and the leaf-to-fruit ratio (vine basis) was unaffected by treatments, the slightly lower anthocyanin berry content in the SMP vines may have derived from increased shoot density and, hence, more shade cast in the fruiting area. Winter pruning was performed in less than 25 hr/ha in the hedged vines, thereby cutting labor demand from 54 to 70% compared with HP. Thus, if all other vineyard operations are also mechanized, a single high-wire Barbera vineyard with a mostly erect canopy can be maintained in less than 70 worker hr/ha. Such a performance, coupled with overall unchanged yield and grape composition, represents a solid and reliable approach in a wine market that demands greater efficiency and competitiveness.

The Effect of Nutrient Enrichment of Either the Bank or the Surface Water on Shoreline Vegetation and Decomposition

Riparian ecosystems can harbor great diversity and provide important ecological functions such as improving water quality. The impact of eutrophication on riparian ecosystems, however, is unclear. We conducted a mesocosm experiment to study the effects of nutrient loading on riparian ecosystems. We specifically asked whether the source of nutrients in the riparian zone affects the complex interactions that occur between surface water and adjacent wetlands. We also studied litter decomposition in the wetland component of the mesocosms, because litter accumulation in fens is assumed to control succession toward floating mats. Each mesocosm consisted of an upland component, referred to as the bank, and a water compartment. The bank and water compartments were planted with typical riparian zone and open water fen species prior to the addition of nitrogen (N) and phosphorus (P) in different combinations to either the bank or the surface water. Nutrient addition (mainly P) resulted in increased plant production and higher expansion rates of plants on the bank and in the water. There were also clear interactions in plant responses between the bank and water. Only eutrophic species increased shoot densities after fertilization. Nutrient addition further resulted in higher litter production, especially on the banks, and stimulated decomposition. Both the plant responses and the litter experiment indicated that eutrophication would accelerate succession to floating mats. Such floating fen mats are not likely to have the typical species-rich combination of desirable species; however, as our results suggest that they would be dominated by a few eutrophic species.

Sand Topdressing Applications Improve Shear Strength and Turfgrass Density on Trafficked Athletic Fields

Drain tile installation into a native-soil athletic field and subsequent sand topdressing applications are cost-effective alternatives to complete field renovation. However, if cumulative topdressing rates exceed root system development, surface stability may be compromised. The objective of this research was to evaluate the effects of cumulative topdressing, over a compacted sandy loam soil, on the fall wear tolerance and surface shear strength of a kentucky bluegrass (Poa pratensis)–perennial ryegrass (Lolium perenne) stand. Research was initiated in East Lansing, MI, on 10 Apr. 2007. A well-graded, high-sand-content root zone (90.0% sand, 7.0% silt, and 3.0% clay) was topdressed at a 0.25-inch depth [2.0 lb/ft2 (dry weight)] per application, providing cumulative topdressing depths of 0.0, 0.5, 1.0, 1.5, or 2.0 inches applied from 11 July to 15 Aug. 2007. Fall traffic was applied twice weekly to all treatments from 10 Oct. to 3 Nov. 2007. In 2008, topdressing applications and traffic, as described earlier, were repeated on the same experimental plots. Results obtained from this research suggest that the 0.5-inch topdressing depth applied over a 5-week period in the summer will provide improved shoot density and surface shear strength in the subsequent fall. Results also suggest that topdressing rates as thick as 4.0 inches accumulated over a 2-year period will provide increased shoot density, but diminished surface shear strength.

Trait‐dependent modification of facilitation on cobble beaches

Fundamental gaps remain in our knowledge of how positive species interactions, such as facilitation and mutualism, structure and maintain populations and communities. Foundation species create extensive biogenic habitats, but we know little of how their traits, such as density, age, and patch size, modify their ability to facilitate other species. We tested the role of facilitator traits in cobble beach plant communities in New England, USA. In this system, intertidal beds of the cordgrass Spartina alterniflora facilitate populations of halophytic forbs at higher shore elevations by buffering wave action, stabilizing cobbles, and limiting physical disturbance. Using descriptive and experimental techniques, we tested the hypotheses that (1) the density and height of cordgrass shoots modify the strength of the cordgrass-forb facilitation, and (2) cordgrass peat alone contributes to the facilitation of forbs. Increased shoot density, as well as the combination of cordgrass peat and shoots, positively affected two life history stages (seedling and adult) of the abundant forb Suaeda linearis, demonstrating that cordgrass traits modify the strength of facilitation in this system. Since the expression of traits varies within and among patches of any given foundation species, traits can and should be used to predict the strength of facilitation, to guide the development of conservation strategies, and to develop more accurate models of species interactions.

Disturbance, rainfall and contrasting species responses mediated aboveground biomass response to 11 years of CO2 enrichment in a Florida scrub‐oak ecosystem

AbstractThis study reports the aboveground biomass response of a fire‐regenerated Florida scrub‐oak ecosystem exposed to elevated CO2 (1996–2007), from emergence after fire through canopy closure. Eleven years exposure to elevated CO2 caused a 67% increase in aboveground shoot biomass. Growth stimulation was sustained throughout the experiment; although there was significant variability between years. The absolute stimulation of aboveground biomass generally declined over time, reflecting increasing environmental limitations to long‐term growth response. Extensive defoliation caused by hurricanes in September 2004 was followed by a strong increase in shoot density in 2005 that may have resulted from reopening the canopy and relocating nitrogen from leaves to the nutrient‐poor soil. Biomass response to elevated CO2 was driven primarily by stimulation of growth of the dominant species, Quercus myrtifolia, while Quercus geminata, the other co‐dominant oak, displayed no significant CO2 response. Aboveground growth also displayed interannual variation, which was correlated with total annual rainfall. The rainfall × CO2 interaction was partially masked at the community level by species‐specific responses: elevated CO2 had an ameliorating effect on Q. myrtifolia growth under water stress. The results of this long‐term study not only show that atmospheric CO2 concentration had a consistent stimulating effect on aboveground biomass production, but also showed that available water is the primary driver of interannual variation in shoot growth and that the long‐term response to elevated CO2 may have been caused by other factors such as nutrient limitation and disturbance.

Flowering and seedling production of understory herbs in old-growth forests affected by 1980 tephra from Mount St. Helens

To determine the variation in flowering among species and the contribution of flowering to the recovery of forest herbs, we counted the flowering shoots, vegetative shoots, and first-year seedlings in old-growth forests affected by tephra from the 1980 eruption of Mount St. Helens. Using permanent 1 m2 plots with either undisturbed tephra or with the tephra removed, we obtained 2–9 years of flowering data during 1980–2000 for six sites that received 2–15 cm of tephra. Flowering was infrequent for most species. Most of the 12 commonest species had 2%–4% of their shoots flowering (range 0%–15%). Among growth forms, deciduous nonclonal species flowered most. Flowering percentage increased with plant density; thus microsites favorable for growth were also favorable for flowering. Flowering varied considerably among sampling years. Incidence of flowering was higher on shallow than on deep tephra, and higher on natural tephra than in cleared plots. Seedlings were more common on shallow than on deep tephra. The 20-year increase in shoot density on tephra (ratio of 2000 to 1981 values) was positively related among species to the number of seedlings produced per previous-year flowering shoot. Our results indicate how species vary in the quantity and timing of flowering and also in their reliance on seedling establishment versus clonal growth for population increase following disturbance.

Fertilization augments Canada thistle ( Cirsium arvense L. Scop) control in temperate pastures with herbicides

Integrated weed management combines treatments for improved weed control. Few studies have compared integrated approaches for longer-term control of Cirsium arvense in pasture. We evaluated the response of this weed in perennial pasture to one-time mowing or herbicide application (2,4- d ester, 2,4- d+mecoprop+dicamba, clopyralid, or picloram+2,4- d) at the early bud stage of growth, both with and without annual spring fertilization over a 3-year period in central Alberta, Canada. Regardless of fertilization, all herbicide treatments suppressed Cirsium arvense in the year of treatment. Fertilization without control measures decreased Cirsium arvense density but increased biomass, while a one-time mowing treatment increased shoot densities. Although all herbicide treatments tended to reduce Cirsium arvense into the second and third year, picloram+2,4- d at 0.24+0.89 kg ai/ha and clopyralid at 0.22 kg ai/ha provided the greatest weed suppression, particularly when fertilized annually. Overall, longer-term control of Cirsium arvense by herbicides was enhanced with annual spring fertilization, underscoring the importance of an integrated approach to controlling Cirsium arvense in temperate pastures.

Long-Term Population Changes of a Fire-Adapted Plant Subjected to Different Fire Seasons

Abstract Long-term studies of the responses of plant populations to fire can inform adaptive management of ecosystems. I present results of an analysis of responses of a fire-adapted plant, Pityopsis graminifolia (Michx.) Nutt. (silkgrass goldenaster), to season of fire from 2001 to 2005 in a longleaf pine (Pinus palustris P. Miller) sandhill community in north Florida. Replicated May-burned and January-burned plots had been burned biennially from 1986 to 2004. Previous work in the early 1990s showed that populations of this species benefited more from fires during the peak lightning fire season (i.e., May) than from January fires. In 2001, however, shoot densities in both treatments were substantially lower than in 1992 and remained relatively low through March 2005. Shoot densities were significantly higher in May-burned plots than in January-burned plots between 2001 and 2005. May fires significantly increased shoot densities in 2002 (relative to January fires), but did not in 2004. Shoot densities in ...