Dry Spring Research Articles

Vulpia ciliata Dumort. ssp. ambigua (Le Gall) Stace & Auquier (Vulpia ambigua (Le Gall) More, Festuca ambigua Le Gall)

<i>Vulpia ciliata</i> Dumort. ssp. <i>ambigua</i> (Le Gall) Stace &amp; Auquier (<i>Vulpia ambigua</i> (Le Gall) More, <i>Festuca ambigua</i> Le Gall)

A 38‐year study of relations between weather and vegetation dynamics in road verges near Bibury, Gloucestershire

1 The Bibury long‐term data set contains information on annual fluctuations in the abundance of over 100 grasses and forbs in roadside verge vegetation over the period from 1958 to the present. Monitoring has been carried out every July by the same individual. The data set represents a unique long‐term record of the dynamics of a complete plant community. 2 Records for the most abundant taxa (including bare ground and litter) were used to determine the effect of climate variability on the year‐to‐year performance of the selected species. Residuals about the long‐term mean log biomass of each species (de‐trended where the species showed a significant increase or decrease in abundance over time) were correlated against indices of interannual climate variability. Plant and weather records were compared over 3‐month seasonal periods (March–May, June–August, September–November, December–February) or 6‐month seasonal periods (March–August, September–February), with time lags of 0, 1 and 2 years. 3 Principal components analysis (PCA) was used to formulate annual weather indices, using either conventional weather variables (temperature, rainfall and sunshine) or the Lamb catalogue of daily weather types. 4 Between 5% and 70% more correlations were observed than might be expected to occur by chance, depending on the season and the PCA index, indicating markedly non‐random plant–weather relationships. Total vegetation production was positively correlated with minimum spring temperature. The distribution of correlations was generally evenly distributed across the three lag periods. 5 In general, those species favoured by environmental stress or disturbance were promoted following warm dry springs and summers, whereas those favoured by more productive conditions were promoted following a wet growing season.

Reproduction in the Western Box Turtle, Terrapene ornata luteola

Optimal egg size theory was tested using reproductive data from the western box turtle, Terrapene ornata luteola, obtained from 272 X-rays of 124 different turtles, 72 of which were gravid. Clutch size was positively correlated with maternal body size, but egg width was not related to either maternal length or width or clutch size but was related to maternal mass. Pelvic width was significantly correlated with egg width and maternal body size. Egg size varied little, as predicted by optimal egg size theory. Mean clutch size was 2.70 (n = 77) and mean egg width was 2.66 cm (n = 203); clutch size is smaller and egg width is larger than for T. o. ornata. During the three years of the study, two years had unusually wet springs, and egg production was high; one year had a dry spring, and a smaller proportion of females produced eggs; but mean clutch and egg sizes were not significantly different from other years. These data indicate that spring rains may increase the proportion of females laying eggs in the summer, and in years with dry springs, turtles may defer laying eggs completely, rather than reducing annual output.

Thirty years of in situ tree growth under elevated CO2: a model for future forest responses?

Rising concentrations of atmospheric carbon dioxide have been predicted to stimulate the growth of forest trees. However, long‐term effects on trees growing to maturity and to canopy closure while exposed to elevated CO2 have never been examined. We compared tree ring chronologies of Mediterranean Quercus ilex which have been continuously exposed to elevated CO2 (around 650 μmol mol–1) since they were seedlings, near two separate natural CO2 springs with those from trees at nearby ambient‐CO2‘control’ sites. Trees grown under high CO2 for 30 years (1964–93) showed a 12% greater final radial stem width than those growing at the ambient‐CO2 control sites. However, this stimulation was largely due to responses when trees were young. By the time trees were 25–30 y old the annual difference in tree ring width between low and high CO2 grown trees had disappeared. At any given tree age, elevated CO2 had a relatively greater positive effect on tree ring width in years with a dry spring compared to years with more rainfall between April and May. This indicates a beneficial effect of elevated CO2 on tree water relations under drought stress. Our data suggest that the early regeneration phase of forest stands can be accelerated in CO2‐enriched atmospheres and that maximum biomass per land area may be reached sooner than under lower CO2 concentrations. In our study, high CO2 grown Q. ilex trees reached the same stem basal area at the age of 26 y as control trees at 29 y, i.e. three years earlier (faster turnover of carbon?). Reliable predictions of the future development of forests need to account for the variable responses of trees over their entire lifetime. Such responses to elevated CO2 can presently only be assessed at such unique field sites.

Cumulative effects of nitrogen application to winter wheat at Ropsley, UK, from 1978 to 1990

In 1977, a four-course rotation was set up at Ropsley (UK) to study crop response to eight rates of nitrogen application (35–265 kg ha−1). This rotation continued until 1990 when continuous winter wheat was introduced. Results from 1978 to 1990 provide an opportunity to study the initial phase of cumulative effects from different rates of N fertilizer application on the recovery of N by cereals and the retention of N in the soil.From 1978 to 1990, considerable variation in the recovery of nitrogen by winter wheat was observed. Neither rainfall nor drainage, as indicators of possible denitrification or leaching losses, provided a useful explanation for this, possibly because of the relatively dry conditions prevailing after spring fertilizer application. There was no evidence of increased soil N fertility, beyond single year residues, as a result of large N applications over the 13-year period.In order to achieve the economic optimum grain yield, it was necessary to use N applications which produced inefficient recovery of N. Thus, greater return of N in crop residues and immobilization at relatively large N applications (&gt;150 kg ha−1) contributed to an observed build-up in soil organic N over the period of study. Plots receiving, on average, 265 kg ha−1 appeared to gain c. 250 kg ha−1 N over control plots (35 kg ha−1) after 13 years of N application. Reducing the N application rate from the economic optimum to a more biologically efficient N rate (156 kg ha−1) was calculated to result in an average yield loss of 0·305 t ha−1 and cause an estimated £17 ha−1 loss in profit.

Biogenic isoprene emission: Model evaluation in a southeastern United States bottomland deciduous forest

Isoprene is usually the dominant natural volatile organic compound emission from forest ecosystems, especially those with a major broadleaf deciduous component. Here we report isoprene emission model performance versus leaf and canopy level isoprene emission measurements made at the Duke University Research Forest near Chapel Hill, North Carolina. Emission factors, light and temperature response, canopy environment models, foliar mass, leaf area, and canopy level isoprene emission were evaluated in the field and compared with model estimates. Model components performed reasonably well and generally yielded estimates within 20% of values measured at the site. However, measured emission factors were much higher in early summer following an unusually dry spring. These decreased later in the summer but remained higher than values currently used in emission models. There was also a pronounced decline in basal emission rates in lower portions of the canopy which could not be entirely explained by decreasing specific leaf weight. Foliar biomass estimates by genera using basal area ratios adjusted for crown form were in excellent agreement with values measured by litterfall. Overall, the stand level isoprene emissions determined by relaxed eddy accumulation techniques agreed reasonably well with those predicted by the model, although there is some evidence for underprediction at ambient temperatures approaching 30°C, and overprediction during October as the canopy foliage senesced. A “Big Leaf” model considers the canopy as a single multispecies layer and expresses isoprene emission as a function of leaf area rather than mass. This simple model performs nearly as well as the other biomass‐based models. We speculate that seasonal water balance may impact isoprene emission. Possible improvements to the canopy environment model and other components are discussed.

Comparison of canola, Indian mustard and Linola in two contrasting environments. II. Break-crop and nitrogen effects on subsequent wheat crops

The main canola-growing region in Australia is southern New South Wales where previous studies showed higher yield and grain protein of wheat growing after brassicas compared with wheat grown after wheat. This advantage, called the break-crop effect, was studied using winter oilseeds in two field experiments, one in this region and the other in central western New South Wales which is generally drier during the growing season, warmer throughout the year and is currently considered marginal for oilseeds. The effect of nitrogen (N) fertilizer on the response of wheat to previous crops was also investigated by considering both the soil N remaining from fertilizer applied to the break crops, and N applied to the subsequent wheat crops. The experiments were conducted over three years, with two phases of an oilseed-wheat sequence and a wheat-wheat control sequence at each site. Both sites had low baseline levels of soil mineral-N and average levels of root-disease inoculum. At the drier site the inoculum of wheat leaf and root pathogens remained during both phases but there were no break-crop effects. The effect of previous crops on the yield and protein of a subsequent wheat crop could be explained by the amount of residual soil mineral N. At the wetter site, wheat responses to previous crops could be explained by the amounts of residual soil mineral N in one phase of the experiment when there was no root disease of wheat. Under these conditions, yield and grain protein generally increased in response to increasing levels of soil mineral N. The exception was the yield of wheat after Linola which decreased when an excessive amount of residual N resulted in greater vegetative biomass, rapid depletion of soil water and decreased yield. In the other phase, when root disease was present, break crops increased yield of a subsequent wheat crop by 30% and grain protein by 1.3% compared to wheat growing after wheat, and among the oilseeds the brassicas gave a greater break-crop benefit than Linola. Application of fertilizer N to wheat growing after wheat failed to compensate for the disadvantage, indicating that residual N was not responsible for the differences. The break-crop benefit of the oilseeds extended to the second successive wheat crop for the phase in which root diseases were present, with increases of 13% in grain yield. The break-crop effect at the wetter site confirms previous observations of this benefit of oilseeds in general, and brassicas in particular, in southern New South Wales. The absence of any break-crop effect at the drier site suggests that root disease of wheat was less severe, possibly because the inoculum was less infective during the dry springs.

Karst water control and management in the Hanxing mining area of North China

The Hanxing mining area of North China includes three coalfields: Fengfeng, Handan and Xingtai. Six or seven coal seams can be commercially mined in Permo-Carboniferous strata, among which the lower three, accounting for 37% of the total reserves, are threatened with karst water from the underlain Ordovician limestone. Hundreds of water inrush accidents have occurred and over 30 mines have been flooded, resulting in heavy economic losses and casualties. In order to avoid water inrushes and keep the mines safely operational, dewatering in the karst aquifer was considered an essential measure. Unfortunately, this practice has caused serious environmental problems such as surface subsidence (sinkhole), dry spring, and water supply shortage. On the basis of a series of investigations and tests in the last 20 years, an alternative method, mining with water pressure, has been proposed and is the main focus of this paper. By using this method, the karst water in the limestone can be preserved to some extent and the coals can be mined in a relatively safe way.

Soil forming and plant density effects on carrot yield and internal quality.

The effects of soil forming (SF) and plant density (PD) on the carrot yield, mean root weight and internal quality was studied in field experiments in 1993 and 1994. 'Fontana BZ' carrots were grown in flat land, a narrow ridge, a broad ridge, and a compacted broad ridge soil configurations with low (LD) and high (HD) target plant densities, four and seven hundred thousand carrots per hectar. The total and marketable yields were larger in flat land and narrow ridge than in the broad- and compacted broad ridges in 1993 and in 1994 at HD. The number of marketable carrots were highest with the flat land soil configurations in both years. A dry spring in 1993 favored flat land growing conditions; in ridges the fine sand dried quickly. SF did not influence the mean weight of a marketable carrot in 1993, but in 1994 the narrow ridge configuration resulted in heavier carrots than the flat land or broad ridge growing conditions. In the climatically more unfavorable year of 1993, SF and PD affected quality; dry matter was lower in flat land than in the ridges. At HD, the flat land soil configuration produced higher glucose and fructose than carrots grown in the narrow and broad ridges. Dietary fiber and vitamin C were higher in narrow ridge than in compacted broad ridge grown carrots. At LD the flat land and broad ridge produced highest and compacted broad ridge the lowest beta-carotene contents; alpha-carotene was higher at LD than at HD.

Hydrologic and Geomorphic Factors Affecting Conservation of a River‐ Breeding Frog (Rana Boylii)

Organisms that live in highly variable environments, such as rivers, rely on adaptations to withstand and recover from disturbance. These adaptations include behavioral traits, such as habitat preference and plasticity of reproductive timing, that minimize the effects of discharge fluctuation. Studies linking hydrologic regime, habitat preference, and population processes, however, are predominantly limited to fish. Information on other sensitive taxa is necessary to facilitate conservation of multispecies assemblages and restoration of biodiversity in degraded river channels. I studied the functional relationship between physical habitat and reproduction of the foothills yellow‐legged frog (Rana boylii), a California State Species of Special Concern. From 1992 to 1994, I mapped breeding sites along 5.3 km of the South Fork Eel River in northern California and monitored egg survival to hatching. Frogs selected sites over a range of spatial scales and timed their egg‐laying to avoid fluctuations in river stage and current velocity associated with changes in discharge. The main sources of mortality were desiccation and subsequent predation of eggs in a dry year and scour from substrate in wet years, both caused by changes in stage and velocity. At the finest spatial scale, frogs attached eggs to cobbles and boulders at lower than ambient flow velocities. At larger scales, breeding sites were near confluences of tributary drainages and were located in wide, shallow reaches. Clutches laid in relatively narrower and deeper channels had poor survival in rainy as well as dry springs. Most breeding sites were used repeatedly, despite between‐ and within‐year variation in spring stage of the river. This pattern of site selection suggests that conservation of Rana boylii may be enhanced by maintaining or restoring channels with shapes that provide stable habitat over a range of river stages.

Delayed Effect of Monsoon Rains Influences Laying Date of a Passerine Bird Living in an Arid Environment

Much work has established that early spring temperatures modulate the onset of egg laying in resident birds of the north Temperate Zone. Most of this work has been done on the Great Tit (Parus major) and other parids in Holland (Kluijver 1951, VanBalen 1973), England (Lack 1958, Perrins and McCleery 1989) and other European countries (Dhondt et al. 1984, Barba et al. 1995). Little is known, however, about yearly variation in factors that affect the timing of laying in resident passerine species that inhabit different climatic regimes (but see Lambrechts and Dias 1993). We report here on some climatic correlates of laying date in a natural population of the Mexican Jay that has been studied for 24 years in the mountains of southeastern Arizona. This region has a monsoonal climate that differs greatly from that in Europe and much of North America. A hot, dry spring with almost no precipitation is followed by heavy monsoon rains in July and August (39% of annual total, range 18-73%) and a cold winter, including snow. Annual precipitation on the study area averaged 562.1 mm (SD = 131.5, range 252.0-780.5). The pattern of leaf fall and regrowth in southeastern Arizona also differs considerably from that in northwestern Europe. The vegetation on the study area is Madrean evergreen woodland (Brown 1982). Nearly all trees on our study area are evergreen. In particular, the oaks, except in extreme drought years, retain their leaves through winter and grow new ones immediately after dropping the old ones in spring. In a first attempt to identify ecological factors that influence the date of laying and the frequency of second clutches in a natural population of the Mexican Jay, we examine here the importance of certain climatic factors. 'Received 20 March 1996. Accepted 16 July 1996. SHORT COMMUNICATIONS 879

Weed Control in Ecofallow Corn (Zea mays) with Clomazone

Field studies were conducted from 1986 through 1989 to determine the feasibility of using clomazone in a winter wheat-corn-fallow rotation. Clomazone at 1.1 kg ai/ha did not control emerged downy brome in autumn nor did it control redroot pigweed and tumble pigweed that emerged after a spring application of paraquat at 0.6 kg ai/ha. However, clomazone plus atrazine at 0.6 plus 2.2 kg ai/ha provided weed control in winter wheat stubble from October to corn harvest 12 mo later. Corn yields from this treatment were equal to or greater than the conventional practice of applying atrazine plus glyphosate at 2.2 plus 0.4 kg ae/ha in the fall followed by a spring application of metolachlor plus dicamba at 2.8 ai plus 0.3 kg ae/ha. When kochia densities were high, clomazone plus atrazine controlled triazine-resistant kochia better than metolachlor plus dicamba. Atrazine at 2.2 kg/ha applied in autumn followed by clomazone at 0.8 kg/ha in April generally caused more corn injury than when the same rate of clomazone was applied in autumn with the atrazine. The percent of bleached corn plants varied with planter, year, time of application, and rate of clomazone applied. The optimum time to apply clomazone at 0.6 kg/ha was in autumn with atrazine. This treatment caused some corn bleaching but produced the highest corn yield over 3 yr. In a dry autumn and spring, less clomazone injury occurred with a planter equipped with row cleaners than those without. The injury pattern associated with planters was inconsistent when precipitation was average and no injury occurred in a wet autumn and spring.

Carbon isotope fractionation in tree ring early and late wood in relation to intra‐growing season water balance

ABSTRACTWe determined the stable carbon isotope composition (δ1.3C) of cellulose extracted from early and late wood in Douglas fir [Pseudotsuga menziexii (Mirb.) Franco] tree rings. Data were obtained for the period 1962 to 1981, at the start of which the trees were 20 years old. A water balance model was used to calculate daily stand transpiration and water deficit. The model incorporates site factors (soil water availability, slope and aspect) and environmental variables (solar radiation, air temperature and rainfall). There was far greater variability in late wood than in early wood δ1.3C. In wet years, late wood δ1.3C was significantly lighter (by as much as 2δ) than early wood δ1.3C but in dry years this difference was reversed. Differences between spring and summer cumulative transpiration accounted for almost 60δ of the variability in differences between early and late wood δ1.3C. We found excellent correspondence between summer cumulative transpiration and late wood δ1.3C, with estimates of transpiration accounting for up to 93% of the variability in δ1.3C. Correlations between early wood δ1.3C and spring transpiration were generally poor (r2&lt;0.4), but we were able to identify those exceptional years in which there had been a very dry spring. Our results indicate that, while tree ring δ1.3C correlates reasonably well with basal area increment, it is a far better indicator of inter‐ and intra‐annual variability in water availability than radial growth.

Effects of broad-leaf crops and their sowing time on subsequent wheat production

Lupin, field pea, lentil, chickpea, canola, linseed, and barley were sown at different times (late April-early July) to study their effects on subsequent wheat production on a red earth at Wagga Wagga, New South Wales. The cultivars of field pea ( Pisum sativum) included Dunn, Derrimut, Maitland and Dinkum; narrow-leaf lupin ( Lupinus angustifolius) cultivars were Danja, Geebung and Gungurru, and either the L. angustifolius line 75A/330 (1989–1990) or the broad-leaf lupin L. albus cv. Ultra (1991–1992). Only one cultivar of the other crops was grown in any year and after 1989 lentil ( Lens culinaris cv. Aldinga) was replaced by chickpea ( Cicer arietinum cv. Amethyst). The canola (cv. Shiralee (1989–1991), cv. Barossa (1992)) and linseed (cv. Glenelg) rotations received annual applications of 40–50 kg N/ha as urea. Compared to barley and the oilseeds, grain legumes increased soil mineral N supply to the following wheat crop. Over 4 years the mean wheat grain yield response to a broad-leaf crop, relative to barley, was 115% for lupin, 84% for field pea, 88% for linseed and 86% for canola. However, the effects of the various crops on subsequent wheat grain yields and grain protein varied markedly with season. The main advantage of lupin over field pea occurred in years when disease reduced growth of field pea crops. In high rainfall years, wheat yields following lentil and chickpea were lower than those following lupin. The narrow-leaf cultivars of lupin promoted greater wheat yields than either the reduced branching line 75A/330 or the broad-leaf albus cultivar Ultra. Delayed sowing of lupin reduced yield and grain protein of wheat, except when low rainfall curtailed growth of either crop in the rotation. There was little effect of field pea sowing date on wheat grain yield but sowing in late June combined with a dry spring, reduced mineral N supply and grain protein. Late sowing of oilseeds had no consistent effect on wheat grain yield but increased grain protein in most years. Late sowing of barley typically increased wheat grain yield but had little effect on grain protein. The effects of sowing time were mainly attributed to effects on soil N supply and for barley on disease incidence in the subsequent wheat.

Species Composition, Abundance, and Forecasting of Lygus Bugs (Heteroptera: Miridae) on Field Crops in Finland

A long-term survey of Lygus populations was carried out on 8 field crops in southern Finland. Weekly samples were taken with a sweep net throughout the growing season in 16 successive years. The dominant species on all crops was L. rugulipennis Poppius, constituting 92% of the total number of adults. L. gemellatus (Herrich-Schaeffer), L. pratensis (L.), L. punctatus (Zetterstedt), and L. wagneri Remane occurred in small numbers. Bugs were most abundant on wheat. Development of the nymphs was highly dependent on the temperature in June. The total numbers varied considerably from year to year and were gelwrally highest after warm preceding summers and dry autumns and springs. Multiregressional models were developed to forecast numbers of Lygus bugs.

Differences in groundwater response to deforestation ? a continuum of interactions between hydroclimate, landscape characteristics and time

After several decades of catchment research, most forest hydrologists agree that deforestation leads to increased annual catchment yields and augment groundwater recharge and dry-season flow. Nevertheless, millions of people in the semi-arid tropics have good reasons — a deep knowledge of the land after using it for many generations — to state the very opposite, ie that deforestation implies dry springs and streams. In this paper a model is presented which attemps to show that both of the above views are correct, but only if the hydrological consequences of deforestation are related to the diversity in land management, hydroclimate and landscape characteristics. The possibility of closing the dichotomy of opinion between people and professionals is discussed.

Observation: Comparative Live-History of Cheat-Grass and Yellow Starthistle

The objective of this research was to characterize the life-histories of cheatgrass (Bromus tectorum L.) and yellow starthistle (Centaurea solstitialis L.) growing in association. Biweekly demographic attributes were monitored during 1991 (moist spring) and 1992 (dry spring). Data were arranged into life-history tables, and sensitivity analysis was performed to determine key transition phases. The entire cheatgrass seed crop reached the soil surface, 41% of yellow starthistle's seed output was lost during seed rain. Frost heaving reduced winter seedling populations of cheatgrass (53%) more than yellow starthistle (40%). All cheatgrass seedlings surviving the frost heaving period became adults. Yellow starthistle density was reduced by 75% during the juvenile phase. Cheatgrass adults appeared about 6 weeks before yellow starthistle adults. Cheatgrass seed output remained about 7,000 m2 with moist and dry spring conditions. Yellow starthistle seed output was about 21,600 m2 and 5,200 m2 with moist and dry spring conditions, respectively. Reduction of yellow starthistle seed output with dry spring conditions suggest oscillatory community dynamics. Key processes associated with life-history transitions were interference (competition), resource acquisition rates and duration, and reproductive allocation.

Phytoplankton Ecology of North Carolina Estuaries

Numerous phytoplankton-oriented ecological studies have been conducted since 1965 in the extensive North Carolina estuarine system. Throughout a range of geomorphological estuarine types, a basic underlying pattern of phytoplankton productivity and abundance following water temperature seasonal fluctuations was observed. Overlying this solar-driven pattern was a secondary forcing mechanism consisting of a complex interaction between meteorology and hydrology, resulting in periodic winter or early spring algal blooms and productivity pulses in the lower riverine estuaries. Wet winters caused abundant nitrate to reach the lower estuaries and stimulate the blooms, whereas dry winters resulted in low winter phytoplankton abundance and primary production. Dinoflagellates (Heterocapsa triquetra, Prorocentrum minimum, Gymnodinium spp.) and various cryptomonads dominated these cool-weather estuarine blooms. Sounds were less productive than the riverine estuaries, and were dominated by diatoms such asSkeletonema costatum, Thalassiosira spp.,Melosira spp., andNitzschia spp., as were the most saline portions of riverine estuaries. Nutrient-limitation studies found that nitrogen was the principal limiting nutrient in these estuarine systems over a range of trophic states, with phosphorus occasionally co-limiting. Freshwater and oligohaline portions of large coastal plain rivers were often subject to summer blue-green algal blooms. Formation of these blooms on a year-to-year basis was also determined by meteorology and hydrology: wet winters or springs and consequent nutrient loading, coupled with low summer flow conditions and regeneration of nutrients from the sediments. Dry winters or springs resulted in less available nutrients for subsequent summer regeneration, and high flow conditions in summer flushed out the blooms. In recent years, there has been a dramatic increase in reported fish kills attributed to toxic dinoflagellate blooms, particularly in nutrient-enriched estuarine areas. This issue has become a major coastal ecological and economic concern.

Incidence ofVenturia inaequalison apple fruit during the second half of the season under different fungicide and weather regimes

Abstract The incidence of apple scab (Venturia inaequalis) on apple fruit was estimated from December to March in orchards with different levels of leaf and fruit scab and under different fungicide programs. Twenty orchards were monitored at Batlow, NSW, Australia from 1990 to 1993. Moderate levels of primary infection in the 1990/91 season resulted in levels of leaf scab in December/ January, which varied from 0 to 6.2% (mean 0.8%), and fruit scab in December from 0 to 7.1% (mean 0.9%). Fruit scab incidence measured in March was 0.4% (after the normal practice of removal of fruit to increase remaining fruit size). A mean of 1.6 fungicide sprays were applied from January to March, with a range of 0–7 sprays. It is likely that infection criteria would have been met on three occasions during December‐March. No significant increase of scab occurred in seven orchards where no fungicides were applied over this period. In the 1991/92 season, a dry spring resulted in five infection periods, four of which were lo...

Growth Characteristics of Rhizoma Peanut and Nitrogen‐Fertilized Bahiagrass Swards

AbstractThe interaction of plant sensitivity to drought with N deficiency has been suggested as the reason grasses fail to persist under grazing when grown in association with rhizoma peanut (Arachis glabrata Benth.). To test this assumption, aboveground dry matter (DM) accumulation and relative growth rate (RGR) of ‘Florigraze‘ rhizoma peanut and ‘Pensacola’ bahiagrass (Paspalum notatum Flügge) fertilized with 0, 56, and 112 kg N ha−1 were measured from March to November in 1990 and 1991. Rhizoma peanut aboveground DM accumulation was greater both years (4950 and 6100 kg ha−1 1990 and 1991, respectively) in July at the start of the summer rainy season than any N‐fertilized bahiagrass treatment (640 and 1530, 1240 and 2330, and 1570 and 4490 kg ha−1 in 1990 and 1991 for 0‐N, 56‐N, and 112‐N treatments, respectively), due to higher RGR. Although the RGR of rhizoma peanut was similar to or less than that of the bahiagrass treatments for the remainder of the growing season in both years, by August 1991 only the 112‐N bahiagrass treatment (5260 kg ha−1) had total DM accumulation similar to rhizoma peanut (5110 kg ha−1). Rhizoma peanut RGR appeared to be more sensitive to rainfall, and was positively correlated (r = 0.71) with rainfall for as long as 8 wk prior to the sample dates in the dry spring of 1990. Bahiagrass RGR showed no correlation with rainfall either year. This suggests that lack of persistence of bahiagrass in mixed swards is due to severe competition for light with rhizoma peanut in springs when moisture is adequate and to potentially increased stress due to grazing in springs when drought restricts growth of rhizoma peanut.