Reduction In Leaf Area Index Research Articles

Age-related decline in stand water use: sap flow and transpiration in a pine forest chronosequence

We investigated changes in the pattern of stand water use in a chronosequence of four even-aged maritime pine stands, differing in age (10-, 32-, 54- and 91-year old) and growing under similar environmental conditions. Extensive sap flow measurements were carried out during a period of 2 years (2001–2002), using thermal dissipation sensors located 1 m below the live crown and corrected for the radial profile of sap flow. We found significant differences in transpiration among stands. Overstorey transpiration, E T, declined with stand age from 508 mm per year for the 10-year-old stand to 144 mm per year for the 54-year-old stand in 2001. This age-related decline was driven by (i) a decrease in transpiration per unit leaf area (55%), due to a decrease in stomatal conductance in taller trees and (ii) a significant reduction in leaf area index (38%). There was a strong effect of soil moisture deficit on transpiration and an interaction with age. A strong decrease in overstorey transpiration resulted from the prolonged drought in 2002 due to both a lower transpiration rate per unit leaf area and lower leaf area evidenced both by optical and litterfall measurements. The drought effect was greater in the younger stands compared to older stands. In the 32-year-old stand, simultaneous measurements of sap flow and eddy covariance showed that the overstorey transpiration contributed to half of the total ecosystem water loss, E. Moreover, the total ecosystem evaporation estimated for each stand by a water balance approach was not different between stand ages. We suggest, therefore, that the decline in overstorey transpiration with stand age mainly affects partitioning of the components of evaporation between trees, understorey and soil but not the magnitude of total evaporation.

Amazon drought and its implications for forest flammability and tree growth: a basin‐wide analysis

AbstractSevere drought in moist tropical forests provokes large carbon emissions by increasing forest flammability and tree mortality, and by suppressing tree growth. The frequency and severity of drought in the tropics may increase through stronger El Niño Southern Oscillation (ENSO) episodes, global warming, and rainfall inhibition by land use change. However, little is known about the spatial and temporal patterns of drought in moist tropical forests, and the complex relationships between patterns of drought and forest fire regimes, tree mortality, and productivity. We present a simple geographic information system soil water balance model, called RisQue (Risco de Queimada – Fire Risk) for the Amazon basin that we use to conduct an analysis of these patterns for 1996–2001. RisQue features a map of maximum plant‐available soil water (PAWmax) developed using 1565 soil texture profiles and empirical relationships between soil texture and critical soil water parameters. PAW is depleted by monthly evapotranspiration (ET) fields estimated using the Penman–Monteith equation and satellite‐derived radiation inputs and recharged by monthly rain fields estimated from 266 meteorological stations. Modeled PAW to 10 m depth (PAW10 m) was similar to field measurements made in two Amazon forests. During the severe drought of 2001, PAW10 m fell to below 25% of PAWmax in 31% of the region's forests and fell below 50% PAWmax in half of the forests. Field measurements and experimental forest fires indicate that soil moisture depletion below 25% PAWmax corresponds to a reduction in leaf area index of approximately 25%, increasing forest flammability. Hence, approximately one‐third of Amazon forests became susceptible to fire during the 2001 ENSO period. Field measurements also suggest that the ENSO drought of 2001 reduced carbon storage by approximately 0.2 Pg relative to years without severe soil moisture deficits. RisQue is sensitive to spin‐up time, rooting depth, and errors in ET estimates. Improvements in our ability to accurately model soil moisture content of Amazon forests will depend upon better understanding of forest rooting depths, which can extend to beyond 15 m. RisQue provides a tool for early detection of forest fire risk.

Growth, yield, competition and economics of groundnut/cereal fodder intercropping systems in the semi-arid tropics of India

Intercropping legumes with non-legume crops during the rainy season (wet season) is a common practice in the semi-arid tropics of India. Of late, the concept of intercropping has also been utilized in irrigated (dry season) situations. In a 2-year field study during the dry season (February–May), we assessed yield, competition and economics in a groundnut/cereal fodder intercropping system compared with monocropped groundnut. Maize ( Zea mays L.), sorghum ( Sorghum bicolor (L.) Moench) and pearl millet ( Pennisetum glaucum L.) were grown for fodder. One cutting for all and two cuttings (first at 50 days after sowing and second at 95 days after sowing) for sorghum and pearl millet were made. In intercrops one row of cereal fodder was sown between every three rows of groundnut (1:3). The green fodder yields and pod yield of groundnut were lower in intercropped than in monoculture plots. The highest green fodder yield in intercrops was recorded in pearl millet with two cuts (16.5 t ha −1) followed by pearl millet with one cut (11.8 t ha −1) and sorghum with two cuts (10.7 t ha −1). In intercrops the growth and yield of groundnut were affected by cereal fodder and intensity of cutting. A significant ( ∗ P<0.05) reduction in leaf area index (LAI) and crop growth rate (CGR) was observed in the groundnut–pearl millet system over sole groundnut. Decrease in nodule mass at pod filling stages in groundnut ranged from 3.5 to 11.0% when intercropped with cereal fodders compared to sole groundnut crop. Groundnut yield was reduced more due to pearl millet and sorghum with two cuts. However, maize as the associated crop produced 9.0 t green fodder ha −1 and affected the groundnut less with respect to pod yield (5.76% reduction), yield attributes, CGR, LAI and nodule dry mass. Of the two cutting situations under intercropping, one cut gave 9.9% higher yield of groundnut as compared to two cuts. A higher land equivalent ratio (LER) and relative crowding coefficient (RCC) value leads to a crop yield advantage. Accordingly, yield advantage was greater in case of the groundnut/maize association. The competition ratio (CR) is a better indication of performance than RCC. The CRs of pearl millet and sorghum with two cuts were greater than maize but the corresponding CRs of groundnut were less. Thus, pearl millet and sorghum were more competitive, and groundnut under these two crops was affected more. The maximum monetary advantage was also recorded for the groundnut/maize intercropping system.

Effects of water shortage on six potato genotypes in the highlands of Bolivia (I): morphological parameters, growth and yield

The objective of this paper is to characterize drought-adaptation, and to identify indicators of drought-tolerance in potato in the Andean Highlands. Six genotypes, representing a range of contrasting morphological and agronomic characters and belonging to three species, were grown in containers. The treatments were R0 (irrigated control), R1 (progressive drought after tuberisation, with a recovery period) and R2 (as R1, but no recovery irrigation). Genotypes differed in plant height, leaf area index (LAI), proportion of ground covered by green foliage and root expansion. Drought did not significantly affect stem number, and slightly decreased plant height and leaf number. It reduced leaf area index and canopy cover in all genotypes. Dry matter production was greatly affected in cultivars Ajahuiri and Luky, but very little in the other genotypes. Under drought, Alpha and CIP 382171.10 gave the highest yields and Luky the lowest. Depending on genotype and degree of yield reduction, yield losses were associated with a decrease either in radiation intercepted, the efficiency of its conversion into dry matter, or harvest index. Harvest index and root biomass correlated well with tuber yield. Other parameters, such as leaf number, plant height and canopy cover, correlated with tuber yield only in the R2 treatment.

Cultivar and Herbicide Selection Affects Soybean Development and the Incidence of Sclerotinia Stem Rot

Glyphosate‐resistant (GR) soybean [Glycine max (L.) Merr.] is produced on a majority of U.S. soybean hectares. Concerns have been expressed regarding interactions of herbicides such as glyphosate and Sclerotinia stem rot [Sclerotinia sclerotiorum (Lib.) de Bary]. Research evaluated the effect of herbicides on growth, phytoalexin production, incidence of Sclerotinia stem rot, and grain yield of GR and isogenic nonresistant cultivars in 1998 and 1999. Lactofen at 70 g a.i. ha−1 delayed reproductive development, reduced leaf area index (LAI), reduced Sclerotinia stem rot lesion diameter 2 to 26 d after treatment (DAT), increased phytoalexin production 2 to 26 DAT, reduced disease severity index (DSI), and increased yield of ‘Great Lakes 2415’ (GL2415) by 510 kg ha−1 compared with untreated soybean. Glyphosate at 840 g a.e. ha−1 increased disease severity index in ‘Great Lakes 2600’ (GL2600) and ‘Pioneer 93B01’ (P93B01) GR compared with untreated soybean while yield of ‘Novartis S20‐B9’ (S20‐B9) GR treated with glyphosate was 650 kg ha−1 greater than untreated soybean. Thifensulfuron at 4.5 g a.i. ha−1 delayed reproductive development, reduced LAI, and did not affect phytoalexin production of ‘Novartis S 19–90’ (S 19–90) or S20‐B9 GR but increased grain yield of ‘Novartis S 12–49’ (S 12–49) by 450 kg ha−1 compared with untreated soybean. S 12–49 and GL2415 treated with thifensulfuron had lower disease severity index and increased yield compared with GR isolines. Reduction of disease severity index following lactofen may be attributed to increased phytoalexin production, reduced LAI, and delayed reproductive development; however, increased yield was observed for only one of eight cultivars. Producers can reduce Sclerotinia stem rot with postemergence herbicides, but soybean yield did not necessarily increase.

CO 2 and ozone effects on canopy development of potato crops across Europe

This paper describes the effects of elevated CO 2 (550 and 680 μl l −1) and O 3 (60 nl l −1 O 3 as an 8 h mean), alone or in combination, on canopy development and senescence in potato ( Solanum tuberosum L. cv Bintje) across a range of European agro-climatic conditions. The assessments were made within the European CHIP project (CHanging climate and potential Impacts on Potato yield and quality) that was conducted for two growing seasons (1998 and 1999) in free air CO 2 enrichment systems (FACE) and open-top chamber facilities (OTCs) at seven European sites. A comparison of chambered and unchambered experimental plots was included to examine the effects of chamber enclosure. Phenological growth stages, plant height, leaf area index (LAI) and the number of green and yellow leaves were recorded non-destructively throughout the growing season and by a destructive intermediate harvest at maximum leaf area (MLA). In the dynamic growth analysis CO 2 and O 3 effects were studied over three developmental stages: canopy expansion, full canopy and canopy senescence. Chamber enclosures promoted potato crop development (taller plants, more leaves) during the initial growth stages and led to a faster decline of LAI and a higher number of yellow leaves. The growth in ambient plots varied between sites and seasons, as did the scale of the treatment responses. Despite the large background variation, some overall treatment effects could be detected across all sites. Both levels of increased CO 2 reduced final plant height in comparison to ambient concentrations, which indicates a premature ending of the active plant growth. At the stage of full canopy and crop senescence the average number of green leaves was significantly ( P<0.05) decreased by 680 μl l −1 CO 2 (OTC experiments) and LAI showed the same tendency ( P=0.07). As there was however no indication of a decreased leaf formation during initial growth and at full canopy, this must have been due to an earlier leaf fall. In the FACE experiments LAI had already began to decline at the stage of full canopy at 550 μl l −1 CO 2 but not in ambient CO 2 (DAE×CO 2, P<0.05). These observations strongly indicated that elevated CO 2 induced a premature senescence during full canopy. O 3 did not have an overall detrimental effect on crop development during initial growth nor at full canopy, but did induce a faster reduction of LAI during crop senescence (DAE×O 3, P<0.05). Final plant height was not affected by O 3. There were few CO 2×O 3 interactions detected. There was a suggestion ( P=0.06) that O 3 counteracted the CO 2-induced decrease of green leaves at full canopy, but on the other hand during crop senescence the decline of LAI due to elevated O 3 was faster at ambient compared to elevated CO 2 ( P<0.05). These responses of canopy development to elevated CO 2 and O 3 help to explain the treatment responses of potato yield within the CHIP project at sites across Europe.

Relationship between leaf area index and yield in double-crop and full-season soybean systems.

Previous research indicates a correlation between leaf area index (LAI) and yield of full-season soybean [Glycine max (L.) Merrill], which is a single crop planted early in the season. Leaf area index values of at least 3.5-4.0 in the reproductive stages are required for maximum potential yield. It is unknown how yields of double-crop soybean, which is planted late into harvested small grain fields, respond to changes in leaf area index. We hypothesized that double-crop soybean would be more sensitive to defoliation than full-season soybean. This study used linear and linear plateau models to describe the yield response of full-season and double-crop soybean to reductions in leaf area index through manual defoliation, and evaluated the yield response of double-crop soybean to reductions in leaf area index through natural insect defoliation. From 15 manual defoliation experiments over 3 yr, significant linear decreases in yield occurred in both full-season and double-crop soybean when leaf area index values were below 3.5-4.0 by developmental stages R4 to R5, whereas yields usually reached a plateau at higher leaf area index levels. Average yield loss was 769 +/- 319 kg ha(-1) for each unit decrease in leaf area index below the plateau; average maximum yield was 3,484 +/- 735 kg ha(-1). From eight field experiments over 2 yr, insect defoliators had no effect on double-crop soybean yield; leaf area index levels were above 4.0 by the developmental stage when leaf area index estimates were taken (R3 to R6). Therefore, double-crop soybean that maintains leaf area index values above the 3.5-4.0 critical level by mid-reproductive developmental stages can tolerate defoliating pests.

Herbicide Effects on Visible Injury, Leaf Area, and Yield of Glyphosate-Resistant Soybean (Glycine max)1

The failure of glyphosate to control all weeds throughout the entire growing season has sometimes prompted growers to use herbicides other than glyphosate on glyphosate-resistant soybean. Field studies were conducted in 1999 and 2000 to investigate potential crop injury by several herbicides in glyphosate-resistant soybean and to determine the relationships between soybean maturity group, planting date, and herbicide treatment on soybean injury, leaf area index (LAI), and yield. Glyphosate-resistant soybean generally recovered from early-season herbicide injury and LAI reductions; however, some treatments reduced yield. Yield reductions were more common in double-crop soybean than in full-season soybean. In full-season soybean, most yield reductions occurred in the early-maturing ‘RT-386’ cultivar. These yield reductions may be attributed to reduced developmental periods associated with early-maturing cultivars and double-crop soybean that often lead to reduced vegetative growth and limited LAI. Reductions in LAI by some herbicide treatments were not necessarily indicative of yield loss. Further yield reductions associated with herbicide applications occurred, although soybean sometimes produced leaf area exceeding the critical LAI level of 3.5 to 4.0, which is the minimum LAI needed for soybean to achieve maximum yield. Therefore, LAI response to herbicide treatments does not always accurately indicate the response of glyphosate-resistant soybean yield to herbicides.Nomenclature: Glyphosate; soybean, Glycine max (L.) Merr.Additional index words: Double-crop soybean, full-season soybean, soybean maturity group.Abbreviations: EPSPS, 5-enolpyruvylshikimate-3-phosphate synthase (EC 2.5.1.19); fb, followed by; LAI, leaf area index; PRE, preemergence; POST, postemergence; WAP, weeks after planting.

Effects of plant population on leaf area index, cob characteristics and grain yield of early maturing maize cultivars (FAO 100–400)

Maize ( Zea mays L.) growth in non-typical maize growing regions has several limitations with respect to agronomic characteristics of cultivars, and their reactions on changes of production system in specific climate. Two long term field experiments were carried out to investigate the effect of plant populations (PP) on the leaf area index (LAI), grain yield and cob characteristics of maize cultivars in Maribor, Slovenia. In the first experiment, an increase of PP from 4.5 to 13.5 plants per m 2 did not affect the LAI of four older maize cultivars at the 7–9-leaf stage, but significant differences between cultivars, years and PP appeared at brooming (from 1.58 to 7.07). In this stage, a strong correlation existed between LAI and grain yield ( r=0.87**). Reduction of LAI from brooming to waxy maturity averaged 9.7%. PP explained 93–99% of the variation in grain yield depending on cultivars. Some cultivars lodged at the highest PP in some years. Increased PP significantly changed the following cob characteristics, weight of 1000 kernels, cob length, number of kernel rows, and number of kernels per row. In the second experiment, 11 recent cultivars expressed a relatively small trend of grain yield increases when PP was increased from 7 to 13 plants per m 2. At high PP, the percent of plants decaying after emergence increased up to 27% during the vegetation period. Therefore, we conclude that the information on suitable PP for each maize cultivar is one of the key factors for planning maize production.

Effect of oleiferous brassicas, their staggered sowing and fertility levels on potato (Solanum tuberosum) performance in potato based intercropping systems

A field experiment was conducted during the winter (rabi) seasons of 1995 97 to assess the performance of potato (Solanum tuberosum L.) in potato based intercropping systems (sole potato and 6 intercropping systems created by intercropping 3 Brassica spp. at the time of potato planting and 20 days after potato planting in 3:l replacement series) under 5 fertility levels (No, ,N, N15,, N, S,, and N, S, kg/ha). lrrespective of staggered sowing, intercrop caused significant reduction in leaf area index (LA ), tuberslhill, proportion of A grade tubers, tuber yield and green tubers (%) when compared with sole crop of potato, whereas plant height and shootslhill remained unaffected. Among the intercropping systems, potato staggeredly sown Brassica carinata A. Braun showed its superiority by recording relatively higher values of growth and yield attributes, yield of A grade tubers and tuber yieldlha and minimum value of green tubers(%). lrrespective of time of sowing, Brassica carinata and its system with potato recorded the highest seed yield and tuber equivalent respectively. Nitrogen application induced significant increase in number of shoots and tuberslhill only up to 75 kg Nlha, whereas plant height, LAI, yield of A and B grade tubers and tuber, seed and tuber equivalent yielddha recorded significant increase up to 150 kg Nlha. Application N S caused perceptible increase in the seed yield, B grade tubers and tuber equivalent yield over N alone.

Energy balance of a natural jarrah ( Eucalyptus marginata) forest in Western Australia: measurements during the spring and summer

The jarrah ( Eucalyptus marginata) forest in southwest Western Australia is capable of maintaining vigorous growth through severe dry summers, during which there may be little or no rain for close on 6 months. Hydrologically, the catchments are characterised by very low runoff rates, being 1–10% of rainfall. The work described here aimed to (1) obtain measurements of the various components of jarrah forest evapotranspiration during two contrasting seasons; (2) obtain measurements enabling determination of the components of the energy balance of the forest; and (3) test the hypothesis that these forests have established in these areas in part because they can access the deep soil water to maintain high evaporation rates in the summer months and avoid significant water stress. The results of these measurements are to be used in validation and parameterisation of a model to be described in a separate paper. Measurements of the energy and water balance of this system were taken during 10 days in spring and late summer. Total forest evaporation, tree water use, sensible heat flux above the 25 m canopy and at 2 m above the forest floor, forest floor evaporation, soil heat flux and temperature profiles, canopy heat storage and radiation above and below the canopy were measured. Despite a reduction of leaf area by one-third of the spring value and a reduction of 30% in soil moisture, the forest maintained high leaf conductances during summer, with evapotranspiration rates at around or above 78% of equilibrium evapotranspiration in both seasons. On average, total water use by the trees was slightly, but not significantly, greater in summer than spring (2.8 and 2.5 mm per day, respectively), and in summer soil evaporation was about half its spring rate (0.27 mm per day compared with 0.48 mm per day). Short-wave radiation penetration through the canopy was found to be the same in spring as it was in summer (being 35% of overstorey radiation), despite a 30% reduction in leaf area index (1.32–0.88) from spring to summer. This is probably due to the significance of tree trunks and branches intercepting radiation, relative to the relatively sparse canopy, and the greater component of diffuse radiation during spring. Canopy heat storage, was found to be a significant component in the total hourly energy budget. During the first three daylight hours it was 45% of net radiation, R n, in spring and 69% in summer. During the last three daylight hours the proportions were −17 and −19% in spring and summer, respectively. The data presented illustrate important considerations for modelling this forest system and others like it. The importance of a deep soil profile which retains moisture from winter rainfall of this Mediterranean climate, and provides the forest with an adequate supply through the long dry summer is clearly significant. Canopy conductance must be linked to the availability of moisture from the deep soil profile throughout the dry summer. The forest also appears to have adapted to its climate in such a way as to maximise water use despite seasonal fluctuations in leaf area, although we only have data from a single seasonal cycle.

Downward adjustment of carbon fluxes at the biochemical, leaf, and ecosystem scale in beech‐spruce model communities exposed to long‐term atmospheric CO2 enrichment

Young beech (Fagus sylvatica) and spruce (Picea abies) trees from different provenances or genotypes were grown in competition in large model ecosystems and were exposed to two concentrations of atmospheric CO2 (370 vs 570 μmol mol−1), two levels of wet nitrogen deposition (7 vs 70 kg N ha−1 yr−1), and two native forest soils (acidic vs calcareous) for four years in open‐top chambers. The 2×2×2 factorial experimental design was fully replicated (n=4) with each CO2×N combination applied to each soil type. Exposure to atmospheric CO2 enrichment stimulated daytime net ecosystem CO2 flux (NEC) as measured during sunny days in the middle of the third growing season. Nevertheless, we observed substantial down‐regulation of NEC, with larger adjustments on acidic than on calcareous soil. NEC adjustment was associated with slightly reduced leaf area index (LAI) on the acidic soil (no response on calcareous soil), enhanced soil CO2 efflux from both substrate types, and, most importantly, with down‐regulation of CO2 uptake at the leaf scale. Downward adjustment of light‐saturated single‐leaf photosynthesis (A) and of Rubisco was more pronounced in beech than in spruce and these species‐specific differences increased over time. By year four, A adjustment (except in one specific treatment combination in each species) had become complete in beech but had disappeared in spruce. At no time did we observe a genotype or provenance effect on the downward adjustment of carbon fluxes, and nitrogen deposition rate generally had little effect as well. Overall, our results suggest that tree species and soil quality will have profound effects on ecosystem CO2 fluxes under continued atmospheric CO2 enrichment.

Morphological, phenological and water-use dynamics of sorghum varieties ( Sorghum bicolor) under Striga hermonthica infestation

Leaf number was unaffected due to Striga infestation in the course of crop development in susceptible and resistant sorghum cultivars. Striga effect on stem height of IS 9302 was significant particularly during flowering and grain filling periods. No significant reduction in stem height was observed in SRN 39. Leaf area index (LAI) of IS 9302 Striga infested plants was significantly lower during panicle initiation and flowering. In SRN 39 reduction in LAI started relatively late and was significant only at peak flowering and at final harvest. Specific leaf area (SLA) in both sorghum cultivars was unaffected by Striga infestation. However, the leaf weight ratio (LWR) in the Striga infested plants of both cultivars was increased. Both SLA and LWR indicate that the Striga effect on the assimilatory surface and on the portion of dry matter allocated to leaves was small. The change in allometric coefficient was not high in both Striga infested sorghum cultivars. Only about 14% more dry matter was allocated to roots in IS 9302 Striga infested plants. Both the dynamic changes in leaf morphology and allometric coefficient indicate that the Striga effect may not be mainly through the interference with water balance of the infested host plants. Under Striga infestation there was a delay of about 120 degree-days to reach booting stage in both sorghum cultivars. The delay in developmental shift from booting to flowering under Striga infestation was also more pronounced. Unlike SRN 39, most of IS 9302 Striga infested plants failed to flower. Possible reasons for this delay are discussed. Water use efficiency (WUE) of Striga infected IS 9302 sorghum plants were reduced by about 72% compared to the control. The reduction in WUE of SRN 39 was about 24%. However, the evapotranspiration of Striga infected IS 9302 plants was even larger than the control and almost equal in the SRN 39 infected plants. This implies that dry matter accumulation was reduced, thereby reducing WUE significantly. In this case, the resistant cultivar SRN 39 showed less reduction in LAI, no more dry matter partitioning to roots and a significant improvement in WUE compared to IS 9302 under Striga infestation.

Growth analysis of maize field crops under phosphorus deficiency. II. Radiation-use efficiency, biomass accumulation and yield components.

Biomass accumulation by crops depends on both light interception by leaves and on the efficiency with which the intercepted light is used to produce dry matter. Our aim was to identify which of these processes were affected for maize (Zea mays L., cv Volga) field crops grown under phosphorus (P) deficiency. In the preceding paper (Plenet et al., 2000), it was shown that P deficiency severely reduced leaf growth. In this paper, the effect of P deficiency on the radiation-use efficiency (RUE) was investigated. The experimental work was carried out in 1995, 1996 and 1997 on a long-term P fertilisation trial located on a sandy soil in the south-west of France. Three P fertilisation regimes have been applied since 1972: no- P (P0 treatment) and different rates of P fertiliser (P1.5: 1.5 times the grain P export and P3: 3 times the grain P export). These fertilisation regimes have led to contrasted levels of soil P supply. Only slight differences were observed between the P1.5 and P3 treatment for above-ground biomass accumulation and grain yield. Conversely the grain yield was significantly reduced in P0 (−11%). Above-ground biomass production was severely reduced, with the maximum difference between treatment (−60% in P0) occurring between 400 and 600 °C days after sowing. The lower biomass production in P0 was accounted for by the reduced amount of photosynthetically active radiation (PAR) absorbed by the canopy, which was itself the consequence of the reduced leaf area index (see Plenet et al., 2000). The calculated RUE were found to depend on the plant stage, especially during the pre-flowering period, and on the average air temperature. No effect of P deficiency was observed on the calculated RUE, even during the period when above-ground biomass accumulation was the most severely reduced. These results obtained in field crop conditions strengthen the idea that P deficiency affects plant growth, especially leaf growth, earlier and to a greater extent than photosynthesis per unit leaf area.

Crescimento de plantas de milho submetidas a déficit hídrico em solos de diferentes texturas

O objetivo deste experimento foi quantificar variações no dossel vegetativo de plantas de milho submetidas a dois manejos da água de irrigação em solos de três texturas (argila pesada, franco-argilo-siltosa e franco-arenosa). O experimento foi instalado em área do Departamento de Engenharia Rural, da Universidade Federal de Santa Maria, no ano agrícola 1995/1996. Utilizou-se um conjunto de 12 lisímetros de drenagem com dimensões de 156 cm de comprimento, 100 cm de largura e 80 cm de profundidade, protegidos das precipitações por meio de uma cobertura móvel. Os tratamentos consistiram de dois níveis de manejos da água no solo (irrigado e déficit hídrico terminal) e três texturas de solos. Os resultados demonstraram que a redução no índice de área foliar das plantas de milho submetidas a déficit hídrico foi maior em solos de textura argila pesada e franco-argilo-siltosa do que no solo de textura franco-arenosa. A altura das plantas de milho diminuiu quando a fração de água disponível às plantas foi inferior a 0,57, 0,74 e 0,52, para os solos de textura argila pesada, franco-argilo-siltosa e franco-arenosa, respectivamente.

Responses of communities of tropical tree species to elevated CO2 in a forest clearing.

Communities of ten species of tropical forest tree seedlings from three successional classes were grown at ambient and elevated CO2 in large open-top chambers on the edge of a forest in Panamá. Communities grew from 20 cm to approximately 2 m in height in 6 months. No enhancements in plant biomass accumulation occurred under elevated CO2 either in the whole communities or in growth of individual species. Reductions in leaf area index under elevated CO2 were observed, as were decreases in leaf nitrogen concentrations and increases in the C:N ratio of leaf tissue. Species tended to respond individualistically to elevated CO2, but some generalizations of how successional groupings responded could be made. Early and mid-successional species generally showed greater responses to elevated CO2 than late-successional species, particularly with respect to increases in photosynthetic rates and leaf starch concentrations, and reductions in leaf area ratio. Late-successional species showed greater increases in C:N ratios in response to elevated CO2 than did other species. Our results indicate that there may not be an increase in the growth of regenerating tropical forest under elevated CO2, but that there could be changes in soil nutrient availability because of reductions in leaf tissue quality, particularly in late-successional species.

SHORT COMMUNICATION Growth and Development of White Clover (Trifolium repensL.) as Influenced by P and K Nutrition

The effects of varying P and K supplies on the growth and morphology of white clover were investigated in a pot experiment. Plants were treated with three rates of potassium (K) and four rates of phosphorus (P) in factorial combination, and five harvests were taken between August 1993 and February 1994. During establishment, shoot and leaf development were more adversely affected by P than by K deficiency but, by the final growth period, K deficiency was the more limiting to growth, causing dramatic reductions in leaf area index (LAI), numbers of growing points, and stolon and root masses. The adverse affects of K deficiency on shoot development were attributed to the curtailment of petiole and lamina expansion which have major requirements for K. It was noted that, under K deficiency, proportionately less assimilate was partitioned to leaf production than to stolon and root growth; this may be an adaptation enabling K-deficient plants to exploit nutrient supplies in adjacent localities. The results suggest that, provided white clover can withstand ‘moderate’ P stress during establishment, its persistence in swards will probably be limited more by K deficiency than by low or inadequate P supplies. However, because the experiment was conducted outside the normal growing season and with only a single white clover cultivar, it is inappropriate to make any generalizations regarding the effects of P and K nutrition on white clover in the field.

Biomass allocation and canopy development in spruce model ecosystems under elevated CO2 and increased N deposition.

Ecosystem-level experiments on the effects of atmospheric CO2 enrichment and N deposition on forest trees are urgently needed. Here we present data for nine model ecosystems of spruce (Picea abies) on natural nutrient-poor montane forest soil (0.7 m2 of ground and 350 kg weight). Each system was composed of six 7-year-old (at harvest) trees each representing a different genotype, and a herbaceous understory layer (three species). The model ecosystems were exposed to three different CO2 concentrations (280, 420, 560 μl l-1) and three different rates of wet N deposition (0, 30, 90 kg ha-1 year-1) in a simulated annual course of Swiss montane climate for 3 years. The total ecosystem biomass was not affected by CO2 concentration, but increased with increasing N deposition. However, biomass allocation to roots increased with increasing CO2 leading to significantly lower leaf mass ratios (LMRs) and leaf area ratios (LARs) in trees grown at elevated CO2. In contrast to CO2 enrichment, N deposition increased biomass allocation to the aboveground plant parts, and thus LMR and LAR were higher with increasing N deposition. We observed no CO2 × N interactions on growth, biomass production, or allocation, and there were also no genotype × treatment interactions. The final leaf area index (LAI) of the spruce canopies was 19% smaller at 420 and 27% smaller at 560 than that measured at 280 μl CO2 l-1, but was not significantly altered by increasing N deposition. Lower LAIs at elevated CO2 largely resulted from shorter branches (less needles per individual tree) and partially from increased needle litterfall. Independently of N deposition, total aboveground N content in the spruce communities declined with increasing CO2 (-18% at 420 and -31% at 560 compared to 280 μl CO2 l-1). N deposition had the opposite effect on total above ground N content (+18% at 30 and +52% at 90 compared to 0 kg N ha-1 year-1). Our results suggest that under competitive conditions on natural forest soil, atmospheric CO2 enrichment may not lead to higher ecosystem biomass production, but N deposition is likely to do so. The reduction in LAI under elevated CO2 suggests allometric down-regulation of photosynthetic carbon uptake at the canopy level. The strong decline in the tree nitrogen mass per unit ground area in response to elevated CO2 may indicate CO2-induced reductions of soil N availability.

Production efficiency in sunflower: The role of water and nitrogen stress

We monitores the impact of water stress, nitrogen stress, and the combination of both on the fractional interception of photosynthetically active radiation (fPAR) and growth per unit of intercepted radiation in sunflower canopies over a growing season. The efficiency with which intercepted radiation was converted into biomass (e) was reduced by approximately 35% in the nitrogen-limited canopy and approximately 30% in the water-limited canopy. the relative effect of the stress treatments on e was sensitive to assumptions concerning solar geometry. The impact of stress on fPAR depended more on whether the canopy was nitrogen-and/or water-limited. In the second half of the season, nitrogen stress reduced fPAR by about 70%. Reduction in leaf area index, 45% and 85% for the nitrogen-stressed and water-stressed canopies, respectively, appeared to be a major factor in the fPAR reductions. In the water-stressed canopy, wilting also contributed to the reduced fPAR. Neither of the stress treatments affected the strong correlationbetween fPAR and simple ratio (SR) or the normalized difference vegetation index (NDVI). We used three approaches to calculate fPAR, with two based on a single sun position and one based on the sun position updated at hourly intervals. In general, e was notably higher in the calculations with higher solar elevation angles.

Impact of Hurricane Iniki on native Hawaiian Acacia koa forests: damage and two-year recovery

ABSTRACTDamage to Hawaiian Acacia koa forest by Hurricane Iniki was assessed by comparison with our previous measures of stand structure and leaf area index (LAI) at sites along a precipitation/elevation gradient on western Kauai. Reductions in LAI ranged from 29 to 80% and were correlated with pre-hurricane LAI and canopy height. The canopy damage resulted in a large pulse of litter, ranging from 4 to 19 t ha-1 across our study sites. In the first year post-hurricane, LAI recovery and relative increase in stand basal area were negatively correlated with the fraction of canopy LAI lost. The two major overstorey species, A. koa and Metrosiderous polymorpha, were damaged less than the sub-canopy species, Dodonaea viscosa and Psidium guajava. Although D. viscosa and P. guajava were both severely damaged in the hurricane, the alien species P. guajava had much higher survival than the native D. viscosa, both as adults and as seedlings. However, seedling density of P. guajava was limited by low recruitment. At these sites, there was no drastic change in species composition following hurricane disturbance, and forest structure and productivity had recovered to a great degree within 2 y.