Genetic variation and productivity of Populus trichocarpa and its hybrids. X. Trait correlations in young black cottonwood from four river valleys in Washington

 A common-garden study of Populus trichocarpa Torr. & Gray was established in spring 1986 with 128 clones collected from sites along two mesic (Hoh and Nisqually) and two xeric (Dungeness and Yakima) river valleys in Washington. Two replicate plantations, one in Puyallup and the other in Wenatchee, Wash., were established with this material. Over 2 years data were taken on stem growth, leaf/crown characters, spring/autumn phenology, and the incidence of Melampsora occidentalis leaf rust. Combining clones from all four sources, correlation/regression analyses were used to examine clonal stability of traits between test sites and trait relationships with stem growth; broad-sense heritabilities (H ²) and genetic correlations revealed the genetic strength of these traits. At Puyallup, many leaf/crown traits predicted stem growth moderately to very well (r ²>0.50), e.g., total leaf area (TLA) to diameter gave an r ² of 0.91 and current-terminal leaf size, of 0.79. Some regressions were quadratic, suggesting a threshold level in a trait (e.g., leaf size) beyond which stem growth levels off. Upper-crown TLA was more closely related to height than TLA of the lower sylleptics, but the reverse was true for diameter. A decline in r ² values from upper to lower crown positions was sharper for correlations of TLAs with height than with diameter. Thus, leaf area allocation seems to differentially affect stem growth. When autumn leaf fall (LF) and rust incidence (R) were regressed with growth, r ² values ranged from 0.58 to 0.71, but those of spring flush (SF) were only 0.10 to 0.12. Early LF and high R, both negatively affecting growth, had a strong geographic component as it occurred mainly on lower-elevation Yakima clones. At Wenatchee, field conditions were harsher and microsites more variable, so trait/growth relationships were weaker. Genetic correlations with growth revealed similar trends as phenotypic analyses. Unlike leaf/crown traits, clonal scores of LF, SF, and R were fairly stable across the two test sites (r ²: 0.58–0.80). These traits also showed strong genetic control (H ²: 0.96–0.98). The trait/growth relationships as well as trait stability within clones have implications for selecting clonal stock in poplar culture and conservation. Received: 23 February 1998 / Accepted: 7 May 1998     

Growth and development of Avena fatua (Wild-oat) in the field

The growth and development of field-grown Avena fatua plants were studied for autumn and spring sowings in two consecutive years. The duration of various growth stages from sowing until anthesis was quantified in terms of thermal time (accumulated degree days) or photothermal time (degree days modified by photoperiod). Base temperatures and photoperiods for developmental phases were estimated as those which minimised the coefficient of variation among sowing dates. Relationships were derived between leaf emergence, canopy height, plant leaf area, and photothermal time. Stem extension and flowering occurred earlier in autumn-sown plants than spring-sown plants. Autumn-sown plants produced more leaves on the main stem, and had greater leaf area and above-ground biomass at anthesis than spring-sown plants.     

The effects of herbicides, applied atone and in sequence, on the control of wild-oats (Avena fatua) and broad-leaved weeds, and on yield of winter wheat

Experiments were done to examine the effects of controlling wild-oats and autumn-germinating broad-leaved weeds in winter wheat, early in winter or late in spring. The herbicides used were barban (winter), chlortoluron or isoproturon (winter), and benzoylprop-ethyl, at the recommended doses and at half doses. Sequential treatments of two herbicides at half doses were also examined. All treatments were given a routine broad-leaved herbicide treatment in spring. Yields of wheat were influenced more by the time of weed removal than by the degree of control achieved. Grain yields at three sites with dense autumn broad-leaved weed populations were greatest following the use of chlortoluron or isoproturon. At three other sites with moderate to dense wild-oat populations (60 to 240 plants/m¹), the use of barban at the crop three-leaf stage gave larger yields than benzoylprop-ethyl in late spring at the early stem elongation stage of the crop. Seed formation from surviving A. fatua was similar with both wild-oat herbicides. The treatment which reduced seed production of A. fatua and maintained crop yield most consistently was barban followed by benzoylprop-ethyl, each at half the normal recommended dose.     

Comparaison de la biomasse et du contenu énergétique d'un Copépode planctonique: Acanthocyclops vernalis (Fischer 1853)

In Lake Dayet er Roumi, the annual cycle of Acanthocyclops vernalis (Copepoda) is composed of two different cohorts, the first one in autumn, the other, numerically the most important, in spring.Growth rates and size of the individuals are nearly equivalent in the two cohorts, but their chemical composition changes appreciably. Autumn animals have a higher energetic content than spring ones: 2.425 10⁴ Nm · g^–1 and 1.965 10⁴ Nm · g^–1, respectively. The result is a great difference in biomass and production value between the two populations.Because of the difference in population density, the autumn cohort produces much less than the spring one. However, because each autumn specimen has a higher energy content, this difference becomes much smaller, on balance.The difference in numbers between the cohorts could result from the fact that in autumn, but not in spring, A. vernalis, because so rich in energy, is strongly positively selected by predators.

     

Experimental habitat fragmentation and invertebrate grazing in a herbaceous grassland species

A field experiment was conducted to examine the effects of habitat fragmentation on herbivore damage to individually tagged leaves of Betonica officinalis rosettes. Fragments of different size and corresponding control plots were established at three study sites in nutrient-poor calcareous grasslands in the northern Swiss Jura mountains. Leaf damage was recorded three times over the growing season (late spring, summer and early autumn). Five years after the initiation of the fragmentation, the density of rosettes did not differ between fragments and control plots. The number of leaves per rosette was higher in fragments than in control plots in summer but not in late spring and early autumn. The extent of leaf damage, expressed as proportion of leaf area removed by invertebrate herbivores, increased over the vegetation period. Leaf damage was greater in fragments than in control plots at two study sites, whereas the opposite (less strongly expressed) was found at the third site. Number of species and density (individuals per m²) of potential herbivores (gastropods and grasshoppers) were recorded in all fragments and control plots. Effects of fragmentation on the number of species and densities depended on plot size and differed between gastropods and grasshoppers. Leaf damage in fragments increased with increasing density of gastropods if the third site, which had lowest leaf damage, was excluded. Such a positive relationship was neither found in control plots nor for grasshopper densities. Thus, movement of gastropods in fragments was probably restricted which resulted in increased feeding pressure at least in two sites. However, even if our fragmentation experiment was well designed and replicated, the interpretation of these experimental results remains difficult because there was large site-to-site and seasonal variation.     

The potential of lentil (Lens culinaris) as a grain legume crop in the UK: an assessment based on a crop growth model

A crop growth model developed in Canterbury, New Zealand was used to assess the potential of lentil (Lens culinaris) as a grain legume crop in the UK. The model was validated using five sowing dates at Durham (54.77°N, 1.58°W) in 1999. Predicted time to flowering was within 7 days of actual time to flowering and predicted seed yields were within 9% of actual yields. Actual yields ranged from 1.40 to 1.65 t ha‐¹. Seed was of high quality. The model was used to predict rate of development and yields of spring and autumn sown lentils at eight sites along a transect from NW Scotland (Stornoway, 58.22°N, 6.32°W) to SE England (East Mailing, 51.28°N, 0.45°E) chosen to encompass important environmental gradients in the UK. In general, for a 1 May sowing with 150 or 250 mm plant available water (PAW) and a 1 October sowing with 150 mm PAW, predicted mean values over the period 1987–95 for maximum crop growth rate, maximum leaf area index, radiation intercepted, total dry matter produced and seed yield were closely positively related to monthly mean values for mean daily air temperature and increased along the transect from NW to SE UK. Time to flowering generally decreased along the transect from NW to SE UK ranging from 28 June to 9 July and from 20 May to 14 June with the May and October sowings respectively. For the 1 May sowing with 250 mm PAW, predicted mean seed yield ranged from 1.00 to 1.90 t ha‐1. For all sites, yield was very stable over the 9 yr period. For the 1 May sowing with 150 mm PAW, predicted mean seed yield ranged from 0.97–1.23 t ha‐1. Yields for the four more southerly sites were more variable at the lower PAW and, in exceptionally dry years, were substantially lower than average. For these sites, autumn sowing increased seed yields in exceptionally dry years and gave similar or greater mean seed yields to spring sowing with 250 mm PAW. For East Mailing, predicted yields for autumn sowing were on average 2.78 t ha‐1. Also, for Stornoway, because of its relatively high overwinter temperatures, the model predicted substantial increases in yield with autumn sowing. It is concluded that lentil has considerable potential as a grain legume crop in the UK.     

Drought effects on the dynamics of leaf production and senescence in field-grown Medicago arborea and Medicago citrina

Forage shrub production in the Mediterranean region is frequently limited by soil water availability. To study plant responses to water deficit under such conditions is important for improving crop management and for selecting better yielding forage shrub species. Pre-dawn leaf water potential (Ψ_pd), plant leaf area (PLA), leaf area per stem (LA_s), leaf appearance rate (LAR₁;), leaf senescence rate (LSR), individual leaf area (LA) and maximal leaf elongation rate (LER) were studied throughout the year for Medicago arborea (MA) and Medicago citrina (MC) under irrigated (control) and low rainfall field conditions, at the experimental field site of the University of the Balearic Islands in Spain. With irrigation, the highest LA and LER were observed in autumn and spring and the lowest in winter and summer. LAR; was similar for both species in autumn and winter. Throughout the spring, LAR₁ was higher for MC compared to MA. PLA was similar for both species during the autumn, winter and spring seasons; however, during the summer PLA of MA was significantly reduced by 53%. This decline was attributed to higher leaf senescence during seed maturity. As a consequence, MC maintained higher leaf area (∼ 5 m² plant⁻¹) than MA (3 m² plant⁻¹). Under natural field conditions, soil water deficit increased from February to late August. The main effect of water stress was a marked reduction in LAR₁, LA and LER reflected in lower LA_s and PLA. Leaf area was severely reduced for both species during the summer, but much more intensively in MA, which developed full leaf senescence. Thus, MC maintained higher PLA than MA (0.5 m² compared to 0.0 m²). Throughout the year, but especially in the driest months, MC was superior to MA in leaf growth parameters and PLA.     

Variation in growth attributes of contrasting populations of Trifolium repens

A range of growth attributes was measured in seedlings of 10 Trifolium repens populations, differing in leaf size and origin, grown in three temperature and two glasshouse environments. Growth rates of large leaf types of Mediterranean origin were higher than those of smaller leaf types at 10°C. However, the greater temperature response of the smaller leaf types resulted in higher growth rates for S.100 and S.184 than for a large leaf type from Israel at 20°C. The increase of growth rate with temperature was associated with changes in leaf area ratio and net assimilation rate between 10° and 15°C but only with changes in net assimilation rate between 15° and 20°C. Within each temperature environment, population differences in growth rate were related to differences in net assimilation rate rather than leaf expansion. At low temperature a greater proportion of dry matter was distributed to leaf tissue in large leaf types particularly those of Mediterranean origin but they showed a proportionately smaller increase in allocation to leaves with increasing temperature compared with small leaf types. In the glasshouse environments growth rates in spring were more than double those in the autumn. This difference was associated with net assimilation rates which were about five times greater in the spring environment. However, leaf area ratios in the spring were only half those in the autumn. These differences in leaf area ratio between the glasshouse environments were closely related to differences in specific leaf area but not to differences in distribution of dry matter to leaf tissue which was greater in the spring environment.     

Influences of microclimatic conditions and water relations on seasonal leaf dimorphism of Prosopis glandulosa var. torreyana in the Sonoran Desert,California

Summary Seasonal measurements of microclimatic conditions were compared to seasonal indices of leaf structural components and plant water relations in Prosopis glandulosa var. torryana. P. glandulosa had two short periods of leaf production which resulted in two distinct even aged cohorts of leaves. The two leaf cohorts (summer, winter) were concurrent in the summer and fall, contrasting to previous studies on other species in which one leaf form replaces a previous leaf type. The structural characteristics of these two cohorts differed significantly in two replicate year cycles. The leaves of the spring cohort were larger in weight and area but similar to the summer cohort in specific leaf weight and leaflet number. The second growth period leaves constituted only a small proportion of the total plant leaf area. The dimorphism between the two cohorts was best associated with plant water relations and not energy load. Second growth period leaves maintained turgor to greater water deficits but lost turgor at higher leaf water potentials. Seasonal osmotic adjustment occurred for first growth period leaves but not second growth period leaves. The small leaves produced during the hot climate were most likely the result of low turgor potential during development rather than an adaptation to tolerate stressful environments.     

Effects of riparian leaf dynamics on periphyton photosynthesis and light utilisation efficiency

1. Streambed light regimes change dramatically when riparian trees gain leaves in spring and lose them in autumn. This study examined the effect of these changes on periphyton photosynthetic characteristics, primary production, and light utilisation efficiency in two eastern Tennessee streams. 2. Photosynthesis–irradiance responses were measured at intervals covering leaf emergence and abscission in spring and autumn. Photosynthetic efficiency (α^chl) increased with declining streambed irradiances during spring leaf emergence, but returned to pre‐emergence values after autumn leaf fall. The onset of photosaturation (I_k) displayed the opposite pattern, decreasing during leaf emergence and increasing after leaf fall. Both α^chl and I_k were closely associated (P < 0.01) with daily integrated streambed irradiance, as were periphyton carotenoids. Internal shading by photoprotective carotenoids is hypothesised to account for lower α^chl when streambed irradiances are high. 3. An in situ shading experiment confirmed that the temporal changes observed in periphyton photosynthetic characteristics and carotenoids were primarily the result of changing light levels and not other environmental factors (e.g. nutrients, temperature). 4. Daily chlorophyll‐specific primary production (PP^chl) was calculated with P–I models and recorded streambed irradiances. In both streams, PP^chl was the highest in early spring when trees were leafless, and then declined markedly as leaves emerged, reaching a minimum in summer. PP^chl increased after leaf abscission, but was still lower than it was in early spring, when the sun was higher and daylength was longer. A hyperbolic tangent equation fit to PP^chl and daily integrated irradiance (r²=0. 85) suggested that primary production was light saturated at 4–8 mol m^–2 d^–1. 5. Light utilisation efficiency (Ψ) increased 10‐fold during leaf emergence. Photosaturation at high irradiances and photoacclimation at lower irradiances were responsible for a negative hyperbolic relationship between Ψ and daily integrated irradiance.     

Distribution and leaf growth of Thalassodendron pachyrhizum den Hartog in Southern Western Australia

The seagrass Thalassodendron pachyrhizum den Hartog grows on limestone reef platforms. Monthly leaf biomass was measured over 2 years and showed a strong seasonal variation with maximum biomass of 500 g m⁻². This seagrass loses all its leaves except for a bud and this characteristic was used to obtain a conservative estimate of productivity by change in standing stock. Leaf growth during the growing season was 6.6 mg Cg⁻¹ day⁻¹. Leaf length frequencies showed that new leaves formed during autumn (March–April). They grew from autumn until spring (November) and began to senesce in summer, followed by leaf fall in late summer (February–March).The growth of rhizome shoots “invading” free substratum space and the growth of new stems was measured for a 300-day period; about 9 leaves were produced in this period.     

Leaf mineral concentrations of Erica arborea, Juniperus communis and Myrtus communis growing in the proximity of a natural CO2 spring

Leaf mineral concentrations of co‐occurring Erica arborea, Juniperus communis and Myrtus communis were measured at bimonthly intervals throughout a year in a natural CO₂ spring and in a nearby control site with similar soil chemistry in a Mediterranean environment. There were different responses to the elevated [CO₂] (c. 700 μL L^?1) of the spring site plants depending on the element and the species. In the CO₂ spring site K, Ca, Mg, Mn, Al, Fe, and Ti leaf concentrations and the ratio C/N showed significant greater values in at least one or two of the three species. Leaf S concentration were greater in all three species. Leaf concentrations of N, Sr, Co, and B were lower in at least one or two species, and those of C and Ba were lower in all the three studied species near the CO₂ spring. P, Na, Zn, Si, Cu, Ni, Cr, Pb, Mo, V and Cd leaf concentrations and the specific leaf area (SLA, measured in Myrtus communis) did not show any consistent or significant pattern in response to the elevated [CO₂] of the spring site. There was a slight trend towards maximum concentrations of most of these elements during autumn–winter and minimum values during the spring season, especially in Myrtus communis. Multivariate principal component analyses based on the leaf elemental concentrations clearly differentiated the two sites and the three species. Lower concentrations at the spring site were not the result of a dilution effect by increased structural or nonstructural carbon. In contrast to most experimental studies of CO₂ enrichment, mainly conducted for short periods, several of these elements had greater concentrations in the CO₂ spring site. Nutrient acclimation and possible causes including decreased nutrient export, increased nutrient uptake capacity, photosynthetic down‐regulation, Mediterranean water stress, and higher H₂S concentration in the spring site are discussed.     

Consequences of the Elimination of Old Leaves upon Spring Phenological Events and the New Leaves Nutrient Concentration in a Wintergreen Woody Species in the Southern Hemisphere

Previous studies analyzed the importance of old leaves conservancy for wintergreen species plant growth only after early spring old leaves elimination. However, carbon and nutrient resources for growth could have already been translocated from old leaves to shoots during autumn. In this work, the effect of old leaves absence on the leaf mass per area (LMA, g m⁻²) and nutrient concentration of new spring leaves, shoot growth, and flowering was studied in Aristotelia chilensis, an Andean Patagonic woody wintergreen species of Argentina. Plants were studied after autumn defoliation (AD) or late winter defoliation (WD) and results were compared to those of undamaged control plants (CO). The new leaves LMA and mineral nutrient (N, P, K, and Mg) concentration values did not decrease in AD or WD compared to CO plants. Conversely, CO plants showed higher flowering intensity and shoot lengthening compared to AD or WD plants. There were not remarkable differences regarding the defoliation time, though non-flowering shoots grew in a lesser degree than the flowering shoots in WD plants. It was concluded that A. chilensis old leaves cohort is an important source to shoot growth and flowering but their absence does not affect the new leaves structure or nutritional status from early spring in either AD or in WD plants. New leaves formation probably is guaranteed by resources (carbon and nutrients) previously stored in stems or even in the buds containing the preformed leaves since March, by the end of summer. Provided the availability of complete resources for the new leaf flush independently of the old leaves A. chilensis would restore the carbon balance as soon as possible to resume the growth of heterotrophic tissues at normal rates. Endogenous response to counterbalance the old leaves absence on non-flowering shoots was more effective when there was greater lag time between defoliation and shoot growth resume. Flowering and non-flowering shoots compete for the available resources when A. chilensis have not yet expanded leaves and shoots supporting reproductive structures were stronger sinks compared to non-flowering shoots in WD plants.     

BIOMASS AND PRODUCTION OF PONTEDERIA CORDATA AND POTAMOGETON EPIHYDRUS IN THREE CONNECTICUT RIVERS

Above- and below-ground biomass of the emergent Pontederia cordata and the floating-leaved Potamogeton epihydrus was measured during the growing season in three interconnected rivers in Connecticut, U.S.A. Maximum biomass of Pontederia, averaging 1,212 g m^-2 dry weight (524 g m^-2 above-ground, 688 g m^-2 below-ground), occurred 100–150 days after major spring growth began. Peak biomass of Potamogeton averaged 94 g m^-2 (81 g m^-2 above-ground, 14 g m^-2 below-ground) and was attained in 45–85 days. New growth of Pontederia in spring arose from, and was heavily subsidized by, the large biomass of living overwintered rhizomes and roots, which averaged 497 g m^-2 in early June. This new growth appeared to have been produced in only one season, but in reality it contained energy fixed the current season, plus energy carried over from previous years. Net production of Pontederia calculated for only one growing season averaged 1,049 g m^-2. Potamogeton also perennated from rhizomes, but the biomass of these organs in spring was low, averaging 11 g m^-2 in late May. Biomass of Potamogeton in summer consisted primarily of tissue produced during the current season. Rhizomes and roots comprised a much greater proportion of the plant in Pontederia than in Potamogeton. The ratio of new living below-ground/above-ground biomass of Pontederia rose from zero in spring to an average of 1.71 in autumn. For Potamogeton, the below-ground/above-ground ratio averaged 0.37 in late spring, 0.20 in midsummer, and 0.41 in autumn. The overwintered below-ground biomass of Pontederia alive in spring was 42–79% of the new living below-ground biomass the previous autumn. Net photosynthetic efficiency during the period between initiation of major growth in spring and attainment of peak biomass averaged 1.3% for Pontederia and 0.3% for Potamogeton.     

Life History,Reproduction, Growth,Population Dynamics and Production of Gammarus aequicauda (Crustacea: Amphipoda) at Extremely Low Salinities in a Mediterranean Lagoon

Aspects of the biology of Gammarus aequicauda were studied at extremely low salinities (0.3–5.7 psu) in Monolimni Lagoon, N. Aegean Sea. Samples were collected monthly from February 1998 to February 1999. Breeding occurred continuously but peaked in late spring, late summer and autumn and three cohorts were produced. The spring and summer cohorts showed fast growth (0.15 mm d^–1), accelerated maturity and life span of about three and seven months respectively, while the overwintering cohort showed a life span of about nine months. The largest individual had a body length of 23.6 mm. Mean brood size was 54.5 early embryos, while the intramarsupial loss was 46%. Population density sharply increased in late spring, summer and autumn. Secondary production calculated by Hynes' method gave a mean annual density of 1077.4 ind. m^–2, a mean annual crop (B) of 2.93 g DW m^–2, an annual production (P) of 35.03 g DW m^–2 and a P: B ratio of 11.96. Gammarus aequicauda showed a life‐history pattern similar to those previously reported for this species at higher salinity environments with comparatively large final body length and high growth rate during summer, brood size and P: B ratio, but with high embryo loss as well. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Patterns of growth in the early life history of the round herring Etrumeus teres

Early life history traits of young‐of‐the‐year (YOY) round herring Etrumeus teres, caught in Tosa Bay (south‐western Japan), were studied using otolith microstructure analysis for the 2000–2003 year classes. Hatch dates ranged from October to March, and were restricted to either autumn or winter within each year class. YOY ranged from 50 to 123 mm total length (L_T) and from 57 to 192 days in age. The relationship of L_T to otolith radius was linear. Individual growth rates (G_I) were backcalculated between the 70th and 150th days (the size range of most YOY caught) using the biological intercept method. G_I ranged from 0·3 to 1·4 mm day^?1 and decreased in most cases as season progressed irrespective of year class, although G_I in winter cohorts were significantly higher than in autumn cohorts. Otolith growth rates (G_O) ranged from 2·13 to 12·25 μm day^?1 for autumn spawned YOY and from 3·12 to 12·41 μm day^?1 for YOY spawned in winter. The G_O trajectories followed three consistent patterns: (1) an increase in increment widths after first feeding through the second week of larval life, then (2) a plateau in increment spacing before increment widths increased again until reaching the maximum growth rate, followed by (3) a gradual decrease in increment widths until the end of the fifth month. The three stages occurred irrespective of spawning season, although YOY spawned in October and December had higher G_O during stages (1) and (2) than YOY spawned in February and March, whereas higher G_O was observed for late‐winter cohorts in stage (3). Otolith growth from YOY spawned in December and January showed an intermediate pattern between YOY hatched in the early autumn (October to December) and late winter (February to March). The G_O trajectories were cross‐matched to the calendar date to estimate time series of otolith growth rates (G_OTS) for each year. A parabolic trend was found with maximum G_OTS in autumn and spring and minimum values in winter. This trend was significantly correlated to daily sea surface temperature variations. The differences in otolith growth trajectories suggest that the otolith microstructure of E. teres may be used as a natural tag for identifying autumn and winter spawned cohorts.     

Intrinsic water‐use efficiency of temperate seminatural grassland has increased since 1857: an analysis of carbon isotope discrimination of herbage from the Park Grass Experiment

A 150‐year‐long record of intrinsic water‐use efficiency (W_i) was derived from community‐level carbon isotope discrimination (¹³Δ) in the herbage of the unfertilized, unlimed control treatment (plot 3) of the Park Grass Experiment at Rothamsted (England) between 1857 and 2007. ¹³Δ during spring growth (first cut harvested in June) averaged 21.0‰ (±0.5‰ SD) and has not shown a long‐term trend (P=0.5) since 1857. ¹³Δ of summer/autumn growth (second cut harvested between September and November) increased from 21.3‰ to 22.0‰ (P < 0.001) between 1875 and 2007. W_i during spring growth has therefore increased by 33% since the beginning of the experiment, and W_i of summer/autumn growth has increased by 18%. The variation in ¹³Δ was mainly related to weather conditions. Plant available soil water explained 51% and 40% of the variation in spring growth ¹³Δ and summer/autumn growth ¹³Δ, respectively. In the 1857–2007 period yields have not increased, suggesting that community‐level photosynthesis has not increased either. Therefore, the increased W_i probably resulted from a decreased stomatal conductance. Vapour pressure deficit (VPD) during spring growth (March–June) has not changed since 1915, meaning that instantaneous water‐use efficiency (W_t) in spring time has increased and transpiration has probably decreased, provided that leaf temperature followed air temperature. Conversely, VPD in the months between the first and second cut has increased by 0.07 kPa since 1915, offsetting the effect of increased W_i on W_t during summer and early autumn. Our results suggest that vegetation has adjusted physiologically to elevated CO₂ by decreasing stomatal conductance in this nutrient‐limited grassland.     

TEMPORAL AND SPATIAL PATTERNS OF SPECIFIC LEAF WEIGHT IN SUCCESSIONAL NORTHERN HARDWOOD TREE SPECIES

Temporal and spatial patterns of specific leaf weight (SLW, g/m²) were determined for deciduous hardwood tree species in natural habitats in northern lower Michigan to evaluate the utility of SLW as an index of leaf photosynthetic capacity. No significant diurnal changes in SLW were found. Specific leaf weight decreased and then increased during leaf expansion in the spring. Most species, especially those located in the understory, then had relatively constant SLW for most of the growing season, followed by a decline in SLW during autumn. Specific leaf weight decreased exponentially down through the canopy with increasing cumulative leaf area index. Red oak (Quercus rubra), paper birch (Betula papyrifera), bigtooth aspen (Populus grandidentata), red maple (Acer rubrum), sugar maple (A. saccharum), and beech (Fagus grandifolia) generally had successively lower SLW, for leaves at any one level in the canopy. On a given site, comparisons between years and comparisons of leaves growing within 35 cm of each other showed that differences in SLW among species were not due solely to microenvironmental effects on SLW. Bigtooth aspen, red oak, and red maple on lower-fertility sites had lower SLW than the same species on higher-fertility sites. Maximum CO₂ exchange rate, measured at light-saturation in ambient CO₂ and leaf temperatures of 20 to 25 C, increased with SLW. Photosynthetic capacities of species ranked by SLW in a shaded habitat suggest that red oak, red maple, sugar maple, and beech are successively better adapted to shady conditions.