Mid-growing Season Research Articles

Radial profile of sap flow velocity in mature Xinjiang poplar (Populus alba L. var. pyramidalis) in Northwest China

Estimation of the transpiration rate for a tree is generally based on sap flow measurements within the hydro-active stem xylem. In this study, radial variation of sap flow velocity (J s ) was investigated at five depths of the xylem (1, 2, 3, 5 and 8 cm under the cambium) in three mature Xinjiang poplar (Populus alba L. var. pyramidalis) trees grown at the Gansu Minqin National Studies Station for Desert Steppe Ecosystem from May to October 2011. Thermal dissipation probes of various lengths manufactured according to the Granier’s design were installed into each tree for simultaneous observation of the radial patterns of J s through the xylem. The radial patterns were found to fit the four-parameter GaussAmp equation. The peak J s was about 27.02±0.95 kg/(dm2 ·d) at approximately 3 to 5 cm deep from the cambium of the three trees, and the lowest J s appeared at 1 cm deep in most of the time. Approximately 50% of the total sap flow in Xinjiang poplar occurred within one-third of the xylem from its outer radius, whereas 90% of the total sap flow occurred within two-fifth of the xylem. In addition, the innermost point of the xylem (at 8-cm depth), which appeared as the penultimate sap flow in most cases during the study period, was hydro-active with J s,8 of 7.55±3.83 kg/(dm2 ·d). The radial pattern of J s was found to be steeper in midday than in other time of the day, and steeper diurnal fluctuations were recorded in June, July and August (the mid-growing season). Maximum differences between the lowest J s (J s,1 or J s,8 ) and the highest J s (J s,3 or J s,5 ) from May through October were 12.41, 17.35, 16.30, 18.52, 12.60 and 16.04 g/(cm2 ·h), respectively. The time-dependent changes of J s along the radial profile (except at 1-cm depth) were strongly related to the reference evapotranspiration (ET 0). Due to significant radial variability of J s , the mean daily sap flow at the whole-tree level could be over-estimated by up to 29.69% when only a single probe at depth of 2 cm was used. However, the accuracy of the estimation of sap flow in Xinjiang poplar could be significantly improved using a correction coefficient of 0.885.

Interannual and seasonal variability in evapotranspiration and energy partitioning over the alpine riparian shrub Myricaria squamosa Desv. on Qinghai–Tibet Plateau

The Qinghai–Tibet Plateau is a sensitive area of global climate changes, and riparian ecosystems are thought as “hotspots” for climate change adaptation, but little work has been conducted regarding the alpine riparian ecosystems on the Qinghai–Tibet Plateau. We measured evapotranspiration (ET) and surface energy fluxes over the riparian shrub Myricaria squamosa Desv., which is widely distributed on the Qinghai–Tibet Plateau but has not been studied until now. The results indicated that annual ET was 390mm and 503mm for the period of 2010 to 2011 and 2011 to 2012, respectively, which was higher than the amount of precipitation during the same period. Cumulative ET was lower than the cumulative reference evapotranspiration during the entire experimental period, whereas ET in August was higher than reference evapotranspiration. ET in the growing season occupied over 80% of annual ET with a maximum daily ET of 7.2mmd−1, and the ET in the non-growing season was quite low because of the frozen soil. In general, temperature and net radiation were the key variables controlling daily ET rates for M. squamosa. Annual sensible heat flux (H) consumed 60% of net radiation (Rn) and latent heat flux (LE) 40% during the three years of the study. LE occupied the main part of Rn from July to September. H was the highest in May and June, then sunk in the mid-growing season, and rebounded the other peak at late September and early October. Daily ground heat flux was positive from April to mid-September, and it was an important heat source of land surface in the winter and spring. This study highlighted that as an alpine riparian ecosystem in a semiarid region, ET and surface energy partitioning of the M. squamosa community are strongly affected by the freeze–thaw cycle, groundwater fluctuation, precipitation pulses and soil water content. We speculate that climate warming has a significant impact on ET process and surface energy partitioning of the M. squamosa community by influencing the freeze–thaw cycle and soil water content.

Temporal and spatial variability of methane emissions in a northern temperate marsh

Although methane (CH4) fluxes from northern wetlands in Asia have been described in previous research at different temporal and spatial scales, integrated studies at the ecosystem scale were scarce. In this study, CH4 fluxes were measured using eddy covariance (EC) technique and the chamber method in a cool temperate marsh in northeast China during the growing season (May–September) of 2011. CH4 emissions were highly variable, both temporally and spatially during the measurement period. According to the EC observation data, CH4 fluxes showed a significant diurnal cycle during the mid-growing season with nighttime average flux about 67% of the average daytime values. Daily cumulative CH4 fluxes varied from 54 to 250 mg CH4 m−2 d−1 with an average flux of 136.2 mg CH4 m−2 d−1. The observations of chamber method showed that CH4 emissions differed markedly among the three main plant communities. Average flux at the Carex lasiocarpa site was about 4 times and 13.5 times of that at the Glyceria spiculosa site and Deyeuxia angustifolia site, respectively. The spatial variability of CH4 flux was mainly controlled by the varying water table level as well as the spatial distribution of different vascular plants, while the seasonal dynamic of CH4 emission could be best explained by the change of surface soil temperature and air pressure. A comparison was made between EC measurements and the upscaled chamber based model. The results from the model overestimated CH4 emission by 28% compared to the EC data. Considering the large variability of methane emission, it is necessary to conduct continuous observations on CH4 emission from northern wetlands at different temporal and spatial scales to comprehend the variability and also to predict responses to climate change.

Allocation of nonstructural carbohydrates for three temperate tree species in Northeast China

Aims The content and allocation of nonstructural carbohydrates(NSC) in trees reflect whole-tree carbon balance regimes,and are crucial to determine growth / survivorship trade-off of trees and model tree carbon balance between uptake and investments in structure or loss.Our objective was to examine the concentration and allocation patterns of NSC for three temperate tree species,i.e.,Korean pine(Pinus koraiensis),Dahurian larch(Larix gmelinii) and Mongolian oak(Quercus mongolica).Methods During the mid-growing season(July of 2010),all biomass tissues,including foliage,branch,stem and root,were randomly sampled from three dominant trees for each species.The stem samples were taken from mid-canopy and breast height,and divided into sapwood and heartwood,while the root samples were divided into fine(diameter 2 mm),medium(2–5 mm) and coarse roots(5 mm).All samples were dried,ground,and analyzed for NSC concentrations(including soluble sugar and starch) with a modified phenol-sulfuric acid method.Important findings The concentrations of NSC and its component differed significantly among species and tissues.Concentration ranges were 0.65–8.45,1.96–5.95 and 3.00–13.90 g.100 g –1 DM for soluble sugar,starch,and NSC,respectively.On average,the contents of NSC and its components followed the order of: larch oak pine.Concentrations in the foliage and roots were higher than those in other tissues.Within the stems,the longitudinal variations in the concentrations of NSC and its components were insignificant,whereas the differences between sapwood and heartwood varied with species and NSC components.There was no significant difference in soluble sugar concentration between sapwood and heartwood,but significant differences in starch and total NSC concentration.The concentrations of NSC and its components varied insignificantly with root diameters for larch and pine,but significantly for oak.Oak invested more soluble sugar to aboveground growth,whereas the two conifers did more to roots.Nevertheless,starch was mainly reserved in stems,and the intra-tree allocation pattern of starch exhibited an opposite trend to soluble sugar,leading the total NSC to be relatively balanced between roots and branches.In the stems,heartwood was the major reserve of NSC and starch,while sapwood was the major reserve of soluble sugar for the two conifers.In the roots,coarse root was the dominant reserves of NSC and its components.We concluded that the inter-and intra-specific variations in the NSC and its components in this study reflect differences in growth strategies and within-tree carbon source / sink strength for the three temperate tree species.

Cattle Selection for Aspen and Meadow Vegetation: Implications for Restoration

There is concern over the decline of aspen and the lack of successful regeneration due to excessive browsing of aspen suckers by cattle and other wild and domestic ungulates. We conducted a 2-yr study on Lassen National Forest, California, to aid development of cattle grazing strategies to enhance aspen regeneration. We evaluated seasonal biomass, nutritional quality, and utilization by cattle of aspen suckers, aspen herbaceous understory vegetation, and meadow herbaceous vegetation within six aspen–meadow complexes. Aspen suckers had greater nutritional quality compared to aspen understory and meadow vegetation regardless of season or year. Nutritional quality declined with season in all three vegetation types. Early-growing season foraging by cattle focused on meadow and aspen understory vegetation. Mid-growing season decreases in meadow and aspen understory nutritional quality coincided with a marked increase in utilization of aspen suckers. By late-growing season, utilization on aspen suckers was significantly greater than aspen understory or meadow vegetation. Managers can use early-growing season grazing to reduce aspen consumption by cattle, set stocking rates so that adequate herbaceous vegetation is available throughout the growing season, provide nutritional supplements to reduce demand for nutritious aspen suckers, construct protective fencing, and implement grazing systems that insure years with mid- and late-growing season rest from heavy browsing. Existe la preocupación por la disminucion del alamo y la falta de resultados en la regeneracion debido al excesivo ramoneo de las yemas de los alamos por el ganado y otro ungulados silvestres y domesticos. Realizamos un estudio de dos años en el Bosque Nacional Lassen en California en EUA para ayudar a desarrollar estrategias de pastoreo del ganado que estimulen la regeneracion del alamo. Evaluamos la biomasa estacional, calidad nutricional y utilización por el ganado de las yemas del aspen, la vegetación herbacea debajo del alamo y la vegetación herbacea de pradera dentro de seis complejos de alamo-pradera. Las yemas del alamo tuvieron la mas alta calidad nutricional comparada con la vegetación debajo del alamo y la vegetación de pradera independientemente, de la temporada del año. La calidad nutricional disminuyo con la temporada en los tres tipos de vegetación. Al inicio de la temporada de crecimiento el ganado se inclino por la vegetacion debajo del alamo. A mitad de la temporada de crecimiento disminuyo la calidad nutricional de la pradera y la vegetacion de abajo del alamo coincidiendo, con un marcado incremento en la utilizacion de las yemas de alamo. Al final de la temporada de crecimiento la utilizacion de las yemas de alamo fue significativamente mayor que la de la pradera y de abajo del alamo. Los administradores pueden implementar pastoreo al inicio de la temporada de crecimiento para reducir el consumo de alamo por el ganado, establecer la carga animal previendo la disponibilidad adeucada de vegetacion herbacea a través de la temporada de crecimiento, usar suplemento nutricional para reducir la demanda de la nutritiva yema de alamo, construir cercos de proteccion e implementar sistemas de pastoreo que aseguren años con descansos de ramoneo intenso a mediados y finales de la temporada de crecimiento.

Effects of cloudiness change on net ecosystem exchange, light use efficiency, and water use efficiency in typical ecosystems of China

As a weather element, clouds can affect CO 2 exchange between terrestrial ecosystems and the atmosphere by altering environmental conditions, such as solar radiation received on the ground surface, temperature, and moisture. Based on the flux data measured at five typical ecosystems of China during mid-growing season (June–August) from 2003 to 2006, we analyzed the responses of net ecosystem exchange of carbon dioxide (NEE), light use efficiency (LUE, defined as Gross ecosystem photosynthesis (GEP)/Photosynthetically active radiation (PAR)), and water use efficiency (WUE, defined as GEP/Evapotranspiration (ET)) to the changes in cloudiness. The five ecological sites included Changbaishan temperate mixed forest (CBS), Dinghushan subtropical evergreen broad-leaved forest (DHS), Xishuangbanna tropical rainforest (XSBN), Inner Mongolia semi-arid Leymus chinensis steppe (NMG), and Haibei alpine frigid Potentilla fruticosa shrub (HB). Our analyses show that cloudy sky conditions with cloud index ( k t) values between 0.4 and 0.6 increased NEE, LUE, and WUE of the ecosystems at CBS, DHS, NMG and HB from June to August. The LUE of tropical rainforest at XSBN was higher under cloudy than under clear sky conditions, but NEE and WUE did not decrease significantly under clear sky conditions from June to August. The increase in GEP with increasing diffuse radiation received by ecosystems under cloudy skies was the main reason that caused the increases in LUE and net carbon uptake in forest ecosystem at CBS, DHS, and alpine shrub ecosystem at HB, compared with clear skies. Moreover, for the ecosystem at CBS, DHS, and HB, when sky condition became from clear to cloudy, GEP increased and ET decreased with decreasing VPD, leading to the increase in WUE and NEE under cloudy sky conditions. The decrease in R e with decreasing temperature and increase in GEP with decreasing VPD under cloudy skies led to the increase in LUE, WUE, and net carbon uptake of semi-arid steppe at NMG, compared to clear skies. These different responses among the five ecosystems are attributable to the differences in canopy characteristics and water conditions. From June to August, the peaks of the k t frequency distribution in temperate ecosystems (e.g., CBS, NMG, and HB) were larger than 0.5, but they were smaller than 0.4 in subtropical/tropical forest ecosystems (e.g., DHS and XSBN). These results suggest that the pattern of cloudiness during the years from 2003 to 2006 in the five ecosystems was not the best condition for their net carbon uptake. This study highlights the importance of cloudiness factor in the prediction of net carbon absorption in the Asia monsoon region under climate change.

Age-specific responses to climate identified in the growth of Quercus alba

The objective of this research was to determine whether the dendroclimatic responses of young Quercus alba (aged 29–126 years) differ from those of old Q. alba (149–312 years). We collected Q. alba increment cores across a range of size classes from Buffalo Mountain Natural Area Preserve, an oak-hickory forest in southcentral Virginia, USA. Tree cores were crossdated and raw ring widths were detrended to remove the influence of increasing circumference with age, microsite, and local stand dynamics. Standardized ring widths were averaged to develop two master chronologies from the 20 oldest and youngest trees. Ring-width indices were correlated with temperature, precipitation, and Palmer Drought Severity Index (PDSI). Annual tree-ring growth in old and young Q. alba was significantly correlated with precipitation from the previous growing season, but was not significantly correlated with temperature. Only the old trees showed a significant correlation between annual ring width and PDSI. These results may indicate that growth in old trees is more sensitive to drought than in young trees. If future climate change includes the predicted increase in mid-growing season droughts, tree-level responses are likely to be age-dependent with older trees experiencing relatively greater reductions in growth.

Influence of early dry season fires on primary production in western Serengeti grasslands, Tanzania

Short-term, i.e. 4-9 weeks aboveground net primary production (ANPP) temporal patterns during the first post-fire year in western Seren-geti National Park, and potential differences in the factors limiting ANPP between burnt and non burnt grasslands were examined and es-tablished. Fire stimulated growth at early post-fire stages, even during the dry season, July-October and led to larger increments in green phytomass compared to the non burnt grassland at the onset of short rains, Octo-ber-December. Further, ANPP in burnt plots correlated well with the ratio leaf/total standing phytomass suggesting that the accumulation of standing dead material can be a limiting factor to ANPP in burnt grass-lands. However, ANPP in burnt plots was unrelated to rainfall contrary to earlier arguments, but reached peak earlier and declined early in the rain season, perhaps due to the interactive effects of fire and grazing in the area. In non burnt plots, the temporal change in ANPP was more related to rainfall availability, at least until mid-growing season. Also, the phytomass structure differed between burnt and non burnt grasslands, and together with litter did not recover to non burnt levels within the first post-fire year. The study has demonstrated that the desire of the fire man-agement program in Serengeti National Park, which is to supply green forage to both migra-tory and resident populations during dry season is being fulfilled.

Calcium and phosphorus interact to reduce mid-growing season net nitrogen mineralization potential in organic horizons in a northern hardwood forest

Acid deposition can deplete soil calcium (Ca) and be detrimental to the health of some forests. We examined effects of soil Ca and phosphorus (P) availability on microbial activity and nitrogen (N) transformations in a plot-scale nutrient addition experiment at the Hubbard Brook Experimental Forest in New Hampshire, USA. We tested the hypotheses that (1) microbial activity and N transformations respond to large but not small changes in soil Ca, (2) soil Ca availability influences net N mineralization via the immobilization of N, rather than via changes in microbial activity, and (3) the response to Ca is constrained by P availability. Seasonality was a primary influence on the microbial response to treatments; N cycling processes varied from May to October and treatment effects were only detectable in the mid-growing season, in July. Neither microbial activity (C mineralization) nor gross N mineralization responded to Ca or to P, in either horizon. In the Oa horizon in July net N mineralization was reduced by high Ca and by Ca + P, and gross nitrification was increased by P addition. In the Oe horizon in July net N mineralization was reduced by Ca + P. These results partially supported our hypotheses, suggesting that soil Ca depletion has the potential to increase mid-growing season N availability via subtle changes in N immobilization, and that this effect is sensitive to soil P chemistry. The horizon-specific nature of the responses that we detected suggests that the proportions of Oe and Oa horizons comprising the surface organic layer will influence the relative importance of these processes at the ecosystem scale. Our results highlight the need for further attention to seasonal changes in controls of microbial mineralization/immobilization processes, to functional differences between organic horizons, and to interactions between Ca and P in soils, in order to learn the specific mechanisms underlying the influence of Ca status on nutrient recycling in these northern hardwood ecosystems. ► Calcium addition reduced net N mineralization in the Oa horizon, by increasing N immobilization. ► Phosphorus availability interacted with Ca to reduce net N mineralization in Oe and Oa horizons. ► Soil acidification may be increasing net N mineralization in forests of the northeastern USA. ► These effects are likely sensitive to soil P and proportions of Oe and Oa in the forest floor.

Impact of cloudiness on net ecosystem exchange of carbon dioxide in different types of forest ecosystems in China

Abstract. Clouds can significantly affect carbon exchange process between forest ecosystems and the atmosphere by influencing the quantity and quality of solar radiation received by ecosystem's surface and other environmental factors. In this study, we analyzed the effects of cloudiness on net ecosystem exchange of carbon dioxide (NEE) in a temperate broad-leaved Korean pine mixed forest at Changbaishan (CBS) and a subtropical evergreen broad-leaved forest at Dinghushan (DHS), based on the flux data obtained during June–August from 2003 to 2006. The results showed that the response of NEE of forest ecosystems to photosynthetically active radiation (PAR) differed under clear skies and cloudy skies. Compared with clear skies, the light-saturated maximum photosynthetic rate (Pec,max) at CBS under cloudy skies during mid-growing season (from June to August) increased by 34%, 25%, 4% and 11% in 2003, 2004, 2005 and 2006, respectively. In contrast, Pec,max of the forest ecosystem at DHS was higher under clear skies than under cloudy skies from 2004 to 2006. When the clearness index (kt) ranged between 0.4 and 0.6, the NEE reached its maximum at both CBS and DHS. However, the NEE decreased more dramatically at CBS than at DHS when kt exceeded 0.6. The results indicate that cloudy sky conditions are beneficial to net carbon uptake in the temperate forest ecosystem and the subtropical forest ecosystem. Under clear skies, vapor pressure deficit (VPD) and air temperature increased due to strong light. These environmental conditions led to greater decrease in gross ecosystem photosynthesis (GEP) and greater increase in ecosystem respiration (Re) at CBS than at DHS. As a result, clear sky conditions caused more reduction of NEE in the temperate forest ecosystem than in the subtropical forest ecosystem. The response of NEE of different forest ecosystems to the changes in cloudiness is an important factor that should be included in evaluating regional carbon budgets under climate change conditions.

Eight years of carbon dioxide exchange above a mixed forest at Borden, Ontario

This paper summarizes results from 8 years (1996–2003) of eddy covariance-based ecosystem CO 2 exchange measurements at the Borden Forest Research Station (44°19′N, 79°56′W). The site represents a mid-latitude, 100-year-old, mixed deciduous and coniferous forest dominated by red maple, aspen and white pine. The years 1996 and 1997 were relatively cold, had a late spring and received below average photosynthetic photon flux density (PPFD). This contrasts with an early spring, warmer soil and air temperatures during 1998–1999, and with distinctly wet year of 2000 and dry years of 2001–2003. The combination of early spring, warmer air and soil temperature and relatively high level of PPFD was associated with higher net ecosystem productivity (NEP) that peaked during 1999. Photosynthetic capacity was reduced and NEP showed a mid-growing season depression during the dry years of 2001–2003. Annual average ecosystem respiration (R) determined from a light response model was ∼30% less than R derived from a logistic respiration equation, relating night time CO 2 flux and soil temperature. However these independently determined R values were well correlated indicating that the site is unaffected by fetch and spatial heterogeneity problems. Based on the combined 8 years of growing season daytime data, an air temperature of 20–25 °C and a vapor pressure deficit (VPD) of 1.3 kPa were found to be the optimal conditions for CO 2 uptake by the canopy. Over the 1996–2003 period, the forest sequestered carbon at an average rate of 140 ± 111 gC m −2 y −1. The corresponding gross ecosystem photosynthesis (GEP) and R over this period were 1116 ± 93 gC m −2 y −1 and 976 ± 68 gC m −2 y −1, respectively. The annual carbon sequestration ranged from 19 gC m −2 in 1996 to 281 gC m −2 in 1999. However, these estimates were sensitive to frictional velocity threshold ( u * TH ) used for screening data associated with poor turbulent mixing at night. Increasing u * TH from 0.2 m s −1 (based on the inflection point in the nighttime CO 2 flux vs. u * relationship) to 0.35 m s −1 (determined using a u * TH selection algorithm based on change-point detection) modified the 8-year mean NEP estimate from 140 ± 111 gC m −2 y −1 to 65 ± 120 gC m −2 y −1. Both approaches show that the Borden forest was a low to moderate sink of carbon over the 8-year period.

Energy balance and turbulent flux partitioning in a corn–soybean rotation in the Midwestern US

Quantifying the energy balance above plant canopies is critical for better understanding of water balance and changes in regional weather patterns. This study examined temporal variations of energy balance terms for contrasting canopies [corn (Zea mays L.) and soybean (Glycine max L. Merr.)]. We monitored energy balance for 4 years using eddy-covariance systems, net radiometers, and soil heat flux plates in adjacent production fields near Ames, Iowa. On an annual basis, soybean exhibited 20% and 30% lower sensible heat flux (H) and Bowen ratio than corn, respectively. As canopies developed, a gradual shift in turbulent fluxes occurred with decreasing H and increasing latent heat flux (LE), but with a more pronounced effect for corn. Conversely, during mid-growing season and as both canopies progressively senesced, H in general increased and LE decreased; however, soybean exhibited slightly greater LE and much lower H than corn. These temporal variations in magnitude and partitioning of turbulent fluxes translated into a pronounced energy imbalance for soybean (0.80) and an enhanced closure for corn (0.98) in August and September. These discrepancies could be directly associated with differences in momentum transport as shown by friction velocities of 0.34 and 0.28 m s−1 for corn and soybean, respectively. These results support influential roles of plant canopy on intensity and mode of surface energy exchange processes.

Responses of Litter-Dwelling Arthropods to four Different Thinning Intensities in Douglas-Fir Forests of the Pacific Northwest, USA

We investigated the response of litter-dwelling arthropods to the effects of four forestry thinning intensities (Control, Light Thin, Light Thin with Gap, and Heavy Thin). With the balance between timber demand and maintaining biodiversity in the forest ecosystem in mind, we examined the effects of thinning on the abundance, richness, and diversity of arthropods as an indicator of how ecological processes affect forest litter-dwelling fauna. Study sites were 40- to 60-year-old stands of typical Douglas-fir plantation in the Willamette National Forest, Oregon, USA. To examine the seasonal response of the litter-dwelling arthropods, litter debris and humus samples were collected in October 2000 (wet late-growing season, Late 2000), June 2001 (wet early-growing season, Early 2001), and August 2001 (dry mid-growing season, Mid 2001) and extracted with Tullgren funnels. The abundance and diversity of litter-dwelling arthropods decreased as thinning intensity increased. The decreases in both abundance and diver...

The impact of invasion and subsequent removal of an exotic thistle, Cynara cardunculus, on CO2 and H2O vapor exchange in a coastal California grassland

Changes in vegetation structure and composition, particularly due to the invasion of exotic species, are predicted to influence biosphere-atmosphere exchanges of mass and energy. Invasion of Cynara cardunculus (cardoon or artichoke thistle), a perennial, non-native thistle in coastal California grasslands presently dominated by non-native annual grasses, may alter rates of ecosystem CO2 exchange and evapotranspiration (ET). During spring and summer 2006, we compared midday maximum net ecosystem CO2 exchange (NEE) and ET among adjacent grassland plots where Cynara was present and where it was absent. Measurements of NEE supported the prediction that deeply-rooted Cynara increase midday ecosystem C-assimilation. Cynara-mediated shifts in NEE were associated with increases in ecosystem photosynthesis rather than changes in ecosystem respiration. Furthermore, the presence of Cynara was associated with increased ET during the growing season. An increase in aboveground live biomass (a proxy for leaf area) associated with Cynara invasion may underlie shifts in ecosystem CO2 and water vapor exchange. Following mid-growing season sampling during April, we removed Cynara from half of the Cynara-containing plots with spot applications of herbicide. Three weeks later, midday fluxes in removal plots were indistinguishable from those in plots where Cynara was never present suggesting a lack of biogeochemical legacy effects. Similar to woody-encroachment in some semi-arid ecosystems, Cynara invasion increases midday ecosystem CO2 assimilation and evapotranspiration rates and has the potential to increase C-storage in California coastal grasslands.

Elevated CO2 and defoliation effects on a shortgrass steppe: Forage quality versus quantity for ruminants

Abstract We assessed the effects of elevated atmospheric CO2 on ruminant forage quality and nutrient yields during 4 years in semiarid shortgrass steppe where grazing by domestic livestock is the primary land-use. A defoliation and a nitrogen fertilization treatment were superimposed on CO2 treatments in large open-top chambers. CO2 effects on forage soluble and fiber (celluloses, lignin) constituents were small, even though mid-growing season yield and end of season production increased. However, large negative effects of elevated CO2 were evident in crude protein concentrations and digestibility of forages. While the effects were more negative mid-growing season than autumn, a reduction in already poor quality autumn forage may be more critical to animals. Crude protein concentrations of autumn forage on the elevated CO2 treatment fell below critical maintenance requirements 3 out of 4 years, compared to 1 of 4 for ambient and control treatments. Forage digestibility declined 14% mid-season and 10% in autumn with elevated CO2. Negative effects of elevated CO2 on animal performance mediated through forage quality are likely to be greater than the positive effects of increased quantity, because quality drops to critically low levels that can inhibit utilization. Further, elevated CO2 shifted the proportional availability of protein and energy to a species of lower overall quality and the species most negatively affected by drought. Current-year defoliation increased both quality and production of protein and energy compared to non-defoliated plots, but no CO2 by defoliation treatment interactions were observed. Nitrogen fertilization increased crude protein concentrations and digestibilities, but not in the least nutritious species that increased with elevated CO2 or in autumn when quality was lowest.

Hyperspectral reflectance of emergent macrophytes as an indicator of water column ammonia in an oligohaline, subtropical marsh

Narrow spectral band reflectance indices of two emergent macrophytes ( Typha latifolia and Borrichia frutescens) were found to correspond to changes in water column total ammonia (TA=NH 3+NH 4 +) of a constructed oligohaline (2–5 ppt) subtropical marsh (8 ha) designed as a shrimp farm treatment wetland in south Texas, USA, near the western Gulf of Mexico. A handheld hyperspectral radiometer measured plant leaf reflectance across the visible (VIS, 300–700 nm) and near-infrared (NIR, 700–1100 nm) spectrum during the mid-growing season (July–August). Reflectance of the two species and water quality (TA; total phosphorus (TP); salinity, S) were measured along a dynamic nutrient gradient from wetland inlet to outlet. Mean TA concentration was 1.22 mg N l −1 ranging between 0.92 and 1.71 mg N l −1. Salinity fluctuated between 2.5 and 4.5 ppt, while total phosphorus was between 0.01 and 0.86 mg P l −1. T. latifolia, the more dominant wetland cover: (a) reflected 1–5% less in the VIS compared to B. frutescens, (b) generally reflected 7–8% more in the NIR, (c) exhibited an average red-edge (RE) (718 nm) 15 nm greater than B. frutescens, and (d) had an average leaf normalized difference vegetation index [NDVI=( R 774− R 681)/( R 774+ R 681), where Rλ is reflectance at wavelength λ] of 0.82, versus 0.65 for B. frutescens. A multivariate linear regression equation containing photochemical reflectance index (PRI) and RE for T. latifolia explained 93% of TA variation ( P<0.001) when 0.10<TP<0.20 mg P l −1. For the entire range of water quality conditions, an equation containing PRI, RE and R 493/ R 678 for T. latifolia explained 72% of TA variation ( P<0.001). These results indicate that hyperspectral radiometry can quantify wetland ammonia concentrations in a treatment wetland, which could lead to development of remote techniques for continuously monitoring treatment effectiveness.

Carbohydrate metabolism of vegetative and reproductive sinks in the late-maturing peach cultivar 'Encore'

Activities of NAD(+)-dependent sorbitol dehydrogenase (SDH), sorbitol oxidase (SOX), sucrose synthase (SS), acid invertase (AI), and neutral invertase (NI) in 'Encore' peach (Prunus persica L.) fruits and developing shoot tips were assayed during the growing season to determine whether carbohydrate metabolizing enzymes could serve as indicators of sink strength. In fruit flesh, SS activity was detected during Stage I of growth, when cells were actively dividing, and SDH activity was detected during Stage III, when cells were actively enlarging. Acid invertase activity was detected during Stage I and showed a closer correlation with relative increase in fruit weight during the growing season than SS activity. During seed filling and pit hardening (Stage II), when relative fruit growth rate was slowest, activities of carbohydrate metabolizing enzymes in fruit flesh were not detectable. No SOX activity was detected during Stages I and II. The highest sucrose content occurred near the end of fruit development when the activities of sucrose metabolizing enzymes were low. In developing shoot tips, the sorbitol:sucrose ratio was 2:1 (w/w) and SDH activity was low at the beginning and end of the season when vegetative growth was slowest. The sorbitol:sucrose ratio changed to 1:1 (w/w) along with an increase in SDH activity in shoot tips during the mid-growing season. In 'Nemaguard' peach, SDH exhibited higher activity in root tips than in other organs. Among the sorbitol- and sucrose-metabolizing enzyme activities, only SDH activity was positively correlated with shoot growth in 'Nemaguard' plants.

Carbohydrate Metabolism of Vegetative and Reproductive Sinks in the Late-maturing Cultivar Encore

Activities of major sorbitol and sucrose metabolizing enzymes and carbohydrate contents were followed during the growth season of `Encore' peach fruits and developing shoot tips. In fruit flesh, sucrose synthase (SS) was present during stage I of growth, when cells are actively dividing, and NAD+-dependent sorbitol dehydrogenase (SDH) during stage III, when cells enlarge actively. Acid invertase (AI) revealed the best correlation with relative growth rate (RGR) of fruits during the entire season. Activities of all carbohydrate metabolizing enzymes were not detectable in fruit flesh during seed filling and pit hardening, when RGR of fruits was the slowest. The highest content of sucrose was in concert with low levels of sucrose metabolizing enzyme activities toward the end of fruit development. In shoot tips, SDH had low activity at the beginning and end of the season when vegetative growth was slowest and a 2:1 sorbitol: sucrose ratio was present. Mid-growing season was, on the other hand, characterized by an increase in SDH activity and a 1:1 sorbitol: sucrose ratio. In `Nemaguard' seedlings, only SDH activity showed a positive correlation with shoot growth. Also, in root tips of `Nemaguard' seedlings SDH exhibited its highest activity, even higher than AI activity in the same roots. We concluded that in peach sucrose may represent the major carbon form used for fruit growth, while sorbitol seems to play a predominant role in vegetative growth.

Parameterization and testing of a coupled photosynthesis-stomatal conductance model for boreal trees.

A coupled photosynthesis-stomatal conductance model was parameterized and tested with branches of black spruce (Picea mariana (Mill.) B.S.P.) and jack pine (Pinus banksiana Lamb.) trees growing in the Northern Study Area of the Boreal Ecosystem-Atmosphere Study (BOREAS) in Manitoba, Canada. Branch samples containing foliage of all age-classes were harvested from a lowland old black spruce (OBS) and an old jack pine (OJP) stand and the responses of photosynthesis (A(n)) and stomatal conductance (g(s)) to temperature, CO(2), light, and leaf-to-air vapor pressure difference (VPD) were determined under controlled laboratory conditions at the beginning, middle, and end of the growing season (Intensive Field Campaigns (IFC) 1, 2, and 3, respectively). The parameterized model was then tested against in situ field gas-exchange measurements in a young jack pine (YJP) and an upland black spruce (UBS) stand as well as in the OBS and OJP stands. Parameterization showed that Rubisco capacity (V(max)), apparent quantum yield (alpha') and Q(10) for sink limitation were the most crucial parameters for the photosynthesis sub-model and that V(max) varied among different measurement series in the laboratory. Verification of the model against the data used to parameterize it yielded correlation coefficients (r) of 0.97 and 0.93 for black spruce and jack pine, respectively, when IFC-specific parameters were used, and 0.77 and 0.87 when IFC-2 parameters were applied to all IFCs. For both measured and modeled g(s), the stomatal conductance sub-model, which linearly relates g(s) to (A(n)h(s))/c(s) (where h(s) and c(s) are relative humidity and CO(2) mole fraction at the leaf surface, respectively), had significantly steeper slopes and higher r values when only the VPD response data were used for parameterization than when all of the response data were used for parameterization. Testing the photosynthesis sub-model against upper canopy field data yielded poor results when laboratory estimates of V(max) were used. Use of the mean V(max) estimated for all upper canopy branches measured on a given day improved model performance for jack pine (from a nonsignificant correlation between measured and modeled A(n) to r = 0.45), but not for black spruce (r = 0.45 for both cases). However, when V(max) was estimated for each branch sample individually, the model accurately predicted the 23 to 137% diurnal variation in A(n) for all stands for both the upper and lower canopy. This was true both when all of the other parameters were IFC-specific (r = 0.93 and 0.92 for black spruce and jack pine, respectively) and when only mid-growing season (IFC-2) values were used (r = 0.92 for both species). Branch-specific V(max) estimates also permitted accurate prediction of field g(s) (r = 0.75 and 0.89 for black spruce and jack pine, respectively), although parameterization with all of the response data overestimated g(s) in the field, whereas parameterization with only the VPD response data provided unbiased predictions. Thus, after parameterization with the laboratory data, accurately modeling the range of A(n) and g(s) encountered in the field for both black spruce and jack pine was reduced to a single unknown parameter, V(max).

Ambient biogenic hydrocarbons and isoprene emissions from a mixed deciduous forest

Experiments were conducted during the growing season of 1993 at a mixed deciduous forest in southern Ontario, Canada to investigate the atmospheric abundance of hydrocarbons from phytogenic origins, and to measure emission rates from foliage of deciduous trees. The most abundant phytogenic chemical species found in the ambient air were isoprene and the monoterpenes α-pinene and β-pinene. Prior to leaf-bud break during spring, ambient hydrocarbon mixing ratios above the forest remained barely above instrument detection limit (∼20 parts per trillion), but they became abundant during the latter part of the growing season. Peak isoprene mixing ratios reached nearly 10 parts per billion (ppbv) during mid-growing season while maximum monoterpene mixing ratios were close to 2 ppbv. Both isoprene and monoterpene mixing ratios exhibited marked diurnal variations. Typical isoprene mixing ratios were highest during mid-afternoon and were lowest during nighttime. Peak isoprene mixing ratios coincided with maximum canopy temperature. The diurnal pattern of ambient isoprene mixing ratio was closely linked to the local emissions from foliage. Isoprene emission rates from foliage were measured by enclosing branches of trees inside environment-controlled cuvette systems and measuring the gas mixing ratio difference between cuvette inlet and outlet airstream. Isoprene emissions depended on tree species, foliage ontogeny, and environmental factors such as foliage temperature and intercepted photosynthetically active radiation (PAR). For instance, young ( 3 months old) leaves. During the latter part of the growing season the amount of carbon released back to the atmosphere as isoprene by big-tooth and trembling aspen leaves accounted for approximately 2% of the photosynthetically fixed carbon. Significant isoprene mixing ratio gradients existed between the forest crown and at twice canopy height above the ground. The gradient diffusion approach coupled with similarity theory was used to estimate canopy isoprene flux densities. These canopy fluxes compared favorably with values obtained from a multilayered canopy model that utilized locally measured plant microclimate, biomass distribution and leaf isoprene emission rate data. Modeled isoprene fluxes were approximately 30% higher compared to measured fluxes. Further comparisons between measured and modeled canopy biogenic hydrocarbon flux densities are required to assess uncertainties in modeling systems that provide inventories of biogenic hydrocarbons.