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251.
Klaus S. Larsen Andreas Ibrom Claus Beier Sven Jonasson Anders Michelsen 《Biogeochemistry》2007,85(2):201-213
We measured net ecosystem CO2 flux (F
n) and ecosystem respiration (R
E), and estimated gross ecosystem photosynthesis (P
g) by difference, for two years in a temperate heath ecosystem using a chamber method. The exchange rates of carbon were high
and of similar magnitude as for productive forest ecosystems with a net ecosystem carbon gain during the second year of 293 ± 11 g C m−2 year−1 showing that the carbon sink strength of heather-dominated ecosystems may be considerable when C. vulgaris is in the building phase of its life cycle. The estimated gross ecosystem photosynthesis and ecosystem respiration from October
to March was 22% and 30% of annual flux, respectively, suggesting that both cold-season carbon gain and loss were important
in the annual carbon cycle of the ecosystem. Model fit of R
E of a classic, first-order exponential equation related to temperature (second year; R
2 = 0.65) was improved when the P
g rate was incorporated into the model (second year; R
2 = 0.79), suggesting that daytime R
E increased with increasing photosynthesis. Furthermore, the temperature sensitivity of R
E decreased from apparent Q
10 values of 3.3 to 3.9 by the classic equation to a more realistic Q
10 of 2.5 by the modified model. The model introduces R
photo, which describes the part of respiration being tightly coupled to the photosynthetic rate. It makes up 5% of the assimilated
carbon dioxide flux at 0°C and 35% at 20°C implying a high sensitivity of respiration to photosynthesis during summer. The
simple model provides an easily applied, non-intrusive tool for investigating seasonal trends in the relationship between
ecosystem carbon sequestration and respiration. 相似文献
252.
The significance of photosynthetic and transpiration rates for the perception by plants of light gradients in leaf canopies has been investigated with regard to nitrogen allocation and re-allocation. A gradient of photon flux density (PFD) over a plant's foliage was simulated by shading one leaf of a pair of primary leaves of bean ( Phaseolus vulgaris L. cv. Rentegever). Photosynthetic rate was manipulated independently of PFD and, to some extent, also of transpiration, by subjecting the leaf to different CO2 concentrations. Transpiration rate was changed independently of PFD and photosynthetic rate by subjecting the leaf to different vapour pressure differences (VPD). A reduced partial pressure of CO2 reduced specific leaf mass (SLM) as did a decreased PFD, but did not change leaf N per unit area (NLA ) and light saturated rate of photosynthesis (Amax ). A reduced VPD caused several effects consistent with the effect of PFD. It decreased NLA and Amax and increased the chlorophyll to N ratio in old and young leaves. Furthermore, it decreased the chlorophyll a to b ratio and inhibited leaf growth in young leaves. The transpiration stream is partitioned among the leaves of a plant according to their transpiration rates. The results suggest that relative rates of import of xylem sap into leaves of a plant play an important role in the perception of partial shading of a plant, a situation normally found in dense vegetations. The possible role of cytokinin influx into leaves as controlled by transpiration rate, is discussed. 相似文献
253.
254.
TIMO DOMISCH LEENA FINÉR SEPPO NEUVONEN PEKKA NIEMELÄ ANITA C. RISCH JOUNI KILPELÄINEN MIZUE OHASHI MARTIN F. JURGENSEN 《Ecological Entomology》2009,34(3):369-377
Abstract 1. We monitored three different‐sized wood ant (Formica aquilonia Yarrow) mounds over a 3‐year period in Finnish boreal forests dominated by Norway spruce (Picea abies Karst.), to assess the seasonal temperature dependency of ant activity. Additionally, we also monitored Norway spruce trees around the mounds for descending honeydew foragers. 2. The amount of collected honeydew and prey and its composition, as well as the carbon (C), nitrogen (N), and phosphorus (P) in honeydew and invertebrate prey was also investigated. 3. The number of warm days (average temperature above 20 °C) and the amount of precipitation differed among the years. Ant activity at the mounds (but not on the trees) was highly correlated with air temperature throughout the ant‐active season (May–September), but ant activity in spring and autumn was lower than in summer at similar temperatures. During all 3 years, honeydew played a major role in wood ant nutrition (78–92% of dry mass). Invertebrate prey was mainly Diptera (on average 26.2%), Coleoptera (12.5%), Aphidina (9.3%), and Arachnoida (8.5%). 4. The total amounts of C, N, and P input brought into the ant mounds in the form of food (both honeydew and prey) on the stand level were 12.6–39.0, 1.6–4.6 and 0.1–0.4 kg ha?1 year?1, respectively, which is equivalent to 2–6%, 12–33% and 27–58% of the fluxes in annual needle litterfall in typical boreal Norway spruce forests. Thus, wood ants can play a significant role in short term and local N and P cycling of boreal forest ecosystems. 相似文献
255.
ABSTRACT. Forest fragmentation can create negative edge effects that reduce the reproductive success of birds nesting near the forest/nonforest interface, and threaten bird populations deeper in remnant forest habitats. Negative edge effects may be more pronounced in landscapes that are moderately fragmented, particularly where agriculture is the primary land-use fragmenting forests. Information about the extent and strength of edge effects at a site can help guide conservation actions, and determine their effectiveness. We examined edge effects for birds breeding in a nearly contiguous forest fragmented by relatively narrow agricultural corridors in Illinois (USA). We measured rates of nest predation and brood parasitism for Acadian Flycatchers ( Empidonax virescens ) over a continuum of distances from the edge of an agricultural inholding. Nest predation and brood parasitism were highest near the edge and decreased with increasing distance from the edge. Given the cumulative effects of nest predation and brood parasitism on reproductive success, we determined that forest within 600 m of the inholding was sink habitat. We found, however, that deeper forest interior areas currently serve as source habitat, and that conversion of the entire 205 ha agricultural corridor to forest would add 1350 ha of source habitat for Acadian Flycatchers. Such results provide support for a local conservation strategy of forest consolidation and establish baseline measures necessary to determine the relative effectiveness of any subsequent reforestation efforts. 相似文献
256.
The hepatoprotective potential of a protein-enriched fraction (PEF) isolated from the maggots of housefly (Musca domestica) was evaluated in rats against carbon tetrachloride (CCl4)-induced acute hepatic damage. Activities of serum aspartate aminotransferase and alanine aminotransferase increased by 4-
and 13-fold induced by CCl4, were significantly inhibited by pretreatment with 50, 100 and 200 mg PEF/kg. The formation of malondialdehyde was also significantly
decreased in PEF-treated group compared with CCl4-treated group. The treatments with PEF also elevated total protein levels significantly. These results were further supplemented
by histopathological examination of liver sections. Hyperplasia of kupffer cells was observed after treatment with PEF (100
and 200 mg/kg). We conclude that PEF is effective in this model of liver damage. 相似文献
257.
Biosynthesis,accumulation and emission of carotenoids, α-tocopherol,plastoquinone, and isoprene in leaves under high photosynthetic irradiance 总被引:1,自引:0,他引:1
Lichtenthaler HK 《Photosynthesis research》2007,92(2):163-179
The localization of isoprenoid lipids in chloroplasts, the accumulation of particular isoprenoids under high irradiance conditions, and channelling of photosynthetically fixed carbon into plastidic thylakoid isoprenoids, volatile isoprenoids, and cytosolic sterols are reviewed. During leaf and chloroplast development in spring plastidic isoprenoid biosynthesis provides primarily thylakoid carotenoids, the phytyl side-chain of chlorophylls and the electron carriers phylloquinone K1, alpha-tocoquinone and alpha-tocopherol, as well as the nona-prenyl side-chain of plastoquinone-9. Under high irradiance, plants develop sun leaves and high light (HL) leaves with sun-type chloroplasts that possess, besides higher photosynthetic CO2 assimilation rates, different quantitative levels of pigments and prenylquinones as compared to shade leaves and low light (LL) leaves. After completion of chloroplast thylakoid synthesis plastidic isoprenoid biosynthesis continues at high irradiance conditions, constantly accumulating alpha-tocopherol (alpha-T) and the reduced form of plastoquinone-9 (PQ-9H2) deposited in the steadily enlarging osmiophilic plastoglobuli, the lipid reservoir of the chloroplast stroma. In sun leaves of beech (Fagus) and in 3-year-old sunlit Ficus leaves the level of alpha-T and PQ-9 can exceed that of chlorophyll b. Most plants respond to HL conditions (sun leaves, leaves suddenly lit by the sun) with a 1.4-2-fold increase of xanthophyll cycle carotenoids (violaxanthin, zeaxanthin, neoxanthin), an enhanced operation of the xanthophyll cycle and an increase of beta-carotene levels. This is documented by significantly lower values for the weight ratio chlorophylls to carotenoids (range: 3.6-4.6) as compared to shade and LL leaves (range: 4.8-7.0). Many plant leaves emit under HL and high temperature conditions at high rates the volatile compounds isoprene (broadleaf trees) or methylbutenol (American ponderosa pines), both of which are formed via the plastidic 1-deoxy-D: -xylulose-phosphate/2-C-methylerythritol 5-phosphate (DOXP/MEP) pathway. Other plants by contrast, accumulate particular mono- and diterpenes. Under adequate photosynthetic conditions the chloroplastidic DOXP/MEP isoprenoid pathway essentially contributes, with its C5 isoprenoid precusors, to cytosolic sterol biosynthesis. The possible cross-talk between the two cellular isoprenoid pathways, the acetate/MVA and the DOXP/MEP pathways, that preferentially proceeds in a plastid-to-cytosol direction, is shortly discussed. 相似文献
258.
Daniel Obrist 《Biogeochemistry》2007,85(2):119-123
Plants accumulate significant amounts of atmospheric mercury (Hg) in aboveground biomass, likely sequestering over 1,000 Mg
of atmospheric Hg every year. This large mercury uptake could be strong enough to affect tropospheric Hg levels and might
be partially responsible for seasonal variations in atmospheric Hg observed at Mace Head, Ireland. The fluctuations of Hg
concentrations coincide temporally with the annual oscillation of carbon dioxide (CO2) in the Northern Hemisphere, which is a result of seasonal growth of vegetation. Therefore, declining Hg concentrations in
spring and summer may be attributed in part to plant uptake of atmospheric Hg. Further, the increase of Hg concentrations
during non-active vegetation periods might partially be due to plant-derived Hg emitting back to the atmosphere during carbon
mineralization. The implications of these propositions are that past and future changes in biomass productivity and organic
carbon pools may have had—and may continue to have—significant effects on atmospheric Hg levels. Specifically, large losses
in soil and biomass carbon pools in the last 150 years could have contributed significantly to observed increases in atmospheric
Hg pollution. The roles of vegetation and terrestrial carbon pools should receive detailed consideration on how they might
attenuate or exacerbate atmospheric Hg pollution. 相似文献
259.
Forests soils should be neither sinks nor sources of carbon in a long-term perspective. From a Swedish perspective the time
since the last glaciation has probably not been long enough to reach a steady state, although changes are currently very slow.
In a shorter perspective, climatic and management changes over the past 100 years have probably created imbalances between
litter input to soils and organic carbon mineralisation. Using extant data on forest inventories, we applied models to analyse
possible changes in the carbon stocks of Swedish forest soils. The models use tree stocks to provide estimates of tree litter
production, which are fed to models of litter decomposition and from which carbon stocks are calculated. National soil carbon
stocks were estimated to have increased by 3 Tg yr−1 or 12–13 g m−2 yr−1 in the period 1926–2000 and this increase will continue because soil stocks are far from equilibrium with current litter
inputs. The figure obtained is likely to be an underestimation because wet sites store more carbon than predicted here and
the inhibitory effect of nitrogen deposition on soil carbon mineralisation was neglected. Knowledge about site history prior
to the calculation period determines the accuracy of current soil carbon stocks estimates, although changes can be more accurately
estimated. 相似文献
260.
Phaeocystis spp. cell and colony mass fluxes and their contribution to the vertical particulate organic carbon (POC) export from a wide
range of stations were quantified by short-term sediment traps. The compilation of available data, ranging from polar to sub-arctic
and boreal regions, revealed that Phaeocystis colonial and single cells frequently are observed in shallow sediment traps at 30–50 m depth (average of 7 ± 11% of POC export).
A strong vertical export decline between 40 m and 100 m diminished the contribution of Phaeocystis spp. cell carbon to vertical export of POC to only 3 ± 2% at 100 m depth, with two exceptions (deeper mixed stations). Estimates
of potential corresponding mucus contribution increased the average Phaeocystis spp. contribution to <5% of POC export. The vertical flux attenuation efficiency is higher for Phaeocystis spp. than for diatoms. The overall contribution of Phaeocystis spp. to vertical carbon export based on direct investigations of vertical organic carbon export is small. 相似文献