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81.
82.
83.
Christoph Nehrbass‐Ahles Flurin Babst Stefan Klesse Magdalena Nötzli Olivier Bouriaud Raphael Neukom Matthias Dobbertin David Frank 《Global Change Biology》2014,20(9):2867-2885
Tree‐rings offer one of the few possibilities to empirically quantify and reconstruct forest growth dynamics over years to millennia. Contemporaneously with the growing scientific community employing tree‐ring parameters, recent research has suggested that commonly applied sampling designs (i.e. how and which trees are selected for dendrochronological sampling) may introduce considerable biases in quantifications of forest responses to environmental change. To date, a systematic assessment of the consequences of sampling design on dendroecological and‐climatological conclusions has not yet been performed. Here, we investigate potential biases by sampling a large population of trees and replicating diverse sampling designs. This is achieved by retroactively subsetting the population and specifically testing for biases emerging for climate reconstruction, growth response to climate variability, long‐term growth trends, and quantification of forest productivity. We find that commonly applied sampling designs can impart systematic biases of varying magnitude to any type of tree‐ring‐based investigations, independent of the total number of samples considered. Quantifications of forest growth and productivity are particularly susceptible to biases, whereas growth responses to short‐term climate variability are less affected by the choice of sampling design. The world's most frequently applied sampling design, focusing on dominant trees only, can bias absolute growth rates by up to 459% and trends in excess of 200%. Our findings challenge paradigms, where a subset of samples is typically considered to be representative for the entire population. The only two sampling strategies meeting the requirements for all types of investigations are the (i) sampling of all individuals within a fixed area; and (ii) fully randomized selection of trees. This result advertises the consistent implementation of a widely applicable sampling design to simultaneously reduce uncertainties in tree‐ring‐based quantifications of forest growth and increase the comparability of datasets beyond individual studies, investigators, laboratories, and geographical boundaries. 相似文献
84.
Louise Ilum Sørensen Juha Mikola Minna-Maarit Kytöviita Johan Olofsson 《Ecosystems》2009,12(5):830-842
Mammal grazing is composed of three mechanisms—removal of foliar tissue (defoliation), return of nutrients via dung and urine
(fertilization), and trampling. To evaluate the relative role of these mechanisms in the effect of reindeer grazing on soil
biota in northern grasslands, we subjected experimental plots in a sub-arctic alpine meadow to defoliation, fertilization
(using NPK-solution), simulated trampling, and their factorial combinations once a year from 2002 to 2004 and measured the
response of plants and decomposers (including microbes, nematodes, collembolans, and enchytraeids) in 2004. Trampling affected
both plant and decomposer communities: the coverage of the moss Pleurozium schreberi and the sedge Carex vaginata, as well as the abundance of collembolans and enchytraeids were reduced in trampled plots. Trampling and fertilization also
interacted significantly, with fertilization increasing the abundance of bacteria and bacterial-feeding and omnivorous nematodes
in trampled plots only, and trampling decreasing fungal biomass in non-fertilized plots only. Defoliation had no overall effects
on plants or decomposers. Nematode genera were not affected by the experimental treatments, but nematode and plant communities
were significantly associated, and all decomposer biota, except collembolans, were strongly affected by the spatial heterogeneity
of the study site. Our results indicate that trampling may have larger and defoliation and fertilization smaller roles than
anticipated in explaining reindeer grazing effects in sub-arctic grasslands. However, even the effects of trampling seem to
be outweighed by the spatial heterogeneity of decomposer abundances. This suggests that in sub-arctic grasslands spatial variation
in abiotic factors can be a more important factor than grazing in controlling soil biota abundances.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
Author contributions LIS was involved in planning of the study, fieldwork, lab work, analysis, and wrote the article with contributions from all
other authors; JM and MMK conceived the study and contributed to the data analyses; MMK further contributed to the field work
and JM refined the final appearance of the text; JO started and was responsible for managing the field experiment, collecting
the plant data and gave advice on statistical analysis. 相似文献
85.
A. Christopher Oishi Sari Palmroth Kurt H. Johnsen Heather R. McCarthy Ram Oren 《Global Change Biology》2014,20(4):1146-1160
Soil CO2 efflux (Fsoil) is the largest source of carbon from forests and reflects primary productivity as well as how carbon is allocated within forest ecosystems. Through early stages of stand development, both elevated [CO2] and availability of soil nitrogen (N; sum of mineralization, deposition, and fixation) have been shown to increase gross primary productivity, but the long‐term effects of these factors on Fsoil are less clear. Expanding on previous studies at the Duke Free‐Air CO2 Enrichment (FACE) site, we quantified the effects of elevated [CO2] and N fertilization on Fsoil using daily measurements from automated chambers over 10 years. Consistent with previous results, compared to ambient unfertilized plots, annual Fsoil increased under elevated [CO2] (ca. 17%) and decreased with N (ca. 21%). N fertilization under elevated [CO2] reduced Fsoil to values similar to untreated plots. Over the study period, base respiration rates increased with leaf productivity, but declined after productivity saturated. Despite treatment‐induced differences in aboveground biomass, soil temperature and water content were similar among treatments. Interannually, low soil water content decreased annual Fsoil from potential values – estimated based on temperature alone assuming nonlimiting soil water content – by ca. 0.7% per 1.0% reduction in relative extractable water. This effect was only slightly ameliorated by elevated [CO2]. Variability in soil N availability among plots accounted for the spatial variability in Fsoil, showing a decrease of ca. 114 g C m?2 yr?1 per 1 g m?2 increase in soil N availability, with consistently higher Fsoil in elevated [CO2] plots ca. 127 g C per 100 ppm [CO2] over the +200 ppm enrichment. Altogether, reflecting increased belowground carbon partitioning in response to greater plant nutritional needs, the effects of elevated [CO2] and N fertilization on Fsoil in this stand are sustained beyond the early stages of stand development and through stabilization of annual foliage production. 相似文献
86.
Yasmin Vázquez-Santos Yuriana Martínez-Orea Javier Álvarez-Sánchez Noé Manuel Montaño Sara Lucía Camargo-Ricalde Silvia Castillo-Argüero 《Phyton》2021,90(2):605-619
The aim of this study was to analyze the performance of Acaena elongata colonized by arbuscular mycorrhizal
fungi (AMF) to different phosphorus (P) concentrations, as a measure of AMF dependency. A. elongata, is a species from soils where P availability is limited, such as temperate forests. Our research questions were: 1) How do
different P concentrations affect the AMF association in Acaena elongata, and 2) How does the AMF association
influence A. elongata’s growth under different P concentrations? A. elongata’s growth, P content in plant tissue,
AMF colonization and dependency were measured under four P concentrations: control (0 g P kg−1
), low
(0.05 g P kg−1
), intermediate (0.2 g P kg−1
) and high (2 g P kg−1
) in different harvests. A complete randomized
block design was applied. A. elongata’s growth was higher under -AMF in intermediate and high P concentrations, and the lowest growth corresponded to +AMF in the low and intermediate P concentration. We observed
a negative effect on the root biomass under +AMF in intermediate P concentration, while the P concentration had
a positive effect on the leaf area ratio. The AMF colonization in A. elongata decreased in the highest P concentration and it was favored under intermediate P concentration; while the low and the high concentrations generated a cost-benefit imbalance. Our results suggest that the performance of some plant species in soils with
low P availability may not be favored by their association with AMF, but a synergy between AMF and intermediate P concentrations might drive A. elongata’s growth. 相似文献
87.
William A. Jensen 《Sexual plant reproduction》1998,11(1):1-5
This year marks the 100th anniversary of the discovery of double fertilization by Nawaschin in St. Petersburg, Russia and,
independently, Guignard in France. This discovery came at the end of a period of controversy about fertilization in angiosperms
and ushered in a new period of intense research. Still, by 1950, there were many unanswered questions about double fertilization
because of limitations of the light microscope. The introduction of the electron microscope stimulated new research and helped
resolve some of the questions. My own research with the electron microscope and that of people who worked in my laboratory
is recounted and some of the still unanswered questions raised.
Received: 30 October 1997 / Accepted: 5 December 1997 相似文献
88.
Levitan DR 《Evolution; international journal of organic evolution》2008,62(6):1305-1316
Sea-urchin species differ in susceptibility to sperm limitation and polyspermy, but the influences of gamete traits on reproductive variance, sexual selection, and sexual conflict are unknown. I compared male and female reproductive success of two congeners at natural densities in the sea. The eggs of the species occurring at higher densities, Strongylocentrotus purpuratus, require higher sperm concentrations for fertilization but are more resistant to polyspermy compared to S. franciscanus. Both species show high variance in male fertilization success at all densities and high variance in female success at low densities, but they differ in female variance at high densities, where only S. franciscanus shows high female variance. The intensity of sexual selection based on Bateman gradients is high in males of both species, variable in S. franciscanus females, and low in S. purpuratus females. Strongylocentrotus franciscanus females experience sexual selection at low densities and sexual conflict at high densities. Strongylocentrotus purpuratus may rarely experience sperm limitation and may have evolved to ameliorate sexual conflict. This reduces the variance in female fertilization, providing females with more control over fertilization. Sperm availability influences sexual selection directly by determining sperm-egg encounter probabilities and indirectly through selection on gamete traits that alter reproductive variances. 相似文献
89.
R. Paul Schreiner 《Plant and Soil》2005,273(1-2):219-234
Soil and crop management practices may influence biomass growth and yields of cotton (Gossypium hirsutum L.) and sorghum (Sorghum bicolorL.) and sequester significant amount of atmospheric CO2in plant biomass and underlying soil, thereby helping to mitigate the undesirable effects of global warming. This study examined the effects of three tillage practices [no-till (NT), strip till (ST), and chisel till (CT)], four cover crops [legume (hairy vetch) (Vicia villosa roth), nonlegume (rye) (Secale cerealeL), hairy vetch/rye mixture, and winter weeds orno covercrop], and three N fertilization rates (0, 60–65, and 120–130 kg N ha –1) on the amount of C sequestered in cotton lint (lint + seed), sorghum grain, their stalks (stems + leaves) and roots, and underlying soil from 2000 to 2002 in central Georgia, USA. A field experiment was conducted on a Dothan sandy loam (fine-loamy, kaolinitic, thermic, Plinthic Kandiudults). In 2000, C accumulation in cotton lint was greater in NT with rye or vetch/rye mixture but in stalks, it was greater in ST with vetch or vetch/rye mixture than in CT with or without cover crops. Similarly, C accumulation in lint was greater in NT with 60 kg N ha –1 but in stalks, it was greater in ST with 60 and 120 kg N ha –1 than in CT with 0 kg N ha –1. In 2001, C accumulation in sorghum grains and stalks was greater in vetch and vetch/rye mixture with or without N rate than in rye without N rate. In 2002, C accumulation in cotton lint was greater in CT with or without N rate but in stalks, it was greater in ST with 60 and 120 kg N ha –1 than in NT with or without N rate. Total C accumulation in the above- and belowground biomass in cotton ranged from 1.7 to 5.6 Mg ha –1 and in sorghum ranged from 3.4 to 7.2 Mg ha –1. Carbon accumulation in cotton and sorghum roots ranged from 1 to 14% of the total C accumulation in above- and belowground biomass. In NT, soil organic C at 0–10 cm depth was greater in vetch with 0 kg N ha –1 or in vetch/rye with 120–130 kg N ha –1 than in weeds with 0 and 60 kg N ha –1 but at 10–30 cm, it was greater in rye with 120–130 kg N ha –1 than in weeds with or without rate. In ST, soil organic C at 0–10 cm was greater in rye with 120–130 kg N ha –1 than in rye, vetch, vetch/rye and weeds with 0 and 60 kg N ha –1. Soil organic C at 0–10 and 10–30 cm was also greater in NT and ST than in CT. Since 5 to 24% of C accumulation in lint and grain were harvested, C sequestered in cotton and sorghum stalks and roots can be significant in the terrestrial ecosystem and can significantly increase C storage in the soil if these residues are left after lint or grain harvest, thereby helping to mitigate the effects of global warming. Conservation tillage, such as ST, with hairy vetch/rye mixture cover crops and 60–65 kg N ha –1 can sustain C accumulation in cotton lint and sorghum grain and increase C storage in the surface soil due to increased C input from crop residues and their reduced incorporation into the soil compared with conventional tillage, such as CT, with no cover crop and N fertilization, thereby maintaining crop yields, improving soil quality, and reducing erosion. 相似文献
90.
利用DIG末端标记技术和免疫组化技术分析了小鼠精子体外结合内化外源DNA的效率。试验结果表明,不同小鼠个体的精子结合外源DNA的阳性率有明显差异(P<0.01),平均为13%。利用考马斯亮蓝染色评价了小鼠精子顶体反应发生的情况,筛选出TYH培养液为较合适的体外受精液。利用小鼠体外受精技术,将体外转染GFP基因并获能的小鼠精子与成熟卵母细胞进行体外受精,受精卵进行体外培养,表达GFP胚胎的阳性率为4.7%。验证了精子介导制备转基因小鼠胚胎的可行性,并建立了利用精子载体法制备转基因小鼠胚胎的平台。 相似文献