951 research outputs found
Grapevine and Soil Water Relations with Nodding Needlegrass \u3ci\u3e(Nassella cernua)\u3c/i\u3e, a California Native Grass, as a Cover Crop
Nodding needlegrass [Nassella cernua (Stebbins & R.M. Love) Barkworth], a California native perennial grass, was tested for its effects on grapevine and soil–water relations in a drip-irrigated vineyard in Parlier, CA. Vine water status and in-row and between-row soil moisture (at 0.3 m, 0.6 m, 0.9 m, 1.2 m, and 1.5 m) were monitored semiweekly from June to September. There was no overall significant difference in leaf water potential between treatments. In-row soil moisture was lowest at depths of 0.6 m to 0.9 m within the nodding needlegrass treatment but was lowest from 0.3 m to 0.9 m within the clean cultivation treatment. Compared with clean cultivation, nodding needlegrass in-row soil moisture was significantly higher at depths of 0.3 m and 0.6. m and did not differ at depths of 0.9 m and 1.2 m. In contrast, in-row soil moisture was significantly higher under clean cultivation compared with nodding needlegrass at 1.5 m. Between-row soil moisture was significantly higher under clean cultivation compared with nodding needlegrass at every depth. Combining in-row and between-row data, overall vineyard soil moisture was slightly lower, by 1.2% points, in the nodding needlegrass treatment compared with clean cultivation. There was no interaction between treatment and depth for between-row soil moisture, indicating that the vines used little water from the between-row area. The lack of difference between treatments in the rate of soil moisture depletion over the season indicates that nodding needlegrass used little water during the summer. Based on these results, nodding needlegrass appears to be suitable as a permanent cover crop in California drip-irrigated vineyards where competition for summer water is a concern
On the Minimum Distance of Generalized Spatially Coupled LDPC Codes
Families of generalized spatially-coupled low-density parity-check (GSC-LDPC)
code ensembles can be formed by terminating protograph-based generalized LDPC
convolutional (GLDPCC) codes. It has previously been shown that ensembles of
GSC-LDPC codes constructed from a protograph have better iterative decoding
thresholds than their block code counterparts, and that, for large termination
lengths, their thresholds coincide with the maximum a-posteriori (MAP) decoding
threshold of the underlying generalized LDPC block code ensemble. Here we show
that, in addition to their excellent iterative decoding thresholds, ensembles
of GSC-LDPC codes are asymptotically good and have large minimum distance
growth rates.Comment: Submitted to the IEEE International Symposium on Information Theory
201
Spatially Coupled LDPC Codes Constructed from Protographs
In this paper, we construct protograph-based spatially coupled low-density
parity-check (SC-LDPC) codes by coupling together a series of L disjoint, or
uncoupled, LDPC code Tanner graphs into a single coupled chain. By varying L,
we obtain a flexible family of code ensembles with varying rates and frame
lengths that can share the same encoding and decoding architecture for
arbitrary L. We demonstrate that the resulting codes combine the best features
of optimized irregular and regular codes in one design: capacity approaching
iterative belief propagation (BP) decoding thresholds and linear growth of
minimum distance with block length. In particular, we show that, for
sufficiently large L, the BP thresholds on both the binary erasure channel
(BEC) and the binary-input additive white Gaussian noise channel (AWGNC)
saturate to a particular value significantly better than the BP decoding
threshold and numerically indistinguishable from the optimal maximum
a-posteriori (MAP) decoding threshold of the uncoupled LDPC code. When all
variable nodes in the coupled chain have degree greater than two,
asymptotically the error probability converges at least doubly exponentially
with decoding iterations and we obtain sequences of asymptotically good LDPC
codes with fast convergence rates and BP thresholds close to the Shannon limit.
Further, the gap to capacity decreases as the density of the graph increases,
opening up a new way to construct capacity achieving codes on memoryless
binary-input symmetric-output (MBS) channels with low-complexity BP decoding.Comment: Submitted to the IEEE Transactions on Information Theor
Terminated LDPC Convolutional Codes with Thresholds Close to Capacity
An ensemble of LDPC convolutional codes with parity-check matrices composed
of permutation matrices is considered. The convergence of the iterative belief
propagation based decoder for terminated convolutional codes in the ensemble is
analyzed for binary-input output-symmetric memoryless channels using density
evolution techniques. We observe that the structured irregularity in the Tanner
graph of the codes leads to significantly better thresholds when compared to
corresponding LDPC block codes.Comment: To appear in the proceedings of the 2005 IEEE International Symposium
on Information Theory, Adelaide, Australia, September 4-9, 200
Quasi-Cyclic Asymptotically Regular LDPC Codes
Families of "asymptotically regular" LDPC block code ensembles can be formed
by terminating (J,K)-regular protograph-based LDPC convolutional codes. By
varying the termination length, we obtain a large selection of LDPC block code
ensembles with varying code rates, minimum distance that grows linearly with
block length, and capacity approaching iterative decoding thresholds, despite
the fact that the terminated ensembles are almost regular. In this paper, we
investigate the properties of the quasi-cyclic (QC) members of such an
ensemble. We show that an upper bound on the minimum Hamming distance of
members of the QC sub-ensemble can be improved by careful choice of the
component protographs used in the code construction. Further, we show that the
upper bound on the minimum distance can be improved by using arrays of
circulants in a graph cover of the protograph.Comment: To be presented at the 2010 IEEE Information Theory Workshop, Dublin,
Irelan
New Codes on Graphs Constructed by Connecting Spatially Coupled Chains
A novel code construction based on spatially coupled low-density parity-check
(SC-LDPC) codes is presented. The proposed code ensembles are described by
protographs, comprised of several protograph-based chains characterizing
individual SC-LDPC codes. We demonstrate that code ensembles obtained by
connecting appropriately chosen SC-LDPC code chains at specific points have
improved iterative decoding thresholds compared to those of single SC-LDPC
coupled chains. In addition, it is shown that the improved decoding properties
of the connected ensembles result in reduced decoding complexity required to
achieve a specific bit error probability. The constructed ensembles are also
asymptotically good, in the sense that the minimum distance grows linearly with
the block length. Finally, we show that the improved asymptotic properties of
the connected chain ensembles also translate into improved finite length
performance.Comment: Submitted to IEEE Transactions on Information Theor
Exact Free Distance and Trapping Set Growth Rates for LDPC Convolutional Codes
Ensembles of (J,K)-regular low-density parity-check convolutional (LDPCC)
codes are known to be asymptotically good, in the sense that the minimum free
distance grows linearly with the constraint length. In this paper, we use a
protograph-based analysis of terminated LDPCC codes to obtain an upper bound on
the free distance growth rate of ensembles of periodically time-varying LDPCC
codes. This bound is compared to a lower bound and evaluated numerically. It is
found that, for a sufficiently large period, the bounds coincide. This approach
is then extended to obtain bounds on the trapping set numbers, which define the
size of the smallest, non-empty trapping sets, for these asymptotically good,
periodically time-varying LDPCC code ensembles.Comment: To be presented at the 2011 IEEE International Symposium on
Information Theor
Spider (Araneae) Species Composition and Seasonal Abundance in San Joaquin Valley Grape Vineyards
As part of an investigation to estimate the effect of resident spider populations on Erythroneura variabilis Beamer, spider species composition, relative abundance, and seasonal occurrence were determined. Spiders were sampled monthly during the 1992 and 1993 growing seasons; their numbers were pooled and analyzed for species diversity using the Renkonen index of similarity and cluster analysis. Twenty-seven species of spiders were recorded, representing 14 families. The most common species were Cheiracanthium inclusum (Hentz), Trachelas pacificus (Chamberlin and Ivie), Theridion dilutum Levi, Theridion melanurum Hahn, Oxyopes scalaris Hentz, Oxyopes salticus Hentz, Hololena nedra Chamberlin and Ivie, and Metaphidippus vitis (Cockerell). Three species (C. inclusum, T. dilutum, and T. melanurum) constituted \u3e30% of all spiders collected; however, species diversity varied among vineyard sites. In 4 vineyard sites, hunting spiders (C. inclusum, T. pacificus, Oxyopes spp., and M. vitis) dominated the fauna, representing an average of 79.7% of the specimens collected. In the other 3 vineyards, hunting and web-weaving spiders were more equally represented, averaging 43.5 and 50.0%, respectively,of all spiders collected. Species similarity between vineyards from both years ranged from 19 to 73% based on the Renkonen index. Similarly, cluster analysis showed a wide separation in species composition among sampled vineyards. The discrepancy in species similarity among sampled vineyards is discussed in reference to potential prey density and vineyard cultural practices. Seasonal abundance patterns of the 8 most common species are presented and discussed in reference to their respective phenologies
Can Cover Crops Reduce Leafhopper Abundance in Vineyards?
In 3 of 4 vineyards we studied, late-season leafhopper density was lower on vines in cover cropped plots than in plots with no cover crops. However, the level of leafhopper reduction (about 15%) was rarely economically important and the mechanisms leading to reduction were not clear. For example, there were few differences in the number of leaf hopper predators or parasitoids on the vines in cover cropped versus no cover plots. However, there were significant between-treatment differences in vine growth. Plots with seasonwide maintenance of a cover crop and resident grasses had a reduction in vine vigor. Lower vine vigor has been associated with lower leafhopper densities and, in our studies conducted from 1993 to 1996, those plots with reduced vine vigor often had the greatest reduction in late-season leafhopper density
Influence of Ground Cover on Spider Populations in a Table Grape Vineyard
1. Cover crops and/or resident ground vegetation have been used in California vineyards to increase the number of predators and decrease the number of pestiferous herbivores. The most common resident predators in vineyards are spiders (Araneae). Several observational studies suggest that the addition of cover crops results in an increase in spider density and a decrease in insect pest densities. 2. To test experimentally the effects of cover crops and/or resident ground vegetation (hereafter collectively referred to as ground cover) on spider populations, a 3-year study was undertaken in a commercial vineyard. Large, replicated plots were established with and without ground cover during the growing season. Spider species diversity was analysed on the vines and on the ground cover. 3. On the vines, there was no significant difference in spider species richness or the total number of spiders in plots with and without ground cover. There were differences in the relative abundance of two spiders between treatments, with one species (Trachelas pacificus [Chamberlin & Ivie]) more abundant in plots with ground cover and another (Hololena nedra Chamberlin & Ivie) more common on vines in plots with no ground cover. Annual variation in spider abundance was greater than variation due to ground cover treatment. 4. On the ground cover, the spider species diversity was considerably different from that found on the vines above, suggesting that there is little movement of spiders between the ground cover and the vines. Enhancement of T. pacificus populations on vines with ground covers may be a result of prey species movement between the ground cover and the vines. Spider abundance was sparse on the bare ground. 5. The maintenance of ground cover increased spider species diversity in the vineyard as a whole (vine and ground cover). However, the relatively small changes in spider abundance on the vines indicate there are limitations in the use of ground covers for pest management with respect to generalist predators
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