Rice yields in weed-firee plots were the same across all three irrigation systems in both 2013 and 2014 . Since there was no year by irrigation system interaction, data were pooled and analyzed over years. There was a significant decrease in yield in 2014, but this was found across all three irrigation systems, and may be due to the higher temperatures in 2014 . Research in the southern United States showed that rice yields are reduced by daytime maximum temperatures above 28 C . Rice yields from the weed-recruitment areas were significantly less in the DS-AWD system than in the WS-AWD and WS-Control systems across both years. In spite of a small amount of rice still present at canopy closure, yield losses were 100% in both 2013 and 2014 in the DS-AWD system. The few surviving rice plants did not produce panicles. Percent yield loss was greater in 2013 in the WS-Control system than in the WS-AWD, though the difference was not significant . In 2014 percent yield loss was greater in the WS-AWD than in the WS-Control. Yield reductions in the weedy plots do not correspond to a clear difference in weed species composition between the two water-seeded treatments, flood and drain tray since there was no interaction between irrigation and year. Species specific relationships to yield loss still remain to be elucidated. The WS-AWD system, with its shortened flooding duration, may be a viable option in reducing water use in rice over the growing season.
Yields are equivalent to yields in the WS-Control, if weed control is excellent and soil fertility is maintained. There are few differences in weed species composition between the WS-AWD and WSControl. There is evidence from this study, however, that the early drainage and subsequent flushing events in the WS-AWD may increase small flower umbrella sedge firequency and density. Since there are small flower umbrella sedge populations in California that are resistant to both photosystem II and ACCase inhibitors , the increased biomass and firequency of the weed under the WS-AWD system may increase the proportion of resistant plants in fields that have resistant populations. For small flower umbrella sedge plants that escape early herbicide applications, the shift to an aerobic environment when the field is drained may have a positive effect on growth rate and seed production. The DS-AWD system is only viable with consistent and complete weed control, due to the reduced ability of rice to compete under dry conditions and the 100% reduction in yield under heavy weed pressure. Furthermore, since the DS-AWD system is overwhelmed by watergrass, the presence of multiple-herbicide-resistant watergrass may reduce some growers’ ability to use the system in California. Alternative irrigation systems may prove to be useful to reduce water use over the growing season, but potential water savings need to be balanced against shifts in weed species.
Dry seeding favors watergrass species. and early drainage may favor late-germinating small flower umbrella sedge. Since herbicide resistance in these species is spread throughout the rice-growing region, any changes to growing practices must incorporate methods for dealing with the added pressure of increased populations in growers’ fields.For over a century, there has been concern that several native California oak species are not naturally regenerating adequately to sustain populations . Blue oak is one of these species . Endemic to California, blue oak distribution extends from the Siskiyou Mountains in the north to the Tehachapi Mountains in the south; however, it grows primarily in the Sierra Nevada foothills and coastal mountain ranges. The majority of the woodlands where blue oak grows are used for grazing and beef cattle production. Although blue oak is long lived and relatively few seedlings and saplings are needed in any one year to replace mature trees that die, research indicates that in portions of its range this natural regeneration is not occurring. Swiecki et al. assessed 15 sites representing the broad range of blue oak and reported that the number of saplings at 13 sites was inadequate to offset recent losses in density and canopy cover caused by natural mortality and tree cutting. Even though blue oaks will sprout after their tops are killed by fire or felling , the ability of seedlings to grow into mature trees is essential for the species to sustain itself and prosper.
One theory suggests that the apparent shortage of oak saplings may not signal a regeneration problem but only a lull in natural recruitment, which occurs in spurts, or pulses. These pulses happen when a rare combination of events, such as a wet, late spring following a good acorn crop, combined with, for example, low populations of seedling-eating animals, occurs. The optimal conditions for regeneration may therefore occur only once or twice in a century. For a very long-lived species such as blue oak, these infirequent pulses may be adequate to sustain populations. At present, however, there is little evidence to support this theory, since aging studies of blue oak stands indicate that seedling recruitment occurs over long intervals rather than during short pulses . The reasons for poor regeneration of blue oak vary by site. They include competition from dense annual grasses, browsing by domestic livestock, and herbivory by grasshoppers, squirrels, gophers, voles, rabbits, deer and other animals. Aggravating the situation is the fact that the regions where blue oak grows best have a Mediterranean climate, with a dry period that normally extends from midspring until early fall. Soil conditions can become exceedingly dry, making it difficult for oaks to become established. The bottleneck, or problematic interval in the regeneration process, is from the seedling to the sapling stage . During most years, a sufficient number of acorns germinate, and small seedlings begin to grow in the understory, but few survive to become established saplings. Swiecki et al. defined saplings as having a diameter at breast height between 0.4 and 1.2 inches . The low seedling survival rate has resulted in a bimodal size distribution in many blue oak stands, with considerable numbers of seedlings and trees but few saplings. For over two decades, UC researchers and others have been developing techniques to artificially regenerate California oaks, including blue oak. Research has included collecting, storing and planting acorns; producing oak seedlings in container and bare-root nurseries; and planting and maintaining seedlings in the field . Overall, the research demonstrates that sapling-sized oaks can be established artificially — in less than 5 years — but the substantial management required is costly. As a result, these techniques are not being used for large areas. An alternative oak regeneration strategy is to promote the advancement of naturally occurring seedlings on-site, helping them to reach the sapling stage.
This strategy could produce considerable savings because no effort or cost would be expended to collect acorns or to grow and plant seedlings. An additional advantage is that only genetically adapted plant material would be used, alleviating concerns about using off-site planting stock that is not adapted to local conditions. Given these economic, ecologic and genetic advantages, landowners may be more likely to adopt natural regeneration practices than artificial regeneration.To test the strategy of enhancing natural blue oak regeneration, hydroponics flood tray we initiated a study in spring 2007 at six sites broadly representing the range of blue oak in California . The northernmost site was near Red Bluff in Tehama County, and the southernmost site was in Santa Barbara County about 18.6 miles west of Cuyama. At each site, 144 naturally occurring blue oak seedlings between about 1 and 23 inches tall were identified and tagged. We selected seedlings on each site such that half were under the canopy of existing trees and half were outside the drip line of the trees and in the open. Treatments. The 72 seedlings per canopy treatment at each site were arranged in 18 groups of four seedlings each. Except for a few cases where closely spaced seedlings were difficult to locate, seedlings within each group of four were no closer than 4 feet apart and no farther apart than 20 feet . In spring 2007, one member of each group of four was randomly selected to be covered with a 4-foot tree shelter. Tree shelters are solid, double-walled plastic cylinders that are placed over individual seedlings. They were developed in England in the early 1980s and are reported to protect seedlings from browsing and to stimulate above ground growth . We eliminated the surface vegetation within approximately 2 feet of a second seedling in each group by spraying with contact herbicide and reapplied the herbicide each subsequent spring. We covered the third seedling of each group with a tree shelter and sprayed for weed control. The fourth seedling was a control without protection or weed control. Data collection. Before we installed the treatments, we recorded each seedling and its height . In the falls of 2008, 2009 and 2010, we assessed each seedling for survival and total height. In cases where the top of the seedling had died, we recorded the height from the base to the highest living point as indicated by green foliage or green tissue under the bark. When we found seedlings that had died, we tried to identify the cause , but this proved difficult so no results are reported here. We collected management history for each site from the landowners and average annual precipitation in the 2007–2008, 2008–2009 and 2009–2010 growing seasons , from local weather databases . Statistical analysis. The seedling data was analyzed as a doubly nested randomized block experiment with sites as the main plots, shade as the subplots and factorial combinations of tree shelters and weed control as the sub-sub plots. Before analysis, the data was averaged over shade, shelter and weed control treatments for each site. Differences were considered significant at the P ≤ 0.05 level. Each response variable for each year was tested for significance, as were all two-way interactions. When we found significant differences for the sites, we performed least significance difference tests to determine which sites were significantly different from the others . We also examined all significant two-way interactions to determine their cause. Finally, we computed partial correlations to find out if initial seedling height was related to subsequent growth and survival.Height growth. Height growth can be critical to the survival of blue oak seedlings, because they are only relatively resistant to browsing damage from cattle, or clipping of the above ground portion of the seedling, when they reach 6.5 feet . Without protection, seedlings may languish in a stunted state, due to repeated browsing, for decades . In our study, tree shelters significantly increased seedling height growth at all sites . However, responses were not consistent over sites, and there were highly significant site/shelter interactions for height growth each year. For instance, at the San Luis Obispo County site, two seedlings in the shelter treatment grew 4 feet — to the tops of the shelters — during 2008. This represented an annual height growth of over 2 feet for each of these two seedlings. At the Yolo County site, on the other hand, no seedlings grew more than 1.6 inches during 2008, and the average change in height for each of the treatments, including the shelter treatment, was negative. There were also significant shade/ shelter interactions for height growth each year, because the positive effects of the tree shelters were much less for seedlings under the canopies than they were for seedlings in the open. Partial correlations, adjusted for site, of initial height and height growth were highly significant each year — the taller the seedlings were initially, the more they grew. There was much greater height growth in the last year of the study , which corresponded to an above-average rainfall year. Height growth in 2010, averaged over all sites and treatments, was approximately double that in 2009 and more than triple that in 2008. The difference in height growth for sheltered compared with unsheltered seedlings was also greatest in 2010. Survival. The differences in survival were less pronounced than they were for height growth, although there were significant site differences in survival every year . In 2008 and 2010, there were also significant differences in survival for weed control treatments, with seedlings receiving weed control having greater survival than those not receiving it. In 2009, those receiving a weed control treatment had higher average survival, but the differences were not significant.