Theodore C. Foin

Crop traits related to weed suppression in water-seeded rice (Oryza sativa L.)

Abstract Resistance to herbicides and the lack of viable control options have led to an interest in increasing the role of crop competition as a weed management tool in water-seeded rice production. Weed-suppressive rice cultivars have been suggested as a tool that could improve weed control and reduce the reliance of growers on herbicides. Field studies were conducted at Biggs, CA, in 1999 and 2000 with six to eight semidwarf rice cultivars to identify water-seeded rice traits related to the suppression of watergrass growth. Cultivars S-201 and M-302 were the most suppressive in both years. The dry weight (DW) of watergrass grown with the most suppressive cultivar was only 16% in 1999 and 57% in 2000 of the DW of watergrass grown with the least suppressive cultivar. Rice leaf area and root DW in weed-free plots were linearly related to watergrass DW in both years. Weed-suppressive traits were not inversely correlated with rice yields in monoculture; competitive cultivars also had high yields. This study suggests that an indirect selection program, based on traits that can be identified early in the season under weed-free conditions, has great potential for developing more competitive cultivars for water-seeded rice. Nomenclature: Early watergrass, Echinochloa oryzoides (Ard.) Fritsch ECHOR; late watergrass, Echinochloa phyllopogon (Stapf) Koss. ECHPH; rice, Oryza sativa L.

A provincial model of Phanerozoic marine diversity

Much new empirical evidence on the levels of Phanerozoic paleoprovinciality and of species diversity within paleocommunities now permits a reevaluation of marine diversity patterns. Data on paleoprovincial patterns are assembled from the literature and evaluated by means of a stochastic computer simulation model. The simulation is based on the statistics of modern patterns of diversity and endemism extrapolated conservatively to the paleoprovincial patterns and on estimates of species duration from the fossil record. The species diversities associated with the paleoprovincial patterns are then corrected for temporal changes in species packing in communities as determined by Bambach (1977) from studies of paleocommunities. The model thus has an empirical basis throughout. Furthermore it is free of biases that can arise due to the differential preservation of taxa in space and time. The Paleozoic and Mesozoic were characterized by low provinciality and low average species diversity, on the order of 38,000 to 40,000 species, although there were significant fluctuations in standing diversities. In the Cenozoic, provinciality rose markedly, primarily through the appearance of latitudinal provincial chains, and average species diversity rose to about 240,000. Today it stands over 350,000; this is an order of magnitude greater than the Paleozoic average.

The influence of tidal channels on the distribution of salt marsh plant species in Petaluma Marsh, CA, USA

Tidal channels influence the distribution and composition of salt marsh vegetation in a San Francisco Bay salt marsh. Two channel networks in the Petaluma Marsh, Sonoma County, CA, were mapped and characterized using global positioning and geographic information systems. Plant species abundance was sampled on transects placed perpendicular to and extending away from the channel banks. The vegetation showed significant increases in species richness along channel banks and larger areas of effect which increased approximately linearly with channel size. Composition of species assemblages varies with distance from the channel bank and channel size. These results demonstrate that salt marsh plant assemblages, composed of both major and minor species, are distributed with respect to the channel network in Petaluma Marsh.

Analysis and Prediction of Population and Community Change: A Grassland Case Study

The mechanisms of successional change were analysed in a mediterranean coastal grassland undergoing post-grazing succession at Sea Ranch, northern California, U.S.A. The study included field experiments and observations on the five most abundant species, including four perennial grasses and one annual grass. The results explain the observed trends in the populations, and allow prediction of future change. Patchiness in species distributions, dispersal ability, and localised disturbances strongly affected the outcomes of species interactions. Competitive ability was found to have two distinct components, inhibition and invasion, which were not well correlated. Overall, the analyses show that it is possible to link population and community ecology effectively using appropriate field methods, at least in simple communities.

A plant growth model for integrated weed management in direct-seeded rice. III. Interspecific competition for light

The model DSRICE1 was developed for analyzing integrated weed management strategies for direct-seeded rice. We have shown that DSRICE1 predicts monoculture rice growth well and accounts for water-depth effects on growth. Here, the model is used to simulate competition for light between rice and two weeds, Echinochloa oryzoides (early watergrass) and Ammannia spp. (redstem). Except for minor differences in phenology, weed growth was simulated as described for rice. Direct competition for light depended on the species’ vertical distributions of leaf and stem areas (live and dead) and their extinction coefficients. Water also attenuates light, so species’ early height growth rates were important because they determined when plants emerged into full light. Structural sensitivity analyses of rice in competition with the two weeds revealed that waterdepth effects and leaf area distributions strongly affected competition, and shading by dead leaf and stem dry mass reduced total production. Validation was based on independent data sets for redstem and watergrass competition using several statistical tests and indices. For rice‐redstem competition, DSRICE1 simulated rice growth well because redstem competitive effects were small, but predictions of redstem growth were good only when observed heights were matched in simulations. Redstem competitiveness depended on height growth rate, perhaps due to its small seed size. For rice‐watergrass competition, the growth of both species was predicted well, except that watergrass growth in plots with early-season drainage was underpredicted. Watergrass parameters were similar to those for rice except for faster height growth and higher photosynthesis rates. In a model application, simulations in which rice seeding was delayed for a time after flooding led to greater yield losses from redstem than from watergrass because delays reduced the advantage of rice over redstem. The usefulness of DSRICE1 for drained fields will be improved by better simulation of plant growth responses to drainage, but rice competition with redstem and watergrass in continuously-flooded fields was simulated well. # 1999 Elsevier Science B.V. All rights reserved.

A simple empirical model of salt marsh plant spatial distributions with respect to a tidal channel network

Abstract Previous work has shown that the distribution of plant species in a salt marsh near Petaluma, CA, is strongly influenced by the location and size of tidal channels. We developed a simple mathematical model to measure the “channel influence” at each point as a cumulative function based on inverse squared distance to channel, length of potentially influential channel, and channel order (a surrogate for channel cross-sectional area.) Plant species distributions predicted with this channel influence function compared favorably to known distributions in Petaluma Marsh. Using this function and probabilities estimated from transect vegetation data, we simulated the two-dimensional pattern of species abundance across the entire site. Comparisons of predictions with independently generated field maps showed comparable patterns of cover for most species. The results indicate that the distribution of vegetation in Petaluma Marsh, both major zones of marsh dominants and patches of minor species, can be described by a single, empirical factor: cumulative distance from tidal channels of different size.

Compensatory responses of late watergrass (Echinochloa phyllopogon) and rice to resource limitations

Abstract The development of optimal weed management strategies that rely, in part, on crop interference will require an understanding of how weeds compensate for limitations in above- and belowground resources. Trade-offs in the leaf morphology and biomass partitioning of rice and late watergrass were investigated under glasshouse conditions in 1999 and 2000. Both species responded to shade with increased height, reduced biomass, greater partitioning of biomass to leaves, and greater leaf area ratios. At the lowest light level (18% sunlight), plants of both species showed little response to nitrogen (N). However, height, tillers, biomass, and leaf area increased for plants grown at 50% and full sunlight as N increased from 0 to 224 kg N ha−1. Late watergrass exhibited more plasticity in specific leaf area and root weight ratio than rice in response to shade. This plasticity contributed to the ability of late watergrass to maintain a higher percent of its tillers and total dry weight than rice when sunlight was reduced by 50%. These results support the hypothesis that except at low light levels, limited N further reduces the growth of shaded late watergrass plants. Thus, weed management strategies that limit the plasticity of late watergrass by manipulating light and N availability are likely to be more effective than strategies that rely on manipulating a single resource. Nomenclature: Late watergrass, Echinochloa phyllopogon (Stapf) Koss ECHPH; rice, Oryza sativa L.

The Role of Sedimentation in Estuarine Marsh Development within the San Francisco Estuary, California, USA

Abstract Friedrichs and Perry (2001) have hypothesized that exogenous sedimentation is a vital part of a feedback system (with tidal height, marsh elevation and local vegetation) that keeps the marsh surface in equilibrium with sea level. This study investigated the relationship between local sedimentation rates, presence of overlying vegetation, distance from tidal channel, and local elevation in two marshes located in the San Francisco Estuary. Exogenous sedimentation rates measured using sediment traps on marsh plains at approximately MHW were found to be much lower than expected. Sedimentation rates were highest closest to tidal channels, regardless of overlying vegetation, but declined rapidly on the inland portions of the marsh and were never high enough to build the marsh plain at rates greater than 0.91–1.37 mm m−2 yr−1. Maintenance of tidal marsh elevations solely by exogenous sedimentation within the San Francisco Estuary seems unlikely. Observations that local marshes keep pace with sea level rise implicate local productivity as the source of increased sediments.

A plant growth model for integrated weed management in direct-seeded rice: I. Development and sensitivity analyses of monoculture growth

A new model, DSRICE1, was developed to analyze weed management strategies in direct-seeded rice (Oryza sativa) systems. Previous rice models have not accounted for important cultural and weed management factors in direct-seeded systems, such as growth from seeds and water-depth effects on plant growth. Here we describe the development and sensitivity analysis of DSRICE1 for monoculture rice growth under water-seeded conditions. DSRICE1 is largely process-based and includes all standard weed management practices except fertility. Simulation inputs include latitude, daily solar radiation, daily maximum and minimum temperatures, water depth, and seed rate. Phenology depends on thermal units. Growth begins with seed storage mobilization to seedlings, and photosynthesis starts when the first leaf is extended. Canopy light dynamics depend on leaf and stem area distributions for both live and dead dry mass, and on water depth when submerged. Water-depth effects were explicitly simulated as reflection and attenuation of light. Model analyses revealed that parameter sensitivities varied over time. Some parameters were always important, while the effects of others were limited to particular parts of the season. Judged over the whole season, the most important parameters were for photosynthesis and light capture. Unlike in most monoculture simulations, early height gain rate was important in DSRICE1 because it determined when plants emerged from the water into full light. Analyses of model structure and specifications revealed that predictions were significantly affected by the use of skewed live leaf area distributions and the non-rectangular hyperbola for the light response curve, and the inclusion of waterdepth and dead canopy dry mass effects on canopy light dynamics. The cropping system and management processes simulated in DSRICE1 had important effects on model predictions of rice growth. Explicit consideration of these factors distinguishes DSRICE1 from other rice growth models, and may lead to better simulation analyses of system interactions with plant growth and weed management strategies. # 1999 Elsevier Science B.V. All rights reserved.

A plant growth model for integrated weed management in direct-seeded rice II. Validation testing of water-depth effects and monoculture growth

Manipulating water depths and timing is a key management practice in rice cropping systems, but rice models have not simulated water-depth effects on plant growth. A new plant growth model, DSRICE1, simulates most cultural and weed management practices except fertility. Water-depth effects on plant light capture are mechanistically simulated as reflection and attenuation of light by water. Light attenuation by water in the model depended on water depth and the light extinction coefficient of water, kH2O (m ˇ1 ). DSRICE1 was validation tested for prediction of monoculture growth, and specifically for early-season water-depth effects. Analyses revealed that attenuation by water limited irradiance and reduced seedling growth, and indicated which plant traits contributed to growth during submergence. Reflection did not affect rice growth and may be ignored. In empirical validation tests, DSRICE1 predictions were compared with data from 14 independent data sets. For growth up to 40 days after seeding (DAS) (11 experiments), simulations without water-depth effects failed validation tests by overpredicting rice shoot dry mass (DM). In contrast, DSRICE1 simulations with water-depth effects included were accurate with kH2O from 1 to 4, kH2Oa 3 was best overall, and predictions improved with experiment-specific kH2O values. DSRICE1 also accurately predicted differences in shoot DM per plant in a 1985 water-depth study, while again simulations without water-depth effects did not. Thus, water-depth effects were required to simulate early rice growth accurately. In whole-season empirical validation tests (14 experiments), DSRICE1 simulated shoot, stem, leaf, live leaf, and live stem DM accurately, especially considering the range of data used and the fact that no calibration was needed. DSRICE1 was also corroborated by the fact that it accounts for many physical factors and plant traits that affect submergence tolerance. The model or the techniques it uses may be useful in analyses of cultivar tolerance to submergence. In some respects, DSRICE1 was subjected to more rigorous validation testing than previous rice models, and potentially explains more interactions between rice and weed growth and management. This approach may broaden and improve simulation analyses of integrated weed management in direct-seeded rice systems. # 1999 Elsevier Science B.V. All rights reserved.