Successional dynamics in plant community assembly may result from both deterministic and stochastic ecological processes. The relative importance of different ecological processes is expected to vary over the successional sequence, between different plant functional groups, and with the disturbance levels and land-use management regimes of the successional systems. We evaluate the relative importance of stochastic and deterministic processes in bryophyte and vascular plant community assembly after fire in grazed and ungrazed anthropogenic coastal heathlands in Northern Europe. A replicated series of post-fire successions (n = 12) were initiated under grazed and ungrazed conditions, and vegetation data were recorded in permanent plots over 13 years. We used redundancy analysis (RDA) to test for deterministic successional patterns in species composition repeated across the replicate succes-sional series and analyses of co-occurrence to evaluate to what extent species respond synchronously along the successional gradient. Change in species co-occurrences over succession indicates stochastic successional dynamics at the species level (i.e., species equivalence), whereas constancy in co-occurrence indicates deterministic dynamics (successional niche differentiation). The RDA shows high and deterministic vascular plant community compositional change, especially early in succession. Co-occurrence analyses indicate stochastic species-level dynamics the first two years, which then give way to more deterministic replacements. Grazed and ungrazed successions are similar, but the early stage stochasticity is higher in ungrazed areas. Bryophyte communities in ungrazed successions resemble vascular plant communities. In contrast, bryophytes in grazed successions showed consistently high stochasticity and low determinism in both community composition and species co-occurrence. In conclusion, stochastic and individualistic species responses early in succession give way to more niche-driven dynamics in later successional stages. Grazing reduces predictability in both successional trends and species-level dynamics, especially in plant functional groups that are not well adapted to disturbance.

No abstract

Co-ordinating herbicide applications with the suppressive ability of the crop has the potential to improve weed control and optimize herbicide use in water-seeded rice. However, the successful integration of herbicide applications and crop development depends on the timing and duration of competition between rice and weeds. The critical period of competition between rice and Echinochloa species was examined in field and glasshouse experiments from 1996 to 2000. In 1999 and 2000, Echinochloa species seeded 30 days after rice in field experiments did not survive and rice yields were not reduced when plots were kept weed-free for 30 days or longer. In a basin experiment conducted in 1998, E. phyllopogon seeded with the crop was unaffected by light competition alone but the relative importance of shading by the crop increased when E. phyllopogon was seeded after rice. Management strategies that delay the germination or growth of Echinochloa species might confer a competitive advantage to rice and reduce the need for herbicide applications. However, yields in the field experiments were reduced by at least 18% after only 30 days of competition in both years, suggesting that it may be difficult to integrate currently available herbicides with crop growth in water-seeded rice.

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.

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.