A prediction from the herbivore optimization hypothesis is that for every combination of site/habitat type and plant community type there is a grazing intensity that causes a maximum increase in above-ground net primary productivity compared with the ungrazed control. NPP is defined as the rate of change in green, herbaceous biomass per unit time per unit area. We tested this hypothesis in the primary summer range of a growing population of wood bison (Bison bison athabascae) within the Mackenzie Bison Sanctuary, Northwest Territories, Canada. Plots (0.5 x 0.5 m) in graminoid meadows dominated by awned sedge (Carex atherodes) were either clipped at 3 cm, exposed to wood bison grazing, temporarily protected for 3 weeks, or permanently protected. This resulted in the removal of 100%, 0-79%, 0-79% or 0%, respectively, of shoot tissue available to wood bison. NPP of meadows clipped twice at 3 cm in 1986 was the same as control NPP at 5 study sites. In 1987, only the 2 most productive study sites of 1986 were intensively examined: plots clipped once in early summer increased in NPP by 120% and 133% compared to controls; NPP of meadows grazed by wood bison increased by 200% compared to controls at the most productive site, but remained the same as controls at the less productive site. Therefore, the herbivore optimization hypothesis was accepted at the 2 most productive sites in 1987, but rejected at all 5 study sites in 1986. In 1987, the standing crop of dead material was 258% and 142% higher in controls than in grazed plots at the 2 most productive sites. We think this dead material was responsible for the lower NPP observed in control plots.
Accurate and unbiased radiative energy transfer models are critical to our understanding of ecosystem primary productivity, carbon cycling, and climate change. Much of the current research in this area is based on models parameterized for grasslands and broadleaf forests. However, many temperate montane and boreal forests are dominated by conifers, which present unique challenges to modellers. We propose two fundamentally different strategies by which plant canopies optimize solar radiation interception. Laminar canopies (e.g., grasslands, broadleaf trees) are .solar panels. that directly intercept incoming radiant energy. By contrast, conifer canopies are conical anechoic (.without echo.) surfaces that intercept radiant energy by scattering it through the canopy. The properties of anechoic surfaces are well known in acoustical and electrical engineering, but have not been applied in environmental biophysics. We discuss the physical principles of anechoic surfaces, and demonstrate how these principles apply to conifer trees and canopies. A key feature of anechoic interception is low radiance over all wavelengths, which is an emergent property of the system. Using empirical data from boreal forest stands in Riding Mountain National Park (Manitoba, Canada), we demonstrate that conifer canopies have very low near-infrared radiance compared to laminar broadleaf canopies. Vegetation index values for conifers are thereby reduced, resulting in underestimates of primary productivity and other biophysical parameters. We also discuss the adaptive significance of boreal conifer geometry, and consider factors driving selection of laminar versus anechoic canopy architectures.
Uranium shows relatively conservative behaviour in seawater because of the formation of stable carbonato complexes, whereas particulate uranium, involved in suspended particles with the particle size of more than 0.45 m, is a minor constituent. It was found that particulate uranium, with a range from 0.24 to 39 Bq·1–1, varies spatially and temporally. Its highest concentration occurs in the tropical region of the western North Pacific during the winter of 1983, corresponding to the 1983 El Nifio event. A leaching experiment revealed that major species of particulate uranium are labile organic complexes. Mass balance considerations suggest that particulate uranium in open ocean waters correlates with the presence of particulate organic matter (POM). A high peak of particulate uranium in 1983 may indicate that POM, i.e., primary productivity, increased in the western tropical Pacific during the El Niño event.
The diversity dynamics of the Anisian ammonoids is analyzed in terms of generic richness and turnover rates in one North American (Nevada) and two western Tethyan (Eastern Lombardy, Balaton Highland) regions. Two pulses of diversification are outlined: one in the middle Anisian (Pelsonian) and another near the end of the late Anisian (late Illyrian). The Pelsonian global diversification is interpreted as an effect of global sea-level rise. In the early late Anisian the ammonoid generic richness definitely decreased both in the western Tethys and in Nevada. The latest Anisian peak of ammonoid diversity was low in Nevada, which is explained by the uniform local sedimentary environment and the absence of major global changes. In the western Tethys the late Illyrian diversity peak was very prominent: ammonoid generic richness, turnover and proportion of originations were very high. This explosive peak is interpreted in terms of major changes of two regional environmental factors: coeval volcanic activity and the control of nearby carbonate platforms. The late Illyrian volcanic ash falls provoked a dramatic increase of ammonoid generic richness by fertilization, i.e. supplying nutrients and iron, thus increasing primary productivity in the ocean. Carbonate platform margins offered diverse habitats with new, empty niches; the microbial mats supplied suspended organic matter for the higher trophic levels and eventually the ammonoids. In the western Tethyan regions platform growth re-appeared after the end-Permian crisis, and significantly increased in the late Illyrian. This was closely followed by the remarkable increase of ammonoid generic richness. Many of the genera which originated during the late Anisian seem to be ecologically connected to the platform or peri-platform environments. It is suggested that this explosive diversity peak is a manifestation of the co-evolution of the Tethyan carbonate platforms and the ammonoids.
Authors:W. Qi, X. Zhou, M. Ma, J. M. H. Knops, W. Li, and G. Du
Despite a long history of alpine meadows studies, uncertainty remains about the importance of environmental factors in structuring their assembly. We examined the functional and phylogenetic structure of 170 alpine Tibetan meadow communities in relation to elevation, soil moisture and shade. Functional community structure was estimated with both communityweighted mean (CWM) trait values for specific leaf area (SLA), plant height and seed mass and functional diversity (Rao’s quadratic index) for their traits individually and in combination (multivariate functional diversity). We found that shade induced by woody plants significantly increased the phylogenetic diversity and functional diversity of SLA of co-occurring species, suggesting that woody plants behave as “ecosystem engineers” creating a different environment that allows the existence of shade tolerant species and thereby facilitates the coexistence of plant species with different light resource acquisition strategies. We also found evidence for a clear decrease in phylogenetic diversity, CWM and functional diversity related to plant height in the two extreme, both the dry and wet, soil moisture conditions. This indicates that both drought and excess moisture may act as environmental filters selecting species with close phylogenetic relationships and similar height. Moreover, we detected significant decreases in both CWM and functional diversity for seed mass along elevational gradients, suggesting that low net primary productivity (NPP) limits seed size. Finally, because of different individual trait responses to environmental factors, the multivariate functional diversity did not change across environmental gradients. This lack of multivariate response supports the hypothesis that multiple processes, such as environmental filtering, competition and facilitation, may operate simultaneously and exert opposing effects on community assembly along different niche (e.g., water use, light acquisition) axes, resulting in no overall functional community structure change. This contrast between individual and multivariate trait patterns highlights the importance of examining individual traits linked with different ecological processes to better understand the mechanisms of community assembly.
Authors:G. Ónodi, Gy. Kröel-Dulay, E. Kovács-Láng, P. Ódor, Z. Botta-Dukat, B. Lhotsky, S. Barabás, J. Garadnai, and M. Kertész
. , Schmidt , I.K. , Sirca , C. , Sowerby , A. , Spano , D. and Tietema , A.
2007 . Response of plant species richness and primaryproductivity in shrublands along a north-south gradient in Europe to seven years of experimental warming
Authors:Zoltán Lantos, István Vető, Mária Földvári, and Péter Kovács-Pálffy
productivity in the Toarcian Tethys -- A novel approach based on TOC, reduced sulphur and manganese contents. -- Palaeogeogr. Palaeoclim. Palaeoecol., 132, 355-371.
Estimation of primaryproductivity in the Toarcian Tethys -- A novel