Predicting Cation Ratios in Corn from Saline Solution Composition

The response of Zea mays (L.) cv. Pioneer 3906 to nutrient culturesthat varied in Na/Ca ratios was studied at 5 different salinitylevels; 0, 0.1, –0.2, 0.3 and –0.4 MPa. Principlesof ion exchange theory were used to relate the cation composition(Ca, Mg and Na) in the shoots to activity ratios in the rootmedia. The data are expressed using the Gapon convention wherethe selectivity coefficient is related to the equivalent fractionof the exchange ions. Plots of the cation ratios of Ca/Na, Ca/Mgand Mg/Na can be represented by a single line regardless ofsalinity level. This information supports the concept derivedfrom physiological studies that the uptake of Ca, Mg and Naby salt-stressed plants is passive. The data suggest that theratios of these cations in corn shoots can be predicted forvarious salinities from solution composition of the root mediaand shoot ion concentrations at a single salinity level. Key words: Gapon equation, ion exchange, selectivity, cation uptake     

Soil conservation in Polylepis mountain forests of Central Argentina: Is livestock reducing our natural capital?

Mountain forests and their soils provide ecological services such as maintenance of biodiversity, provision of clean water, carbon capture and forage for livestock rearing, which is one of the principal economic activities in mountain areas. However, surprisingly little is known about livestock impact in South American mountain forest soils. With the aim of understanding how livestock and topography influence patterns of forest cover, soil compaction, soil loss and soil chemical properties, we analysed these parameters in 100 Polylepis australis woodland plots situated in the humid subtropical mountains of Central Argentina. We used distance from the nearest ranch as an objective index of historical livestock impact and measured standard topographic variables. Our main results reveal that distance from ranch in all cases partly explains tree canopy cover, soil loss, soil compaction and soil chemical properties; suggesting a strong negative effect of livestock. Intermediate altitudes had more tree canopy cover, while landscape roughness – a measure of the variability in slope inclination and aspect – was negatively associated to soil impedance and acidity, and positively associated to soil organic matter content. Finally, flatter areas were more acid. We conclude that livestock has had a substantial influence on forest soil degradation in the Mountains of Central Argentina and possibly other similar South American mountains. Soil degradation should be incorporated into decision making when considering long‐term forest sustainability, or when taking into account retaining livestock for biodiversity conservation reasons. Where soil loss and degradation are ongoing, we recommend drastic reductions in livestock density.     

Fate of Listeria monocytogenes during manufacture and ripening of semi-hard cheese

Semi-hard cheeses were experimentally elaborated with pasteurized milk from sheep, goat and cow (15: 35: 50) and inoculated to contain 1.9 times 10⁵ Listeria monocytogenes /ml in cheeses 1 and 2 and 4 times 10³ L. monocytogenes /ml in cheeses 3 and 4. Counts of L. monocytogenes were determined by direct surface plating of samples on listeria selective agar medium. The results show the substantial survival of L. monocytogenes present in milk during manufacture and ripening of this type of cheese.     

Crown dieback events as key processes creating cavity habitat for magellanic woodpeckers

Abstract Woodpeckers are considered keystone species for webs of cavity nesters and habitat and resource specialists that strongly depend on availability of trees suitable for cavity excavation. Most studies carried out in northern hemisphere temperate coniferous forests emphasize the importance of old growth stages of forests or large dead trees as habitat for cavity builders. We present a study of Nothofagus pumilio tree selection by the magellanic woodpecker (Campephilus magellanicus) that incorporates dendroecological data on long‐term growth trends of trees that provides new insights into the processes that create suitable habitat for cavity excavating species. We analysed 351 cavity and neighbouring control trees in terms of age and radial growth patterns, as well as external tree characteristics. In addition, from a subsample of these trees we developed tree‐ring chronologies for each group using standard methods in order to analyse potential differences in radial growth patterns between cavity and non‐cavity trees. Multivariate models that account for differences between paired cavities versus control trees indicated that growth decline and the degree of crown dieback were the primary variables explaining magellanic woodpecker tree selection for cavity building. In contrast to previous work, neither diameter (above a certain threshold) nor age, were important determinants of selection. Furthermore, trees that became present cavity are those that had synchronously declined in radial growth during the 1943–44 and 1956–57 droughts and the 1985–86 massive caterpillar defoliation. Insect outbreaks and extreme climatic events may episodically reduce vigour, induce partial crown mortality, trigger increased fungal attack and heart rot formation at different tree heights on the bole in a group of trees and thus increase availability of soft substrate and their likelihood of cavity excavation by the magellanic woodpecker. These results underscore the importance of drought/biotically‐induced canopy dieback events in creating habitat for woodpeckers and their dependent cavity users.     

Insect herbivory in climber–host shrubs associations: Benefit or detriment?

Spatial proximity between different plant species could modify the sign (positive or negative) of plant–herbivore interaction. The chance of a plant being detected and colonized by herbivorous insects depends not only on the plant's own traits but also on the identity of the neighbouring plants that grow with it. The closest proximity between plants occurs in climbers and their host. We conducted a field experiment to assess the effect of spatial association between a climber plant, Vicia nigricans (Fabaceae), and two host shrubs, Berberis buxifolia (Berberidaceae) and Schinus patagonica (Anacardiaceae), on insect herbivory levels, reproductive output and growth. The presence and identity of the host shrubs affected the herbivory levels of the climber V. nigricans, but not the reproductive output. For the climber, the probability of being attacked by insects could depend on the characteristics of the host shrub. Taking the opposite perspective, climber association affected different traits of the host shrubs. The association with the climber decreased leaf damage (positive), tended to decrease leaf production (negative) and did not affect reproductive output (neutral). Our findings suggest that spatial association between plant species could change the sign of the interactions between plants and insects affecting different traits. By taking into account the perspective of both plants involved in the association, this study shows and emphasizes that plant–animal interactions strongly depend on the community context.