Onset Of Severe Drought Research Articles

The Discovery of African Dust Transport to the Western Hemisphere and the Saharan Air Layer: A History

AbstractThere is great interest in wind-borne mineral dust because of the role that dust plays in climate by modulating solar radiation and cloud properties. Today, much research focuses on North Africa because it is Earth’s largest and most persistently active dust source. Moreover, this region is expected to be greatly impacted by climate change, which would affect dust emission rates. Interest in dust was stimulated over 50 years ago when it was discovered that African dust was frequently transported across the Atlantic in great quantities. Here we report on the initial discovery of African dust in the Caribbean Basin. We show that there were three independent “first” discoveries of African dust in the 1950s through the 1960s. In each case, the discoverers were not seeking dust but, rather, they had other research objectives. The meteorological context of African dust transport was first elucidated in 1969 with the characterization of the Saharan air layer (SAL) and its role in effecting the efficient transport of African dust over great distances to the Western Hemisphere. The link between dust transport and African climate was established in the 1970s and 1980s when dust transport to the Caribbean increased greatly following the onset of severe drought in the Sahel. Here we chronicle these events and show how they contributed to our current state of knowledge.

Selfing and Drought-Stress Strategies Under Water Deficit for Two Herbaceous Species in the South American Andes.

Angiosperms are highly diverse in their reproductive systems, including predominantly selfing, exclusive outcrossing, and mixed mating systems. Even though selfing can have negative consequences on natural populations, it has been proposed that plants having a predominantly selfing strategy are also associated with fast development strategies through time limitation mechanisms that allow them to complete their life cycle before the onset of severe drought. This relationship might be affected by the challenges imposed by global change, such as a decrease in pollinator availability and the earlier and more severe onset of droughts. In this work, our aim was to investigate whether selfing is correlated with a dehydration avoidance strategy, and how this could affect drought resistance and survival in two species with different types of selfing: pollinator-independent delayed selfing (Schizanthus grahamii) and pollinator-dependent selfing (Schizanthus hookeri), representing a gradient in selfing rates. We hypothesize that delayed selfing species and highly selfing populations will show “fast” plant traits whereas we will find no pattern in more outcrossed populations of the pollinator-dependent species. However, we predicted that high selfing populations would have lower survival rates when exposed to chronic drought early in their development since fast traits imply physiological compromises that will affect their drought survival. To evaluate these hypotheses, we characterized different physiological and morphological traits in response to two contrasting treatments (moist and dry) in a total of six populations of the two species. We found a relationship between the delayed selfing species and a dehydration avoidance strategy and also with low drought survival. Our work offers evidence to support the importance of abiotic factors, such as drought, on the possible variation in selfing rates on natural populations, and the effect that this mating system could have in their ability to face new environmental conditions such as those imposed by climate change.

Seedling recruitment and survival of an endangered limestone endemic in its natural habitat and experimental reintroduction sites

The largest and most fecund population of the endangered Purshia subintegra is restricted to limestone mesas in Verde Valley, Arizona, USA, where habitat destruction is imminent. To examine factors limiting its distribution and potential for expansion, we compared recruitment and survival of seedlings growing in soils from occupied and unoccupied habitat in caged field experiments and compared survival of caged and wild seedling cohorts from 1998 to 2003. In field tests, seeds germinated in soils from occupied and unoccupied habitats. Seedling survival, however, was greatest in currently occupied habitat and dropped to zero in some unoccupied habitats with the onset of severe drought. Among 16 factors measured, soil moisture significantly explained between 62% and 71% of the variation in recruitment in both wild and caged plots. Shrubs conferred protection to wild seedlings, but decreased caged seedling survival. For 5 yr following germination, caged seedlings had greater survival than natural seedling cohorts indicating that reintroduction was comparatively more successful than natural recruitment. Expansion of P. subintegra into novel habitats is limited by soil moisture capacity, and this condition varied during the experimental time frame. Reintroductions to limestone mesas are possible and most promising if cages and supplemental watering are used.

AUTUMN NITROGEN FERTILISER IN A DRYLAND SHEEP SYSTEM

Applying 50 kg N/ha to dryland ryegrass-white clover pasture in autumn in each of four years resulted in an increased carrying capacity over the control for the last two years of 137 kg liveweight per ha at minimum in December, with an early spring peak of 211 kg extra liveweight per ha. Despite higher stocking rates on N-treated pastures in the later years. feed on offer at grazing was greater from early winter until the onset of severe drought, typically 8 months later. This improved level of feeding lifted ewe bodyweights post lambing by at least 4 kg. These higher ewe body weights were associated with significantly higher birth weights of twin lambs. Daily growth rates of these lambs. in a flock averagmg better than 175% lambing, were in excess of 260 g/head when ewe bodyweights were over 60 kg post lambing. Keywords: Nitrogen fertiliser, sheep grazing, lamb growth rates