Abiotic conditions, neighbour interactions, and the distribution of Stipa tenacissima in a semiarid mountain range

Abiotic conditions and interactions among plants are important factors determining plant geographical distributions. With the aim of analyzing the distribution of a perennial grass species in a semiarid mountain range, we experimentally tested above- and belowground effects of shrubs on the grass along a gradient of aridity. We measured growth, physiology and reproductive output of Stipa tenacissima tussocks surrounded by intact or manipulated neighbour shrubs. Performance of Stipa improved under mesic conditions, but the net effect of shrubs on Stipa did not change along the gradient. Belowground, we did not find significant effects, suggesting belowground resource partitioning between tussocks and shrubs. Aboveground net effects of shrubs on Stipa were generally positive and independent from abiotic conditions. The improved performance of Stipa at the mesic end of gradient suggests that its absolute physiological optimum lies out of its realized range. Overall, the arid end of the geographical distribution of Stipa in this mountain range may be determined by environmental severity. The coincidence of the upper limit of Stipa with the lower tree limit suggests that the more mesic end of Stipa distribution may depend on other factors; most likely competition with trees, which, unlike shrubs, could be excluding Stipa tenacissima from forests.     

Irrigation expansion and dynamics of desertification in the Circum-Aral region of Central Asia

The aim of this paper is to examine the causes and dynamics of desertification in one of the world's worst disaster areas, the Aral Sea region. During the 1960s, a large-scale irrigation campaign aimed at achieving independence in cotton production was launched in Soviet Central Asia. From 1960, ever-increasing water withdrawal from the two inflowing rivers—the Amudarya and Syrdarya—has resulted in the dramatic decline of the level, area and volume of the sea. Desiccation was accompanied by the development and further acceleration of various desertification processes. The study reveals that, for different reasons, the predominant direction and trends of desertification have been changing during each of the four identified periods from 1961 to 1995. The main desertification processes recorded in the Circum-Aral region (‘Priaraliye’ in Russian) were a decline in the groundwater level, increased mineralization and chemical pollution of watercourses, soil salinization, the spread of xerophytic and halophytic vegetation, and deflation and aeolian accumulation, with the development of salt storms. Recent improvements in the situation are also discussed, along with their causes. Zonation of Priaraliye is carried out and an outlook for the future is given.     

Aridity and the Potential Physiological Response of C 3 Crops to Doubled Atmospheric Co 2 : A Simple Demonstration of the Sensitivity of the Canadian Prairies

Since cultivated annual C₃ field crops cover about50% of the land surface of the Canadian Prairie grassland eco-climatic zone, this vegetationinfluences the aridity of the climate during the growing season. The physiological response of these cropsto a doubling of the atmospheric concentration of CO₂ may be a doubling of canopyresistance. If this physiological effect is not counteracted by interactive feedbacks, such as increasedleaf area, evapotranspiration rates could be reduced. To demonstrate the sensitivity of thearidity of the Prairie climate to this potential physiological effect, representative spring wheatgrowing-season soil moisture and Bowen ratio curves for a doubled canopy resistance(2 × CO₂) scenario were compared with a control (1 × CO₂) scenario.Lower evapotranspiration in the 2 × CO₂ scenario: (1) Increased root-zone soilmoisture levels, and (2) weakened the atmospheric component of the hydrologic cycle by raisingBowen ratios, which reduces the convective available energy, and reduces the regionalcontribution to the atmospheric water vapour over the Prairies. A weakened hydrologic cycleimplies less rainfall, and possibly, lower soil moisture levels. Thus, the net impact of a doublingof the atmospheric concentration of CO₂ on the aridity of the Canadian Prairies is uncertain.This simple sensitivity demonstration did not consider most of the potential feedback mechanisms,nor interactions of other processes. Nevertheless, the result illustrates that the physiologicaleffect should be explicitly included in climate change models for the Canadian Prairies.     

Hydroclimatology of the Volta River Basin in West Africa: Trends and variability from 1901 to 2002

Long-term historical records of rainfall (P), runoff (Q) and other climatic factors were used to investigate hydrological variability and trends in the Volta River Basin over the period 1901-2002. Potential (E_p) and actual evaporation (E), rainfall variability index (δ), Budyko’s aridity index (I_A), evaporation ratio (C_E) and runoff ratio (C_Q) were estimated from the available hydroclimatological records. Mann-Kendall trend analysis and non-parametric Sen’s slope estimates were performed on the respective time series variables to detect monotonic trend direction and magnitude of change over time.Rainfall variability index showed that 1968 was the wettest year (δ = +1.75) while 1983 was the driest (δ = −3.03), with the last three decades being drier than any other comparable period in the hydrological history of the Volta. An increase of 0.2 mm/yr² (P < 0.05) was observed in E_p for the 1901-1969 sub-series while an increased of 1.8 mm/yr² (P < 0.01) was recorded since 1970. Rainfall increased at the rate of 0.7 mm/yr² or 49 mm/yr between 1901 and 1969, whereas a decrease of 0.2 mm/yr² (6 mm/yr) was estimated for 1970-2002 sub-series. Runoff increased significantly at the rate of 0.8 mm/yr (23 mm/yr) since 1970. Runoff before dam construction was higher (87.5 mm/yr) and more varied (CV = 41.5%) than the post-dam period with value of 73.5 mm/yr (CV = 23.9%). A 10% relative decrease in P resulted in a 16% decrease in Q between 1936 and 1998. Since 1970, all the months showed increasing runoff trends with significant slopes (P < 0.05) in 9 out of the 12 months. Possible causes, such as climate change and land cover change, on the detected changes in hydroclimatology are briefly discussed.