Most ecological studies in North Africa reveal a process of continuous degradation of rangeland ecosystems as a result of overgrazing. This degradation appears across the decreasing of perennial grass diversity. Indeed, the majority of steppe ecosystems are characterized by a low density of perennial grass species at present. This study evaluated the effects of temperature and water limitation on the seed germination of Stipagrostis ciliata (Desf.) de Winter, a perennial grass species. The seeds were collected from the Bou Hedma Park, Sidi Bouzid Governorate, Central Tunisia. The thermal time and hydrothermal time models were used to describe the seed germination of S. ciliata under different water potentials and temperatures. The germination response of S. ciliata seeds in darkness was evaluated over a range of temperatures (15°C, 20°C, 25°C, 30°C and 35°C) and across a wide range of osmotic potentials (0.0,-0.2,-0.6,-1.2,-1.6 and-2.0 MPa) of the polyethylene glycol (PEG 6000) solutions at each temperature level. Among the tested temperatures, 25°C was found to be the optimal temperature to the germination of S. ciliata seeds. The final germination percentage (75.2%) was obtained with distilled water. The progressive decrease of osmotic potential of the PEG 6000 solutions inhibited the seed germination. However, the number of days to first germination was increased with a reduction of osmotic potential. A significant positive relationship was identified between final germination percentage of S. ciliata seeds and osmotic potential of the PEG 6000 solutions, with R 2 ranging from 0.5678 to 0.8761. Furthermore, a high degree of congruency between predicted and observed germination time course curves was observed. In general, S. ciliata exhibits a significant adaptation capacity for water limitation and high temperature in arid ecosystems.

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This paper proposes a nonlinear average model of three levels NPC inverter. This model takes into account the semiconductor non-linearity and static characteristics of these devices. The proposed technique uses experimental results to evaluate the different devices parameters. The developed nonlinear average model allows dissipated power and junction temperature evaluation. Unlike the circuit model (fine model), the proposed method has less computational burden. It can be applied to three levels Neutral-Point-Clamped inverter (NPC).

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