Ecosystems dominated by herbaceous vegetation can highlight plant-soil-water relationship and its effect on production processes. Therefore, one of the challenges in ecology is to quantify climate change effects on primary production of annuals. Results/Conclusions:

We use field data and a recently published spatio-temporally explicit model to study factors affecting long-term variation in annuals production in two water-restricted ecosystems: semi-arid and mediterranean. Both ecosystems are dominated by annual vegetation. The model was operated in both patch and landscape scales and was executed along 30 years (1979-2008) at the semi-arid site and along 21 years (1986-1990; 1993-2008) at the mediterranean site. Model predictions were validated against harvested samples that were taken from each site at the end of the growing season, in 15 seasons (1994-2008) at the semi-arid site (0.632<0.85; p<0.0001) and 6 seasons (2003-2008) at the mediterranean site (0.53<R²<0.66; p<0.001). The long-term prediction has contributed to the delineation of production boundaries under different seasonal weather conditions. 
The model was programmed for random simulations, based on climatological forecasts, of the following five climate change scenarios: 1) reduction of seasonal rainfall depth at the range 5-35%; 2) change in rainfall frequency along the wet season; 3) increase of seasonal mean temperature by 1.5Cº and evaporation rate by 10%;  4) change in rainfall distribution along the wet season; and finally 5) increase of rainfall amount at the beginning of the season at, in both Mediterranean (ME) and Semi arid (AR) sites.
The results show: 1) no significant reduction in primary production occurred as a result of a decrease in rainfall amount; 2) random change in rainfall frequency along the wet season showed significant effect on habitats stability at AR. However, it did not affect significantly the productivity in ME; 3) the other three scenarios did not affect primary production.
According to our simulations, the predicted changes in herbaceous production, as a result of the forecasted climate change, appears to be rather limited comparing to the inter-annual variations in productivity under the current climatic conditions. One may assume that water stress that the herbaceous vegetation suffers during present rainfall fluctuations in both Mediterranean and semi-arid environments make it adapted to climatic changes. Therefore, only long dry spell will significantly affect the primary production of herbaceous vegetation in these areas