Alireza Taab 
Weeds and their Ecological Functions 

Weeds are problematic both in agricultural and nonagricultural areas so that they potentially cause economic losses. Weed interference in crops also date back to the beginning of agriculture and they have been able to persist, in spite of long term control operations. They are able to adapt and survive in the environment due to their specific characteristics. Thus, to control weeds efficiently knowledge of weed biology, in particular their survival mechanisms, ecological responses and interactions, is of important value in weed management systems. "Weeds and their Ecological Functions" provides information on weeds functional behaviors in their cultivations including weed seed dormancy, germination and seedling emergence, examples of beneficial and detrimental interactions between weeds and other organisms, weed behaviors in perennial crops and response of weeds to cover crops. This information is useful for students and weed scientists to analyze some of the functional behavior of weeds and to develop integrated weed management systems. Nowadays, there is a remarkable variability in environmental factors like temperature and precipitation because of climate changes around the globe. Weed and crop growth are substantially influenced by changes in the environment. Therefore, increased knowledge is needed to develop predictive tools to quantify the consequences of the changes. Moreover, weed control in the early stage of growth is of high importance. Factors regulating seedling emergence characteristics of the weeds in the field need to be well understood. The aspects of the seed dormancy system such as the effect of maternal environmental factors on primary dormancy, dormancy induction and dynamics, germinability and seedling emergence characteristics of weeds, are reviewed and discussed. Discussion is provided on the possible ecological and agricultural implications of weed seed ecophysiology. In addition, examples of seedling emergence behavior of summer and winter annual weed species are presented. Information is provided on the presence of some weeds in perennial crops that are not always harmful. Some weeds especially those characterized as perennial or invasive are highly competitive and could cause problems to the crops. However, the presence of some other weeds should be clearly reconsidered regarding their utility as cover species and their beneficial impacts. Such an approach could certainly maintain and enhance the profitability of sustainable agricultural systems like organic or low input agriculture. Cover cropping is a technique to improve soil fertility and to ensure yield stability. An overview is given on cover cropping typologies and their significance in different agricultural contexts, including their effects on physical and chemical soil characteristics, as well on the biocenotic components of the agroecosystems. The main characteristics of the better-suited species for cover cropping purposes are also reported. There are possible interactions between weeds and other organisms in the environment. To understand the role of weeds in agroecosystems, weed interactions with other biotic compartments including nematodes, plant pathogens, arthropod communities, and a crop are also presented.