Light limitation shapes the community seed mass of annual but not of perennial weeds

How seed mass determines plant performance is dependent on the type of herbaceous community. Here, we hypothesized that life span is a decisive driver of the seed mass response to an environmental gradient. We examined whether separating community data into annuals and perennials sheds new light on seed mass importance in herbaceous weed vegetation of arable fields. We studied the seed mass response to a gradient of light limitation and tested the prediction that seed mass will increase with light limitation but that the trend will differ in annuals compared to perennials. In summary, only the seed mass of annuals reacted to the light limitation gradient. The seed mass community-weighted mean (CWM), controlled for crop type and seasonality, was positively linked with crop coverage, i.e., a proxy for light limitation. The seed mass CWM of perennials exhibited a random distribution. In annuals which are strongly dependent on seeds, large seeds are advantageous under dense crop canopies. We showed that considering the relevance of a trait for a particular strategy can improve our understanding of community assembly. This approach can help to explain some differences among published studies regarding the effect of an environmental gradient on the community seed mass.