Rare events of massive plant reproductive investment lead to long‐term density‐dependent reproductive success

Abstract

Abstract The level of reproductive investment and density‐ and distance‐dependent (DDD) processes are major determinants of plant reproductive output. The reproductive investment of a plant population varies temporally, but whether and how DDD processes are affected by population‐level reproductive investment is a puzzle. We used a spatially explicit approach in order to examine DDD effects on Sorbus aucuparia crop sizes for a continuous period of 16 years. Our special interest was to investigate whether and how DDD processes affect long‐term individual plant reproductive success (PRS) and whether such processes vary between years of relatively high and low population‐level reproductive investment, measured as fruit crop size. Our study revealed that DDD processes of fruit crop relate to population‐level reproductive investment. In most years, including all years with low and most years with moderate reproductive investment, no positive or negative DDD was found for PRS. However, significant negative density‐dependent effects were found during most years of high and some years of moderate reproductive investment. During these years, the individual reproductive success decreased with increasing density of conspecifics. The overall accumulated long‐term negative density‐dependent pattern of PRS was determined by few sporadic years of high reproductive investment, rather than by the most frequent years of low or moderate reproductive investment, when the DDD effects were usually weak. Synthesis. Our study highlights the ecological relevance of relatively infrequent processes which affect plant reproductive success, stressing thus the importance of long‐term ecological research.