Unravelling conflicting density- and distance-dependent effects on plant reproduction using a spatially explicit approach

Density- and distance-dependent (DDD) mechanisms are important determinants of plant reproductive success (PRS). Different components of sequential PRS can operate either in the same or in different directions and thus reinforce or neutralize each other, and they may also operate at different spatial scales. Thus, spatially explicit approaches are needed to detect such complex DDD effects across multiple PRS components and spatial scales. To reveal DDD effects of different components of early PRS of the Iberian pear (Pyrus bourgaeana) sampled over three consecutive years, we used marked point pattern analysis. Our special interest is to identify conflicting processes that regulate populations at different spatial scales, for example whether DDD on fruit initiation and on fruit development acted in opposite directions. To evaluate the significance of observed mark correlation functions based on empirical data (e.g. fruiting success), we compared them to expectations given by spatially explicit null models. Diverse DDD processes affected several aspects of PRS in a variable extent over the three seasons. First, fruit initiation was higher for individuals with more neighbours at small distances (i.e. up to 40m). However, P.bourgaeana fruit development decreased with increasing number of nearby neighbours, but these effects cancelled for overall fruit set that did not show DDD effects. Secondly, the absolute number of fruits produced (crop sizes) by trees showed positive density dependence in 2011 and 2012 but not in 2013. Finally, the total number of seeds produced did not show DDD effects, indicating that conflicting demographic processes can disrupt the initial spatial pattern of tree investment in reproduction.Synthesis. Understanding complex spatial effects of DDD processes requires dissection of component processes to attain the complete picture since contrasting DDD processes may be hidden behind a single cumulative measure of reproductive success. The combination of novel and classic mark correlation functions used here constitutes a powerful spatially explicit tool that can be broadly applied to unravel conflicting mechanisms of DDD regulating the persistence of sessile organisms at a range of spatial scales. Our findings help to explain why some authors failed to find expected DDD of PRS and highlight the importance of detailed multiyear field studies on plant reproductive success.