Acorn crop size and pre-dispersal predation determine inter-specific differences in the recruitment of co-occurring oaks

Abstract

The contribution of pre-dispersal seed predation to inter-specific differences in recruitment remains elusive. In species with no resistance mechanisms, differences in pre-dispersal predation may arise from differences in seed abundance (plant satiation) or in the ability of seeds to survive insect infestation (seed satiation). This study aimed to analyse the impact of pre-dispersal acorn predation by weevils in two co-occurring Mediterranean oaks (Quercus ilex and Quercus humilis) and to compare its relevance with other processes involved in recruitment. We monitored the patterns of acorn production and acorn infestation by weevils and we conducted experimental tests of acorn germination after weevil infestation, post-dispersal predation and seedling establishment in mixed forests. Monitoring and experimental data were integrated in a simulation model to test for the effects of pre-dispersal predation in recruitment. In both oaks pre-dispersal acorn infestation decreased with increasing acorn crop size (plant satiation). This benefited Q. ilex which exhibited stronger masting behaviour than Q. humilis, with almost a single and outstanding reproductive event in 6 years. Acorn infestation was more than twice as high in Q. humilis (47.0%) as in Q. ilex (20.0%) irrespective of the number of seeds produced by each species. Although germination of infested acorns (seed satiation) was higher in Q. humilis (60%) than in Q. ilex (21%), this could barely mitigate the higher infestation rate in the former species, to reduce seed loss. Conversely to pre-dispersal predation, no inter-specific differences were observed either in post-dispersal predation or seedling establishment. Our results indicate that pre-dispersal predation may contribute to differences in seed supply, and ultimately in recruitment, between co-existing oaks. Moreover, they suggest that seed satiation can barely offset differences in seed infestation rates. This serves as a warning against overemphasising seed satiation as a mechanism to overcome seed predation by insects.

Ecological Implications of Acorn Size at the Individual Tree Level in Quercus suber L.

“…It is well known that predispersive acorn predation by insects is one of the constraints for the sexual regeneration of oak [13], so our results show that the endogenous consumption of seeds gives seed size an important role in the chance of seed's surviving predation.…”

“…Researchers have been interested in that variability, producing many works about the relationship between acorn size and germination or seedling production in cork oak [7][8][9] and in other species [10][11][12][13][14]. However, all of those studies of this trait have been carried out at the stand level not at the individual tree level.…”

Temporal trends in the enhanced vegetation index and spring weather predict seed production in Mediterranean oaks

“…The lack of such an effect in our oak stands could be due to several reasons: (i) the low number of acorns produced in these moderately young forests (coppiced stands of ca. 40-60 years; see Espelta et al2008Espelta et al , 2009 relative to the number produced in older forests, ii) oaks in the mild Mediterranean climate of our study area can even photosynthesise in autumn and early winter after the seeding season (Sánchez-Humanes 2009;Ogaya et al 2014), thus replenishing a certain amount of the resources spent in reproduction and allowing to preserve more leaf area, and (iii) the frequent droughts in our study area (see the values for SPEI9 in Fig. 1) may be a much more relevant factor accounting for the changes in leaf area than the amount of resources invested in acorn production.…”

“…masting, is a widely geographically and taxonomically extended reproductive behaviour (Kelly and Sork 2002). Masting events have cascading effects on several forest functions and processes (Ostfeld and Keesing 2000) such as seedling establishment (Negi et al 1996;Espelta et al 2009), the Communicated by Joy Nystrom Mast.…”

“…more favourable weather conditions for photosynthesis would lead to larger harvests (e.g. Espelta et al 2008Espelta et al , 2009). On the other hand, the resource budget model, or the resource accumulation hypothesis (Sork et al 1993;Isagi et al 1997), suggests that plants produce extraordinary seed crops when a resource threshold is reached after several years of progressive accumulation.…”