Shrub Interaction with Oak Seedlings Under High Herbivory Pressure: The Role of Ontogeny, Drought, and Plant Competition

Abstract

Land use changes occurring during the last decades have caused new forest management defeats in Mediterranean ecosystems, such as overpopulation of wild ungulates. This is causing severe impacts on forest regeneration, especially from increased seedling mortality altering the demography of keystone tree species. This study aims to unravel whether oak seedlings could be facilitated by non-palatable shrubs under herbivory and drought stress to improve assisted oak regeneration. For this purpose, we planted one- and two-year-old seedlings of an evergreen oak (Quercus ilex L.) inside and outside herbivore exclosures in a Mediterranean open shrubland, considering three factors: herbivore exposure, seedling age, and presence of non-palatable shrubs (Cistus ladanifer) above planted seedlings. We evaluated seedling survival and performance monthly along the growing season (February-September), and measured physiological traits in May, just prior to the drought season. Our results revealed that herbivores, mostly wild boar (Sus scrofa), were the main cause of mortality from February to late May, while drought was responsible of mortality from early June to September. Almost all seedlings died within the first year of plantation after a severe long summer. Leaf predawn water potential and leaf stomatal conductance of one-year-old seedlings were lower under shrubs than in open microsites, a negative effect of shrubs that was not evident in the two-year-old seedlings. Plant photosynthetic performance was higher under shrubs than in open microsites, but only for two-year-old seedlings. Interestingly, the net effect of shrubs as facilitators (measured monthly through chlorophyll content) was greater in areas with no herbivores, probably due to the strong reduction of competition with herbs under shrubs (5-fold taller herbaceous layer in herbivore-excluded areas compared to those exposed to herbivory). Our results suggest that facilitation of Q. ilex seedlings by C. ladanifer shrub is weak under herbivory pressure and strongly depends on seedling age as shrubs seem to mostly compete for water with one-year-old seedlings and alleviate photoinhibition in two-year-old seedlings. This study reveals the possible use of two-year-old seedlings under very specific conditions, but some important inconveniences may arise as they are more stressed in the open microsites, show similar rates of mortality by herbivores, and require more resources (e.g., time, energy and water) before plantation than one-year-old seedlings. We conclude that the age of plant material must be taken into account when planning oak restoration practices that can potentially benefit from the facilitative effects of shrubs under moderate biotic and abiotic stress.

Keywords: Plant-plant interaction, oak regeneration, wild ungulates, management, tree physiology