Facilitation of Quercus ilex recruitment by shrubs in Mediterranean open woodlands

Abstract

Question: Insufficient tree regeneration threatens the long‐term persistence of biodiverse Mediterranean open oak woodlands. Could shrubs, scarce due to decades of management (clearing and ploughing), facilitate holm oak recruitment at both acorn and seedling stages? Location: Open oak woodlands in Central Spain. Methods: Plots with four acorns were planted: (1) under the canopy of the spiny shrub Genista hirsuta, (2) in a small cage, protecting against ungulates, (3) in a shaded cage, protecting against ungulates and sun, and (4) in open grassland. Sets of these four treatments were spatially grouped according to a randomised block design, with 16 blocks near (< 10 m) and 16 away from (> 20 m) parent trees to test for distance‐related survival. Plots were regularly checked for seed removal. After emergence one seedling per plot (97 in total) was selected and its survival monitored. Results: Three months after sowing, 199 of 512 acorns were removed, predominantly by rodents. Acorn removal occurred at each treatment but was highest under shrubs. Eight months after sowing, seedling survival was highest under shrubs (50%), followed by shaded cages (16%), open grassland (4%) and cages (0%). Main mortality cause was drought (90%), killing most seedlings between June and July. No seedlings died from ungulate browsing. Conclusion: Shrubs demonstrated clear net facilitative effects for Quercus ilex recruitment, despite higher seed removal. Shading appears the crucial factor facilitating seedling survival. We therefore propose that lack of shrubs contributes largely to tree recruitment failure in Mediterranean open woodlands; management should aim at conserving shrubs.

Limitation de l’installation du chêne vert (Quercus ilex subsp. ballota (Desf.) Samp.) dans un écosystème de savane forestière méditerranéenne

“…Desiccation during summer drought was also the main mortality cause for establishing oak seedlings, while no seedlings died from insect or ungulate herbivory. Previous studies have shown that shrubs, absent in savannas but omnipresent in forests, facilitate oak seedling survival particularly by shading (Smit et al, 2008). Protection of first year's tree seedlings against summer drought is of crucial importance for survival in harsh Mediterranean environments (Castro et al, 2004;Gómez-Aparicio et al, 2006).…”

Diversification of Pinus halepensis forests by sowing Quercus ilex and Quercus pubescens acorns: testing the effects of different vegetation and soil treatments

“…This effect has been well documented in the literature, which usually reports a net positive effect on survival due to the attenuation of the harsh climatic conditions prevailing in open areas, such as limitation of photo inhibition due to high radiation and attenuation of extreme temperatures and transpiration (e.g., Valladares and Pearcy 1997;Gómez-Aparicio et al 2005a;Castro et al 2006;Smit et al 2008). The absence of shrub influence can be explained by a lack of a ''shading effect'', as shading was already partly provided by the upper pine stratum intercepting 30% of incoming radiation.…”

Shrub facilitation is an important driver of alpine plant community diversity and functional composition

“…Shrub facilitation has been more commonly assessed in other habitats such as in arid or semi-arid ecosystems including Mediterranean forest (Gómez-Aparicio et al 2004;Smit et al 2009) and deserts (Silvertown and Wilson 1994;Brittingham and Walker 2000;Lopez and Valdivia 2007) and, to some extent, in tundra or steppe (Raffaele and Veblen 1998;Olofsson 2004). In the alpine areas of Australasia including in Papua New Guinea (Körner et al 1983) and Australia (McDougall and Wright 2004;Venn et al 2009) shrubs may play an important role in enhancing biodiversity by facilitating the establishment and growth of other species (McDougall and Wright 2004;Venn et al 2009).…”