Land use changes occurring during the last decades have caused new challenges in forest management of Mediterranean ecosystems, such as overpopulation of wild ungulates. This is causing serious impacts on forest regeneration, particularly increased seedling mortality of dominant 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 ( L.) in a Mediterranean open shrubland, considering three factors: herbivore exposure (with or without herbivores), seedling age (one- vs. two-year-old seedlings), and shrub cover (seedlings planted under non-palatable shrubs or in open microsites). 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 (), 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 of two-year old seedlings was higher under shrubs than in open microsites. 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 difference in herbaceous plant cover between open areas and under shrubs. Our results suggest that facilitation of seedlings by 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 (ungulate induced mortality was similar for one and two-year old oak seedlings) 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.

中文翻译：

---

高食草压力下灌木与橡树幼苗的相互作用：个体发育、干旱和植物竞争的作用

过去几十年发生的土地利用变化给地中海生态系统的森林管理带来了新的挑战，例如野生有蹄类动物的数量过多。这对森林再生造成严重影响，特别是优势树种的幼苗死亡率增加。这项研究旨在揭示在食草和干旱胁迫下，橡树幼苗是否可以通过不适口的灌木来促进，以改善橡树的辅助再生。为此，我们在地中海开阔的灌木丛中种植了一岁和两岁的常绿橡树（L.）幼苗，考虑了三个因素：食草动物暴露（有或没有食草动物）、幼苗年龄（一对两岁）一岁的幼苗）和灌木覆盖（种植在不好吃的灌木下或开放的微型场地中的幼苗）。我们在生长季节（二月至九月）每月评估幼苗的存活率和表现，并在干旱季节之前的五月测量生理特征。我们的结果显示，食草动物（主要是野猪）是 2 月至 5 月下旬死亡的主要原因，而干旱是 6 月初至 9 月死亡的主要原因。在经历了严酷的漫长夏季后，几乎所有的幼苗在种植的第一年就死亡了。一岁幼苗的叶片黎明前水势和叶片气孔导度在灌木下低于开放微场地，灌木的负面影响在两年生幼苗中并不明显。灌木下两年生幼苗的植物光合性能高于开放微场地。有趣的是，在没有食草动物的地区，灌木作为促进者的净效应（通过叶绿素含量每月测量）更大，这可能是由于开阔地区和灌木下草本植物覆盖的巨大差异。我们的结果表明，在食草压力下，灌木对幼苗的促进作用很弱，并且很大程度上取决于幼苗的年龄，因为灌木似乎主要与一岁幼苗竞争水，并减轻两岁幼苗的光抑制。这项研究揭示了在非常特定的条件下使用两年龄幼苗的可能性，但可能会出现一些重要的不便，因为它们在开放的微型场地中承受更大的压力（有蹄类动物引起的死亡率对于一年龄和两年龄的橡树幼苗来说是相似的）和与一岁幼苗相比，种植前需要更多的资源（例如时间、能源和水）。我们的结论是，在规划橡树恢复实践时必须考虑植物材料的年龄，这些实践可能受益于灌木在中等生物和非生物胁迫下的促进作用。