The use of shrub cover to preserve Mediterranean oak dehesas: a comparison between sheep, cattle and wild ungulate management

Abstract

Questions Oak savanna‐like systems (dehesas) are highly valuable and protected systems that are strongly compromised by a continuous failure of oak recruitment. Our question is whether different long‐term management types involving cattle, sheep and wild ungulates affect the abundance and diversity of safe microsites (shrubs) for oak recruitment. We also assess microsite location and species‐specific differences among the nurse shrubs in order to provide management guidance. Location Dehesas of holm oak (Quercus ilex) in central Spain. Methods We examined oak regeneration in four different microsites (tree canopy, shrub cover, open and tree–shrub) and across three distinctive and representative management types for a period of, at least, 30 yr: (1) traditional management (i.e. extensive sheep rearing); (2) high commercially competitive management (extensive cattle); and (3) wildlife management (wild ungulate grazing, mostly deer). Results Wildlife management showed higher abundance and diversity of shrubs. Predicted seedling density under wildlife management was three‐fold higher than sheep, and almost nine‐fold higher than under cattle management. Highest oak density was found under shrubs that grew beneath trees, twice as high than under trees, ten‐fold higher than under shrubs outside the tree canopy, and almost 40‐fold higher than in open microsites. However, the proportional facilitative effect of shrubs was higher in open areas than under trees. Three shrub species (Erica australis, Genista hirsuta and Rosmarinus officinalis) showed significantly higher probability of facilitating oak seedlings in comparison to other shrub species, indicating species‐specific differences in plant facilitation. Most young oak trees (saplings and juveniles) exceeded the average shrub height, suggesting that facilitation occurs mostly at the seedling stage. Conclusions Only dehesas under reasonable wildlife management showed a clear three‐layered structure with a significant representation of the shrub layer, which increased seedling density. Current cattle management was the least sustainable in terms of oak regeneration. Even though shrub cover, in general, facilitated oak seedlings, not all species were equivalent facilitators. Only three low‐palatable and large‐sized shrub species were keystone facilitators that should be especially considered in restoration and management practices. Further agriculture policies should promote reduction of cattle densities, rotational grazing and mixed species management to enhance the regeneration of these systems.

Differential tree recruitment in California oak savannas: Are evergreen oaks replacing deciduous oaks?

“…Tree cover was also an important microsite for regeneration but showed lower recruitment rates and greater herbivory damage (both occurrence and intensity) than under shrub cover. These results confirm that shrubs provide a complementary facilitation against browsers since abiotic facilitation (mainly reduction in water stress by shading; Smit et al, 2008) is common under trees and shrubs, or possibly greater under the dense shade of trees (Perea et al, 2016).…”

“…Callaway (1992) found shrub facilitation on Q, douglasii seedlings but not on Q, lobata seedlings. However, most Mediterranean oak species, both evergreen and deciduous, have been proved to benefit from shrubs, particularly in dry environments (Gomez-Aparicio et al, 2004;Smit et al, 2008), or when browsing pressure is high (Perea and Gil, 2014b;Perea et al, 2016).…”

“…However, throughout the world, a lack of natural regeneration has been reported in many oak woodlands, forests and savannas of Asia, Europe and North America (Muick and Bartolome, 1987;Plieninger, 2006;Tyler et al, 2006;Zavaleta et al, 2007;Campos et al, 2013;Lopez-Sanchez et al, 2016). This shortage of tree recruitment is critical and represents a long-recognized threat for the persistence of many scatteredoak woodlands -known as oak savannas -worldwide (Griffin, 1971;McClaran and Bartolome, 1989;Manning et al, 2006;Fischer et al, 2009;Bergmeier et al, 2010;Perea et al, 2016). The failure of oak recruitment has been mostly attributed to the high intensity and permanent herbivory pressure (browsing) exerted by wild and domestic ungulate overgrazing over the last decades (Bartolome et al, 1987;Pulido et al, 2001Pulido et al, , 2010Plieninger et al, 2003;Tyler et al, 2006Tyler et al, , 2008Perea et al, 2014;Lopez-Sanchez et al, 2014, 2016, although other factors have also been identified, including competition with grasses (Griffin, 1971;Gordon and Rice, 2000), and increased drought stress (Gomez-Aparicio et al, 2008a;Mahall et al, 2009;McLauglin and Zavaleta, 2012;Davis et al, 2016) among others.…”

“…Previous work on oak regeneration in Mediterranean environments has shown that regeneration microsites are crucial for successful recruitment. At many sites, shrub cover has been found to facilitate oak recruitment (Callaway, 1992;Callaway and Davis, 1998;Pulido and Diaz, 2005;Zavaleta and Kettley, 2006;Perea et al, 2016), mostly due to the "nursing" effects of some shrubs against herbivory and desiccation (Callaway and D'Antonio, 1991; Gomez-Aparicio et al, 2008b; Perea and Gil, 2014a, 2014b). However, other studies have found no evidence of shrub facilitation for certain oak species (e.g., Q, lobata), while regeneration of other sympatric species (Q, douglasii and Q, agrifolia) seems to be strongly associated with the presence of shrubs (Callaway, 1992;Callaway and Davis, 1998).…”

Thorny Shrubs Limit the Browsing Pressure of Large Herbivores on Tree Regeneration in Temperate Lowland Forested Landscapes

“…Recently, awareness of MTAs as important biodiversity refuges has increased [33,34]. In contrast with studies which have presented the results of interactions between large domesticated herbivores and the regeneration of forest tree species [35,36], our study aimed to reveal the interactions among large wild herbivores, thorny bushes, and the natural regeneration of forest tree species. The aim of this study was to test the effects of large herbivores and thorny shrubs on tree regeneration in a current landscape in order to understand how patterns may play a key role.…”