Tree-shrub associations in grazed woodlands: first rodents, then cattle?

Abstract

Spatial associations of tree saplings with spiny or toxic plants in grazed woodlands are generally explained by associational resistance, i.e., protection against grazing via a well-defended neighbor. In this study, we tested whether directed seed dispersal and post-dispersal seed removal by wood mice are additional explanations for the observed spatial association between thorny shrubs and trees, hence before associational resistance. We performed three studies in grazed woodlands in the Netherlands to test this idea. Our first seed dispersal experiment with tagged acorns showed indeed that wood mice disperse acorns directed towards shrubs. The majority of these dispersed acorns were, however, consumed. Our second experiment revealed that post-dispersal removal of cached acorns was higher under shrubs than in grassland and under trees, but also indicated the importance of within-shrub position: shrub centers were faster depleted than shrub edges. Also the number of freshly emerged seedlings and older saplings differed between micro-habitats, being higher under trees and at shrub inner-edges than at shrub outeredges, shrub centers, and in grassland. The spatial associations with shrubs got stronger with the age of saplings, which probably reflect accumulated grazing effects over time, being higher in the open than in shrubs. We conclude that directed acorn dispersal and post-dispersal acorn removal by wood mice are two additional explaining mechanisms for the spatial associations between thorny shrubs and trees in grazed woodlands, before associational resistance. Our findings change the view that large herbivores are the sole main drivers behind tree recruitment patterns in grazed woodlands.

Seed predation patterns favor the regeneration of dominant species in forest gaps compared with the understory in an oak-pine mixed forest

“…We also found that the CU proportion was significantly lower than CG when checked at 30 days and the next spring, although initially higher in CU than in CG after 5 days. Previous studies have shown that not only dispersal but also removal of seeds is higher under dense vegetation such as shrubs (Iida 2006;Pérez-Ramos and Marañón 2008;Smit et al 2006, Smit andVerwijmeren 2011), which is again mostly attributed to reduced predation risk (Manson and Stiles 1998). Our results indirectly have demonstrated that cache dynamics were lower in gaps than in the understory, which was beneficial to seeds of light-demanding species.…”

“…Ample evidence suggests that the predation risk may affect the seed-hoarding behavior of rodents (Li and Zhang 2003;Lu and Zhang 2004) and subsequent seedling recruitment (Duncan and Chapman 1999;Li and Zhang 2003). Thus, studies of the seed dispersal patterns in and around open woodlands are important for evaluating the role of seed dispersal by small rodents during the natural regeneration of woody plants (Scheper and Smit 2011;Smit and Verwijmeren 2011).…”

Facilitation of holm oak recruitment through two contrasted shrubs species in Mediterranean grazed woodlands

“…In addition, large sapling abundance in Retama formations was much more responsive to small sapling abundance compared to that of Cistus formations (Table ), suggesting a cumulative effect under Retama . Smit & Verwijmeren () reported similar results in a grazed wood–pasture system in presence of blackthorn, arguing that a cumulative grazing effect can lead to a strong spatial association between saplings and shrubs over time. The associational resistance theory states that unpalatable species (spiny or toxic) can facilitates palatable species by protecting against herbivores (Olff et al.…”

“…Seedling emergence beyond the canopy edge tended to be higher under shrubs than in open spaces. This result suggests an active dispersal of acorns toward shrubs, rather than a passive dispersal, which is common under tree canopies (Smit & Verwijmeren ). In the study area, holm oak mostly relies on rodents for the dispersal of acorns in open woodlands (Muñoz et al.…”

Leaf litter affects the survival and predation rates for large and small Pinus seeds in the Qinling Mountains, China

“…Numerous biotic and abiotic factors, e.g., predation, habitat, and season, may affect the seed hoarding behavior of rodents (Li and Zhang 2003;Lu and Zhang 2004;Lai et al 2013), and subsequent seedling recruitment (Duncan and Chapman 1999;Li and Zhang 2003;Scheper and Smit 2011;Smit and Verwijmeren 2011;Yu et al 2013a). In particular, the substrate upon which an animal feeds strongly influences foraging behavior (Kotler et al 2001).…”