Ecological anachronisms in the recruitment of temperate light‐demanding tree species in wooded pastures

Abstract

Summary1. Light-demanding trees and thorny shrubs in temperate plant communities may reflect adaptations to now-extinct large grazers, such as aurochs and tarpans, rendering these adaptations ecological anachronisms. 2. We explored the ecological functions of plant traits of Quercus robur and Prunus spinosa in areas grazed by cattle and horses, the domesticated descendants of aurochs and tarpans. Specifically, we tested the hypothesis that grazing induces a shifting mosaic of grassland, shrub thickets and woodlands through the key process of associational resistance: the protection of palatable young trees by thorny shrubs.3. An exclosure experiment with transplanted Q. robur seedlings revealed that Q. robur grew best in grassland exclosures and on the edge of thorny shrub thickets, which may be viewed as an optimal balance between sufficient protection from large herbivores and sufficient light availability. 4. A cross-site comparison of four floodplain woodlands in north-western Europe showed that Q. robur can regenerate in the presence of large herbivores through spatial association with P. spinosa . However, we found that expansion of P. spinosa shrubs and Q. robur coincided with periods of low rabbit abundance and not with livestock density. From this, it appears that the process of associational resistance does not work with rabbits. 5. Synthesis and applications . With extensive grazing by large (domesticated) grazers in temperate floodplains, a shifting mosaic of grassland, shrubs and trees may develop that has high conservation value. Palatable, light-demanding Q. robur seedlings can successfully regenerate in spiny P. spinosa shrubs through associational resistance. This process does not offer protection from abundant small herbivores, such as rabbits, that can inhibit the recruitment of shrubs and trees in this mosaic vegetation. In floodplain meadows frequent flooding may be an efficient way to reduce rabbit populations, with dry conditions in summer and wet in winter. When floodplain meadows are combined with adjacent higher grounds, large herbivores can escape the floods through migration.

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

“…The site is situated on a riverine floodplain along the Overijsselse Vecht and consists of a mosaic of short grazed grasslands (dominated by Festuca Rubra and Agrostis capilaris), thickets of dense shrubs (dominated by blackthorn), and isolated or small groups of trees (dominated by pedunculate oak). The oaks are spatially associated with blackthorn shrubs (Bakker et al 2004); dense thickets of shrubs surround these trees, with palatable grasses reappearing under the older trees. The Junner Koeland area is one of the few remaining examples of the natural vegetation that was once present along most large rivers in temperate Western European lowlands before canalization and intense agriculture (Bakker et al 2004).…”

“…Grazed woodlands are ecosystems consisting of a mosaic of grassland patches, thickets of spiny shrubs, and isolated or clustered trees, often growing within these shrub thickets (Bakker et al 2004). Within this high structural diversity, a rich biodiversity in both plant and animal species can develop (Olff et al 1999;Vera 2000).…”

“…Within this high structural diversity, a rich biodiversity in both plant and animal species can develop (Olff et al 1999;Vera 2000). Several studies show that low-intensity grazing plays an essential role in maintaining the rich structure and associated biodiversity in these systems (Olff et al 1999;Bakker et al 2004;Bakker et al 2006;Smit et al 2006a). Introducing large herbivores is therefore an increasingly used method throughout Europe since the last two decades (Van Uytvanck et al 2008a).…”

“…Grazers are hypothesized to play a key role in this succession by damaging palatable tree saplings within grassland but not within spiny shrubs. Several empirical studies have indeed shown that unpalatable species-spiny or toxic plants-protect tree saplings from ungulate damage (Smit et al 2006a;Van Uytvanck et al 2008b), resulting in spatial associations between young palatable trees (e.g., pedunculate oak Quercus robur) and spiny or toxic plants (e.g., spiny blackthorn Prunus spinosa) (Bakker et al 2004;Smit et al 2005;Van Uytvanck et al 2008a). This process in which an unpalatable species facilitates a palatable species by reducing the amount of herbivory is called associational resistance (Hamback et al 2000;Milchunas and Noy-Meir 2002) and is considered an important factor in the proposed cyclic succession in grazed woodlands (Olff et al 1999).…”

“…The high spatial variety in microhabitats in grazed woodlands, with alternating patches of grassland, shrubs, and trees, creates high variability in conditions for tree recruitment. For example, the differences in light intensities (Arrieta and Suárez 2005) and grazing pressure (Bakker et al 2004) between microhabitats largely affect the establishment success during the seedling and the sapling stage. However, also processes at the seed stage, such as seed dispersal and seed predation, may vary between these different microhabitats, and so strongly affect emergence and survival.…”