What limits forest plant migration along wooded corridors?

Europe’s natural forest is heavily fragmented by agricultural land. Such isolation threatens the long-term persistence of forest biodiversity. The concept of patch-corridor-matrix system suggests that isolated patches can be ecologically connected by a corridor network. As forest-specialist plants (termed also as “ancient forest plants”) are adapted to a stable forest environment and are considered to be poor long-distance dispersers, their use of wooded corridors hasn’t got much attention.

We asked why? As in the long run even slowly dispersing species should eventually immigrate into corridors. Maybe the edge effect limits the establishment? Or is the positive expectation on the functionality of corridors driven by the response of habitat generalist species?

To avoid the early-successional underestimation of arrival success, we concentrated our study on older (>50 years) wooded corridors. The isolation effect was evaluated by sampling two types of corridors: directly connected to source forest and isolated mid-field corridors, and margins of ancient forests were used as a comparison habitat (as they are also prone to edge effects).

Forest connecting alley near Vara, Estonia (pic by Taavi Paal)

Overall, forest generalists prevailed over a small number of forest specialists in wooded corridors. The connectivity/isolation effect ruled over the edge effect as the number of forest specialists in connected corridors was analogous to forest edges. The expectation of ineffective immigration was met only in isolated corridors which harboured a few forest specialists, but still many generalists. Tested in other terms, forest specialists depended on historical stability of the surrounding forest landscape, while forest generalists reflected the status of present-day landscape configuration. The poorly detected edge effect was probably suppressed by overall canopy closure of overstorey and the width of the corridor. In semi-open corridors forest specialists were outcompeted by open habitat plants.

What can be done in order to improve the environment of corridors for forest specialists? As forest specialists cannot cope with rapid changes in forest landscape configuration, wide corridors with suitable shaded conditions and directly connected to ancient forests should be maintained for many decades. Ecologically functioning of wooded corridors can be ensured with the width of at least two lines of mature trees and with overhanging side branches. Ground disturbances should also be kept to a minimum (e.g. ditch construction, frequent mowing) as they promote generalist species. However, as the formation of shade providing overstorey takes decades, historical alleys should be sustainably managed and their connectivity with forests should be improved.

Citation: Paal, T., Kütt, L., Lõhmus, K. & Liira, J. 2017. Both spatiotemporal connectivity and habitat quality limit the immigration of forest plants into wooded corridors. Plant Ecology (in press). doi: 10.1007/s11258-017-0700-7 (link to full text)

Anemone nemorosa growing in an alley (pic by Taavi Paal)

Abstract:

Extensive afforestation of agricultural areas has increased the importance of green corridors as a dispersal network. We tested the effect of spatiotemporal connectivity, edge effect and habitat structural quality of wooded corridors on the long-term immigration success of forest specialist plants relative to the performance of forest generalists. In agricultural landscapes of central and southern Estonia, we sampled 28 historically connected and 52 isolated tree lines and alleys with a minimum age of 50 years, and 93 edges of ancient forests. The regional pool of common forest plants was compiled using species’ frequency data in 91 ancient forests. Both landscape connectivity and habitat quality affected the richness of response groups, but specialists and generalists responded to different drivers. Forest specialists required long-term neighbourhoods of ancient forest and benefited from a direct connection between forest and corridor. Habitat generalists reacted positively to the recently modified structure of the landscape. When a corridor was connected to forest, the dual edge in the corridor did not result in an increased negative edge effect on forest specialist arrival. Even if both specialists and generalists required wide corridors with optimum shade, forest specialists also benefited from mature overstorey and outward overhanging branches, whereas forest generalists used disturbance-created microhabitats. We conclude that only wooded corridors with long-term connectivity to seed source forests and widely branched tree canopies will function as a green infrastructure supporting forest-specific biodiversity.