Drivers of natural colonisation and regeneration within planted woodlands in England: towards an integrated approach to increase resilience

Increasing woodland cover is at the heart of global and national strategies to combat climate change. While tree planting is a rapid and scalable means of woodland creation, it remains expensive and prone to high mortality, highlighting the need for additional more cost-effective methods including natural colonisation and regeneration. A key question is whether trade-offs between the advantages of rapid planting and natural colonisation can be avoided by using a combined approach. Yet few studies have quantified ongoing colonisation and regeneration within planted woodlands and the habitat and management drivers influencing these processes. This study explores the relative importance of seed source (parent candidate) trees, woodland habitat, management, browsing, and scrub variables in influencing natural colonisation and regeneration within 57 planted woodlands across East Anglia, UK. We test for these effects on three species of colonising tree sapling, and the total number and species richness of all tree saplings (colonising and regenerating). Fraxinus excelsior and Acer campestre, showed parent candidate (PC) variables to have greater influence on colonising sapling abundance than woodland or management variables, while effects were much weaker for Quercus robur. Woodland age was a positive predictor for A. campestre saplings, more equivocal for F. excelsior, and had no influence on Q. robur. However, woodland age had a strong positive influence on species richness of tree saplings and on percentage cover and species richness of scrub. Canopy cover was the strongest predictor of total tree sapling abundance. Management had negative effects on Q. robur, total number of saplings and scrub coverage. Species compositional analyses showed most sapling taxa were associated with older, unmanaged, more diverse plantations, with greater canopy cover and scrub species richness. We conclude that substantial natural colonisation occurs within planted woodlands, complementing regeneration and tree planting in enhancing woodland creation. Further research on the potential benefits of combining active and passive methods of afforestation is needed to ensure woodland creation and management is optimised for biodiversity and ecosystem services.