Using historical woodland creation to construct a long-term, large-scale natural experiment: the WrEN project

Abstract

Natural experiments have been proposed as a way of complementing manipulative experiments to improve ecological understanding and guide management. There is a pressing need for evidence from such studies to inform a shift to landscape-scale conservation, including the design of ecological networks. Although this shift has been widely embraced by conservation communities worldwide, the empirical evidence is limited and equivocal, and may be limiting effective conservation. We present principles for well-designed natural experiments to inform landscape-scale conservation and outline how they are being applied in the WrEN project, which is studying the effects of 160 years of woodland creation on biodiversity in UK landscapes. We describe the study areas and outline the systematic process used to select suitable historical woodland creation sites based on key site- and landscape-scale variables - including size, age, and proximity to other woodland. We present the results of an analysis to explore variation in these variables across sites to test their suitability as a basis for a natural experiment. Our results confirm that this landscape satisfies the principles we have identified and provides an ideal study system for a long-term, large-scale natural experiment to explore how woodland biodiversity is affected by different site and landscape attributes. The WrEN sites are now being surveyed for a wide selection of species that are likely to respond differently to site- and landscape-scale attributes and at different spatial and temporal scales. The results from WrEN will help develop detailed recommendations to guide landscape-scale conservation, including the design of ecological networks. We also believe that the approach presented demonstrates the wider utility of well-designed natural experiments to improve our understanding of ecological systems and inform policy and practice.

Keywords: Ecological networks; WrEN project; landscape‐scale conservation; natural experiment; woodland creation.

Figure 1

Location of woodland study sites in the Scotland (n = 67) and England (n = 39) WrEN study landscapes, showing location of these study landscapes in the UK.

Figure 2

An example WrEN woodland from the England study landscapes showing the use of historical mapping to establish the age of woodland creation. The woodland labeled as “Eleven Acre Covert” appeared on the maps between 1890 and 1900, making it approximately 115 years old (EDINA © Crown Copyright 2014. An Ordnance Survey/EDINA supplied service http://digimap.edina.ac.uk/).

Figure 3

Distribution (rank order) of the four site‐scale variables: (i) patch area; (iia) patch characteristics – tree species richness; (iib) patch characteristics – tree density; and (iii) patch age in years since creation for the 106 WrEN woodland sites in Scotland (n = 67) and England (n = 39).

Figure 4

Distribution (rank order) of the four landscape‐scale variables: (iv) surrounding habitat – proportional cover of woodland within a 3‐km buffer; (v) spatial isolation – distance to nearest broadleaved woodland in meters; (via) surrounding matrix – proportional cover of agricultural land within a 3‐km buffer; (vib) surrounding matrix – proportion of seminatural land cover with a 3‐km buffer for the 106 WrEN woodland sites in Scotland (n = 67) and England (n = 39).

Figure 5

PCA biplots showing local‐level variables of WrEN sites in Scotland (A) and England (C) and landscape‐level variables in Scotland (B) and England (D). Variables with PC loadings <0.25 are not shown.