Biomass allometry for shrubs at a UK rewilding site

The planting and growth of trees and forests are a major component of meeting international net zero emissions targets. A more passive and cost-effective approach to meeting these targets is the natural regeneration or colonisation of native woody species, including shrubs. There has been much research into how trees and forests sequester carbon and increase in biomass, with a wealth of biomass calculations and allometric equations available. However, there is a contrasting lack of this information for shrubs. Biomass equations for shrubs need to differ to those for trees due to their inherent structural differences, and also recognise that we do not have the same levels of data to train models on this massively diverse vegetation type. Diameter at breast height (DBH) is often cited as the single most influential metric to derive biomass, but this can be impractical or unclear for many shrubs: a different metric must be used. Hawthorn (Crataegus monogyna; Jacq.) is a multi-stemmed, multi-branched thorny shrub native to Europe, North Africa and western Asia. Despite its broad geography, there is little information describing this species’ allometry or biomass.

 

Our aim was to generate an allometric equation for the above-ground biomass of distinct hawthorn shrubs by adapting existing allometric equations and using field-collected measurements to generate a coefficient that accounts for the diverse structural relationships brought about by the multi-stemmed nature of hawthorn compared to trees.

 

Our study site was a 150ha rewilding site in Bedfordshire, East of England. In the 35 years since farm management was withdrawn, shrubs and other vegetation have been allowed to naturally colonise the previously arable fields. Hawthorn is common here in an unmanaged distinct form, as well as in overgrown hedgerows and merged canopies with other shrubs and species. In a first visit, we measured shrub height and crown diameter measurements for 36 distinct hawthorn shrubs up to 5m in height. In a second visit, we destructively sampled 27 of these and an additional 55 hawthorn shrubs and measured wet weight in situ.

 

To analyze this data, we employed a Maximum Entropy allometric model using Bayesian inference. This approach enabled us to quantify the likely range of biomass values based on our allometric measurements while accounting for the inherent uncertainty caused by incomplete or missing data. Preliminary results from this model are promising, and we are working towards linking these findings with Earth Observation (EO) data to extend the application of shrub allometry to larger spatial scales. By integrating field-derived measurements with EO techniques, we aim to estimate biomass not only for individual shrubs but also for entire shrubland areas, providing a broader perspective on carbon storage potential.

 

Additionally, we welcome feedback on the types of allometric relationships we have assumed and invite discussions on how our newly developed models might find applications in other fields. These efforts could pave the way for improved understanding and management of shrubland ecosystems in the context of global carbon accounting.