Wood mould boxes enhance biodiversity and green infrastructure for organisms living in hollow trees
About this good practice
Hollows form when trees grow old. These hollows are successively colonized by many organisms. But many species living in hollow trees are threatened today. They often have a low dispersal capacity and are dependent on old trees with holes and cavities within a near distance. Today many findings of rare hollow-living species have too few hollow trees close by to maintain the population over time.
In many areas with old oaks (which are the ones with highest probability of being hollow) there are no middle aged or young trees. Therefor there will be a gap in time with no oaks with cavities as the old trees dies. If there are a lot of younger trees, veteranisation is a good practise to create cavities (see Good Practice Veteranisation). But in sites with not enough young trees, yor where veteranisation is not possible for other reasons, wood mould boxes are a solution.
A wood mould box looks like o large bird box and is filled with wood chips, saw dust, leaves, twigs and some water. It can be placed on the ground, on legs or up in trees and be built in different sizes. Those used in county of Västra Götaland are 250 litres, but models from 80-1000 litres have been used in Sweden.
This method allows us, within a relatively short period of time, to improve the conditions for species dependent on holes and cavities in trees. With this method, we can both improve sites, bridge geographical distances in the landscape as well as the generation gap of the trees.
Expert opinion
Resources needed
Material: oak planks. Attachment: tree or directly on the ground. Filling material: wood chips, leaves, twigs and water. 5-10 boxes per area. Purchase of 20 boxes in 2017, 7500 Euro. Labor cost for placement of 12 boxes, 3000 Euro. Additional cost for annual inspection and replenishment of material.
Evidence of success
Studies in Sweden, Poland and England have shown that wood mould boxes successfully imitate the microhabitats that exist in hollow trees. 70% of the beetle species that were found in hollows in old oaks were also present in the boxes. The hermit beetle has a weak dispersal capacity but was still found in 7% of the boxes. In an inventory after 4 years 39 wood mould boxes were surveyed for beetles. 93 saproxylic beetle species were found, 8 of the red listed. As a mean 9 species / box were found.
Potential for learning or transfer
The problem of a generation gap (lack of suitable successors) to old trees is probably common around Europe. Therefor there is also a lack of hollow trees. This increases the probability of extinction for species depending on holes and cavities in old trees. In areas without many young trees suitable for veteranisation, wood mould boxes can be used.
Wood mould boxes can be used by different stakeholders and be applied in different regions or countries. They do not weaken the trees which allows them to be used in parks and other areas with a lot of people. As it is a relatively cheap method, it can be used by towns, counties, and conservation organisations. This good practice can help many species depending on cavities survive until enough trees have become old.
This good practice is therefore considered to have a high learning and transfer potential.
