Selecting tree species for landfill restoration

 

The case for tree species selection

Tree species selection should form one of the final decision making processes in land reclamation, with choice governed by a proper assessment of the ability of the newly restored site to support plant growth, and an evaluation of the limiting factors which will hinder the flexibility of choice and expectation. Unfortunately, it is often the case that tree species are chosen early in the restoration process with little regard to site and soil constraints.

Even if restored well, landfill sites often suffer from comparative exposure, drought and infertility. Badly restored, compaction and waterlogging can also compromise tree growth. Species choice is therefore crucial and it is vital that only species are chosen that have a proven ability to tolerate relatively harsh conditions.

Species suitability

Our research has shown that some tree species, notably those like alder, poplar and willow that tolerate waterlogged soil conditions, may be more able to penetrate cracks that exist in the underlying landfill clay cap.  Unless there is an adequate soil cover over a unprotected clay cap, these species may pose a small risk to cap integrity and should not be planted. 

The tables below gives guidance on species suitability. In general, so-called pioneer tree species should be chosen over more demanding ‘climax’ species.

Broadleaved species suitability for different site types
++ Very suitable, + Suitable, - Not suitable

Species	Heavy soils	Calcareous soils	Acidic soils	Exposure	Air pollution	Comments
Ash	-	++	-	-	-	More fertile sites only
Common alder	++	+	+	+	++	Nitrogen-fixing
Crack willow	++	++	-	-	+
Downy birch	+	+	+	++	++	Tolerates low fertility
English oak	+	+	+	+	+	More fertile sites only
False acacia	+	+	++	-	++	Nitrogen-fixing South only
Field maple	+	++	+	+	+
Goat willow	+	+	+	-	++
Grey alder	++	+	+	+	+	Nitrogen-fixing
Grey poplar	++	++	+	++	++
Hawthorn	+	+	+	++	+	Tolerates browsing
Italian alder	+	++	-	-	++	Nitrogen-fixing
Norway maple	+	++	-	++	+
Red alder	++	-	+	++	+	Nitrogen-fixing
Red oak	+	+	++	+	+
Rowan	+	+	+	++	+
Silver birch	-	-	++	++	++	Tolerates low fertility
Swedish whitebeam	++	+	+	+	+
Sycamore	+	++	+	++	++
Turkey oak	++	+	+	+	+
Whitebeam	+	++	++	+	+
White poplar	++	-	+	+	++
Wild cherry	-	+	-	-	+	More fertile sites only

Conifer species suitability for different site types
++ Very suitable, + Suitable, - Not suitable

Species	Heavy soils	Calcareous soils	Acidic soils	Exposure	Air pollution	Comments
Corsican pine	+	++	++	++	++	Below 250 m O.D.
European larch	+	-	+	+	-
Japanese larch	+	-	+	+	+
Scots pine	-	-	++	++	-

Research overview

This research is an amalgamation of several current and completed research programmes.  A decision support software package (roots) has been developed which will desseminate best practice guidance for species selection for a range of site types.

This research is ongoing.