Exploring Noble Hardwoods in Sweden

I recently had the great pleasure to be invited as a guest speaker and guide for a special day exploring noble hardwoods, hosted by the Swedish University of Agricultural Sciences (SLU) at Alnarp.

I presented a short lecture, and then contributed to discussions in the field for the remainder of the day. The group consisted of students studying for the excellent MSc in Forest Science (read more) at SLU Alnarp, plus forestry technicians, local landowners, and academics from the university.

The noble hardwoods of interest on this day were Turkish hazel (Corylus colurna), wild service (Sorbus torminalis), and walnut, both common (Juglans regia), black (J. nigra), and various walnut hybrids. Noble hardwoods are defined as hardwood tree species which often have very high economic value, but are usually only minor components in the forest. Other noble hardwood species include:

• Italian alder (Alnus cordata)
• black alder (Alnus glutinosa)
• field maple (Acer campestre)
• Norweay maple (Acer platanoides)
• sycamore (Acer pseudoplatanus)
• silver birch (Betula pendula)
• hornbeam (Carpinus betulus)
• sweet chestnut (Castanea sativa)
• narrow-leaved ash (Fraxinus angustifolia)
• common ash (Fraxinus excelsior)
• common walnut (Juglans regia)
• apple (Malus sylvestris)
• wild cherry (Prunus avium)
• common pear (Pyrus pyraster)
• rowan (Sorbus aucuparia)
• service tree (Sorbus domestica)
• field elm (Ulmus minor)
• wych elm (Ulmus glabra)
• white elm (Ulmus laevis)

We visited the arboretum and park in the campus of SLU Alnarp which includes a great diversity of interesting tree specimens, plus a nursery containing many forestry trials. Later we visited a private forest to see silvicultural trials of walnut and wild service. We covered a huge range of topics during the day, ranging from soils and climate change, to timber qualities and pruning methods.

Some key take home learnings:

1. Many of the group were surprised by the stature of the Turkish hazel and its economic value.
2. Turkish hazel is a great species to plant into a forest to replace any mature trees which may be have been removed or died. They can be planted into small gaps and being shade tolerant will thrive in the shelter provided by the other trees. They will often grow rapidly but as they near the height of surrounding trees their growth will slow, as they tend to be a ‘sub-canopy’ species.
3. We learnt about how important it is to ensure that a ‘blue halo’ (meaning no crown crowding) is maintained around the crown of many noble hardwood species, especially as they are often not the dominant species in a forest.
4. Wild service is very attractive to browsing deer but is actually very difficult to kill, as it will take a lot of punishment from animals (or the land manager ignorant of its high value!).
5. Wild service can be worth up to five times more than walnut (which itself if many times more valuable than all other hardwoods) when grown for the veneer market.
6. ‘Twenty is plenty’ is a good mantra when growing wild service, meaning that it is better to have only 20 trees per hectare mixed with other species. This allows the forest manager time to really concentrate on nurturing these 20 to be the very best quality.
7. Wild service can be pruned quite hard but will often respond by growing many epicormic shoots. A good way to prevent these (and to protect from moose!) is to tightly wrap the newly-pruned stem with mesh. Then hang a bright red ribbon from the tree to protect it from accidental damage during forest operations. See photo above.
8. Hybrid walnuts can grow very vigorously and tend to have much better apical dominance than either common or black walnuts. Pruning to remove branches is critically important to improve timber quality.
9. I talked about ‘No Pain, No Gain’ in walnut silviculture, meaning that walnut can be demanding to grow (e.g. site selection, pruning requirements, cost of plants), but that the rewards after these efforts can very substantial.

I am very grateful to Professor J P Skovsgaard for involving me in this fascinating day, and to the group for welcoming me and contributing to such stimulating discussions about noble hardwoods.