Knepp ‘Ancients for Arbs’ with Jon Hartill

Tom Burns who is the Ranger/Woodsman of the Knepp Estate hosted a two-day course entitled Knepp ‘Ancients for Arbs’ course in May this year, run by Jon Hartill. The venue was exceptional and it was a great opportunity to explore the Knepp Estate during my time there too.

Jon started the two-day course off with a series of lectures in the newly completed training barn and focused on the Variant and Invariant initially around plant evolution, biomechanics of plants and their function and broached the topic of biophysical optimisation. He shared many theories around the physical laws of plants including Drag force, Fick’s Law, Beers Law, Hagen-Poiseuille Bending movements and the Euler-Greenhill equation.

He expanded on concepts recently discussed around air embolisms from pruning cuts and the impact of large pruning cuts on trees creating lots of dysfunction and how this might impact trees based on their specific hydraulic strategies and the phenological timing of pruning works.

Lecture 2 explored the material properties of green wood and delved into the work by Karl J Niklas and Hanns-Christof Spatsz on worldwide correlations of mechanical properties of green wood density. Wood is a composite structure made up of many different materials and creates an elastic modulus of the wood.

It is important to understand the key concepts of wood properties around stress which could be tensile, compressive and torsional. As well as strain, proportional limits, plastic deformation, strain energy, elastic modulus, shear modulus and viscoelastic material. Jon went on to expand on how these concepts are relevant concerning cable bracing, fungal interactions, hollow trees, environmental events like storms, adaptive growth responses like thignomorphogensis, pruning works and even inspections of trees using decay detection devices.

Key take-home messages from day one included

  • Stems & branches twist more easily than they bend 
  • They will break more easily in torsion
  • The wind is more dangerous than gravity 
  • Fibre buckling is not a weakness
  • Shape, size and geometry are all important 
  • The direction of loading is very important 
  • T/ratio – not just degree of hollowing, length of hollowing 
  • No sense in looking at any element in isolation 
  • Tree root biomechanics change to aid anchorage
  • Trees can adapt annually

Key reading recommendations

  • Plant Biomechanics: An Engineering Approach to Plant Form and Function by Karl j Niklas 
  • Plant Physics by Karl j Niklas 
  • Plant Allometry: The Scaling of Form and Process by Karl j Niklas 
  • The Evolutionary Biology of Plants by Karl J Niklas
  • Ecological Strategies of Xylem Evolution by Sherwin John Cariqust (1975 & 2023)
  • Tropical trees and forests: An architectural analysis by Francis Halle

On day two Jon started with the session on wood anatomy looking at tree ring allometry and biomechanical outcomes. This was a deep dive into the makeup of trees and the differences between angiosperms and gymnosperms as well as looking at ring-porous and diffuse-porous tree differences.

He went into a lot of detail about water movement between cells and how and where trees store water. He discussed heartwood and ripe wood formation as well as decay mechanisms as well as how to monitor growth rings using tools like a Resistograph and dendrochronology. He went on to expand upon natural wounds like tear-out wounds or natural fractures generally showing limited decay whilst pruning wounds are often decayed as they cut across the vessels. He went through the differences between compression and tension wood as well as flexure wood.

Jon then went on to expand on one of the take-home messages from day one around the importance of size, shape and geometry. He ran through mechanics, hydraulics, light interception and reproduction mechanisms before breaking for lunch.

Key reading recommendations

  • Origins of decay in living deciduous trees: The role of moisture content and a re-appraisal of the expanded concept of tree decay by Lynne Body & Alan Rainer (1983 research paper)
  • Allometric scaling theory – Allometric theory and mechanical stability of large trees: proof and conjecture by Karl J Niklas and Hanns-Christof Spatz 2006

After lunch, we got to explore the Knepp Estate with Tom leading the way and explaining the history of the site and his involvement with the trees and forestry elements of the estate. We had a chance to see some of the veteran trees that have been historically managed by various methods including mulching, reductions, and cable bracing and you could chart the history of various pruning cuts like cornet cuts and more natural fracture cuts. It was also a chance to see some of the amazing wildlife and even got to see glow worm larvae.

Hopefully, they will be running further courses shortly so look out for the follow-up session to this in September and get in touch with Tom Burns for more information.

A few images from the afternoon session