Is heartwood or sapwood more susceptible to decay?

by Treehugger arb
(UK)

QUESTION:

Is heartwood or sapwood more susceptible to decay? im getting conflicting information. On the one hand im reading that sap wood is more resistant because there is little to no oxygen and so that is the defence as fungi need oxygen but im also reading that heartwood is more resistant as it has less living cells to feed from.

I would really like to know what some mycologists can tell me about this subject.

Thanks


ANSWER:

To provide a clear and comprehensive answer about heartwood and sapwood decay, I'll break down the key characteristics and explain why there's some complexity to this question.

Heartwood and sapwood actually have different characteristics that influence their susceptibility to decay:

Sapwood:
- Contains living cells and active transport systems
- Higher moisture content
- Contains more nutrients
- More recently formed wood
- Generally MORE SUSCEPTIBLE to decay

Heartwood:
- Consists of dead cells
- Contains extractives and chemical compounds that resist decay
- Less moisture content
- Denser cellular structure
- Generally MORE RESISTANT to decay

The conflicting information you've encountered stems from different mechanisms of decay resistance:

1. In sapwood, the high moisture and nutrient content can indeed support fungal growth. However, active cellular processes can also provide some initial resistance.

2. In heartwood, the key resistance comes from:
- Decreased moisture levels
- Presence of phenolic compounds
- Tyloses (membrane-like structures) that block pathways
- Accumulated chemical defenses developed during the tree's life

Mycologists typically agree that heartwood is more resistant to decay due to these chemical and structural defenses. The oxygen argument for sapwood is somewhat misleading, as fungi can adapt to varying oxygen levels.

The specific decay resistance also depends on:
- Tree species
- Environmental conditions
- Type of fungal organism
- Age of the wood

So in most cases, heartwood is considered more decay-resistant compared to sapwood.

Best,

Nick