Ash before oak, we’re in for a soak…
We frequently get asked at this time of year why ash (Fraxinus excelsior) trees are so late to come into leaf and if this is a sign of declining health, or a symptom of disease like chalara ash dieback (Hymenoscyphus fraxineus).
Figure 1: Ash (right) with no leaves and sycamore (left) in full leaf in late May 2023
A feature of trees which most of us are aware of are growth rings inside the trunk. These appear as repetitive bands of light and dark wood, with each one marking a temporary halt in growth during an unfavourable part of the year: winter in the UK!
In deciduous trees, this growth is first initiated by swelling buds in early-spring, and spreads downwards through the twigs and branches, and finally into the trunk (early-season growth is a top-down process). The timing and rate of this process varies from species to species, though deciduous trees can roughly be divided into two categories: diffuse-porous and ring-porous trees.
Figure 2: Comparison of vessel size and arrangement in growth ring of: (1) maple (diffuse-porous) and (2) ash (ring-porous)
In diffuse-porous species - such as birch, maple, beech, lime and poplar - the downward spread is steady, with growth occurring at the base of the main trunk often not until months after it is initiated in the twigs. Because of this, new wood in diffuse-porous species is produced at fairly a consistent rate and density throughout the growing season, with an even distribution of many narrow water conducting vessels (Fig. 1). The narrowness of the vessels in diffuse-porous species has the drawback that they restrict the volume and speed water can be moved from the roots up to the leaves.
On the other hand, new growth in ring-porous species - such as ash, elm and oak - spreads downwards rapidly from the twigs and appears at the base of the trunk within days. The wood produced by ring-porous trees in early-spring as a result is generally of lower density and contains fewer vessels – though these are significantly wider and longer than those found in diffuse-porous species (Fig. 1). Wide and long vessels can transport water up to the leaves at high-volumes and speeds; for example, movement of water in ring-porous species has been found to be in the region of 20 m per hour, compared to 1-3 m per hour in diffuse-porous species.
You might ask why all trees are not ring-porous if this adaption is so superior during the growing season…
Well, although more efficient, the strategy of ring-porous trees is also significantly riskier. Where diffuse-porous trees may use 10 growth rings to transport water up to the leaves, ring-porous trees often transport the majority of water through the newest growth ring alone. Since this can be only a few millimetres wide in mature trees, any damage to this outer ring can have a significant impact upon a trees hydraulic capacity.
Large and long vessels are also at a disadvantage during winter, when they are mostly inactive. Through the action of freezing and thawing, air bubbles are more likely to develop inside large water filled vessels; come spring these can take weeks to disappear and can render them useless - much like air bubbles in a central heating system. For this reason, ring-porous trees like ash rely on the rapid growth of new wood in early-spring - before the leaves emerge - so when they do there is a new ‘plumbing’ system purged and ready to supply them with the necessary water.
Figure 3: Ash buds breaking in late May
So for the casual observer, although ring-porous trees, like ash and oak, are usually the last to produce leaves, often not until late-spring, if you were to measure trunk growth at this time you would find that ring-porous trees have actually grown significantly more.
There is an old rhyme that which suggests that the timing of leaf flush in oak and ash can be used to help predict the long-term forecast, it goes “oak before ash we'll just have a splash; ash before oak we're in for a soak”. Oak were definitely first this year so it looks like we’re in for a good summer!