April..A blog by Andy Hirons, Spring has sprung (probably)

Gone are the dreary days of winter! Like many, I always find spring a fantastic time of year, full of hope and expectation. It is also a fascinating time of year to look at trees. All sorts of things are happening, but one of the interesting things is that they don’t all happen at the same time.

Different species begin leafing out at quite different times of year. If you walk round parks or gardens right now, some trees will be in full leaf, whilst others are still shyly emerging from their winter dormancy (Figure 1). Why is this?

Figure 1: If you wander around gardens at this time of year, you see that trees leaf out at quite different times. As seen here in Batsford Arboretum. Some trees appear in full leaf, others are just emerging, whilst others (background) are still in bud. Photo © Andrew Hirons

 

Well, really it’s the outcome of the cost-benefit analysis that trees have been carrying out over aeons. Deciduous trees have to determine when it is safe to expand their leaves. Early emergence has the advantage of extending the growing season, but it has the potential to render the young, tender leaves vulnerable to late frosts. This can be particularly problematic in temperate oceanic (maritime) climates, like the UK, that have a very protracted spring. Just look at what has happened this year: things started to get warm and then it went cold again.

For temperate winter-deciduous trees, control over leaf emergence is a vital component of the life-strategy. Although the precise mechanisms of this control are still not fully understood, it is clear that most temperate species are very sensitive to temperature and, often, its interaction with day length.

Two components of temperature influence the timing of leaf emergence (assume I am talking about temperate winter-deciduous trees from here on): sufficient chilling in winter and warm temperatures in spring. In order to break dormancy, most trees have a chilling requirement that must be met. Variation exists across species, but temperatures above about 12 oC seem not to contribute to chilling, freezing is not necessary though.

If trees are exposed to warm temperatures before their chilling requirement is met, then they will be very reluctant to start growing: it might just be a couple of warm late-winter days lulling them into a false sense of security. This safety mechanism helps prevent trees bursting forth only to be caught out by a late frost.

After chilling requirements are met, the accumulation of heat energy plays an important role in the timing of leaf emergence. This is often referred to as a forcing requirement or thermal time and is quantified as a cumulative measure of time above a critical temperature, something known as degree-days. This forcing requirement is under strong genetic control so is an important reason for different species coming out a different times. This variation in the timing of leaf emergence is exaggerated in gardens with lots of non-native species as many of the trees have evolved in different climates, even if they are somewhat analogous to our own (Figure 1).

Importantly, leaves emerge sooner if they are given a longer chilling requirement. They have more ‘confidence’ that winter is over and spring has sprung. In other words, cold winters require less spring warming than milder winters, but trees will eventually break bud as the accumulated spring warmth overcomes the dormancy. However, mild winters can effectively reduce the length of the growing season for species, such as beech Fagus sylvatica, that have a high chilling requirement. Over time, this can make them less competitive in regions with mild winters and can lead to changes in forest composition: just one of the potentially harmful effects of a changing climate. This interaction between chilling and spring warmth is also responsible for species leafing-out in different orders in different years.

A further cause of variation is the position that species occupy in the forest. Understorey species, such as hawthorn Crateagus monogyna, leaf out earlier so that they can make the metaphorical hay, while the sun shines (Figure 2). Pretty soon the canopy species leaf-out and make it much more difficult for the understorey species to photosynthesise efficiently. This is a crucial time of year for the productivity of the understorey, a bit like Christmas for the retail sector.

 Figure 2: Understorey species, such as hawthorn Crateagus monogyna, (as seen here) need to leaf out early to make the most of their time before the canopy closes above them. Here the canopy trees are mostly ash Fraxinus excelsior. Photo © Andrew Hirons.

 

Of course, there are lots of other things happening to trees at this time of year. For many species, it is an important time of year for flowering. In fact, many species are planted precisely for this reason. Flowering cherries are certainly candidates for this as they can give simply stunning displays in spring (Figure 3), but don’t forget to look out for the more understated flowers, such as paperbark maple Acer grisium that are also around (Figure 4).

 

 Figure 3: Flowering cherries, such as this Prunus ‘Shu Jaku’ at the Batsford Arboretum are fantastic during spring. Photos © Andrew Hirons

 Figure 4: Many flowers in spring are rather understated and harder to spot, such as this paperbark maple Acer grisium. Photo © Andrew Hirons