Spring into summer – when to grow a leaf?

The solstice has just come, but summer still seems far away in parts of the Rocky Mountains. Many of the high peaks are still covered in snow. New growth and new leaves await bare ground and warm conditions. But why? Deciding when to deploy a leaf is a risky decision for a plant – too soon, and the leave may freeze and die, or simply be too cold to photosynthesize – too late, and opportunity will have been wasted, letting other competitors gain an advantage. Leafing out is like making an educated guess, or perhaps a gamble, about what the weather of the coming days and weeks will be like.

I have been fascinated in past years to observe the range of different strategies that species take in making this critical decision. This year I have been watching a few aspen stands more closely, as the snow begins to melt and the valleys fill in with life.

Down low, summer has fully arrived, with the aspen trees fully leafed out. It seems as though the weather is the main driver of leaf-out date.

But a few hundred meters higher, the story is not so simple. In the foreground is an aspen tree with leaves just breaking from their buds – and in the background, not a few meters away in the exact same microenvironment, a spray of mature dark green leaves.

In more extreme environments, it’s reasonable to imagine that the weather (or long-term climate) would play a stronger role in determining leaf-out dates. But here, the same pattern appears. A few thousand feet above the valley floor, there are still a few scraggly aspen growing out of the scree.

In some cases, stems are still bare, or have only a few delicate young leaves beginning to emerge.

And adjacent to these stems, or on nearby slopes, there are other aspens with nearly mature leaves.

So what’s going on? Clearly microenvironmental conditions aren’t the whole story. My guess is that each of these leaves is part of a different clone, and that the leaf-out date is determined by genetic differences between these clones. But can weather overpower genes? Or can genes overpower weather? And is each strategy somehow better in its local context? Answering this question will help us understand one of the major economic choices that plants face – and one whose answer will help better predict winners and losers in changing climates. This summer we are going to be installing large observational grids of climate loggers and DNA samples up and down mountains to tease apart these effects. I don’t know what we will find, but am looking forward to finding out.

A small postscript – the economic game for quaking aspen is not entirely a simple story of leaves. The species is relatively unique in the temperate zone for being able to photosynthesize via its greenish-white bark in late-winter conditions. This may allow it to gain significant resources (or offset metabolic losses) during times of year other species have no options for growth. I have also noticed that the greenness of bark changes as much between stems as the date of leaf-out, but don’t know whether this variation is independent of leaf level variation, or also linked to genetic variation. We will soon find out!

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