Spring happens every year, and every year it is amazing.  This is what happened to one small oak tree in 2021.

MARCESCENT LEAVES

Why do things hang on when there is no hope?  In the lines of Robert Frost: “The leaves are all dead on the ground/ Save those the oak is keeping” (the poem is “Reluctance”).

Usually, seasonally deciduous trees drop their leaves before the harsh season arrives – cold and/or dry – when the leaves cannot survive.  The process of leaf fall is abscission, the thinning and breaking of cell walls where the leaf meets the stem.

But some leaves are marcescent, remaining on the tree after death.  They have an abscission layer right were it is supposed to be, but it remains strong enough to keep the leaves on the tree.  These are the leaves “the oak is keeping.”

The leaves leave eventually.  They don’t stay on among the new green leaves.  When do they fall?  One source claims that the initiation of new leaves (the swelling of leaf buds) is the signal that sets the old leaves free.  The authors suggest that some early-season hormone (probably from the growing buds) travels through the tree and stimulates the completion of abscission.  Poof, no more brown leaves.

Their claim became the hypothesis for my observations in 2021.  I expected the dead leaves to remain fast until the buds started to expand.  Then the transition from brown to green would be quick, brown down, green up.

What actually happened?

There were quite a few leaves on a small red oak on February 28.  I photographed the tree nearly every day until May 27 to see what happened.  I also photographed some buds high on the tree (too high to measure directly), and measured the lengths of some buds on low branches that had no marcescent leaves.  

By March 15 or so, half the leaves present at the end of February had fallen.  In another two weeks, roughly a quarter of them remained. By April 10, it was down to about ten percent.  

ABSCISSION

But it’s not quite that simple.  Early on, the leaves broke off at the petiole (leaf stalk), not the abscission point where the petiole meets the stem.  The petiole had weakened more than the abscission layer.  On March 27, there were petiole stubs still on this stem, even though a large majority of the leaf blades had fallen.  Technically, the leaves had not yet abscised.  

But abscission was coming.  Two days later, one petiole from the stem had cleanly abscised.  By April 6, another was gone (the little wasp had nothing to do with it).  Once the abscission layer had weakened sufficiently, the stub of the petiole could break free, leaving the typical leaf scar. 

The weakening of the abscission layer is obvious in this April 14 photo

On April 16, we got several inches of wet snow.  The next day, all the petioles were gone. 

The late snow had accomplished what all the snows of winter had not.  Only when the tree had weakened its abscission layers could the petioles fall off.  

Several hormones affect abscission, some by inhibiting it, some by stimulating it.  Petioles were breaking before the hormonal pathway had kicked in.  But eventually, it did, and then the once stubborn stubs became stubborn no more.

BUDS

What were the buds doing as the leaves were falling?

Buds low on the tree, where there were no marcescent leaves, remained essentially the same length through May.  The fluctuations were mostly (entirely?) from my inconsistency in measurement, though there might have been some actual swelling and shrinking with humidity and internal moisture (if internal moisture changes, I don’t know).  By the end of May, there was no sign that these buds were going to do anything at all.  Was the tree sacrificing its lower branches?  They had to go sometime, maybe this was it.

Buds higher on the tree, where the marcescent leaves were present, changed little through March, but began to swell in April.  (I used a distinctive lenticel on the branch as a unit of measurement because the buds were out of my reach.)  

But not all buds changed at the same rate.  Some were only swelling while others had already burst with new tissues showing.  These tissues turned out to be stems and leaves, no flowers.  The buds that swelled only a little stopped swelling and remained intact by the end of May.  Will they burst later?  We’ll see.

Most marcescent leaf blades were gone before the buds began to swell.  But true abscission happened later.  The buds still hadn’t begun to lengthen when the first petioles fell, but the first sign of enlargement was evident shortly before the snow pulled off the last of the leaves.  I suspect that hormones were stirring well before the buds started to expand, initiating the formation and growth of tissues at essentially the same time as the marcescent leaves were being set free.

NEW LEAVES

The new leaves and stems were tinged with red, the brief pulse of color before they were fully expanded (see Robert Frost: “Nothing Gold Can Stay”).  And they expanded really fast.

The new stem elongated impressively in ten days. The new leaves took on their distinctive oak shape and became bright green.

Some buds among the new leaves had still not burst. They had begun to swell with the others, but then stopped. What is their fate?

Buds low on the tree showed no sign of growth, and the branch they were on – a significant branch on this small tree – had drooped about 25 centimeters from March to May. What is going to happen to the buds and this branch?

There remain many things to observe.

A FEW MORE WORDS

Marcescent leaves are remnants of a bygone year, a season of productivity that went dormant for the winter. They have no reason to linger when the new season bursts forth, which is when they finally fall off. But perhaps we shouldn’t be surprised that there are a few that don’t play by what we think are the rules. Here are two brown leaves that were hanging on well into May.

Many arthropods (insects, arachnids, and others) live and hide among the tissues of trees. Some blend in so well that we miss them. I almost missed this spider among the buds in April. It was among these buds for three successive days, then was gone. Perhaps it is still somewhere on the tree. I don’t expect I’ll find it.

One more thought: This red oak tree, though small, has a lot going on: lingering leaves, buds that burst, buds that swell, buds that remain small, branches that burst forth with green, and branches that droop and remain gray. Such small trees might be useful – and accessible – as subjects for explorations of developmental physiology. If that is your field of interest and expertise (it is not mine), go for it.

Further reading

Earl Berkley 1931. Marcescent leaves of certain species of Quercus. Botanical Gazette 92: 85-93.

Robert Hoshaw and Arthur Guard 1949. Abscission of marcescent leaves of Quercus palustris and Q. coccinia. Botanical Gazette 110: 587-593

https://www.jstor.org/stable/2472663?seq=1#metadata_info_tab_contents

https://www.woodlands.co.uk/blog/woodland-activities/leaves-in-winter-marcescence/