Monthly Archives: January 2018

“When Autumn Leaves Start to Fall”…or Not!

By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester

It’s a sure sign of autumn at Tryon Creek State Natural Area (TCSNA) when the leaves start to fall.  However, the time of leaf fall can vary dramatically between species.  I became aware of this during the past 5 years of my phenology study.  This phenology study involves visiting more than 60 specific selected plants at TCSNA every week to ten days throughout the year.  I make notes on the stage of development of each plant on each visit.  For example, I note stages of development like “buds open enough to see leaf veins”, “flower’s stamens visible” or (in the autumn) “last leaf has fallen.”  This year, for example, every one of the vine maples (Acer circinatum) in the study had lost all their leaves by November 22nd.  In contrast, several thimbleberries (Rubus parviflorus) still had numerous green leaves as late as December 13th.  This is a difference in leaf fall between species of more than 3 weeks.  Sometimes the difference in the date of leaf fall varies dramatically even on the same plant.

This year I found numerous examples of extra-long leaf retention in several plants.  These plants appear to have differences in time of leaf fall for one of two different reasons.

 

Adventitious Buds

Adventitious buds are buds that form from normal plant tissues.  Oftentimes they arise from the cambium, the layer of soft, actively dividing cells just under the bark of woody plants.  Normally the cambium cells just differentiate into either bark or woody stem cells.  Most plants, like Douglas-fir (Pseudotsuga menziesii) and bigleaf maple (Acer macrophyllum), recognize that the energy they need for growth is coming from the sun up above.  Their secret to success is growing tall, where they can reach above most plants, and bask in the full sunlight.  To do this, these plants want to direct as much energy as possible into the shoots that are growing straight up towards the sun.  The apices of these plants “control” the growth of side buds and shoots by producing a chemical named auxin.  The chemical structure of auxin is shown below.

 

Photo 1

The chemical structure of auxin

 

This auxin is produced at the tip of the dominant stems.  As the auxin is transmitted down the stem, the message that this chemical delivers to the lower buds is “don’t grow” and “don’t develop new buds” depending upon the exact circumstances.  When the top of the tree, or the tip of a branch, is killed or broken off, the auxin no longer flows down the stems, and eager lower buds start to grow out.  In some cases, brand new buds are formed along the stem, and take off like a rocket!

One of my favorite examples of this at TCSNA is found on a Pacific yew (Taxus brevifolia) tree along the Cedar Trail.  The tree blew over during a storm and the trunk lay across the trail, although some roots were still in the ground.  On January 22, 2017 I cut off the trunk to allow the trail to be more accessible.  But the big news was months later.  All along the remaining trunk, brand new little yew branches were growing out, no longer inhibited by the top of the tree, as shown in the picture below.

 

Photo 2

Me sawing off the top of a yew tree which feel across Cedar Trail. Note the trunk is bare of sprouts.

 

Photo 3

Same yew trunk on August 10th showing new sprouts

 

Another example started last spring when a trail maintenance crew cut the top off a lot of shrubs along the Red Fox trail.  This produced results similar to what happened on the Pacific yew.  One of the plants they cut back was the hazel (Corylus spp.) shown below.  The black arrow is pointing to a stem which has lost all of its leaves, and in fact, almost all of the plant has lost its leaves.    In contrast, the purple arrow points to one of the two stems on the plant which is still holding onto its leaves.

 

Photo 4

Hazel (Corylus spp.) bush on November 29, 2017

 

So what’s the difference?  Note the blue and red circles in the picture.  The blue circle shows where a major stem of the plant was cut off by the trail maintenance crew between May 30 and June 6.  On this particular plant, the leaves had started growing about March 8 and were approximately 2 inches long at the time the bush was trimmed back.  The removal of this main stem liberated buds on the lower parts of the stem.  These liberated buds began to grow (note purple arrow and red circle).  So these adventitious shoots and their leaves were much younger than the shoots and leaves of the other parts of the plant.  These young leaves stayed on the plant much longer than the “normal” leaves.

Two other examples of this phenomena are shown below.  The first is a red huckleberry (Vaccinium parvifolium) which was also pruned off (note red arrow).         

 

Photo 5

Adventitious shoots on a red huckleberry near Red Fox Trail.

 

 

 

Photo 6

Normal shoots on a red huckleberry near Red Fox Trail.

 

 

 

The final example is an Indian plum (Oemleria cerasiformis) plant.  Here the branch tip appears to have been randomly broken off, rather than cut off, but the end result is the same.  All the leaves on this plant, except some leaves on the small shoot shown in the left picture had fallen off by September 28th.  The fact that this one leaf is still green and healthy 10 weeks after all the “normal” leaves had fallen off the plant is astounding.

 

 

Photo 7

Leaf on an adventitious shoot on December 13th on an Indian plum growing near Cedar Trail.

 

Photo 8

Leaf on a normal shoot on December 6th on the same Indian plum as above, growing near Cedar Trail.

 

 

Buds:  Preformed or Neoformed?

The second reason that some leaves might hang on longer than others is due to when the leaves are formed.  For example, in overwintering Douglas-fir buds, each of the needles that will appear the following growing season is already formed.  You can see these “baby needles” as little whitish bumps (red arrow) in the picture below after all the bud scales (blue arrows) have been cut off.

 

 

Photo 9

Dormant Douglas-fir bud after removing the protective scales.

 

 

In contrast to these “pre-formed” buds, some plants have a different (“neoformed”) growth strategy.  In these plants, new leaves (“primordia”) will be initiated during the spring and summer as long as growing conditions are favorable.  These primordia will immediately start to develop into leaves.  Thus on one plant, there will be leaves of vastly different ages.  Not surprisingly, the older leaves will drop off before the younger ones.  One example of this at TCSNA is the red elderberry (Sambucus acemose).  I took the pictures below of one plant on December 6, 2017 at TCSNA.

 

 

Photo 10

Buds. but no leaves on lower branches on a red elderberry along Cedar Trail.

 

Photo 11

Healthy leaves on the upper branches of a red elderberry along Cedar Trail.

 

 

This continual development of new leaves is illustrated in the chart below of the length of 3 different leaves throughout the growing season, until somehow the plant’s stem was severed.

 

Photo 12

Leaf length of the 7th, 8th, and 9th leaves on a red elderberry stem.

 

 

Photo 13

Tip of red elderberry stem (in winter) which is dead beyond the pair of red-brown buds

 

 

The elderberries’ strategy is successful although when the end of the growing season comes, the youngest segment of the twig hasn’t had a chance to produce viable buds, and the twig beyond the last pair of viable buds just dies.

 

There’s more than one road to success

Mother Nature has produced many species of plants, which use a variety of strategies to achieve success.  Two of the strategies involve leaf and shoot development.  As illustrated above, the first strategy involves focusing the plant’s energy on the development of just a few leading shoots and suppressing the others.  However, Mother Nature knows that bad things happen, and if those leading shoots are killed or injured, tissue lower down on the plant can create branches that will take over and keep the plant alive.  The second strategy involves having a stem that just keeps elongating and producing new leaves as long as the weather is good.  Both strategies have been successful, and add more interest to our forest.

 –All photos by Bruce Rottink

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