Do Stressed Trees Produce More Seeds?

Do Stressed Trees Produce More Seeds?

Even those who say they don’t know the first thing about trees are likely to appreciate the fact that giant trees emerge from tiny seeds. Really, what sort of grump doesn’t smile at the image of a tree growing from a seed? It’s certainly a worthy inspiration, but it’s not the only miracle. Indeed, before there can be any trees growing from seeds, there must be seeds growing on trees. And if you consider everything that has to go right for a tree to make even one seed, let alone hundreds of thousands of them at a clip, you’ll be even more impressed.

Before it produces a seed, a tree first has to become established and survive to reproductive maturity, the age of which varies widely, from 5-10 years for shade intolerant / shorter-lived species to 30-40 years for shade tolerant/ long-lived species. Once mature, the tree must make sufficient food to stay alive and compete with its neighbors, fuel growth in length and width (to remain healthy), and make new buds containing next spring’s growth. Then, the tree must endure a dangerously cold and often ice-encased winter dormancy and potentially uncooperative spring weather for those buds to open and develop into leaves, shoots, and flowers.

Should all that fall into place without something freezing, eating, or breaking the buds, the flowers still must be pollinated amidst the vagaries of weather and the uncertain fortunes of insects or wind. Then the male and female reproductive cells have to unite through fertilization. And if that all goes swimmingly, the fruits and seeds still must grow and ripen – and avoid being destroyed by adverse weather or the various insects, mammals, and birds that depend on consuming them for their own survival – if the seeds are to be successfully shed with even a chance (still no guarantee) to become a tree. Yikes.

Clearly then, heavy seed production is a big investment for a tree because it consumes large amounts of energy at the expense of other tree needs. Put simply, trees cannot grow as much in heavy seed years as they do in other years, and in many species, the seed crop of one year negatively affects the seed crop of the following year. They rarely have big seed years back to back. And beyond expensive, it’s also a risky scheme. Those energetic investments can and do go for naught at so many different points along the way. Remember, too, that whatever energy a tree devotes to flowering and fruiting it cannot put toward other important physiological processes, like staying alive or growing, for example. So, when trees initiate flowering and fruiting but fail, they waste energy that could have been invested in leaves, branches, trunks, and roots.

Knowing all this, it makes sense that a vigorous tree would produce more successful seed crops than a stressed tree. But curiously, stressed trees do sometimes produce a reactive burst of flowering which – if all goes to plan – leads to enhanced fruit and seed production. Now why would a stressed tree add stress by running down its reserves that could otherwise be used for growth or to recover from the original stress?

One possibility is that, high costs and multiple risks notwithstanding, trees need to produce seed to pass along their genes and perpetuate their kind. This is the biological imperative. It is so important that heavy seed production may be triggered in some trees as a reaction to significant, threatening stress. It’s as if the trees interpret stresses like summer drought or insect defoliation as a sign of the beginning of the end, and they react to impending doom by shifting their precious and hard-won resources away from their own growth and focus on the next generation. In other words, under this hypothesis, stressed trees put what energy they have left into flowering to make baby trees before they can’t.

Perhaps the heavy flowering and fruit production that has been so widespread in Vermont this year was triggered by the double-whammy of stresses that occurred the previous spring and summer when damaging late frosts were followed by a significant July drought. These combined stresses may have been sufficient to trigger an increase in the number of preformed flower buds initiated last summer, which then burst forth so noticeably this spring and summer as flowers, fruits, and now seeds.

Michael Snyder, a forester, is Commissioner of the Vermont Department of Forests, Parks and Recreation.

 
Discussion
  1. Julie Rothermund → in Magnolia, Texas (just outside of Houston)
    Mar 22, 2012

    Now I understand why our elm trees have produced hundreds of thousands (seriously) of seeds/now seedlings in our 10 huge flower beds and alongside the driveway in the grass - our unprecedented drought in 2011. Obviously, they have to come out, which will be quite a job and it will make me feel a little sad knowing how much energy they used up producing them.  The big trees look pretty good, so I think they’re going to make it.

  2. James Pierce → in Vienna, VA
    Nov 12, 2016

    Our 30-year old, 30-foot tall dogwood began to get dead branches last year, and this past spring we found three new dogwood saplings nearby.  They flourished all year with no special care from us (we were overseas) and are now well-established. I like to think the parent tree was feeling its mortality.

  3. Jim Dilon → in Zephyr Ontario
    Sep 04, 2017

    Last summer was one of the driest we’ve seen in quite a while.  This year the cedars are LOADED down with seed pods.  The branches sag with the weight as they would after a liberal dumping of snow.  Spruce, pine and hemlock trees crowns are heavy with more cones than I can remember ever seeing before.  I thought this was a portent of a nasty winter ahead, but maybe this is a result of last seasons drought. we’ll just have to wait and see.  Hmm, time to check the wood shed.

  4. Dave Mance → in Shaftsbury, VT
    Sep 05, 2017

    I’m noticing the same thing in southern Vermont with pretty much every tree species. Bitternut hickory, beech, maple, apple, hawthorn, are all loaded with seed and that’s just what i can see looking out my office window. We had some significant drought conditions down here last summer, too.

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