Essential Differences

Heard a bluebird about a week ago.  There are very few phenomena which can be described indifferently as occurring at different seasons of the year, for they will occur with some essential difference.  Journal, Nov 3, 1853

Henry David Thoreau's attempt to made a detailed seasonal nature calendar was probably doomed.  After my own "years of observation," I am astonished at how variable events can be.  Not only do the early-blooming trees vary year to year by more than a month, these trees have not been varying together, but each following its own mysterious impulses.

Henry David Thoreau is well-known among modern naturalists for his close, systematic, and long-continued observation of nature in his hometown of Concord, Massachusetts.  Among his ambitions was to record the events of nature precisely enough to predict--almost on a daily basis--the flowering and fruiting of plants, arrival and departure of migrating birds, mating seasons, and other events of the season.  Although Thoreau's interests varied from year to year (one year he hatched and followed snapping turtles), he did one concentrated, conscientious "year of observation" around 1852 in which he determined to record every seasonal event.  He was building his own version of the seasonal calendars that were popular at that time.

I have been doing my own "years of observation" in my own little neighborhood over the last three years or so.  The wild swings of the last few springs have convinced me that nature is predictable only within wide limits.  Last year the warm winter brought out the early-blooming trees far earlier than the year before.  This year we had another warm winter--one of the warmest Februarys on record worldwide, and definitely the warmest ever in this region--and I looked for the same trees to bloom at about the same times as last year.  To my surprise, some bloomed earlier, but some bloomed later.  So nature is still less predictable than I'd thought. 

Silver maples began blooming April 4, 2015, but February 29, 2016 and February 23 this year!

Quaking aspens bloomed around April 10, 2015, but March 26th, 2016, and was in full bloom before March 22 this year.

 Red maples bloomed April 15, 2015, but March 11, 2016, and is just beginning to bloom as I write on March 25th.  (Males above, females below.)

But maybe it's better this way.  A predictable Nature would be a boring Nature.  In truth, the closer you look, the more Nature remains full of surprises.

Evolutionary Thinking

Silver maples (Acer saccharinum) beginning to bloom in late February.

Flowers are red; females have finger-like pistils with pollen-receiving surfaces.

Males each have a puff of numerous, white pollen-bearing stamens.

Over the last two weeks I've watched the silver maples coming into bloom.*  Silver maple (Acer saccharinum) is monoeious--having separate male and female flowers on the same tree.  I had noticed that the female flowers mainly opened a little earlier than the male flowers.  This makes sense as a way to encourage outcrossing--reducing chances of a tree fertilizing itself.  (Inbreeding often results in unhealthy offspring.)  Further, the female flowers were mainly low on the tree--which makes sense considering that pollen is more likely to lose altitude than gain it in drifting from tree to tree.  They tended to be at the ends of branches, also

But this year I also noticed that there were many more male flowers than female on each tree.  Why? I wondered.

It isn't immediately obvious that there shouldn't be equal numbers of male and female flowers, but there certainly might be an advantage to a ratio other than 1:1.  Let's try a little "natural selection thinking."  Natural selection should drive the evolution of the flower sex ratio to one which maximizes reproduction.  What influences might be at work?

Unlike most animals, plants are typically hermaphrodites--simultaneously male and female--therefore any silver maple can be mother of some offspring, and father of other offspring.  And, also unlike most animals, silver maples depend on the wind to carry sperm (packaged inside pollen grains) from tree to tree.  On the one hand, wind pollination is the chanciest way to have sex: surely not more than a few pollen grains in a million finds its random way to a receptive female flower on the right kind of tree.  (My neighborhood walks cover perhaps a square mile in which I count only a half-dozen or so silver maples).  Therefore a lot of investment in pollen gives only a very low return in offspring.  Does that mean any individual tree is better off with a lot of female flowers? 

On the other hand, it is the female, like in most animals,** that actually has to bear the offspring, and all the costs associated with that.  (Male flowers usually drop off the trees at the end of flowering, but female flowers are only just getting started: they have to grow into the fruit and seeds.)  Having said that, silver maples disperse their young with wings--depending on the same winds that disperse their pollen.  That means silver maple trees don't need to create large, nutritious fruits that will attract animal carriers, the way many plants do.  So it isn't clear which gender has the better return on investment: the male that must produce large amounts of pollen, or the female that must grow the fruit and seeds.

On the other other hand, it does no good for all the trees to have only female flowers: where would the pollen to fertilize them come from??  Natural selection works not for a particular individual, but for the average individual.***  To put it another way,  a silver maple tree is just a gene's way of making more of itself.  In order for this to happen, somehow or other natural selection MUST advantage the reproduction of the average individual: any competing adaptations that didn't would simply lose out in the race to dominate the genes of the next generation.  It seems as though the balance of forces acting on silver maples results in very male-heavy trees--though just why is difficult to say. 

What does the science say?  As a genus, maples have a lot of variety in their flowering patterns, and silver maple is referred to as "labile," meaning that expression of flower types and ratios is flexible.  I found nothing specific to silver maples, nor monoecious plants in general; but dioecious (separate sex) plants have been investigated.  A study of the deioecious alpine herb Rumex nivalis found that, in the presence of a higher density of male plants, offspring tended to be female; but if neighboring males were few, male offspring were favored.  This is a flexible response, rather than one fixed by genes more narrowly.  Another study hypothesizes that sex ratios (again in dioecious plants) may interact with stress tolerance through hormones that influence both these things.   A large comparative survey of 243 species looked at a number of hypothesized influence on sex ratios.  This study found that male-biased species were more common, and more often associated with long-lived plants like trees, and fleshy fruits dispersed by animals (neither of which is true of silver maples), while female-based plants tended to be herbaceous, clonal (often reproducing by runners, etc), have wind-dispersed pollen and plants having sex chromosomes.  This study finds that costs of reproduction, mechanisms of pollen and seed dispersal, and chromosomal sex determination can all play roles.  The short answer as to why silver maple is so male-heavy hereabouts? no one knows.

Silver maples in full bloom, beginning of March.

*Tragically early, just like last year: the delicate flowers and developing fruits will be doomed by days of temperatures in the teens or twenties, like those over this weekend.  Are these warm winters mainly a consequence of climate change, or just a fluke of weather? 

**A conspicuous exception being the seahorse, in which the male holds the babies in a special brood pouch.  Just as intriguingly, most of the gender-related trends in other animals (such as which gender selects the mate) are reversed in seahorses. 

***Which is why it is perfectly possible for natural selection to act against individuals, as long as it benefits individuals on average!  There are nifty examples of this in David Sloan Wilson's amazing book, Evolution for Everyone. 

Spoke too soon...

 This week it became clear that I was wrong in describing how well the red maple flowers had weathered snow and temperatures that plunged into the twenties.  Because this week most of the female flowers have dropped from the trees, virtually ending their chance of having any kids this year. 

 April 2nd, just before the snow. 

 female red maple flowers.

 male red maple flowers.

 

I missed the signs of trouble on April 5th. And the

male flowers (bottom) were on their way out, anyway.

 On the 9th & 13th the flowers were blackening & shrivelling.

By the 21st so many had fallen that I wonder if any will survive at all.

Red maple’s flowering so early appears to have been a bad “decision.”  Of course, seasonal weather from year to year varies quite a bit, and so mistakes will be made—sometimes growth will begin later than is optimal for taking full advantage of the frost-free growing season, while some years—such as this one—growth will begin too early and a year of reproduction will be lost.  We can expect that the genes of these trees are optimized to split the difference between these two timing risks; that is what natural selection does. 

On the other hand, climate change is making warmer winters more common, and too-early blooming more common as well: what then?  Probably there are variations in timing among red maple trees--I know of one near my house that was weeks later in blooming than the others—and if this tree and others like it reproduce more successfully year after year, it’s more-successful, late-timing genes will spread through the red maple population: in other words, red maples will evolve to better fit the new conditions.  That, too, is what natural selection does!

The silver maples (Acer saccharinum) bloomed earlier and were farther along when the snow and freezing weather hit, and may have fared better.  Time will tell, but it looks like some developing fruit survived. 

Silver maple ice-rimed on the 3rd.

Silver maple beginning to leaf out a few days ago,

and with some maturing fruit (bottom)

Early Bloom

I was just about to talk about the long period of late winter and early spring when nothing much seems to be happening in the plant world, when I came upon a twig on the sidewalk.  Silver maple, I was pretty sure.  Positively packed with swelling buds, as usual.  And did I see flowers peeking out of a few?  Already?!  And in WINTER, for God's sake!  Last year silver maples didn't flower until the beginning of April, more than four weeks later!

Over the next day or two I paid a visit to every silver maple I knew about, and also a good many quaking aspens, which bloom at about the same time as silver maple.  Sure enough, silver maples are popping more than four weeks earlier than last year, and quaking aspens aren't far behind.  The prolonged warm weather we've had lately is probably to blame.  How?

 Silver maple (Acer saccharinum) on Moraine St almost in full bloom on 2/29.

Silver maple on Tuesday, 3/1.  The white filaments sticking out all over are stamens shedding pollen.

 Pussy willow at the high school.

 Some of the quaking aspen (Populus tremuloides) buds above look larger than their neighbors

because they are expanding to flower.  Below are unopened buds.

Quaking aspen at the top of my street is almost in full bloom on Tuesday 3/1.

Why the early blooming?

Trees go dormant in winter on a schedule fixed roughly by the length of uninterrupted darkness.  (By interrupting darkness, streetlights sometimes play havoc with tree life cycles.)  When nights become long enough in the fall, processes are set in motion that cause leaves to turn color and drop, and dormancy to begin.  But dormancy doesn't end according to night length: instead, the tree must experience a certain minimum number of hours of sufficiently cold temperatures.  Just how cold, typically a little above freezing, and for how long, depend on the tree species.  This cold requirement reduces the chance that a tree will become active and grow leaves and flowers during an early thaw, only to be killed by a later cold snap.  But once the tree has clocked the required number of cold hours, any prolonged warm spell can break dormancy and send the tree into action for the new growing season.  

This early flowering was caused mainly by a fluke of the weather.  But over the long term we should expect plants to flower and leaf out earlier in a warming world.  This has consequences that could prove disastrous.  For example, a British bird called the great tit depends on a good supply of winter moth caterpillars to feed their growing chicks.  The winter moths, in turn, eat the leaves of English oak. The birds have always laid their eggs at the right time so that caterpillars are at their peak just as the chicks are at their hungriest. However, warming climate has led to earlier growth for the oaks and caterpillars, while the birds have been left behind.  Laying their eggs too late, the birds fledge fewer and lighter young, putting stress on the great tit population.  It bodes ill that most species studied so far react differently to climate change than the species they depend on.

Food chain being stretched to the breaking point?

Great tit, English oak, and the winter moth and caterpillar. 

(All photos from Wikimedia Commons.)

Spring 3b: Maples

Silver maple (Acer saccharinum) flowered very early-- at the same time as quaking aspen.  I place it here with other maples for comparison.  Red maple (Acer rubrum) bloomed more than a week later than silver maple, and sugar maple and Norway followed close behind.

 Red maple is the only maple around here that is typically dioecious--male and female flowers on separate trees.  Pictures are all of a young female tree.  [Whoops! so is ash-leaved maple, below.]

Silver maple April 29th, May 4th, 5th, 7th, 11th and 16th.

Red maple (always same twig) April 28th, 29th, May 4, 7th, 11th and 16th.

Sugar maple (Acer saccharum) flowers and leafs out at almost the same time--rather like Norway maple.  Sugar maple, like silver maple, is monoecious--having male and female flowers together on the same tree.  Unfortunately, there are no low twigs on any of the trees I monitor.

May 7th, 11th, 13th and 16th.

(Sorry for the contortions.)

Norway maple, an alien invasive introduced as a street tree, gets included because it is so common here.  Norway maple has rather confused flowers: a single flowers cluster may have some that are male, some female, and some that are both.   (Look in the early photos for the central styles of the female and the outer, powdery, pollen-bearing stamens of the males.)

Norway maple (Acer platanoides) May 4th (2), 5th (2), 11th, 13th and 16th.

Only well after flowering did I discover a few ash-leaves maples (aka boxelder) at the high school.  Unlike other maples, ash-leaved maple has compound leaves similar to the (unrelated) ashes.   This makes five species of maples in this area!

Ash-leaved maple (Acer negundo)

May 8th (3) and 14th (2).