What Triggers the Blooming of a Tree?

One of the events I look forward to in June is the blooming of the iconic tree of our area – a canillo. It is Miconia rubiginosa. It has a distinctive look – trunk a little gnarled, bark rough and reddish, leaves in clumps at the ends of the branches.

When it blooms, in June as I said, the whole grove lights up.

But this year it bloomed in early May, nearly six weeks earlier than usual. (Sorry, but my pictures from that day are horrible. The blooms, though, looked like these, above and below, taken in June last year.)

Why such an early bloom?
Plants may not bloom at exactly the same time every year, but it struck me that this year’s early blooms might possibly have something to do with the heavy rains in April.

It has been argued that either moisture availability or sunlight could limit photosynthesis in trees and thus limit the time of year when there is enough energy in the plant available to produce flowers, fruits, and seeds. A study by Zimmerman and colleagues found that, on a community level in tropical forests, sunlight was more important than moisture in triggering the blooms. But at a species level, things were different. Some species found in both seasonal and nonseasonal forests bloomed at roughly the same time of year whereas other species did not. Overall, most species in both places produced flowers, fruits, and seeds when irradiance was highest, but there were some that bloomed when less light and more moisture were available.

My take-away point from this study is that, in general, sunlight provides the energy necessary for a plant to start the reproductive process. However, different species may use different levels of sunlight, whether determined by day length or cloud cover, as a trigger to set flowering in motion. So, does M. rubiginosa look for June day length or June light level to determine when it will bloom?

We are close to the equator. Day length does change throughout the year, but by relatively little. So, for instance, the total day length for the first week in May is about 12 hours and 48 minutes (calculated at the sunrisesunset). The total day length for the first full week in June is about 13 hours. The average difference in day length, then, from early May to early June is 12 minutes. A relatively small difference. Perhaps the flower-triggering clock in M. rubiginosa was a little sloppy this year.

Maybe it’s not so early
However, it is also true that the heavy rains in April caused a reduction in light level for the month. Enough to bring it down nearly to the level that it was in May of 2009. If light level, rather than day length is the trigger, maybe the clock was not so sloppy. Could it be that April’s low light level triggered an early May bloom?

I’ve found no literature on when M. rubiginosa usually blooms, and probably many conditions must be met for the tree to bloom. Maybe it’s just a coincidence that we had much more rain than usual, and much less light than usual, in April of this year and that M. rubiginosa bloomed in early May.

Or perhaps, after all, the clock was simply off. Hey, it happens in this house all the time.