Over the past 16 years, annual rainfall in our area has averaged 183 inches. That’s a little over 15 feet. On average, most of this rain comes in the months of August through October, with 25-31 inches, or more than 2 feet, of rainfall each month.
So far this year we’ve had 134 inches (11 feet) and we’re two weeks away from August.
Some days, the standard 5-inch rain gauge doesn’t hack it.
Just saying.
In June we experienced a record rainfall of 43 inches. The previous record was 41 inches and the average for this area is 22 inches in June. Here’s a graph of the erratic 2010 rainfall (red line) compared to a 16-year average (blue line).
Back in May, I noted the early blooming of Miconia rubiginosa after the heavy rains in April.
Now, in July, I’ve seen two plants in bloom that I normally see bloom in August: a tree with large bunches of yellow flowers, Senna hayesiana, and a miniature relative of the Elephant Ear herb, Xanthosoma mexicanum. I’m also seeing flower buds on an unidentified Miconia that usually blooms in September. I’m sure more’s going on along this line than I’m seeing.
Maybe these large swings in time of blooming are relatively normal for the tropics. But I’m starting to think that it’s more than coincidence that different plants from three unrelated families (Senna = Fabaceae or bean family, Xanthosoma = Araceae or arum family, Miconia = Melastomataceae or “black mouth” family) are blooming immediately after uncommonly large monthly rainfalls.
It is now clear that The Thing which appeared on my tree, Allophylus psilospermus, is indeed an example of witch’s broom (or witches’ broom in the plural, if you write for the fungus and lichen journal Mycologia).
To refresh your memory,
A Witch’s broom is a disease or deformity in a woody plant, typically a tree, where the natural structure of the plant is changed. A dense mass of shoots grows from a single point, with the resulting structure resembling a broom or a bird’s nest.
First recorded instance in Sapindaceae
The Allophylus tree is in the family Sapindaceae, the soapberry family, and I went looking for other instances of witch’s broom in that family. But Dr. Pedro Acevedo, of the Smithsonian Institution and a specialist in Sapindaceae, wrote to me that this is “…the first time that I know of the occurrence of witches’ brooms in Sapindaceae.”
The agent that causes this witch’s broom is not known, either, unsurprisingly. An expert in fungus, Dr. Meike Piepenbring, wrote to me that she has seen several witches’ brooms on avocado and on native plants here in Panama, but not on Allophylus. She found the presence of, not fungi, but aphids. After studying the literature, she concluded that the most probable agent is for the witch’s brooms that she saw is mites, but she has not yet seen these mites herself.
Ted MacRae, in a comment on my earlier notes on this witch’s broom, had suggested that I might look for mites, and I did. I found one on the first “broom” I examined. I have since, very unscientifically, looked through more than a dozen or so brooms, chopping them up with pruning shears, and have found a few more mites, a couple of tiny beetles, a weird-looking white flat thing that I couldn’t begin to identify, several very small and various spiders, and no fungal spores, at least not any detectable with my 16X hand lens. I don’t have much hope of discovering the causal agent all on my own.
Not so innocent brooms
One little broom all by itself doesn’t look so bad, especially since it has taken the basic shape of the inflorescence and become carried away with it, so that it looks like it might give rise to a robust bloom of small flowers.
But once you know that witch’s broom is a disease and that it could harm the tree (for instance, witch’s broom is a grave threat to cocoa production), they’re not so attractive. Here’s a portion of a tree that is infested with these things.
Maybe even these green brooms don’t look so bad to you, but they look pretty nasty after they’ve “died” or whatever they do after they’ve accomplished their purpose. The brown or black ones may be a little harder to see, even after you click the image on the left. So I’ve circled them in the image on the right, which is also clickable.
No self-respecting inflorescence would end up looking like that!
It is my hope that a graduate student somewhere with an interest in Sapindaceae stumbles across this post and decides to write a thesis on witch’s broom in Allophyllus psilospermus! Wouldn’t that be great?
Our rainfall in May of this year was average. In fact, it was nearly exactly average: the 17-year average is 22.4 inches for our local area in western Panama; this May we had 22.7 inches.
On the following graph, the blue line is the 17-year average for each month of the year, and the red line represents the rain we’ve had so far this year. Months that have more than 10 inches of rain occur during our rainy season, thus our rainy season extends from May through November, a period of seven months.
March had rainfall pretty close to average, but February and April were way off the average line. Because April was so far above average (it qualified as a rainy season month this year) and because May was so on-the-nose average, I decided to look at just how often do we have average rainfall in any given month.
What is average?
Here’s a graph representing each of 17 years of rainfall, superimposed on the average (thick black line).
On seeing this graph, one person remarked that “It looks like there’s not a single average year on that graph!”
So I wondered – how many months actually have average rainfall? I created a table showing the deviation from average for each month. (You may have to click on the table to see it more clearly.)
I highlighted the months that differed from the average by 1 inch or less and then totaled those months in the last row. A total of 28 months during that 17-year period had within an inch of average rainfall. This is 14% of the months represented. We can therefore expect that we will have non-average monthly rainfall 86% of the time.
Over that 17-year period, August never had near-average rainfall. September had the greatest deviations from the average – more than two feet of rain (24.4 inches) above average in 1999 and 20.4 inches of rain below average in 2009.
Does El Niño have anything to do with this?
It turns out that the 2009/2010 El Niño dissipated in May of this year. You may have noticed in the table above that from June through September of 2009, rainfall was below average, usually several inches below average. This is the typical pattern of El Niño in Panama – below average rainfall in the rainy season – leading to droughts – and (slightly) above average rainfall in the dry season. With the dissipation of El Niño in May, our rainfall returned to exactly average.
If El Niños result in less rainfall in Panama during the rainy season, then La Niñas result in more rainfall during the rainy season. Note that the September 2-ft excess rainfall was in 1999 and that rainy season months August through November of that year all had higher than average rainfall. That year, 1999, was a La Niña year.
Back in graduate school, when I first learned about El Niño and atmospheric circulation as a basis for understanding the surface currents of the oceans, we learned that El Niño occurs more or less every 30 years. This gives you an idea about how long ago I was in graduate school! It seems these days that we’re either in an El Niño or a La Niña practically all the time. In fact, NOAA’s Climate Prediction Center states that “Conditions are favorable for a transition to La Niña conditions during June – August 2010.”
Grateful as we may be for average rainfall this May, it looks like average conditions won’t last long, and we may be in for some heavy rain this year.
Update: Carla Black has posted a comment giving the rainfall data from Volcan, which is to the west of us and uphill by a great deal, but still on the Pacific slope side of Volcan Baru. You’ll see that their May rainfall, unlike ours, was far below average.
A lesson from this observation is that local conditions can seem to overwhelm global conditions at any particular time. For instance, last month Carla sent me 13-year’s worth of rainfall data from Aprils in Volcan. This exchange was prompted because here in Potrerillos we had an extraordinarily high rainfall this April. When I plotted all 13 Aprils from Volcan vs. the same 13 Aprils in Potrerillos, I found that there was no correlation between rainfall in the two places whatsoever. Yes, there’s a slope to the line, but the r-squared value, which shows whether there is any significance to that slope, is about 0.2, which says that there is a 20% chance that the data are related. Or an 80% chance that they are not related.
So, if it’s raining in Potrerillos, we cannot predict at all whether it will be raining in Volcan. For example, she wrote that it had been raining for six hours in Volcan – at the moment I received her comment it had only begun to rain here and has now done so for about one hour.
Good lesson!
