Africans in Hawaii

                                    11/12/10 KNOW THY ENEMY

                                                                                                                   

When you see photos of the tops of the forest canopy in Hawaii and there are large patches of bright red-orange (and sometimes yellow–orange) they will most likely be the blossoms of African Tulip trees.  Here they are an invasive weed that crowds out many of the native trees and the older orchards.  I’ve written about them and my efforts to control them more than once.  Today I had time to take a closer look so, along with a photograph; I’m going to tell you how they make such a nuisance of themselves.

As the African Tulip trees have very soft wood they are easy to cut down but from each stump 2 to 8 new shoots will sprout in a couple weeks.  In six months the shoots can be six feet tall and 1 ½” diameter.  In addition, like many tropical trees they have wide ranging surface feeder roots and they can send up sprouts and certainly will when the trunk is cut.  Even the logs laid on the ground will grow a sprout about every 4 feet in the right conditions and they will keep on doing so for a couple years in spite of my cutting them off every 3 or 4 weeks.  So far I’ve not applied herbicide to the stumps but I might some day.

Recently on the first sunny day after 3 days of rain I was sitting on the North shore of the Hakalau River mouth looking south.  The macro view was of a steep canyon wall covered in dark green of many different species.  In between, in the air over the river and the park on the far side, the air was sparkling with the drifting white winged seeds of the African Tulip.  The seeds have the shape of a Chinese Elm – a disk with a wing all around it – but they are 2 or 3 times the diameter.  It was a calm day so the seeds were descending with the drift at about 12:1.  Except, I then noticed, about one seed in 30 was rapidly assenting.  As I watched it became apparent that individual seeds were suddenly changing from decent to assent for several seconds and a climb of 12-20 feet then reverting to decent.  I couldn’t tell if an individual seed could do this more than once but I knew from a past observation what was happening.  At some point in the drying of these seeds they are capable of flapping their wings to gain elevation.  Not only that but the tree also has evolved the right mechanisms to open its pods and release those seeds at the proper time to give them the best chance of flying the longest distance.  The days a lot of seeds are released are always sunny days after a rainy period.  I’d love to see this in slow motion cinema so if you know of such footage please let me know.

Another thing African Tulips excel at is the abundant numbers of seeds they produce.  Each 4” x 4” flower produces a pod 7-9” long.  Each pod has a wall down the middle from which the 550 to 900 seeds grow.  The pods split open from one side only so that first the chamber on the open side releases its seeds.  The following day, if the weather is dry, the umbilical wall drops out and the remaining seeds can scatter.

Because the pod usually only splits from one side the empty pods look like little canoes and they work quite well for that purpose.  You can see flower, green pods a half open pod and an empty pod in the picture.