Wednesday, September 15, 2010

Lost Pecan Trees

Looking out my kitchen window, past the top of my neighbor's house, I surveyed the trees of the neighborhood as I washed dishes.  I counted two pecan, two hackberry, one crape myrtle, blooming pink, one sycamore, and one cedar elm tree.  Two years ago I would have counted four pecan trees, but two of those trees had died since then, leaving behind thick trunks and tall, leafless branches that seemed to be locked in perpetual winter.

Driving down Red River south of 45th Street, I was surprised to see another two dead pecan trees standing side by side.  Before this year, they were older, well-established trees of beautiful stature that had survived many droughts.  I wondered if there was a disease, a fungus that was to pecan trees what oak wilt was to oak trees, that was killing the Austin pecan trees.  I began noticing that many pecan trees, including the young pecan trees in the median strip of Airport Road near the Mueller development, had top branches that were missing leaves.

Walking down the Boggy Creek Greenbelt trail, my feet crunched over fallen pecans that were still green, and still tightly wrapped in their protective coat.  Squirrels skittered overhead, seeming both excited and disappointed by the unripe pecans as they chased each other from tree to tree.  I wondered if the squirrels were responsible for the premature pecan fall, or if the trees were dropping pecans to conserve water.  To answer my questions, I found out what I could about pecan tree health.

Although there are fungi (including cotton root rot and downy spot) and insects (including webworms and the twig girdler insect) that damage pecan trees, there isn't a widespread biological epidemic affecting Austin pecan trees.  Instead, the drought of 2008 and 2009, the worst two-year drought in 60 years according to the Austin Tree Experts, was the most likely killer of so many pecan trees.  Those pecan trees didn't leaf out this spring because they didn't have enough water last summer and fall, near the end of two years of drought, when the water was needed to make another year's leaves and to store sugar before winter dormancy.  Winter and spring rains arrived too late for those trees, which were already dead, though it wouldn't be obvious until leaf-out time in April.

Pecan trees (Carya illinoiensis) are native to the floodplains of the Mississippi valley and the bottom lands of east Texas and northeastern Mexico.  The pecan trees growing in Austin represent many different varieties of the tree, some native to central Texas, some native to the Mississippi valley, and some developed for agricultural production.  The varieties that are native to this area are better adapted to survive through drought years, while the non-native and agricultural cultivars need more water and are therefore more likely to die during water shortages.  This explains why I've been seeing dead pecan trees in backyards and along streets, while the pecan trees along the greenbelts (more likely to be native varieties) seemed to have fared okay.

Because pecan trees are adapted to the hot summers and relatively cool winters of river valleys, freezing temperatures can also injure the trees, especially if the freeze occurs before the tree is fully dormant for the winter.  Pecans are among the last trees to drop their leaves in the fall, hanging on to them well into November in Austin.  Last year was colder than usual in Austin (it was actually the coldest winter since 1983-1984 and the 8th coldest winter on record), with more frequent cold fronts and fewer warm days than usual, especially in December and February.  Several hard freezes happened in December, which is after pecan trees should be dormant.  Given how slow pecan trees are to drop their leaves, though, it seems possible that a tree or two might have been caught with still-active phloem (sap-carrying tissue) during one of the December freezes.

Early leaf drop, and the early dropping of unripe nuts, is normal for pecan trees that are water stressed.  In late summer, when temperatures max out and water is scarce, trees can increase their chances of survival by dropping some leaves, each of which is a site of water loss from the tree.  This "drought-deciduous" habit helps many native trees, including live oaks and hackberries, survive the extended Texas summers. 

Pecans reach the "water stage" of nut development in late July to early August, when kernel development begins in pecans that are now full size.  If the tree is stressed in the water stage, up to half of the developing pecans on the tree may be dropped in the interest of the remaining pecans, and in the interest of the long-term survival of the tree itself.  So I guess I don't get to blame the squirrels.

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