Galls are irregular plant growths which can be stimulated by the reaction between plant hormones and powerful growth regulating chemicals produced by some insects, mites, nematodes and fungi. Galls may occur on leaves, twigs, flowers, buds or roots. Many plants serve as gall hosts, but certain plant groups are more attractive to gall producers than others. The Oak family is by far the most popular (with 805 species of gall makers; the next largest family being the Daisy family, with less than 200 gall makers). Galls on oaks are most often caused by small wasps or midges.
Each gall-making species of insect produces a uniquely shaped and colored gall. Thus, it is possible to identify the insect within a gall just by noting the appearance of the gall itself as well as what plant it is on.The growth of the galls takes place in the spring. Gall-making insects lay eggs on the host plant, and the insect larva resides inside the gall that the plant forms. The galls provide the insects within them with both shelter and food. Because many oak leaves persist well into the winter, there is still the opportunity to find galls, though some may be lacking residents at this stage, as many insects emerge as adults in the fall after pupating within the galls.
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Deciduous trees typically lose all of their leaves by late autumn. However, when snowshoeing or skiing through the woods this time of year, one is likely to find a scattering of deciduous trees that still have leaves clinging tightly to their branches. These plants are exhibiting marcescence, the trait of retaining plant parts after they are dead and dry.
Most deciduous trees form a layer of cells called the abscission layer at the base of each leaf’s stem, or petiole, where it attaches to the branch. This layer is composed of thin-walled cells that break easily, allowing the leaf to drop. A thin layer of corky cells seals the tree at the spot where the leaf was attached. Abcission layers are not formed on marcescent trees such as oaks and the American Beech, all members of the family Fagaceae. Therefore, their leaves do not fall off as readily, and many remain attached through the winter.
The evolutionary reasons for marcescence are not clear, though theories include defense against herbivory (e.g. browsing by deer), protection of leaf buds from winter desiccation, and as a delayed source of nutrients or moisture-conserving mulch when the leaves finally fall and decompose in the spring.
Leaf marcescence is most often seen on juvenile trees, and on the lower branches of older trees.
It’s common knowledge that you can often find warblers at this time of year by looking up into flowering oak trees, as the male flowers (catkins) attract an important source of protein for birds in the form of insects. Oaks (all parts) are host to more than 550 species of butterfly and moth larvae as well as many other invertebrates, many of which are attracted to catkins.
It turns out that birds are not the only creatures that visit oak catkins in order to secure a meal. Although we think of Gray Squirrels as consumers of nuts, seeds, fruit and fungi (and bird eggs and fledglings), their preferred food in the early spring includes the nutritious buds and catkins of oaks, elms and maples. Apparently their taste buds are not the same as humans’, as people who have consumed catkins say that their taste leaves a lot to be desired. (Photo: Gray Squirrel eating Red Oak catkins. Thanks to Sadie Brown for photo op).
There are approximately 600 species of oaks in the genus Quercus, all of which are in the Beech family. This genus has two subgroups, the red oaks and the white oaks. The leaves of trees in the red oak group have sharply pointed lobes with bristles. The white oak group has leaves with rounded lobes lacking bristles. Although usually there are some leaves on an oak tree that persist well into the winter, it is helpful to be able to identify a species by its buds alone. Oaks tend to have multiple terminal buds. Northern Red Oak’s terminal buds are large, pointed, cone-shaped and covered with reddish-brown, mostly hairless scales that overlap like shingles, with one edge covered and the other edge exposed.
Of the 2,000 kinds of galls found on North American plants, 800 different kinds form on oaks. One of these is the woolly oak leaf gall, produced by a tiny Cynipid wasp, Callirhytis lanata. This gall is usually attached to the mid-vein on the underside of an oak leaf, and looks like a ball of wool. It may be as large as three-fourths of an inch and is often bright pink or yellow in color, fading to light brown in the fall. Oak trees have lots of tannic acid in them (a defense which makes the tree unpalatable to herbivores), with the highest concentration found in oak galls. (The bitter taste is where the name “gall” originated.) It’s possible, since tannins are somewhat anti-microbial, high-tannin galls such as the woolly oak leaf gall may protect the larva against fungi and bacteria.
There are several species of poisonous mushrooms in the genus Amanita in the Northeast that are referred to as “destroying angels” but the most widely distributed and commonly encountered is Amanita bisporigera. It has a smooth white cap, gills, a skirt-like ring underneath the cap surrounding the stem (annulus) and a swollen stem base enclosed in a cup-like structure (volva). As it ages, this mushroom often acquires an odor reminiscent of rotting meat. Destroying angles are mycorrhizal with oaks – the underground portion of this fungus surrounds a tree’s rootlets with a sheath, and help the tree absorb water and nutrients while the tree provides sugars and amino acids to the mushroom. Destroying angels are among the most toxic known mushrooms, and closely resemble other white mushrooms that are edible. Consult an expert mycologist before consuming any mushroom that even remotely looks like this!