trees

Acorns: Science & Mysteries

Penn State Forest Stewards series article – written by Jim Finley, Professor Emeritus, Forest Resources Management, Center for Private Forests at Penn State.

Oaks are economically important tree species in Pennsylvania and across the East. Acorns, or mast – a word derived from old English which means “forest food,” are important to wildlife. Oak-borne mast production varies from year to year. Much research has sought to predict masting years, when big acorn crops occur, and shed light on what leads to poor years – or early acorn abscission (detachment) – like you may have seen this season.

Science and Mysteries of Acorns - Pennsylvania Forest Stewards article

Oaks separate into two groups, popularly referred to as red and white, and learning to identify the difference between the two groups is easy.

Red oaks – e.g. Northern red oak, pin oak, scarlet oak, and black oak – have small “bristles” on the lobes and tips of their leaves. White oaks – e.g. white oak and rock, or chestnut, oak – lack these bristles, and have rounded leaf lobes.  (CFC note: the image above is of a white oak.)  There are other differences that are more difficult to recognize, such as acorn structure and wood anatomy.

The Process of Creating an Acorn –
and the Trials of 2020

Both red and white oaks produce female and male flowers on the same tree, unlike ash which has male and female trees. The process of producing an acorn starts late in the growing year when the male flowers form as the tree’s growth slows toward the end of summer. That is the end of the first year (year 1) in the process. Then, in the second year (year 2), as the tree comes out of dormancy, female flowers form in the axil of the leaf stem and the twig and remain dormant. As the spring leaves begin to unfold, the male flowers emerge and are very apparent as rather-long, drooping, greenish-yellow catkins. These appear about two weeks before the much smaller female flowers emerge.

White Oak Acorn Production

For the white oaks, as the male and female flowers emerge in year 2 as described above, pollination and fertilization should happen. The pollen from the male flower, which is wind-disseminated, lands on the style, which is part of the female flower. When this happens, the pollen initiates the development of a pollen tube that transfers male cells into the ovule to complete fertilization and the process of acorn formation should start in earnest. For this to happen, it is ideal to have warm days and cool nights. 

If temperatures are not right, fertilization may fail, and the female flowers will abort, which results in low acorn initiation. Alternatively, if temperatures become too hot or drought conditions occur, white oak acorns may abort, which is likely apparent in mid-June to mid- to late-July; perhaps that is the reason for reported early acorn drop this year. The other big threat to white oak acorns is late spring frosts, which also happened this year, and would again remove the fertilized flowers.

Trees and Dry Conditions

Penn State Forest Stewards series article – written by Allyson Muth, Interim Director, Center for Private Forests at Penn State.

Many areas across the northeastern US are experiencing dry and drought conditions, Pennsylvania included. As of July 28, two-thirds of Pennsylvania was in an abnormally dry to moderate drought condition. Yet, tallied across the state, we are very close to the total predicted rainfall for an average year to date. Depending on where you look in Pennsylvania, the regions are somewhere between 25% below, to as much as 26% above annual precipitation. These numbers don’t sound like they would result in two-thirds of the state be in abnormally dry conditions. So, how does that work?

PAFS article tree health during drought

Climate Change

Climate change models for Pennsylvania have consistently predicted the pattern that we’re seeing this summer. Less frequent, but more intense rainstorms, with extended drought periods occurring between those large rain events. The result is highly variable and uneven conditions across the state. Conversations with landowners in the south central region have shared their recent 4-inch rain storm events and subsequent flooding. In many other Pennsylvania regions, rains came early, and we’ve entered a dry summer period with rather infrequent rain events.

Pennsylvania is known for its waterways – over 86,000 miles of streams, creeks, and rivers. We usually have ample water during the growing season. We rarely have the water conservation requirements that the US West and Midwest implement to conserve water for human consumption. But when it’s this dry, do we need to worry about the trees?

Contributing Factors

Healthy, established trees can normally withstand relatively long periods of drought, so long as there are intervening months or years that are more favorable. However, recently planted trees, or trees with small soil footprints (rootprints?) are more susceptible to decline under these dry conditions.