Eating and Hoarding—How Do Animals Influence Woodland Regeneration?

It may come as a surprise to many that Pennsylvania's forests lack a sufficient number of young trees to grow to become the next forest. For at least the past thirty years, the U.S. Forest Service has noted a steady decline in the number of small-diameter trees in our forests. Periodic inventories conducted by the Forest Service show that there are insufficient numbers of seedlings and saplings to form the next generation of forests.

Not even delving into the factors that influence the survival of seedlings and saplings, there are a variety of factors that determine which tree species will regrow (regenerate) in a woodland. In some stands, the dominant species may be one that regenerates best by cutting to promote root suckers or stump sprouts. In many cases there may be seedlings already present, waiting to be "released" with a removal of some overstory trees or control of competing vegetation in the understory. Sometimes those seedlings aren't present yet--the seeds from overstory trees have not yet taken root, or there are seeds stored in the forest floor. The seeds stored in a forest floor could have fallen from above, be blown there from a neighboring forest stand, or be deposited after being carried in a stream's current or floodwaters, and then buried by leaves and duff. Another important way that seeds are stored in the forest floor is by animals. Unlike many understory plants whose small seeds can adhere to fur or feathers and be passively dispersed that way, trees typically have larger seeds, which animals disperse through frugivory or scatter hoarding.

The term frugivory describes an animal species' preference for and consumption of fruits (soft mast). Seeds are dispersed as an animal eats fruit in one location and the seeds are eliminated from its digestive system in another location. This might bring to mind an image of a mammal's scat containing paw paw (Asimina triloba) or persimmon (Diospyros virginiana) seeds. Seeds from blueberry and huckleberry bushes, too, can be dispersed through frugivory by mammals, as well as by birds. It may be easy to recall shrub species that are eaten and dispersed by birds, including Rubus species (blackberry, raspberry, etc.) or small trees like serviceberry (Amelanchier spp.). Likewise, the seeds of black cherry (Prunus serotina) trees can be distributed over long distances by birds and mammals. As summer gives way to fall, it's common to see groups of cedar waxwings and other birds flitting about, devouring the drupes of black cherry. Interestingly, studies have shown that black cherry seeds that have passed through the digestive tract of passerine birds have higher rates of germination.

Scatter hoarding is another common way that both mammals and some bird species help to disperse tree seeds over distances the larger seeds of some species would otherwise not travel. Scatter hoarding is exactly what it sounds like--seeds are gathered by an animal and cached in multiple locations. This type of seed dispersal is key in the reproduction of nut-bearing trees like oaks, hickories, and walnuts. There are benefits to an animal in storing seeds in multiple, scattered locations. Most significantly, it reduces the chance that the total food stores will be wiped out by opportunistic pilferers.

Since acorns are an important food source for a variety of mammals and birds and since oaks are of interest to many foresters and wildlife ecologists, there have been a number of studies about the relationship between acorns and wildlife, including squirrels, mice, and blue jays. Numerous studies have shown that the behavioral decisions of these animals are strongly related to the ecology and distribution of oaks. Particularly, these behavioral responses are related to acorn characteristics. White oak acorns usually germinate during seed fall or shortly after, sending out a root radical in search of a good growing site. Acorns of white oaks are an important food source, some years representing a large proportion of available hard mast. White oak acorns have lower levels of tannins, making them more palatable and easier to digest than red oak acorns, in addition to their being more readily available in autumn. However, the acorns of white oaks contain lower amounts of lipids, which are the primary source of digestible energy, according to 2004 research paper by Michael Steele and others (see references below). They also note that red oaks, on the other hand, have more tannins and fewer lipids. An earlier study by Smallwood and W. D. Peters (1986) found that grey squirrels "selected acorns low in tannin levels in autumn when energy requirements were low and food was relatively abundant." In winter, they selected acorns with higher lipid levels--even when they contained higher levels of the tannins which make them less palatable and digestible. Grey squirrels will selectively store the acorns of red oaks, which are less perishable due to the tannins, and eat the acorns of white oaks soon after they fall.

Just as animals adapt their behavior relating to acorns, Steele and Smallwood (in a 2001 paper) noted that the germination of white oaks in autumn is an adaptation to prevent the acorns from being eaten by mammals, birds, and even insects. As soon as the root radical develops the acorn becomes less digestible--making it a race between the acorn eaters and the acorn. Even still, grey squirrels may open the acorn, bite off the indigestible root radical, and eat or store the white oak acorn.

This relationship between acorn predators and both white and red oaks is key to how oaks are spread throughout forests and is part of the process of regeneration. Because red oaks acorns are stored and sometimes forgotten about--by both squirrels and blue jays--they may be the first oaks to regenerate in a woodland. They also may become dispersed further from the parent tree when dug up and recached by pilferers. This behavior of animals is important to the reproduction of oaks, as well as to regeneration success of other nut-bearing (hickories, walnuts, etc.) or fruit-bearing species (black cherry, paw paw, eastern hackberry, etc.) whose seeds are dispersed by animals. When seeds are spread further from the parent tree, competition with other seedlings for available resources are likely to be reduced. Predation and pilfering are also likely to be reduced.

Interactions between tree seeds and animals play an important role in influencing what grows in a woodland, and points to the complexity of forests. Understanding the ways seeds are dispersed in a forest--whether by gravity, wind, water, or animals--is key to predicting what seedlings are likely to grow. Looking beyond tree species that are commonly considered desirable species, seed sources may also include non-native invasive plants. For this reason, landowners should seek the advice and planning expertise of a forester. A walk in the woods with a service forester from the PA DCNR Bureau of Forestry is a good place to start (search online using the words "service forester" and "PA DCNR"). Because around 70% of Pennsylvania's woods are privately owned, landowners can play an instrumental role in helping neighbors become aware of regeneration concerns and sharing trusted resources with others.

To learn more about the process of regeneration and what actions landowners can take, contact your local PA DCNR service forester or the Center for Private Forests at Penn State at 1-800-235-9473.

For more about animals' roles in seed dispersal, read:

"Oak Dispersal Syndromes: Do Red and White Oaks Exhibit Different Dispersal Strategies?" in Spetich, Martin A., ed. 2004. Upland oak ecology symposium: history, current conditions, and sustainability. Gen. Tech. Rep. SRS-73. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern Research Station. 311 p.

Bartlow, A.W., Lichti, N.I., Curtis, R., Swihart, R.K., Steele, M. A. "Re-caching of acorns by rodents: Cache management in eastern deciduous forests of North America," Acta Oecologica, Volume 92, 2018, Pages 117-122.

Bartlow, A.W., Kachmar, M., Lichti, N.I., Swihart, R.K., Stratford, J.A., Steele, M. A. "Does multiple seed loading in Blue Jays result in selective dispersal of smaller acorns? Integrative Zoology. Volume 6, 2011, Pages 235-243.