One Acorn, Two Acorns, Three Acorns, Four…

How to Evaluate Acorn Crops

a cluster of acorns
Acorn production potentials are unique to each landscape and change continuously. Public domain photo by Rob Norris.

By lying on your back under an oak tree, you can look up and estimate its number of acorns. But why?

“A lot of state wildlife agencies do acorn surveys annually because hunters want to know crop sizes, which fluctuate like crazy from year to year, among different oak species, and among locations,” says USDA Forest Service scientist Katie Greenberg. “Big acorn crops can help to increase reproduction and survival of deer, grouse, turkeys, and other game species that eat acorns. In fact, acorns are considered a keystone species because mice and squirrels also benefit from big acorn crops, and so do other animals – like hawks, foxes, and bobcats – that in turn eat them. Foresters are also interested, since acorns are the seeds for future baby oak trees.”

Land managers survey acorns visually, in various ways. “They lie down or stand under a tree and estimate the number of acorns on the tree crown,” Greenberg says. Although the process sounds simple, it has downsides. “Some of these methods are labor-intensive. Also, when agencies each use different methods, it’s hard to compare them across the whole region.”.

Greenberg and collaborators knew there had to be a better, more efficient way.

Her 2020 article in The Wildlife Society Bulletin builds on earlier studies that build on each other to show how to best quantify acorn production for wildlife and forest management.

In 2007, Greenberg collaborated with the North Carolina Wildlife Resources Commission, using 21 years of its acorn data to verify that when more trees are producing acorns, they’re also producing more acorns per tree.

Because of that strong relationship, land managers can rank the acorn crop (for example, poor, good, or excellent) simply by determining the proportion of oaks in their forest with acorns – a simple yes or no instead of laboriously totting up its number of nuts. This is known as the percentage-bearing-acorns (PBA) method.

Of course, looking at just a few trees won’t do the trick. It’s important to survey hundreds of trees to get an accurate idea of the proportion of oaks with acorns.

The researchers also determined that the PBA method could be easily converted back to the traditional hard mast indices used for years by wildlife agencies.

“It was reassuring to know that land managers could use the new, easier PBA method to replace traditional, more time-consuming methods without losing their past, hard-won information on how crop sizes vary among years and locations, because they were comparable,” Greeenberg says. “Another big advantage is that this method offers a standardized approach to assessing acorn crop sizes. If agencies use the same index method, crop sizes can be compared throughout much of the Southeast.”

The next step was to see if you could use those visual estimates of the proportion of trees with acorns to figure out the actual number of acorns – not just if it was a good or poor crop, Greenberg says. Her most recent study shows how visual surveys can be translated into the actual number of acorns in a forest stand in any given year.

black mesh bags underneath oak trees
These acorn traps are bags into which acorns fall. “We put three traps under each tree,” Greenberg says. “We collect acorns every couple of weeks over the course of the whole autumn, September through November, then take the acorns to the lab and count them.” USDA FS photo by Julia E. Kirschman.

To find that out, she compared seven years of visual acorn survey data with counts of fallen acorns from the same trees in Bent Creek Experimental Forest in North Carolina.

On average, the eyeballing estimates matched the trap counts. But even then, without an idea of how many oak trees are in the forest, having an estimate of acorn counts per square meter of crown is not a whole lot more informative than a general ranking of crop size. “If it’s a great crop but there are only three oak trees in your forest, it doesn’t mean much.”

So, Greenberg took it one step further. With her method, if you know how many oaks of each species and size are in your forest, and you know the proportion of oaks bearing acorns, you can estimate how many acorns are being produced that year. “It’s a more meaningful way to estimate acorn crop sizes, because it provides an estimate of actual quantities of acorns, rather than a general ranking of poor, good, or excellent crop sizes,” says Greenberg.

Some land managers and forest planners are less interested in each year’s acorn crop size but want to know how many acorns their forests can potentially produce on average. In a related study, Greenberg and colleagues developed a way to model a forest’s average acorn crop. If land managers know the number of oaks in their forests and how big they are, they can estimate how many acorns that forest could produce on average.

Later, Greenberg and colleagues added their acorn averaging model into the Forest Vegetation Simulator (FVS) – the growth-and-yield modeling system of the USDA Forest Service that virtually “grows” trees over time. Using the FVS-ACORN module, land managers can predict future average acorn production for their forest. Land managers can also use FVS-ACORN to see how different forest management actions – like regeneration harvests – could affect acorn production on their forests, and figure out how many oak trees to leave to maintain a supply of acorns for wildlife.

“Each of these related studies adds another piece to the puzzle,” Greenberg says. “Together, this research provides tools for land managers to rapidly index acorn crop sizes and convert those rankings into the actual number of acorns in their forest each year, or estimate average acorn production and examine how those numbers might change over time, and with different forest management scenarios.”

Read the full article in The Wildlife Society Bulletin.

Watch Greenberg explain this research in a PBS video.

For more information, email Katie Greenberg at katie.greenberg@usda.gov.

Access the latest publications by SRS scientists.

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