July’s REVIEW piece lands on the subject of maple regeneration. Put simply, how do big trees make baby trees and what factors promote or inhibit that process. This review comes courtesy of a doozy of a 2021 titled paper “Complex drivers of sugar maple (Acer saccharum) regeneration reveal challenges to long-term sustainability of managed northern hardwood forests.” The team of authors, all from the mitten-shaped state to the north, was led by Catherine Henry from Michigan State’s Department of Forestry.
It goes without saying that it requires a whole bunch of seeds to hit the forest floor in order for a single tree to reach maturity decades later. But just how complex is the regeneration struggle for sugar maples? After all, despite a lifespan of 300 years give or take a century, if mature sugar maples do not replace themselves with seedling, sapling, and teenager sugar maples, the ultimate goal of passing one’s genes on to the next generation will fail. Henry et al. examined research plots in 141 different forest stands to dig into factors related to sugar maple regeneration throughout northern Michigan. The study sites were all managed with single-tree selection silviculture for decades, a forestry practice that is commonly regarded to be a great tool for regenerating and recruiting sugar maple. It is important to note potential geographic differences between the study’s region and Ohio; not everything will necessarily apply to our state, but we can learn from their findings as well.
Sugar maples are generally considered to be shade tolerant tree species, and that is a fine way to categorize them from a 30,000 feet above the surface of the earth perspective. Zoomed in up close however, a simple shade tolerant descriptor is insufficient. Sugar maple regenerate best under conditions of intermediate canopy openings, and successful production of seedlings and saplings is optimized in larger single-tree gaps that are maintained or increased through time. The truth is that while maple seedlings are technically shade tolerant, more light is required as regeneration grows into sapling sizes and beyond. Prolonged deep shading stunts out maple regeneration, and it is important to remember that shade doesn’t just come from overstory trees; ferns, dense midstories of beech, and invasive plant infestations can all starve a cohort of seedlings of the light they require to become saplings and ultimately larger trees. In addition to growing space, variables of deer browsing pressure, site quality (related to soils), and competing vegetation were considered.
The very first line of the study’s Results section reads as follows: “Stand-level sugar maple regeneration was highly variable within and among size classes.” It goes without saying that nature contains tremendous variation, and this statement reinforces that idea. Examining one forest stand and anticipating the next forest to behave identically is foolish, and taking one study and assuming that it directly applies to a novel new region is equally foolish. All that said, there are absolutely some lessons to be learned.
Maple regeneration was most successful at intermediate basal area levels and at sites with intermediate quality. Imagine an upside-down U where the peak is in the middle and the start of the curve and end of the curve are low, that’s essentially what the graph would look like. This plays well with the Goldilocks analogy that we like to use for sugar maples – sugar maples favor conditions that aren’t too _______ but also aren’t too ________, just like Goldilocks didn’t dive straight into a bowl of scalding hot or freezing cold porridge. The study had some educated guesses as to why this may be. Excessive basal area (a forest stand that is overstocked) and too many sugar maples in the overstory casts deep shade that even the shade tolerant maple babies can’t survive. Too few mature maples in the overstory may be limited by seed availability and more easily overwhelmed by deer browse pressure (see photo above). Low quality sites for sugar maple, duh, did not have a lot of vigorous healthy sugar maples. But high quality sites were often associated with higher deer densities that likely led to overbrowsing of seedlings and smaller saplings. An additional explanation is that overstory maples grow so quickly on high quality sites that canopy gaps quickly close thus reducing understory recruitment.
The study is published in the journal Forest Ecology and Management, and authors are encouraged to write a rather in-depth closing section called “Implications for management.” In these parting paragraphs, Catherine Henry and her colleagues boldly comment that single-tree selection silviculture – a system that ought “to produce ample sugar maple regeneration” – is failing. While the study’s results were highly variable, factors of deer overbrowsing, site quality, light limitation, and seed availability confounded attempts to successfully recruit sugar maples to the sapling size class in nearly 70% of plots. The solution is not easy or obvious. While the authors point to silvicultural harvest methods that open more growing space and release more light into the understory – namely uneven-aged group selection and even-aged shelterwood harvests – they acknowledge a different approach may exacerbate other problems (denser shrub densities, higher success of undesirable species). Regardless of harvest method, a parting recommendation was that managers take deer population management seriously through increased hunting take or use of exclusion practices, such as wire fencing or natural slash walls.
Bringing this review home to Ohio, what are the over-arching takeaways. While there are undoubtedly more to consider, I’ll quickly point to 3 recommendations. #1 – Deer can destroy even the best laid plans, our state mammal HAS TO BE managed. The best possible silvicultural plan can quickly unravel with too many deer. #2 – Cutting a single tree here and a single tree there is not likely to recruit your next generation of syrup-making trees. #3 – Work with a state or credentialed forester to develop a management plan for your woods. They will understand the complexities and caveats to navigate a timber harvest and help you balance your objectives against the impacts of the past, the conditions of the present, and the goals for the future.