A goal of many landowners is to maintain a “healthy forest”. This has a variety of meanings, depending on the landowner’s objectives. Among other things, a healthy forest contains many healthy trees. A healthy tree is one that creates enough of its own food to grow vigorously and resist insects and disease. Often on woodlots, however, the trees are too crowded to be healthy.
Like all plants, trees need sunlight, water, nutrients, and room to grow. When trees grow too close together, they compete for these resources, and few get enough of any of them to be healthy.
Trees live by photosynthesis. This is how they make their own food from sunlight and carbon dioxide. If a tree doesn’t have enough foliage in relation to its mass, it’s not creating enough carbohydrates for it to thrive. It may just be surviving, or slowly dying.
White pines are particularly sensitive to growing too close together. In a crowded stand of pine, the trees’ lower branches can’t survive in the deep shade created by the closed canopy. This reduces the amount of live foliage, which reduces the live crown.
Live Crown Ratio
Live crown ratio is a term that describes the amount of live foliage on a tree, expressed as a percent of total height. The total height of the tree from the ground to the tip is one hundred percent, so the percentage of that height covered in live foliage is the live crown ratio. A tree should have at least thirty percent live crown ratio to be healthy.
A tree with a low live crown ratio is just surviving or slowly declining, leading to stress, which makes it much more susceptible to insects or disease.
Diseases that impact white pines include needle cast, Caliciopsis canker, and blister rust. These fungal diseases impact white pines in various ways, but the net effects are slow growth, degraded wood quality, or death. In a densely stocked pine stand, there is less air flow and more moisture, which are conducive to the fungal diseases that impact pines.
How to Improve Health and Vigor
To improve the health and vigor of a crowded pine stand, the stand needs to be thinned. Thinning focuses on keeping the healthiest trees and cutting the competitors. The trees in the stand should have a minimum average of thirty percent live crown ratio. If most trees in the stand have less, they won’t respond well to thinning and will be more likely to break or uproot in a windstorm or under heavy ice or snow. Trees left to grow after a thinning should not only have the fullest crowns, they should also have good form – as straight as possible and mostly free of crooks caused by white pine weevil damage. White pine weevil is an insect that lays its eggs on the terminal bud (the tip) of white pines. The larvae feed on the terminal leader, which then dies. A side branch takes over, creating a crook in the trunk. Repeated weevil damage gives a pine a shrubby appearance and destroys the lumber value. If you’re going to invest time and money into thinning a pine stand, the work should not only improve the health and vigor of the remaining trees, it should also result in higher value future timber. Even if timber isn’t the primary goal, it’s beneficial to maintain the option to harvest good quality logs in later years.
The best time to thin a pine stand is when it is around three to six inches in diameter at breast height (DBH, 4.5 feet above the ground), and at least sixteen feet tall. Thinning at this size will ensure that there will be at least one log that is free of weevil damage and plenty of wood to grow over any pruned knots.
At this point the treatment will be pre-commercial, meaning that the thinning does not result in revenue from the cut trees. They can be left on the ground to provide habitat and eventually become soil.
Pre-commercial treatments are an investment in the future stand. Funding for this work can be provided out-of-pocket, from the proceeds of a commercial timber harvest, or through government funding. The Natural Resources Conservation Service (NRCS) can provide funding as well as technical assistance for pre-commercial treatments. Your county Extension forester can also provide you with advice on these practices.
Thinning pine stands can also be done when the trees are of commercial size, generally twelve inches DBH and up. It’s best to start when they’re smaller than this, however, when there are more options regarding trees to grow. If a landowner waits until a commercial harvest, they likely lose opportunities to improve the trees’ form and value. For example, a young weevil damaged stand often has straight trees among the crooked ones. The dominant trees in the stand tend to be weevil-damaged, and they continue to dominate the stand if it is not thinned, resulting in a stand of large weevil-damaged trees of little to no value. Precommercial thinning earlier in the stand’s cycle can remove the damaged trees and favor the straight trees while it’s inexpensive to cut them. Waiting until a stand is of commercial size before thinning can also result in trees with poor live crown ratios, in which case thinning is no longer a viable option and heavier cutting is necessary.
There is more to thinning a pine stand than described here. Interested landowners can seek advice from a professional forester. UNH Extension County Foresters are a great place to start. They can assess the stand, provide advice on its suitability for thinning, and point you toward professionals and resources that can help get the work done.