Forestry 101: All in the numbers

Naturally occurring crown lift is the physiological response to competition for available light and space. (Illustration by Amy Vaughn of Vaughn Creative)

Planting density, or trees per acre, is not a preference, but rather a complement of landowner objectives coupled with an understanding of site resources and an educated forecast of future markets.

Site Index

Site productivity is an initial consideration when contemplating planting densities.

The carrying capacity of the land will drive the ranges of densities considered and landowner objectives must be in alignment to prevent premature stand stagnation or under-utilized site resources. Poorer site indices may lend themselves to lower densities, providing more resources to less stems, reducing competition for water, nutrients and sunlight, therefore maximizing the growth potential for dominant and codominant trees. The bottom threshold of trees per acre still needs to maximize utilization of the carrying capacity of the site.

Conversely, higher site indices exhibit greater carrying capacity and may constitute more trees per acre to fully utilize site-specific resources. But it is necessary to understand, on a given site, the disparity of volume over time between planting densities that begin to converge as the site reaches carrying capacity and the trees near biological maturity. A similar relationship for basal area can be observed. The growth rate for the stand does not decrease with higher trees per acre, it is just distributed across more stems.

Product Markets

Raw material markets, over time, will change due to global demand, mill closures and technological advances. Planting density can provide necessary flexibility.

Depressed pulpwood markets may suggest lower initial densities to minimize or remove thinning operations. Tree improvement programs have capitalized on genetic indicators, reducing branch length, diameter, angle and quantity, as well as minimizing the needle count, enhancing tree form at low densities while maximizing growth rates. But emerging markets may, in time, create additional pulpwood demand and therefore thinning opportunities, subsequently encouraging higher planting densities. Energy, pellet and biofuel markets indicate a preference for thinning pulpwood. More trees per acre will allow for intermediate income from thinning regimes while growing higher value products to maturity.

In addition, Louisiana has seen an influx of new lumber mills. The trees supplying these markets can be slightly different in form and age. Diameters can be smaller, lengths longer and rotation ages younger. While tree improvement programs have done an exceptional job improving tree form at low planting densities, higher densities can reduce live crown ratios further by forcing natural crown lift, fostering better tree form and higher value end-product potential.

These trees can then be utilized in the historical cut-to-length plywood markets, tree-length log markets or pole markets. Given the appropriate age, this diversity of potential products provides some security under variable market conditions.

Tree Form

Lower planting density, on any site, will allow space and resources for maximizing tree growth. Trees may be shorter with large boles and high ratios of live crown. Naturally occurring crown lift may be slower at lower densities due to available sunlight. The lower initial stem density of the plantation may eliminate the need for a thinning operation to remove competing pulpwood, but it could generate more pulpwood per stem at final harvest.

Higher planting densities may require mid-rotation thinning to reduce tree on tree competition, but the tree forms derived from such initial density may be conducive under market dynamics. Competition for space and resources can cause stems to go up rather than out. Trees may be smaller in diameter, but often are taller with reduced live crown ratios, maximizing the log portion of trees to the minimum top size accepted at market.

Harvest Regimes

Thinning operations can be eliminated if the stocking is low enough, removing potential plantation stagnation while maximizing stem growth. Although, some issues may arise around tree form and product merchandizing, at final harvest all stems should be available to all markets in some capacity, while avoiding being held captive to a depressed pulpwood market.

Pulpwood demand may increase with technological advances in engineered wood products and growth in large scale biomass industries. Plantation stagnation from higher planting densities can be avoided by providing feedstock to these pioneering operations as well as existing pulp and paper markets, releasing stands from like species competition and growing product for solid wood facilities.

Additional Considerations

Long-term forecasted weather conditions appear unpredictable and Louisiana has experienced excessive rainfall followed by drought within the past 10 years. Lower initial densities may introduce understocking risks from poor survival due to inundated soil conditions in the flatwoods or xeric conditions in the sand hills, leading to additional capital to recapture the site. A reasonable buffer from failure can be calculated into the use of higher density plantings, often providing security to meet landowner objectives.

Storm hardening of plantations can be a supplementary function of planting density and can work across multiple spacings. Lower densities, with maximum growing space per tree, can grow large trees quickly with more durability per stem against windstorms. Plantations utilizing higher planting densities will allow trees to work collaboratively with each other transferring wind energy from crown to crown as one tree leans into adjacent trees, thereby reducing storm damage. Care must be taken not to overthin these plantations, leaving the ability for residual stems to adequately transfer wind energy.

When comparing planting densities, species should remain constant. Response to space availability and consumption

of resources can vary greatly by species and initial planting densities may require adjustment based on changing the species prescription.

Tree spacing within the row becomes a function of maximizing the distance between the trees while allowing for equipment to enter the stand without damaging residual stems or crop trees. A need or desire to thin low density stands may develop under market demand. Regardless of planting density, the row spacing should align to equipment size and width, driving the spacing within the row. Regional harvesting techniques can be comparatively different and local methodologies should be applied to row spacing layout.

Physiological Spacing

Understanding tree physiology will maximize utilization of site resources and subsequent tree response. Planting density will allow for changing market conditions, prescription modification and risk mitigation. Capitalize on the experience of professional foresters to help establish region specific criteria and set planting densities based on forecasted market dynamics.

(Bruce Colclasure is a forester with SilvaGroup Advisors LLC. He can be reached at bruce@southland.group.)