CROP TREE SPOT SELECTION
by David Lloyd, RPF

Why are Crop Tree Spots important?
Crop tree spots can be defined as those places where your crop trees will grow. In the past stands often supported 200-300 crop trees/hectare. Now, in our most productive working forests, we are seeking 700-1,000 uniformly, fast growing, highly valued stems, with short rotational ages to economic maturity. This transition is no small task and requires deep understanding of the soil/stand interaction dynamics within your plantation areas.

Effective crop tree spot selection will determine how quickly we meet our crop development goals, whether these be short-term, like 70 cm to be cattle deterrent, 1.3 m to exceed ungulate browse height, 1-3 m to achieve free-growing/green-up heights; or longer-term, like duration until revenue flow achievement through commercial thinning, or final harvest size. Seedlings planted in crop tree spots will establish faster and will dominate the future stands. Seedlings planted in ineffective places will remain suppressed and contribute to waste in the final harvest of short rotational stands. Waste results in lost profit! 

Longer durations to plantation performance achievements often increase the costs. They require greater numbers of inputs reduce crop growth rates, increase risk of non-achievement, and lengthen revenue return/amortization periods. In addition, costs associated with extended adjacency periods for surrounding stands, which have already achieved their economic rotation age, must be considered. If you don’t realize what extending amortization curves past age 40 does to the exponential value growth of your investment, create a spreadsheet of it right now!

Concepts for Understanding Tree Growth vs. Crop Tree Spot Selection/Development 
If we can’t grow roots, we can‘t grow trees! Root establishment/early growth is the most important factor in new seedling establishment. Roots must be placed in their new “home” with care for their growth requirements. Our objective must be to establish growth momentum first in the root growth, and then in the foliar growth.

Oxygen is the most important nutrient! Root metabolism requires that oxygen be breathed in at the growth point. Soil porosity must be friable enough to allow oxygen to move through the soil atmosphere. Tree seedlings cannot extract oxygen from water against the force of the hydrogen bonds. Therefore soils must be aerated and humid but not wet to encourage root growth. 

Soil temperatures must be in the 10o to 25oC range to promote root growth. Wet soils will be colder, dry soils may be hotter; both will inhibit root growth and should be avoided throughout the root plug depth profile. 

Two centimetres of organic litter will protect deeper soils from excessive drying. Exposed mineral soils will dry down 10 cm to provide the same amount of humidity protection. Unfortunately, in cold climates soils are often too cold to promote root growth at depths greater than 10 cm.

The thicker the roots are, the more foliar growth will occur next year. Roots have annual rings. A two year-old root can easily be 1 cm thick near the seedling’s stem. These are the root diameters required to support strong foliar growth. Dig up a well-established seedling that is several years old, and compare the root ring size with the initiation of strong leader increments. You’ll find that strong root diameter growth precedes strong leader growth by one year.

Storage sugar needs must be filled by year’s end to ensure effective survival through winter and strong growth for next year. Fulfillment is indicated by waxy green needles, large stem diameters and large bud sizes and numbers. Inadequacy (chlorotic yellow needles) forces the seedlings to re-hydrate/soften and begin photosynthesis too early next spring, making them susceptible to mortality from normal spring frosts.
The most important 10 seconds in a tree’s life are those between the planting bag and the enclosed new root home. In the end, the tree planter is the most important person on the planting site, for it is the planter who delivers the infant organism from the protected nursery community to the final growing site, the new root home. Roots must be protected from the sun’s photooxidation and housed in a humidity-sealed, aerated, non-compressed, nutritious soil environment, all within a few seconds if the planter is to make money. Perhaps the most important thing that planting team leaders can do is to help their planters to establish a rhythm of efficient good behaviour in the delivery of seedlings to their new root home. Only through the continued, diligent application of good behaviour will this tough job be completed properly. Everyone on the planting site needs to know what good behaviour is and be easily able to identify it for each other.

Seedling growth momentum development is illustrated in Figure 1. Note that the ratios remain consistent with stem height to diameter at 45–50:1, and stem height to crown diameter at 2:1 for openly growing seedlings. Narrower ratios indicate increasing vegetation height competition. Note also the growth momentum in bud size, numbers of buds, and stem volume. The first two years are critical in exponential growth establishment. If the exponential momentum is not established in years one and two, then the development will be delayed and the seedlings’ ability to occupy the sites quickly and easily may be lost.

Roots are missing in Figure 1. Effective seedlings have roots colonizing a circle twice as large as that of the waxy green photosynthetic crown foliage. Root growth is more important than top growth in early seedling establishment.

Planted poorly, the spring root growth momentum is missed and the seedling will flush without root growth, so nutrients will not be available for foliar completion to waxy, green, photosynthetic needles. Instead, though satisfactory leader length extension may occur, the needles will be chlorotic yellow and unable to manage their water resources. These cannot photosynthesize sugars effectively. Such seedlings do poorly through the mid-summer quiescence, particularly in drought years, and will not be able to produce sugars to support root growth in the fall. Then the seedlings will go into winter with insufficient stored sugars, and will remain too depleted to grow roots in spring. In the following spring top growth cycles, leader extension will be very restricted, until the roots can finally get ahead of the tops. We call this cycle “planting shock” but more appropriately we should call it “incorrect planting shock”.

Success of our planting program can be measured by the second year leader length, which should exceed 15-70 cm depending on the climatic capability. Leader extensions of 2–5 cm in the second top growth year indicate root growth failure properly due to incorrect planting.

Root home characteristics are illustrated in Figure 2. Where do we normally find the largest trees in our forests? They’re always on high places. Why do we dig our soil pits in low places? Is it because there’s less chance of encountering roots there? High spots have a different vegetation community, which tend to open up earlier in the fall, allowing greater light penetration to establishing seedlings than do low spots. 

In Figure 2, note that the composting, nutrient providing, fermenting layer tends to be thickest on high spots. The composted, organic clay, humic layer is thickest in the low spots. The water table that restricts oxygen availability and chills the soil is closest to the surface in the low spots. The litter layer is uniform across the site, and is critical to maintaining the integrity of the nutrient contribution of the fermenting layer, and should not be scraped off in the planting process. Screefing off the litter layer, only turns the top 2 cm of the fermenting layer into litter, thereby reducing its effectiveness.

Soil temperatures are warmer on the high spots, extending the growing period both throughout each day and each season. Therefore, roots have the greatest seasonal growth capability in the fermenting layer on high spots.

The seedlings planted on high places tend to bear more waxy, green, photosynthetic needles, which tend to be bluish on spruces. As we drop off the side of high places, needle waxiness tends to decrease until foliage is chlorotic yellow in the low, wet places.

In Figure 2, note that a cavity is left below the root plug. Planting holes 1.5 times the plug depth is recommended, leaving an oxygen opportunity beneath the plug where roots are not compressed, or “J’d” and can grow freely into soils below. Planting holes should be narrow, minimizing disturbance to the natural soils. Seedlings should be placed at one corner of the hole and the soil closed gently against it to form a vapour seal 10 cm deep. 

All foliage remains above ground. The most temperature-resistant part of the plant is the root collar area. Foliage is sensitive to both surface soil temperatures and penetration by soil pathogens. Burying foliage in the planting hole is a bad and often lethal habit that some planters use in an attempt to get the roots closer to water. Both ideas are wrong.

New roots tend to grow along the interfaces between the fermenting/humic and mineral soils. Seedlings should be placed with the top of the plug within the litter layer, and roots spanning all three other horizons. As long as the shovel tip contacts mineral soil, roots can be placed exclusively in organic soils and the roots will access the environments that they need.

Always keep seedlings at least a shovel width (10 cm) away from stumps. Never slip the plug down the side of stump. This will place the roots in litter only, and probably doom the seedling to an early death.
Site improvement must be the goal of expensive mechanical treatments. Costs of $0.20-$1.00 per planting spot are being expended, often without improvement of the site, and then are poorly utilized by the planters. Mechanical site improvement should supplement, not degrade, the existing natural high spots. The treatments should raise the profile height and mix mineral with fermenting layer materials. Roots grow well in such spots and poorly in purely mineral spots. Given the high cost of mechanical treatments, it is usually better to pay a few cents more for greater care in placement of the seedlings, and less on mechanical treatment.

Mounds of humic black materials are similar to those of mineral clay, and produce similar results in poor seedling root growth and early frost mortality. 

Mound sizes do not need to be larger than a 30 cm square; just enough to get the seedling established and let the roots grow out to access surrounding areas as needed. Larger mounds waste mechanical energy, and tend to turn up much more non-productive parent material, non-biological soils.
Planting in mechanically treated areas must find the highest profile of mixed fermenting/mineral soils for oxygen, nutrient, and moisture availability. Planting in pure raised mineral soil can cause growth retardation.

Early performance differences are evident from the review of all plantations. Figures 3 and 4 illustrate such differences from seedlings planted at Woss in the spring of 2006. In Figure 3, note the green, waxy needles throughout the seedling, the large stem diameter, and large numbers and size of buds. All of these indicate strong root establishment momentum, and a seedling well prepared to survive its first winter and grow well in its second top growth year.

Figure 4 shows a poorly planted seedling, slipped in against a stump. Foliage is a chlorotic yellow, indicating non-waxed needles with little ability to resist leaching of nutrients or to manage water reserves. Stem diameter, bud sizes, and numbers are small. Food reserves are inadequate to resist winter/spring stresses. Root colonization would be small, probably sparse, and not further than the branch tips, since the needles are not effectively photosynthetic. This microsite offered the opportunity to be a crop tree spot, but the chance was lost by planting too close to the stump.
Establishing high performance plantations through utilizing crop tree spots has an intricate simplicity of artistry. It’s not rocket science, it’s way more complicated than that! We need healthy, biologically capable seedlings that are:
• carefully handled to minimize shocks
and stored sugar loss
• well planted/housed, and in time with
their biological cycles
• planted soon after harvest for advantage
over competing vegetation
• fertilized as suitable for the climate and
soil sites
• protected from animal browse as
required

David Lloyd, RPF, is Manager, Product Research Education at PRT and can be reached at 604-465-5411.