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NEW TECHNIQUES REVEAL NEW ROLE FOR MECHANICAL SITE-PREPARATION

Twenty years ago, the first step in establishing most industrial pine plantations was preparation of the sites with huge machines so that they looked like they could be plowed. More recently use of herbicides have replaced the big machines in site-prep, but continuing research and experience has revealed that both techniques may have a place. To investigate this subject, we interviewed Dr. Howard W. Duzan, Jr., manager of Weyerhaeuser Company's research station in Columbus, Mississippi, and here's what we learned:

JMV:	What's happened recently in mechanical site-prep?
HWD:	Some of the first site-prep that involved was done with bedding. We didn't really understand what it did; we mostly used it in very wet areas to get the trees up out of the water so they would survive. But in the last few years, we have started to see benefits from cultivation that aren't associated with wet sites.
JMV:	For instance?
HWD:	One of the newest techniques is ripping or subsoiling. A ripper is steel shank hooked behind a powerful tractor. You lower it about 12-24 inches into the ground and pull it through the soil. If the soil has the right moisture content, the shank will not only pull a trench through it, but also will shatter it, make cracks out to the side. The trench will soon fill back in with soil, and then the ripped area will have improved soil physical properties. This expands the rootable zone, i.e., allowing seedling roots to penetrate a large area more easily. Studies have shown that trees in ripped areas may put on 50% more roots in the first year than they would in an area that has not been ripped. There are many kinds of implements currently available that can be used for subsoiling or ripping.
JMV:	Is all this extra growth due to loosening the soil?
HWD:	No, ripping or subsoiling can also produce two other effects. 1) It reduces competition from both herbaceous and woody vegetation in a strip along the trench which improves survival and growth. 2) It allows you to plant the seedling deeper. If you have a very dry first year, the seedling can continue growth because part of its roots are lower in the soil profile where some moisture remains.
JMV:	How big is the plus from ripping, and does it eliminate the need for herbicides?
HWD:	At least one study shows that the plus you get from ripping is about the same size as you get from use of herbicides. This is not surprising since both treatments reduce competition. Both expand the rootable zone near the seedlings because they eliminate plants that are occupying part of this zone. However, herbicides don't improve the bulk density of the soil as ripping does. There appears to be a favorable effect of the cultivation of ripping in addition to control of competition. A study conducted by with scientists at Mississippi State University on Weyerhaeuser land showed effects of the combination of ripping and herbicides. Two years after planting, total lateral root development was 543 cm on seedlings with no treatment, 1435 cm on those treated only with herbicides, 1705 cm on those treated only with ripping, and 2980 cm on those treated with both. Crown development is almost directly proportional to root development, and since the needles are the food factory, tree size is directly related to crown development. I want to point out, however, that this is one study on one soil, and results may not be the same everywhere.
JMV:	What is "bulk density?"
HWD:	Bulk density is the weight of a unit volume of dry soil divided by the weight of an equal volume of water. The more pore space (or air pockets) there is, the lighter the soil weight, and therefore, the lower its bulk density. The amount of organic matter in the soil also affects bulk density. The soil in your garden prepared for planting probably has a bulk density less than 1.0. You would probably have a hard time driving a spade into soil with a bulk density of 1.6 or more, and tree roots would have the same problem.
JMV:	What do you do with logging debris left on the site?
HWD:	Most rippers will go through a fair amount of logging debris. You can also put a V-blade on the front of the tractor and make it easier by pushing debris to one side.
JMV:	Do you use ripping on all sites?
HWD:	No, it depends upon the soil you are dealing with. If you have a deep loamy soil, ripping won't help much because you don't have much root restriction in the first place. Soils with high bulk density or compacted layers are the best candidates for ripping. A heavy clay soil shouldn't be ripped when it's too wet. The ripping shank creates a groove with sealed sides that holds water, so the situation is worse than in the beginning. Finally, ripping must be done on the contour; otherwise the small troughs that it creates may lead to serious erosion. Ripping works best of all when there are hard layers in the soil: hard pans, plow pans, or other natural barriers to root growth.
JMV:	Tell us about bedding.
HWD:	A lot of work has been done on bedding. It was first used to get trees out of the water on wet sites, but it does a lot of other things. Most bedding plows pull top soil from a strip six or seven feet wide and pile it up on a three-foot strip in the center where the trees are planted. You concentrate nutrients and improve bulk density in this strip, and you eliminate a good amount of both woody and herbaceous competition. You can't use the practice until logging debris has been removed. The biggest gains from bedding result from improving drainage on wet sites and usually increases site index by three to five feet (up to seven on the wettest sites). The negative about bedding is that you pile 12 inches of well-tilled top soil on top of undisturbed original soil that may have a high bulk density; consequently the trees may grow well for two or three years and then run into problems as the roots attempt to penetrate the original soil profile.
JMV:	And disking.
HWD:	Disking six to eight inches of the surface soil gives good tillage and good initial control of herbaceous competition near the surface. But obviously it doesn't provide the deep benefits that you get from ripping. There may be some survival improvements from disking. However, like bedding, the site must be cleared of residual material prior to disking.
JMV:	You make it sound like the first two years are the most important ones in the life of a plantation.
HWD:	They are. The first and second years of seedling development are important for the later development of the plantation; they are critical to getting fast tree growth. Our desire is to get excellent survival and then get the trees up above herbaceous and woody competition very quickly. Finally, anything you can do to increase tree size in the first two years will produce a healthy and fast-growing stand.
JMV:	We've been talking about site-prep. Does anyone cultivate existing stands?
HWD:	Very little work has been done on cultivation of existing stands; it seems to have few benefits. If you cultivate between the rows of a plantation, you are almost certain to cut off a lot of fine roots, and this will be detrimental to your goals. If the object is to reduce competition from other vegetation there, herbicides will likely do the job better.