| | Figure 1. Field installations of the Cooperative Research in Sustainable Silviculture and Soil Productivity. Gold Stars represent active sites, and the black star represents a site that was burned in 2000. The Fred and Bainbridge sites are 10 years old; the Bryceland site is 9 years old; the Black Lake and Lee Forest sites are less than 5 years old. |
| | | Figure 2. Nitrate produced in the soil as a percentage of the total mineral nitrogen produced (ammonification plus nitrification) measured in plots where the average height is above-average and where the average height is below-average.Although the percent of nitrate production differs with location, the percentage is consistently greater in the plots with the shortest trees. |
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Thomas J. Dean
For many years, sustainable forest management was judged by two simple factors: replacement of harvested trees and consistent yields. Because forestry in the United States originated in response to exploitive logging and land clearing, these were sufficient and effective indicators of sustainability. But today forests produce more than just wood and fiber. Sustainability also includes landscape-level properties such as biodiversity, soil and water conservation, global carbon cycles and socio-economic values.
Forest sustainability depends not only on large-scale factors but also on stand-specific properties such as soil productivity. Soil scientists and foresters recognize that site productivity can be compromised by harvesting and reestablishing trees. Effects accumulate through the years, and Louisiana is harvesting its third crop and planting the fourth.
Given the increasing reliance on high-quality sites to produce larger amounts of wood and fiber products, long-term productivity of individual forest stands has become a priority issue. Concerns over forest productivity extend beyond the obvious problems of surface erosion, rutting and mass wasting that can be initiated by road construction, harvesting and subsequent establishment practices that directly treat the soil. Prudent foresters are keenly aware that soil quality is key to the revenue potential of a site and its ability to produce other forest values. The concern now is whether organic matter removal and subsequent nutrient loss, combined with heavy machines working on forested sites, reduce the productive capacity of the soil despite proper soil conservation practices.
A greater expectation from timberland in Louisiana is anticipated as timber-related activities move from the Atlantic seaboard to the western Gulf region because of urban sprawl. The nearly exclusive use of rapidly growing, resource-demanding genotypes of loblolly pine on industrial lands may also tax the productive capacity of timberland.
Sustainability Project
In 1993, the LSU AgCenter joined with five other entities to evaluate how whole-tree conventional harvesting and subsequent establishment techniques affect the growth of new generation of loblolly pine trees. The five cooperators were the U.S. Forest Service-Southern Research Station, International Paper Co., Temple-Inland Forest Products Corp., Willamette Industries and Louisiana Tech University.
Since then, the cooperative – now called Cooperative Research in Sustainable Silviculture and Soil Productivity (CRiSSSP) – has grown. Roy O. Martin Timber Company joined the cooperative and Willamette Industries was bought by Weyerhaeuser Co. In addition, two new sites have been added, and one site was lost to fire. Trees in the oldest installations are now approaching canopy closure. The oldest CRiSSSP site is in its 10th growing season, and the youngest site is in its third growing season (Figure 1).
If harvesting and establishment affect tree growth, the effects presumably occur through changes in the soil. Therefore, the primary objective of CRiSSSP is to evaluate the effects of soil compaction and organic matter removal at levels created during normal harvesting operations. Rarely does the forest industry plant seedlings after harvest and not apply some treatment to stimulate early growth. Treatments such as vegetation control and fertilization applied singularly or in various combinations to reduce the time required for trees to reach harvest size.
The second objective of CRiSSSP is to measure the capacity of these various establishment practices to overcome negative harvesting effects and even to enhance the growth of the new stand. With careful monitoring and measurements, foresters can determine how harvesting and establishment practices affect the growth of the new stand. Another objective of CRiSSSP is to develop new methods for detecting management effects on forest productivity before they are manifested in the trees.
Experimental Protocols
The principal comparison conducted by CRiSSSP is the height accumulation of loblolly pine planted on plots where the stand was harvested with and without heavy machinery. Where machines were used, whole trees were removed from the plot, and where machines were not used, trees were cut with chainsaws, and only the commercially valuable portions of the stems were removed. Each harvesting method was combined with a series of various site preparation and follow-up treatments, such as burning to eliminate small logging debris. While harvest treatments were the same at each location, site preparation and other treatments varied by site.
Before harvesting a stand, data were collected to determine the total above-ground biomass, nutrient content and standing volume of pine trees and mature hardwood trees. In addition, a subsample of the best trees on the site was felled to determine height growth patterns. CRiSSSP uses tree height as the primary measure of growth response to the harvesting and establishment treatments because it is relatively unaffected by competition.
Preliminary Results
Height growth of the trees in the previous stand was measured by counting the number of rings on disks cut from the stems at regular intervals. The difference in the number of rings between successive disks represents the time required for the tree to grow that distance. Further analyses convert this information to tree height at annual intervals. Height growth of these trees can then be compared with the annual height measurements made of the trees planted on the harvested sites.
At two of the installations, cutting and dragging the whole tree with heavy machinery did not reduce the height growth of new trees planted on these sites when compared with the growth pattern of the best trees in the previous stand, at least during the first decade of growth.
While removing the whole tree from the site with heavy machinery did not seem to reduce height growth of trees in the new stand compared with trees in the previous stand, the heavy disturbance caused by whole-tree machine harvesting sometimes affects the height of the new trees compared to trees planted on other plots where the trees were cut with chain saws and usable portions lifted from the plot. This is seen when the height curve of the trees planted on the plots harvested with minimal disturbance is higher than the height curve of trees planted on plots harvested with maximum disturbance.
Since machines greatly increase harvest efficiency, can the reduced growth that may occur with machine harvesting be offset with additional treatments? According to our results for the first decade of growth since harvesting, the answer appears to be “yes,” if the treatments reduce competition from other vegetation such as seen at the Bryceland site. Adding nutrients through fertilization resulted in significant height increases at the Fred site but had no effect at the Black Lake site. Burning the site to clear small logging debris reduced growth at the Bryceland site for six years after planting. At the Lee Memorial Forest site, burning reduced height growth the first two years after planting.
Finding soil properties affected by harvesting that also affect tree growth has been difficult. While machine harvesting can measurably compact the soil, tree growth may only be affected if the soil was already dense before harvest. Some soil effects that were measurable one year after harvest – such as the total amount of carbon in the soil and the amount of organic nitrogen converted to mineral nitrogen – disappeared one year later. Soil sampling indicates that harvesting at these sites does not cause important nutrients to move below the root zone of the newly planted trees.
Two relationships between soil properties and tree growth, however, show some promise as indicators of site productivity. The Forest Service’s Long-term Soil Productivity study indicates that for loblolly pine plantations in central Louisiana, phosphorus availability before harvest seems to predict tree response to soil compaction and organic matter removal. The CRiSSSP study indicates that the rate in which nitrate is produced in the soil may be associated with stand production. In plots where tree height is below average, the nitrification rate as a percent of the total mineral nitrogen produced in the soil is relatively high. In plots were tree height is above average, the relative nitrification rate is relatively low (Figure 2). The relationship is consistent across locations and plantation age. It is also consistent regardless of the treatment that increased tree height above average.
The potential for using nitrification rate relative to the combined rate of nitrate and ammonium production in the soil as an indicator of site productivity needs further study. But differences in relative nitrification and ammonification rates across a range of site quality has been seen in other studies. Such an indicator would be useful in diagnosing whether harvesting and establishment practices have reduced site potential before growth losses and lower revenues have occurred.
Thomas J. Dean, Professor, School of Renewable Natural Resources, LSU AgCenter, Baton Rouge, La.
(This article was published in the spring 2006 issue of Louisiana Agriculture.) |