Long-Term Soil Productivity

The NFMA of 1976, Sec. 6 (g), part C, states that the USDA, through research and continuous monitoring, must ensure that the effects of each management system have no substantial and permanent impairment of the productivity of the land.So, in 1989, the NFS and FS Research partnered together to design and implement a nationwide, long-term study on the effects of forest management practices on long-term soil productivity.

The LTSP study in the South is 5-13 years old, which is the most critical time for research on site productivity. Much work has been done on early plantation productivity (less than 15 years), but not on stands well past canopy closure, which is when site quality begins to exert its greatest influence on stand growth and productivity.

Figure 1: Graph of Stand Production VS Time in Years

Figure 1 | Graph of Stand Production VS Time in Years

The LTSP study was designed to answer the following questions:

  • What is the inherent carrying capacity of a site?
  • Is this capacity altered by management?
  • If carrying capacity is degraded, can it be restored?
  • How does soil disturbance affect soil functions?
  • Can these processes be monitored with surrogates?

The founding principles of the study are that:

  • Soil is the key factor controlling site productivity
  • The fundamental measure of productivity is vegetative carrying capacity;all other uses dependent on forest
  • Losses of organic matter and/or porosity most common causes of long-term productivity loss
  • Past efforts to directly document productivity loss have been confounded by climate, weeds, stocking, management, etc.

The experimental design is a 3×3 factorial design with three levels of experimentally-induced soil compaction and three levels of organic matter removal. The three compaction levels were:

  1. None, in which no mechanical equipment was allowed on the plots;
  2. Moderate, which consisted of six passes of a wobble-wheel road compactor with 3.6 Mg of ballast;
  3. Severe, which consisted of six passes of the compactor with 6.4 Mg of ballast.

The organic matter removal treatments ranged from merchantable bole-only, where all tops were left on-site, to whole-tree, in which all tops were removed as well as boles, and complete removal, where all aboveground organic matter was removed (trees, understory, forest floor).

Each experimental plot was 0.4 ha in size and planted on a 2.5 X 2.5 m spacing. The main plots were split into two 0.2 ha split-plots, and one half was kept free from woody competition with manual removal and herbicides.

People in cleared area

People in cleared area

Cleared area

Cleared area

People with large equipment

People with large equipment

Image of a hole

Image of a hole

Map of LTSP locations

Figure 3 | Map of LTSP Locations (each red star represents 3–4 locations)

LTSP Locations
Location Forest Year Installed Soil Series
LA 1 Calcasieu RD, Kistachie NF, Louisiana 1990 Malbis
LA 2 Catahoula RD, Kistachie NF, Louisiana 1992 Glenmora
LA 3 Catahoula RD, Kistachie NF, Louisiana 1993 Metcalf
LA 4 Catahoula RD, Kistachie NF, Louisiana 1993 Mayhew
MS 1,2,3 Chickasawhay RD, DeSoto NF, Mississippi 1994 Freest
TX 1,2,3 Davy Crockett NF, Texas 1996 Kurth

Other LTSP Websites