FIELD METHODS

INITIAL LAYOUT

    Four levels of thinning, three levels of pruning, and all their combinations were included in the study design.  The arrangement of plots is shown in Figure 4.  Each combination has three replications within a randomized complete block design.  Four plots were also established for each of the four thinning treatments without pruning for a total of 40 plots.  Each plot has a gross size of 132 by 132 feet and contains an inner plot 66 by 66 feet where all trees are individually  numbered.  Thus, the 0.1 acre measurement plot is surrounded by a similarly treated (including pruning) 0.3 acre buffer zone one-half chain wide.


  Figure 4.  Plot arrangement and treatments.  Density treatments in square feet of basal area per acre:  T1=30, T2=50, T3=70, T4=90.  Pruning treatments as percent of live crown to total tree height:  P1=25, P2=40, P3=50.  Control plots 41-45 (no treatment).   Shaded plots 4, 15, and 17 were severely damaged during the 1974 ice storm.

 


 
 

Figure 5.  Each measurement plot boundary corner (a total of four) is marked with a red stake (NE corner of plot 23 in October of 2000), and all buffer zone boundaries are marked with a blue stake (buffer zone boundary marker for plots 7, 8, 9, and 10 in October of 2000).

TREATMENT PROCEDURES

Thinnings

    Plots were initially thinned at age 12 to 40, 60, 80 and 100 square feet of basal area per acre.  After the second inventory at age 15, basal areas were reduced to 30, 50, 70 and 90 square feet per acre.  Plots were thinned again at ages 24, 27, 30, 35, and 40 to the same density levels (30, 50, 70 and 90 square feet).  Severely damaged plots 15 and 17 have not been thinned since the ice storm at age 16.  Plot 4 recovered by age 35 and was thinned at age 40.  A natural reduction of growth rates observed after age 30 allowed us to extend the thinning period to five years.

Selection of Trees to be Thinned

    After calculation of basal area for each measurement plot and its corresponding buffer area, we identified the trees to be removed in order to maintain the prescribed basal area.  Trees were generally thinned from below.  We applied the following, somewhat overlapping criteria (in order of decreasing importance):
1.  Inferior tree size (diameter and height).
2.  Low increment.
3.  Poor stem form.
4.  Traces of insect infestation.
5.  Damaged stems (logging or lightning).
6.  Damaged or lopsided crown.
7.  Uneven spatial distribution.
8.  Excessive cone production, an indicator of reduced increment.

Prunings

    Only twelve numbered trees were pruned on each plot. Three times as many trees were similarly treated on the surrounding buffer.  These trees were pruned twice, at ages 12 and 15, to 25, 40, and 50 percent of total tree height.


 
 Figure 6. Examples of no pruning (plot 27, age 12) and pruning to 40% of the crown (plot 3, age 16).
 
 REMEASUREMENT PROCEDURES

       Measurement methods and techniques have been maintained throughout the study to assure the compatibility of results from all inventories.  The diameter (DBH) of all trees was measured at a horizontal line on each inner plot tree originally placed at 4.5 feet above ground level.  The measurements are provided in the file tree.dat and explained in the file tree.doc (See DATA FILES).  During the seventh inventory diameters were also measured at 1 foot and 3 feet above ground level (files diam7_13.dat, diam7_13.doc).   Diameter at crown base was measured in the sixth, seventh, and eighth inventories (files crown.dat, crown.doc).  Crown radius was measured in the longest direction and 90 degrees to it in the seventh inventory (files cw_7.dat, cw_7.doc).
    Total tree height (to the top of the tree) and height to the base of the live crown (HLC) were measured using a Zeiss teledendrometer (files tree.dat, tree.doc) for the first seven inventories. Since the eighth inventory, these measurements have been taken using a CRITERION 400 laser hypsometer.  The inherent difficulties involved in determining the position of the upper crown points made these measurements subject to error.  A correction factor for each inventory was applied to all heights to remove bias through calibration of the instruments.
    The same teledendrometer was used to measure height to even-number upper stem diameters (2, 4, 6, etc., in inches) and the diameters themselves to calculate volume according to the Grosenbaugh height accumulation method.  Lower even-number diameters were measured using a diameter tape or caliper.  During the first four inventories, heights and upper stem diameters were measured for up to only 12 trees per plot.  At subsequent inventories, these measurements were conducted for all living trees. 

    For the twelfth and thirteenth inventories a haga vertex hypsometer was used for measuring total tree height.  The instrument was calibrated and verified by measuring the predetermined height of the University of Arkansas at Monticello Forestry Building.  Data for the twelfth inventory can be found in the file DiameterHeightAt45.dat and explained in the file DiameterHeightAt45.doc.  Data for the thirteenth inventory can be found in the file DiameterHeightAt48.dat and explained in the file DiameterHeightAt48.doc.

INSTALLATION OF UNTHINNED CONTROL PLOTS

    Originally, no unthinned plots were established.  The need for such plots was recognized later and at the age of 27 (in the summer of 1984) five control plots (without thinning or pruning) were established on the adjacent untreated part of the plantation.  The size and arrangement of each plot is the same as that of the 40 original plots.  To make growth comparable, hardwood competition was controlled on the plots by injecting Tordon 101-R.

ICE STORM MODIFICATIONS

    A devastating ice storm hit the plots at a vulnerable age (16 years) and period (a year after thinning). A salvage cut left three plots (4, 15, and 17) with basal areas below the intended densities.  One plot (4) recovered at age 35.  The two subsequent scheduled thinnings (at ages 18 and 21) were not conducted for any plot due to the reduction of density from the storm.  Other ice storms occurred in 1979 and 1994 but were less severe.

OTHER MODIFICATIONS

    In 1986 the construction of a new road destroyed one of the control plots (after only one measurement).  This lost plot was replaced in 1986 and assigned the number 44.  It was measured for the first time in 1987.  Drought conditions during the spring and early summer of 1988 along with extremely high late summer temperatures placed many trees under severe stress.  In August, three isolated areas of insect damage were located at the southeastern border of the plots.  The entomologist Dr. Lynne Thompson surveyed the area and found evidence suggesting the presence of southern pine beetles and turpentine beetles. To control the infestation, twenty trees were cut just outside of the study area.  Sixteen trees infested with turpentine beetles within the test plots were sprayed with Pestroy (9 ml/gal of water).  In 1997, six trees (including two damaged by lightning ) had insect damage, possibly due to the southern pine beetle and were salvaged.  Prescribed burns were conducted in 1981, 1984, 1986, 1990, and 1995 to reduce competition from hardwoods, shrubs, and herbaceous vegetation.


 
 

Figure 7.  Consequences (picture taken in 1999) of prior insect damage outside control plots 43 and 45.
 
 

Figure 8.  Regular prescribed burns have done a good job of controlling herbaceous and hardwood competition.  This picture was taken on May 4, 2001 looking south towards plot 9.  The last time the place was burned in December of 2000.

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