©2011 International Society of Arboriculture
279
MATERIALS AND METHODS
Experimental Design and Treatments
The study was conducted in 48 individual, physically isolated soil
cells (phytotrons) located at the Ohio Agricultural Research and
Development Center in Wooster, Ohio, U.S. Each phytotron was
constructed by slicing a narrow trench one meter deep, around
plots 4 m2 in area. The perimeter of each phytotron was lined with
30 mil PVC landfill liner extending one meter vertically into and
20 cm above the ground, which was heat welded at the overlap to
completely enclose the plot. The liner isolated the soil environment
without affecting drainage, allowing for establishment of replicated,
randomized soil environments. The native soil is a Canfield
silt loam (29% sand, 42% silt, 29% clay). A single paper birch
(avg. height: 1.1 ± 0.07 m) was planted in the corner of each plot
on September 20, 2000, and a single Austrian pine (avg. height:
1.0 ± 0.12 m) was added to the opposing corner of each plot on
September 12, 2002. Weeds were controlled with pre-emergence
herbicides (isoxaben and oryzalin or prodiamine) in combination
with manual cultivation and spot treatment with glyphosate.
The experiment was designed as a randomized complete
block, with four treatments: 1) untreated control, 2) paclobutrazol
soil drench, 3) fertilizer, and 4) paclobutrazol + fertilizer.
There were 12 replicate trees per treatment, with each treatment
combination replicated four times in each of three blocks. Paclobutrazol
was applied via soil drench on September 19, 2002,
to both paper birch and Austrian pine using the commercial product
Cambistat 2SC (22.3% paclobutrazol) at the labeled rate of
17 ml per 2.54 cm trunk diameter. Trunk diameter was measured
at 50 cm from ground level. The fertilization treatment consisted
of 3:1:1 N:P:K with 60% of N in slow release form (urea formaldehyde)
and 40% in fast release form (ammonium and nitrate)
applied to the soil surface at a rate of 200 Kg N/ha/yr, with half
the annual rate applied just after bud break in the spring, and
the other half prior to leaf drop in late summer or early autumn.
The fertilization treatment represents an intermediate rate based
on standard recommendations for woody plants (ANSI 2004).
Trees were fertilized in May and September each year through
2005, with fertilization of paper birch and Austrian pine commencing
in May 2001 and September 2002, respectively.
Treatment effects on tree physiology, secondary metabolism,
and insect resistance were measured in 2003 and 2004
for paper birch, and from 2003 to 2005 for Austrian pine. Pine
trees were maintained for an additional year to test for any
lagging effects of treatments on resistance to European pine
sawfly, which feeds on needles that are one year old or older.