United States

Department of A Method to Study Response

Agriculture

Forest Service of Large Trees to Different

Amounts of Available Soil Water1

Southern

Research Station

Research Note

SE-370

June 1995

Donald H. Marx, Shi-jean S. Sung,

James S. Cunningham, Michael D. Thompson,

and Linda M. White

Abstract Introduction

A method was developed to manipulate available soil water

on large trees by intercepting thrufall with gutters placed

under tree canopies and irrigating the intercepted thrufall

onto other trees. With this design, trees were exposed for 2

years to either 25 percent less thrufall, normal thrufall, or 25

percent additional thrufall. Undercanopy construction in these

plots moderately affected patterns of thrufall entry onto the

forest Boor when compared to a plot without construction. In

this pilot study, a variety of aboveground and belowground

measurements were recorded from trees over the 2-year period.

Additional amounts of thrufall increased circumference growth,

litter fall, fine-root biomass, ectomycorrhizal development, and

number of fruiting bodies of ectomycorrhizal fungi. Intercepted

thrufall decreased amounts of ergosterol, a surrogate for soil

and root-origin fungal biomass. Recommendations are made to

improve and simplify the design, including arranging gutters

to intercept more or less thrufall at any time of year.

Keywords: Ectomycorrhizae, global climate change,

root-ingrowth cores, thrufall.

Donald H. Marx is Director (Emeritus Scientist); S&i-Jean,

S. Sung is Plant Physiologist; James S. Cunningham

andMichael D. Thompson are Forestry Technicians; and

Linda M. White is Biological Lab Technician, all from the

Institute of Tree Root Biology, Southern Research Station,

U.S. Department of Agriculture, Forest Service, Athens, GA

30602.

1 This study was supported by U.S. Department of Energy

Interagency Agreement No. DEAI09-76SR-00870.

The use of trade or firm names in this publication is for

reader information and does not imply endorsement by the

U.S. Department of Agriculture of any product or service.

General circulation models suggest that the earth’s

climate may be changing at an unprecedented rate

in history (Hansen and others 1987, Schneider 1989).

Mean annual global temperature is projected to rise

2 to 6 T by the middle or end of the 21st century.

Increasing temperatures in the United States are

predicted to change both the atmospheric supply

of moisture (precipitation) and the atmospheric

demand for moisture (potential evapotranspiration).

Significant increases in drought in the United States

are predicted if precipitation decreases by as little as

10 percent (Rind and others 1990, Waggoner 1989).

Few models agree on the direction of change in

seasonal patterns of temperature or precipitation in

any area of the United States. Some models predict

decreases in precipitation for the Southeastern United

States, and others predict increases (McCabe and

others 1990). Contradictions between circulation

models became more controversial after examining

historic precipitation records. For example,

precipitation in 38 of the 48 contiguous States of the

United States has actually increased significantly since

1955 when compared with the period 1895-1954. Idso

and Balling (1991) suggest that this increase is caused

by increasing emissions of SOz from industry since

1955. Air pollution and its contribution to cloud

and haze formation and cooler daytime temperatures

would tend to increase rainfall and ameliorate drought

trends. However, the impact of air pollution on global

warming has not been factored into most circulation

models.

.--
Precipitation is the component of climate change

likely to have the greatest effect on the composition,

structure, and productivity of forests (Wigley and

others 1984, Winjum and Neilson 1989). This

effect is primarily made possible through controls

on soil supply of available water. Available soil

water is determined by the differences between gains

from precipitation and soil storage and losses from

evapotranspiration. Most forest scientists agree that

availability of soil water is a primary factor limiting

productivity of natural ecosystems (Brown 1978,

Kramer 1983) and plantations. Unfortunately, most of

our knowledge concerning the effect of available soil

water on forest trees is based on studies of seedlings

(Lee and others 1990, Seiler and Cazell 1990, Seiler

and Johnson 1988), stand modifications by thinning

(Brown 1978, Donner and Running 1986, Langdon

and Trousdell 1978, Zahner and Whitmore 1960), or

the relationship between historic rainfall amounts and

tree diameter growth (Bassett 1964, Bay 1963, Marx

and others 1988, Zahner and Myers 1986).

Because climate modeling has not provided credible

regional predictions of precipitation, how do we

study the effects of precipitation on forest trees?

Obviously, we need to create long-term experimental

scenarios that mimic both increasing and decreasing

precipitation. Recent studies on elevating soil water

by irrigation (Jarvis 1985, Linder 1987, Persson 1980,

Whitehead 1985) provide the only knowledge we have

about how large trees respond in the field to different

experimental levels of available soil water. The future

development of reliable simulation models for whole

trees or stands depends on testing with real field data.

Information obtained from experimental treatments

that mimic rainfall changes should be obtained from

aboveground and belowground components of large

trees grown where manipulative field designs provide

above and below normal precipitation levels (i.e.,

available soil water).

In this report, we describe a method that provides

above normal, normal, and below normal amounts

of available soil water by controlling thrufall in

plantation-grown loblolly pine (Pinus laeda L.).

Intercepting thrufall with gutters under tree canopies

produces below normal amounts, while irrigating the

intercepted thrufall onto other trees produces above

normal amounts. We also present preliminary data

from aboveground and belowground measurements

that reflect tree response to the different amounts of

available soil water.

Study Description

Plantation Characteristics

A closed canopy, loblolly pine plantation (planted

in 1980) with good 2.4 x 2.4-meter (m) stocking

on a 5-percent, smooth slope was selected at the

Department of Energy’s Savannah River Site (SRS),

Savannah River Forest Station (USDA Forest Service),

Aiken, SC. Soil is Troup (site index 25 m) classified

as well-drained, loamy, siliceous, thermic Grossarenic

Paleudults. Trees ranged from 7 to 20 centimeters

(cm) in diameter and from 5 to 9 m in height.

In October 1990, soil core sampling protocols were

developed for estimating the standing crop of live

fine-root (roots 5 2 millimeter (mm) diameter)

biomass on the trees in the plantation (Zarnoch

and others 1993). The results showed that 5.7-cm

outside diameter (OD) soil cores were as statistically

representative as 7.9-cm diameter cores and better

than 2.9-cm diameter cores. Using 5.7-cm diameter

cores, no spatial patterns resulting from distance from

tree, tree diameter, or a void caused by a missing

tree were found in fine-root biomass. The lack of

spatial patterns in this plantation indicated that no

statistical basis for spatial stratification in sampling

existed; therefore, random sampling could be used to

estimate live fine-root biomass.

In April 1991, all understory woody vegetation was

removed with hand tools and nonwoody vegetation

was killed with a herbicide. Branches lower than

3 m were removed from all trees. Debris was removed

from the site.

Before the soil water treatments were installed,

the dimensions of the root systems of trees were

measured. The forest floor and upper 10 cm of

mineral,soil were removed from around the base of

several trees to expose lateral roots. Lateral roots (to

0.5-cm diameter) radiated up to 8 m from the base

of the trees, with the majority (90 percent) radiating

4 to 5 m, or within two tree rows of each tree. The

distance that lateral roots radiated suggested that

a two-tree border was necessary around interior

measurement trees in study plots.

2
Chemical analyses of soil collected from the upper

0 to 20 cm in the study area showed the soil to

be low in nutrients. It contained 7, 5, 15, and 80

micrograms per gram (g) of available P, K, Mg, and

Ca, respectively; pH 4.9; 1.4 percent organic matter

content; and 1.0 cation exchange capacity of 1.0 meq

per 100 g.

Construction of Study Plots

Four study plots, each with seven rows of six to seven

trees, were selected in the plantation and randomly

designated as the intercepted thrufall, irrigated

thrufall, control, or site-control-treatment plot. Each

,plot had three rows of three trees in the interior for

measurement trees; two trees bordered measurement

trees on all sides (table 1). Plots were approximately

17-m square. Treatments were not replicated in this

pilot study.

Table l-Tree characteristics in study plots

Treatment

Total plot Measurement trees

No. trees DBH No. trees DBH

e m c m

Intercepted thmfall 43 13.8 9 14.6

Study control 48 14.2 9 14.2

Irrigated thrufall 42 14.7 9 14.4

Site controll 47 14.1 9 13.7

DBH = Diameter at breast height.

1 No construction plot.

No construction occurred on the site-control plot,

while lumber to support gutters or plastic pipe framed

the other three plots (figs. 1 and 2). All lumber was

pressure-treated, and all hardware (nails, bolts, nuts,

etc.) was galvanized steel. Posts (10 cm x 10 cm

x 2.5 m) were placed upright, 45-cm deep into the

soil. They were spaced 4 to 5 m apart and centered

between trees. Lumber (5 x 20 cm) was bolted to

those posts to create a frame with a roof effect. The

frames ran perpendicular to tree rows with a l-m

slope from plot center to plot ends. Frame height was

1.5 m at plot ends and 2.5 m at the center. Either

half or full rings of stainless steel were fastened to the

top side of the lumber to hold gutters or plastic pipes

in place. The holders were spaced so gutters or pipes

placed parallel to tree rows would occupy 25 to 30

percent of the area under tree canopies.

Gutters made of 2-mm (11-gauge) thick sheet

aluminum were placed in the intercepted thrufall

plot. They were half-round, lo-cm wide x lo-cm deep

x 10 m in length and painted with white oil based

paint. Their end view was six sided with a pointed

bottom. Gutters were turned in their ring holders to

empty out pine needles and other debris shed by the

canopies. The low ends of all gutters on both sides of

the plot were fitted into a manifold that transferred

the collected thrufall to a reservoir. Two manifolds

were prefabricated from 20-cm diameter, thick-walled,

white polyvinyl chloride (PVC) pipe. Slits, lo-cm

wide and of various lengths running parallel to the

run of the pipe, were cut from the PVC pipe to hold

the ends of the gutters. Aluminum screens (0.6-cm

mesh) were placed at the ends of the gutters to collect

debris. Both manifolds had screen filters at their

low ends and were connected to a common lo-cm

diameter PVC pipe that drained collected thrufall

into a reservoir 15 m from the plot. The reservoir

was a 3.7-m diameter x 0.9-m deep, commercially

available, round, children’s wading pool (8800 liter

capacity). The estimated required capacity of the

reservoir (pool) was based on the highest weekly

rainfall at the site in the past 30 years. The pool had

metal sheet sidewalls covered on the inside with a

heavy duty plastic liner. It was placed 0.8 m into soil

to create a drainage angle from the manifolds and to

help keep the water cool. A low roof was constructed

over the reservoir (pool) to protect it from debris and

sunlight.

In the irrigated thrufall plot, lo-cm diameter PVC

pipes, instead of gutters, were placed on the frames.

Manifolds and other components of water collection

and interception were not constructed in this plot. To

standardize shade and thrufall patterns, the PVC

pipes were the same size as the gutters. Six irrigation

lines (1.9-cm diameter, thick-walled, white PVC pipe)

were placed on the forest floor approximately 2.4

m apart and perpendicular to the tree rows. Each

line had six irrigation nozzles (Micro-bird spinner

3
Figure l-Plots in the loblolly pine plantation used in the intercepted, normal, and irrigated thrufall study. Site-control plot

without construction (A), study control plot with under canopy construction (B), gutter array under canopy to intercept

thrufall (C), and a manifold that transferred collected tlhrufall from gutters to water reservoir (D). Note rain gauges, litter

collectors, and uniform shade on forest floor.

SP-30-340 nozzle, Rain Bird Sprinkler Manufacturing

Corporation, Tampa, FL); one nozzle was placed 1 m

from each end of the line, and the remaining four

nozzles were equally spaced on the line about 2.4 m

apart. Nozzles were mounted on 1.3-cm diameter

PVC pipe 15 cm from the forest floor, and the radius

of irrigation for each nozzle was 1.3 m. Cut-off valves

were placed on the low end of each line to facilitate

maintenance and to drain the pipes for winterization.

The six irrigation lines were connected to a pumping

system by a common PVC pipe (3.8-cm diameter).

An electric pump (3 horsepower, log-liter per minute

capacity) powered by a gas-powered generator

pumped water to the plot from the reservoir 20 m

away. Water in the reservoir was first filtered through

a gravel-coated, filtration pipe (1’7-cm diameter by

1.5-m length) that lay on the bottom of the reservoir,

and then through a loo-mesh, in-line filter.

Ten-centimeter PVC pipes were placed on the

frames in the control plot to duplicate the shade and

thrufall patterns of the other two treatment plots. To

determine the possible effects of the construction

on tree response, no construction occurred on the

site-control plot.

Physical properties of the soil in each plot were

determined by driving the sharpened ends of 1.3-m

long PVC pipe (lo-cm diameter) 1 m into the soil at

each plot corner when soil water was high. After soil

was dug from around the pipe, it was removed. Soil

horizons (color changes) were identified and measured

from the hole created by the pipe; these depths were

marked on the pipe. Ends of the pipes were closed;

pipe:soil columns were brought to the laboratory

and frozen at -3 OC for 96 hours. While still frozen,

pipe:soil column disks (5-cm thick) were cut from

each horizon with a power saw. Soil in the pipe was

thawed and processed for soil properties. Located

up-slope, the soil properties of the site-control plot

differed from those of the three treatment plots, while

the treatment plots had similar horizon depths, water

flow rates, and water holding capacity (table 2).

4
Figure 2-Water reservoir used to hold collected thrufall for irrigation (A), irrigated thrufall plot with irrigation system (B

and C), and nozzle irrigating collected thrufall onto forest floor (D). Note rain gauges and pans used to measure volume of

irrigation in B and C.

Table 2-Weighted averages of saturated hydraulic conductivity (K) and water holding

capacity (WHC) of soil horizons in study plots

Total profile

Plots Horizon Depth K WHC K WHC

c m m/day Pet m/day Pet

Intercepted A 4.25 16.89 24.5

thrufall El 13.75 16.93 20.7 14.29 21.6

E 2 15.50 11.23 21.5

Study A 5.50 18.58 29.3

control El 13.50 14.70 22.5 14.86 23.1

E 2 15.50 13.67 21.4

Irrigated A 6.25 14.29 30.4

thrufall El 14.75 8.22 21.2 10.44 22.9

E 2 12.50 11.06 20.9

Site A 5.25 6.88 22.9

controll El 13.50 8.37 18.8 8.29 20.4

E 2 15.25 8.70 20.9

1 No construction plot.

5
Monitoring Thrufall and Tree Measurements

Fifteen glass rain gauges (1.9-cm diameter by 12.5-cm

depth) were attached 15 cm apart to 1.5-m long, 5-

by 20-cm lumber to create an array of rain gauges.

Four arrays were placed level on the forest floor and

perpendicular to the overhead gutters or PVC pipes

at random in each plot. These arrays were used to

measure volume and pattern of thrufall as it passed

through the canopies and overhead construction.

Measurements were made after significant rain (> 4

mm) and measured to the nearest 1 mm.

Dendrometers were prefabricated (Liming 1957) and

placed 1.4 m from ground level on stems of each of

the nine measurement trees in each plot. Diameters

were measured monthly. Four l- by l-m litter baskets

with screen (6 mm) bottoms were placed on top of

the gutters or PVC pipes in the interior five rows of

five trees (interior 25 trees) of the treatment plots,

Litter baskets were placed on the forest floor in the

site-control plot. Litter was measured yearly.

In June 1992, the standing crop of live fine roots

was determined on each plot by removing 7.9- by

20-cm deep soil cores from 25 random locations

within the area of the 25 interior trees. Fine roots

were separated from soil over a 4-mm mesh wire

screen. Soil was further examined by hand sorting for

roots passing through the screen. Fine roots were

transported to the laboratory in coolers and processed

within 24 hours. Roots were placed in 250-milliliter

(mL) beakers containing 200 mL water with three to

four drops of wetting agent, and sonification was

performed for 12 minutes to loosen soil and organic

matter. Soil and debris were removed by hand with

forceps over a No. 10 wire sieve. After cleaning, roots

were suspended in water and visually assessed at 4X

magnification for ectomycorrhizal development using

an arbitrary scoring scale of 0 to 3 in 0.5 increments.

Roots were then dried at 65 OC for 24 hours and

weighed to the nearest 0.1 milligram.

Root-ingrowth cores were used to measure soil

water effects on fine-root-growth potential. The

root-ingrowth cores were made 20-cm long from

7.5-cm diameter perforated, rigid plastic tubing

(RN-7480, R-L plastic tubing from Internet Inc.,

2730 Nevada Avenue North, Minneapolis, MN 55427)

with 5- by 5-mm holes in the walls (nine holes per 4

cm2). Cores were put into the 7.9-cm diameter holes

made during sampling (June 1992) for standing live

fine-root biomass. After removal of the fine roots, the

soil was used to refill each core; all cores were covered

with the forest floor.

At monthly intervals from September to December

1992, five cores were removed from each plot and fine

roots were collected from the cores (fig. 3). Cores

were removed from soil by first vertically cutting

along the outside of the core walls with a 25-cm long

knife and carefully lifting them from the core hole.

Fine roots were screened from soil as described for

the June 1992 sample. Cores and screened soil were

returned to their respective holes. The September

sample represented 3 months of fine-root ingrowth,

and the December sample represented 6 months.

In March 1993, soil and roots from all cores were

removed and processed; all cores and screened soil

were returned to their respective holes. At this

time, 5 additional cores were randomly placed in

each plot, raising the total to 30 cores per plot. At

monthly intervals (September to December), six

cores were removed from each plot. Three were

processed for live fine-root biomass, and three were

processed for ergosterol analyses. Ergosterol, a

fungus-origin sterol, is used as a surrogate to estimate

living ectomycorrhizal fungal biomass (Nylund and

Wallander 1992). After on-site soil screening, both the

fine roots and screened soil were placed in separate

plastic bags, transported on ice to the laboratory, and

processed within 24 hours. Soil was partially dried

for 2 hours in a 60 OC oven and screened over No. 10

and No. 25 mesh screen to remove organic matter.

Quick partial drying of soil did not quantitatively

affect ergosterol. Total fresh weight of screened soil

was recorded. Hyphal masses retained on screens

were collected and processed separately. After

visual evaluation for ectomycorrhizal ratings, the

ectomycorrhizae were removed from lateral roots and

analyzed separately.

Ergosterol was determined for soil, hyphal masses,

and ectomycorrhizae using a modified version of

techniques developed by Nylund and Wallander

(1992) and Davis and Lamar (1992). This method

involved extraction with ethanol:dithiothreitol:KOH

and hexane. High-performance liquid chromatography

with a Cl8 reverse phase column and an ultraviolet

detector at 282 nanometers (nm) was used to detect

and quantify ergosterol dissolved in methanol. Hyphal

6
Figure 3-Measurement of fine roots in root-ingrowth cores. Knife is used to cut lateral roots grown into soil in the cores

(A), core removed (B), removal of fine roots by screening soil from cores (C), and example of fine roots produced in 4

months within a core (D).

masses and ectomycorrhizae were processed like soil

with one exception-before extraction, they were

homogenized with mortar and pestle in the extract

solution. Fine-root-ingrowth biomass was determined

for each of these samples by combining the dry

weights of all processed root tissue. A series of control

tests to determine extraction efficacy were performed

by adding known amounts of reagent-grade ergosterol

to soil and roots. The recovery of ergosterol in these

quality control tests was > 95 percent.

Irrigation

The study began in May 1992 by turning gutters

upright to begin intercepting thrufall. Irrigation was

done when > 4 mm of thrufall was detected in rain

gauge arrays under the PVC pipes. The volume of

each irrigation was measured with the four rain

gauge arrays and four 60- by 60- by 5-cm deep pans

randomly placed in the irrigated thrufall plot (fig,

2). At each irrigation, fruiting bodies of fungi, mainly

those of ectomycorrhizal fungi, were counted and

removed from all plots.

In 1992, treatments began in late May and ended in

mid-December when the gutters were turned over

to stop thrufall interception. Litter samples were

collected and the irrigation system was winterized by

draining all pipes and the pump. In 1993, treatments

were maintained from early April to mid-December.

Results and Discussion

Patterns of tree growth, aboveground and

belowground, and litter fall in the site-control plot (no

construction) were similar to those in the control plot.

Therefore, tree growth data from the site-control plot

will not be included in this report. Because thrufall

treatments were not replicated in this pilot study,

the data can only be used to show trends and not

statistical probabilities.

Analyses of rain volumes in the rain gauges in each

plot indicated that the construction changed thrufall

patterns by about 15 to 20 percent when compared

to those from the site-control plot. The coefficient

of variation (CV) for thrufall collected in the site
control, intercepted thrufall, irrigated thrufall, and

the control plots was 15.5, 19.1, 34.9, and 31.1,

respectively. The CV of rainfall collected in the open

was 6.9. Plots with PVC pipes showed the greatest

variation. The pipes tended to collect thrufall on their

undersides, which dripped irregularly along the pipe

lengths, and the rain gauges measured the irregular

pattern. The aluminum gutters dripped considerably

less than the PVC pipes and thrufall patterns for the

site-control plot differed little from the plot with

gutters (intercepted). Other than the dripping

under the PVC pipes, no significant rain shadows

resulted from the construction. Apparently, the height

(1.5 to 2.5 m) of the construction allowed enough

wind-mixing of thrufall between the construction and

the forest floor to minimize rain shadows. Visual

examination of mineral soil in a few locations in each

plot showed that after lo- to 12-mm rains, the soil

was wetted evenly. Obviously, the l- to 1.5-cm deep

forest floor helped disperse the thrufall.

Using data from 15 rain events ranging in volume

from 9 to 109 mm, amounts collected in the open

were compared to amounts of thrufall collected in the

site-control plot. The comparison showed that the

tree canopies intercepted an average of 30 percent

(range from 9 to 50 percent) of rainfall allowing 70

percent to penetrate as thrufall to the forest floor.

The degree of canopy interception did not appear to

be related to volume of rainfall; canopies retained

about 4 to 6 mm of rainfall regardless of final volume

of the rain event.

In 1992, treatments were applied for 61/2 months and

involved 17 irrigations, equaling 1 every 10 days (fig.

4). Based on rainfall records dating back to 1952, the

rainfall at the SRS in these months in 1992 was 15

percent above average. The gutters intercepted 24.3

percent of thrufall when compared to the average

thrufall in the irrigated and control plots. The

irrigated plot received 24.8 percent additional water.

The amount of thrufall affected the percent of thrufall

intercepted by the gutters. Thrufall more than 15 mm

was intercepted at the designed rate more consistently

than thrufall less than 15 mm. Interception of thrufall

less than 15 mm was erratic.

c

- Normal

125 - Intercepted. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............................................................................ 125

I I I r I I I I I I I I I I I

6ilO * 6i30 7121 8110 8126 9i24 1016 1116 12/10

6116 719 7129 a/1 8 9110 9130 10115 11/16

1992 Irrigation Dates

Figure I-Amount of thrufall in millimeters received by

trees in intercepted, normal, and irrigated thrufall plots in

1992. Amount recorded on June 10 represents a Sweek

accumulation.
125 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........................................................ .................

1 I I I I I I I I I I I I I

4114 6111 719 815 8126 9123 11116

516 6116 7126 8118 9/l 10126 12116

1993 Irrigation Dates

Figure 5-Amount of thrufall in millimeters received by trees

in intercepted, normal, and irrigated thrufall plots in 1993.

In 1993, treatments were applied for 81/z months

and involved 14 irrigations equaling 1 every 17 days

(fig. 5). In contrast to 1992, the rainfall in this period

in 1993 was 18 percent below normal. Over 5 months

in this period, less than 125 mm of total thrufall

was measured in the site-control plot. The gutters

intercepted 25.4 percent of thrufall, and the irrigated

plot received 24.2 percent additional thrufall.

Tree circumference growth between trees in the

control plot and the intercepted thrufall plot did not

differ (fig. 6). Trees in the irrigated plot, however,

showed 30 percent more circumference growth midway

into the treatment period. These differences persisted

through the winter (December to March), and stem

growth differences in 1993 were similar to those found

in 1992 (fig. 6). Additional thrufall from irrigation

stimulated stem circumference growth, but less

thrufall did not decrease stem growth when compared

to control.

30 1 130

- 25

1

l Intercepted

o Normal

* Irrigated

t8192 IO/92 12192

-25

&I”‘I I I”’ 0

3193 5193 7193 s/s3 11193

4193 6193 8193 10193 12193

Dates

Figure B-circumference growth of trees in millimeters in

plots with intercepted, normal, and irrigated thrufall in 1992

and 1993.

9
The initial standing crop of live fine-root biomass,

removed from soil cores before ingrowth cores were

installed in June 1992 ranged from 328 to 340 g per

m2 of soil 20-cm deep for all plots. Ingrowth of fine

roots was stimulated by irrigation as early as October

1992 (fig. 7); however, a sharp decrease in fine-root

ingrowth occurred in November 1992. This decrease

was most evident in the control and irrigated thrufall

plots. High thrufall in August preceded high ingrowth

in October, and low thrufall in September preceded

low ingrowth detected in November, which suggests

that a large amount of fine roots died between

October and November. Rates of ingrowth increased

in all plots following increased thrufall. By December,

irrigation had produced twice as much ingrowth

as the other treatments. This pattern continued

through 1993, but overall fine-root ingrowth was much

less than in 1992 (probably due to less soil water).

Ingrowth after 3 months in 1992 was as large as

ingrowth after 6 months in 1993. Neither in 1992 nor

in 1993 did fine-root-ingrowth biomass approach that

found in the original standing crop of fine roots. This

suggests that some fine roots persist from year to

year and do not die and regenerate within the same

growing season.

Ingrowth that occurred after replacement of cores

each month from September to December 1992

was assessed in March 1993. Results indicated

that regardless of thrufall treatments, only modest

amounts of ingrowth occurred after September, and

biomass decreased sharply each month thereafter (fig.

8). It is possible that no growth occurred or growth

occurred and died (turnover) before assessments in

March 1993. These findings also suggest that most

fine-root development in loblolly pine takes place in

the spring and early summer.

l intercepted

0 Normal

* Irrigated

I I I I I no sampling I I I I

9192 10192 11192 12192 9193 10193 11193 12193

Sampling Dates

30

5 0

0 0

5 0

0 0

5 0

Figure i’-Production of he-root biomass in grams per square

meter after different thrufaU treatments in 1992 and 1993.

10
250

73- 300

9 250

z

4 200

.e(

250 /--

Intercepted

Control

.

.

.

.

.

.

- - - .

- - - > .

- . -=------

- . - . -- - - - - - - - l

- . -. -.

- . - .

- . - --..

- . -\

- . - . -

- . _- - -. .

- . \- --a

-*

? I I I I T

9192 1 o/92 1 l/92 12/92 - no treatment -

3193

Date RIG’s Replaced Date Assessed

Figure 8-Production of fine-root biomass in grams per square meter after different thrufall treatments

in ingrowth cores emptied and replaced at different times and then sampled in March 1993.
Assessments of ectomycorrhizal development on

lateral roots from the ingrowth cores showed the

effects of thrufall (fig. 9). A strong and consistent

trend for increasing ectomycorrhizal development with

increasing thrufall appeared in most months of both

years. Black ectomycorrhizae formed by Cenococcum

geophilum, a droughty soil, root-symbiont, were

common in all samples but appeared most frequently

in high numbers in samples from the intercepted

thrufall plot. Considerably fewer fruiting bodies

of known genera (Amanita, Suillus, Lactarius,

Tricholoma, and Cantharellus spp.) of other

ectomycorrhizal fungi occurred in the intercepted

thrufall plot. In 1992, the number of fruiting bodies

in the intercepted, normal (control), and irrigated

thrufall plots were 317, 400, and 649, respectively; 84,

149, and 343 appeared in the same plots in 1993.

These data show both the effects of increased thrufall

in 1 year and the effects of different amounts of

thrufall between years on fruit body production. Over

80 percent of the fruiting bodies were observed in

November and December each year.

Sampling Dates

-I

l Intercepted

0 Normal

* Irrigated

I I I I no sampling I I I I

S/92 10192 11192 12192 S/93 10193 11193 12193

Figure g--Rating for ectomycorrhizae produced on roots

collected from ingrowth cores after different thrufall treatments

in 1992 and 1993.

12
Visual quantitative estimates of ectomycorrhizal

development have several limitations. First, these

estimates do not consider the quantity of lateral

roots supporting ectomycorrhizae; they only estimate

the relative proportion of ectomycorrhizal to

nonmycorrhizal roots on the lateral roots. This,

limitation can be overcome by using a derived

parameter-feeder root potential obtained from

live fine-root-ingrowth biomass (a) and the

ectomycorrhizal rating (b) data in the formula (a x

b) divided by 100. These derived values (fig. 10)

show more succinctly the response of fine roots and

ectomycorrhizae to different amounts of thrufall.

Trees in irrigated thrufall plots had more than twice

the feeder root potential than those receiving less

thrufall each year.

A second limitation of visual estimates involves the

lack of accounting for the amount of fungal tissue in

the ectomycorrhizae, which is related to the relative

size of the ectomycorrhizae. This limitation can

be overcome by using ergosterol analyses. Results

from the ergosterol analyses showed large differences

in the location of fungal biomass and its response

to thrufall treatment and sampling period. With

thrufall treatments combined, ergosterol (fungal

biomass) in soil exceeded the combined biomass in

ectomycorrhizae and hyphal masses by nearly two

times from September through November and by

more than two times (fig. 11A) in the December

sample. Normal and irrigated thrufall showed sharp

increases in soil and total fungal biomass in the

December sample (figs. 11B and llC), suggesting

there was insufficient soil water in the intercepted

thrufall plot to stimulate fungal growth either in

soil or elsewhere. Little difference in fungal biomass

in any samples from September through November

indicated a steady state for fungal growth during

this period. Increased ergosterol, both total and

in soil, found in the December samples correlated

well with increased production of fruiting bodies of

ectomycorrhizal fungi.

Ii

w

I0

8-

I’O

s

0

a

6-

w

0

0

er

&

4 -

73

:

LL 2 -

@ Intercepted

0 Normal

* Irrigated - 8

- 6

- 4

- 2

I I I I no sampling I I I I

9192 10192 11192 12192 9193 10193 Ill93 12193

Sampling Dates

Figure IO-Feeder root potential derived from tile-root

biomass and ectomycorrhizae rating after different thrufall

treatments in 1992 and 199%

-
1,500

- A * Total

0 Soil

; 1,200 - * Hyphal clump

J

25

0 Mycorrhizal root

2 900-

d

$

g 600-

m

I5

300 -

hLL4

al

0

A

1,500

I I

.E

:: - B * irrigated

$ 1,200 - l Normal

5

A intercepted

Ti 900 -

i5

t;

Ek 6WI2

z

43 3wI I

1,500

I I

z

::

- c * Irrigated

A 1,2w -

-sl

l Normal

2

A Intercepted

g 9w5

ti

g 6Wiii

z

5 3wI -

I

Sept

I I

act Nov

I

Dee

1993

Figure II-Ergosterol analyses of soil and roots from root-ingrowth cores after different thrufall treatments in 1993.

Ergosterol in combined thrufall treatments (A), from soil as affected by thrufall treatments (B), and as total from soil,

ectomycorrhizae and hyphal masses as affected by thrufall treatments.

In both years, litter fall in the intercepted thrufall

plot was less than in the other treatment plots. The

1992 and 1993 accumulations of litter biomass were

254 and 248 g per m2 in the intercepted thrufall plot,

276 and 412 g per m2 in the control (normal) thrufall

plot, and 300 and 353 g per m2 in the irrigated

thrufall plot.

Conclusions and Recommendations

The system of gutters arranged under tree canopies in

this pine plantation consistently intercepted predicted

quantities of thrufall without seriously affecting

thrufall patterns on the forest floor. The physical area

of the gutter surfaces was between 25 and 30 percent,

and they intercepted an average of 25 percent thrufall

over 31 treatment periods during the 2 growing

14
seasons. The arrangement of manifolds, screen filters,

drain pipes, water reservoir, and the pump/irrigation

system was also effective. The system intercepted

nearly 100 percent of the thrufall during the 31

irrigations. The simple wooden frames supporting the

light weight gutters and PVC pipes had no obvious

invasive effects on aboveground and belowground tree

growth. The construction design in this pilot study

successfully created thrufall treatments of 25 percent

below normal, normal (control), and 25 percent above

normal.

Several improvements could be made in construction

design:

1. PVC pipes should not be used to mimic physical

effects of the gutters. They create identical shade

but, unfortunately, dripping of thrufall from their

undersides has significant and undesirable effects on

thrufall patterns.

2. The gutter shape should be changed from the

half-round six-sided, pointed bottom design to a

simple tray shape with 2-cm high walls and a flat

bottom lo-cm wide. Less expensive to construct, the

flat gutters would allow easier emptying of debris.

The half-round gutters would not easily lay flat in

their rings in the wooden frames after turning to

empty the debris. With tray gutters, properly placed

wooden blocks mounted in the wooden frames would

simplify emptying and repositioning. Tray gutters

should be made of the same 11-gauge aluminum to

assure sufficient strength to support their weight and

the weight of thrufall and debris over the 4- to 5-m

span between supports.

3. The drainage angle (3-cm fall for each l-m length)

of the current design was too severe. Intercepted

thrufall drained rapidly to the manifolds as did all

the debris. Collecting this debris from the manifold

screens and replacing it in the plot was time

consuming. Tray gutters with a drainage fall of 1 cm

per meter length would suffice. Intercepted thrufall

would still drain effectively but most litter would

remain approximately where it landed and not wash

to the manifold. Turning these gutters to empty them

periodically would allow more normal deposition of

litter on the forest floor. To reduce drainage angle,

the current 1.5 cm-high wooden frames on plot-ends

should be raised so they are only 10 cm shorter than

frames in the plot center.

4. All treatment plots should have tray-shaped

gutters. On intercepted thrufall plots, gutters

would be turned to intercept thrufall. On other

treatment plots, gutters would be inverted SO they

did not intercept thrufall. Because gutters would be

constructed on all treatment plots, thrufall patterns,

shade, etc. would be the same for all treatment

plots. Using gutters on all plots would also allow for

different treatments, such as intercepting thrufall

during only parts of the year or intercepting thrufall

on alternate years. The number of gutters could be

increased to intercept more thrufall or decreased to

intercept less.

5. All construction components in contact

with intercepted and irrigated thrufall must be

noncorrosive. The gravel filter used in the water

reservoir had steel parts that began to rust by the

second year. Such components must be made of

plastic, stainless steel, or other noncorrosive material.

Measurements of tree response in this pilot study

were kept simple because our main objective was

to develop reliable methods to intercept a specific

amount of thrufall from one plot of trees and to

irrigate this same amount of thrufall onto another

plot of trees. Certain trends in tree response to

thrufall amounts, however, were observed. In both

test years, 25 percent below normal thrufall did not

have obvious effects on tree growth, but 25 percent

above normal thrufall did. Stem growth, ingrowth

of fine roots, ectomycorrhizal development, feeder

root potential, and number of fruiting bodies of

ectomycorrhizal fungi were greater on the irrigated

plot than on the control plot. The only measurement

to show an effect of below normal thrufall was the

ergosterol assay. Ergosterol amounts were lower,

indicating less live fungus biomass, in soil and

ectomycorrhizae in the 25 percent below normal

thrufall plot than in the control plot and higher in

the 25 percent above normal thrufall plot than in the

control plot.

This pilot study shows how simple construction can

manipulate thrufall amounts (available soil water) for

extended periods on large, field grown trees. This

gutter and irrigation system for controlling available

soil water could be used to create a range of available

soil water regimes in well-replicated studies on a

variety of forest sites. With data obtained from

aboveground and belowground measurements, a

reliable simulation model on tree response to changes

in precipitation patterns could be created. This

model would be useful in predicting tree, stand, and

ecosystem response to future climate changes.

15
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16