United States Department of Agriculture / Forest Service

Rocky Mountain Research Station

Research Paper RMRS-RP-101

November 2012

Response of Transplanted Aspen to 

Irrigation and Weeding on a 

Colorado Reclaimed Surface Coal Mine

Robert C. Musselman, Wayne D. Shepperd 

Frederick W. Smith, Lance A. Asherin, Brian W. Gee
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Musselman, Robert C.; Shepperd, Wayne D.; Smith, Frederick W.; Asherin, Lance A.; 

Gee, Brian W. 2012. Response of transplanted aspen to irrigation and weeding on a 

Colorado reclaimed surface coal mine. Res. Pap. RMRS-RP-101 Fort Collins, CO: U.S. 

Department of Agriculture, Forest Service, Rocky Mountain Research Station. 20 p.

Abstract

Successful re-establishment of aspen (Populus tremuloides Michx.) on surface-mined 

lands in the western United States is problematic because the species generally 

regenerates vegetatively by sprouting from parent roots in the soil; however, topsoil 

is removed in the mining process. Previous attempts to plant aspen on reclaimed 

mine sites have failed because transplanted root sprouts or seedlings do not have an 

extensive root system to access water and nutrients. This study identified factors that 

limit the survival and growth of aspen on reclaimed surface-mined lands by examining 

planted aspen saplings with supplemental irrigation and removal of competing 

vegetation in a fenced plot. The aspen saplings were grown on reclaimed roto-tilled, 

fresh-hauled soil or on dozer-cleared stored soils. Separate observations were made 

on survival and growth of nearby plots of natural aspen sprouts (fenced or unfenced) 

and on potted aspen seedlings. The best combination of conditions for aspen survival 

used transplanted saplings from local sources on fresh-hauled soil directly removed 

and placed from local aspen stands. Growth was better when competing vegetation 

was controlled by hand-hoeing around individual trees. The plants responded less to 

irrigation, but irrigation with non-saline water may enhance survival and growth in 

years with drought conditions. Aspen trees in an unfenced plot were heavily damaged 

by browsing ungulates.

Keywords: Populus tremuloides, aspen, reforestation, irrigation, mine reclamation, 

fencing, weeding

Authors

Robert C. Musselman, Plant Physiologist, U.S. Department of Agriculture Forest 

Service, Rocky Mountain Research Station, Fort Collins, Colorado.

Wayne D. Shepperd, (retired) Research Forester, USDA Rocky Mountain Research 

Station, Fort Collins, Colorado.

Frederick W. Smith, Professor, Colorado State University, Department of Forest and 

Rangeland Stewardship, Fort Collins, Colorado.

Lance A. Asherin, Forester, USDA Rocky Mountain Research Station, Fort Collins, 

Colorado.

Brian W. Gee, former Graduate Student, Colorado State University, Department of 

Forest and Rangeland Stewardship, Fort Collins, Colorado.
USDA Forest Service RMRS-RP-101. 2012. 1

Introduction

Quaking aspen (Populus tremuloides Michx.) is the most widespread tree species in North America (Baker 1925; Preston 1976; Lieffers and others 2001). Aspen 

is found in most of eastern Canada and the United States (except the Southeast), 

throughout the upper Midwest and Lake States, across sub-boreal Canada and 

Alaska, in the Rocky Mountains from Canada through the United States and into 

northern Mexico, and in mountain ranges paralleling the west coast from Alaska 

through British Columbia south through Washington, Oregon, California, and 

Mexico’s northern Baja California (Preston 1976). The species is most abundant in 

Canada’s central provinces and the U.S. states of Colorado and Utah (Jones 1985; 

Lieffers and others 2001). Aspen is a mid-elevation, shade-intolerant species and 

is a relatively minor component of more widespread conifer forests in much of the 

western United States.

Aspen has several physiological characteristics that permit it to attain great 

geographic distribution. Lieffers and others (2001) outlined several important 

adaptive traits for aspen. It seems to have a very high stress tolerance among the 

wide ranging genus Populus spp. (cottonwoods, poplars, and aspen). Usually, high 

stress tolerance is associated with slow growing species and those with a limited 

reproduction strategy. Aspen appears to rely on vegetative reproduction via root 

suckering more than other Populus spp., and the passing of extensive root systems 

between generations enhances tolerance to climate stress (DesRochers and Lieffers 

2001). Aspen has the ability to adapt leaf size to xeric and mesic conditions (that 

is, smaller leaves for drier sites). The species’ smaller leaf size could keep the leaf 

surface slightly cooler, allowing earlier shut down of stomata and thus tempering 

water stress during drought. Aspen seems to tolerate cold temperature and short 

growing seasons better than most hardwoods (Pearson and Lawrence 1958), and 

its leaf fluttering may be an adaptive advantage in leaf cooling. Aspen appears to 

have higher rates of photosynthesis than other Populus spp. and is comparable to 

that of high yield poplar hybrids.

Although aspen does produce abundant crops of viable seed (McDonough 

1979), it primarily reproduces vegetatively by root suckering throughout most of 

its western range. Occasional seedlings do establish but require bare mineral soil 

and constant moisture to survive (McDonough 1979). Aspen typically grows in genetically identical groups referred to as clones. All stems in a clone sprouted from 

the roots of parent trees and share a common genotype. However, they do not share 

a common root system as connections break down from generation to generation 

as new trees grow new roots.

The aggressive nature of weed invasions at some sites suggests that vegetative 

competition may limit survival and growth of aspen trees. The inability to easily 

control competing vegetation with herbicides around broad-leaved species such as 

aspen presents additional constraints. Having access to a well developed parental 

root system gives aspen sprouts a great advantage over other plants. The parent 

roots supply carbohydrates and access water deeper in the soil profile, allowing 

sprouts to grow rapidly and out-compete other vegetation and to withstand frequent droughty conditions in the West.

Re-establishing aspen on surface-mined lands is therefore problematic, since the 

parent root systems are destroyed when topsoil is removed. Planting containerized 

aspen seedlings in a non-irrigated location in a study near Tabernash, Colorado, 

was not successful (Shepperd and Mata 2005). Transplanting greenhouse or nursery-grown aspen seedlings into the field has similar problems to those of natural 
2 USDA Forest Service RMRS-RP-101. 2012.

seedlings; the small root mass of transplanted seedlings is insufficient to absorb 

enough moisture to maintain the seedlings during periods of summer drought in 

the wild.

In contrast, transplanting sapling-sized aspen in irrigated urban landscapes has 

not been a problem because the abundant supplies of water in lawns and landscape 

beds enable the transplants to thrive. Although aspen are somewhat tolerant of 

drought conditions (Lieffers and others 2001), we expect that irrigation could benefit survival and growth of planted aspen stock because moisture stress negatively 

affects aspen response to nutrient uptake (van den Driessche and others 2003). 

Water deficit stress also reduces stomatal conductance, root hydraulic conductivity, 

and shoot leaf water potential in aspen (Siemens and Zwiazek 2003). Irrigation has 

been shown to increase growth of hybrid poplar, a closely related species (Hansen 

1988; Strong and Hansen 1991).

Therefore, it seems reasonable to hypothesize that supplemental irrigation of 

aspen planted on reclaimed surface-mined lands could increase initial survival and 

allow trees to grow sufficient root systems to ultimately survive without additional 

water and to establish new self-regenerating clones on mined lands. Testing this 

hypothesis, gaining additional knowledge about different planting methods, and 

documenting factors that potentially limit the re-establishment of aspen is crucial 

to re-establishing aspen on surface-mined lands in the arid West.

Experimental Design

This study was conducted on the Peabody Energy’s Seneca Coal Company 

II-W Mine near Hayden, Colorado (Figure 1). The site is typical of the Colorado 

Plateau of western Colorado and eastern Utah. Habitat before mining consisted 

of vegetation dominated by Gambel oak (Quercus gambelii Nutt.), serviceberry 

(Amelanchier alnifolia [Nutt.] Nutt. ex M. Roem.), sage (Artimesia spp.), and 

mountain grassland with scattered stands of aspen and Douglas-fir (Pseudotsuga 

menziesii [Mirb.] Franco var. gluaca [Beissn.] Franco). Original soils in the mined 

area derived from sedimentary sandstone and shale and were rocky and shallow. 

The study site was located at an elevation of 2300 m. The project was designed 

as a case study of the feasibility and effectiveness of using irrigation to establish 

native aspen sprouts transplanted from a nearby un-mined site to a fenced area 

on a reclaimed surface coal mine. We examined the survival, growth, and water 

status of aspen transplants planted on a reclaimed site. Top soils are normally removed prior to mining and stored for a number of months before replacing over 

resurfaced, overburden materials. Half of our study plot has topsoil that had been 

dozer-cleared and stored before placement at the site. The other half of the plot had 

topsoil that had been roto-cleared and fresh-hauled directly to the plot site.

Prior to planting, mine overburden material had been used to re-contour the 

original topography of the study site, and the two types of topsoil had been placed 

in separate locations on the overburden at the site. Roto-cleared topsoil consisted 

of the original vegetation, including aspen trees on the site, chopped and mixed 

into the topsoil with a large roto-tilling machine prior to removal. This mixture 

was directly fresh-hauled from its original site to the experimental plantation site. 

All above-ground vegetation in dozer-cleared soils was bladed aside for disposal 

prior to topsoil removal and storage before final placement. The dozer-cleared soil 

used in this study had been stored for several months before placement at the 

experimental site, evidenced by the indication of decay of vegetation present in 

the soil and the few herbaceous plants that initially grew from this soil. Both soil 
USDA Forest Service RMRS-RP-101. 2012. 3

types were from aspen stands containing aspen roots and were placed to a depth of 

approximately 1 m on top of the overburden at the plantation site. The soils were 

spread by scraper in late summer of 2003 and were final graded in May/June of 

2004 prior to aspen planting in October 2004.

Aspen trees were planted on the reclaimed soils during the fall of 2004 and 

examined for survival, growth, and water status for the 2005-2007 growing seasons. The aspen transplant trees were drip irrigated at three watering levels and 

a non-watered control, and all plots received natural precipitation. In addition to 

the irrigation treatments, beginning at the second growing season, competing vegetation was removed from half the experimental plots on each soil type by hand 

hoeing.

This study was designed to measure the effect of supplemental irrigation on 

bare root aspen saplings that had been transplanted from a naturally regenerating 

un-mined site on the nearby (<3 km) Yoast Mine where the original forest had 

been dozer-cleared in preparation for mining. Aspen saplings between 1 and 2 m in 

height and about 1.5 to 3 cm basal diameter were selected from this site at the end 

of the growing season in 2004 and pruned to leave only the uppermost branches 

intact. In October, 2004, these saplings were dug using a small backhoe, and their 

roots were immediately immersed in a truck-mounted water tank. The dug saplings 

were transported to the experimental site and transplanted the same day into 0.3 

m diameter by 0.75 m deep power augered holes that had been prepared inside a 

fenced planting site at the II-W Mine. All saplings were presumed to be from the 

same genetic clone since they were collected from the same area and had similar 

morphological characteristics. Trees were planted in 8 blocks consisting of 5 rows 

II-W Plantations Source of transplants

Yoast Naturals

Hayden, CO

Figure 1. Map of experimental 

plot location, Seneca Coal 

Mine, Hayden, Colorado.
4 USDA Forest Service RMRS-RP-101. 2012.

of 10 trees, (50 trees total) spaced on a 1.5 m x 1.5 m grid (Figure 2). Four blocks 

were placed in each of the two types of topsoil.

Water was delivered to the transplanted aspen saplings during the 2005-2007 

growing seasons by drip irrigation via a data-logger-controlled system that timed 

the daily application of water through calibrated emitters. The four water treatments (high, medium, low, and non-irrigated control) were randomly assigned to 

one of the four blocks in each of the two soil types, with all 50 trees in each block 

receiving the same amount of water (Figure 2). The gravity-fed drip system, supplied by a 7500 l tank located 63 vertical m upslope from the test site provided an 

adequate head to maintain water pressure greater than 4100 mb in all lines. The 

tank was filled by Seneca Coal Company workers as needed, generally once or 

twice a week. Drippers delivered water at 4.4 l/minute, and were programmed in 

2005 to deliver water daily at 0, 1.4, 2.7, and 5.6 l/day/tree for the control, low, medium, and high irrigation levels, respectively, equivalent to 0.0, 9.1, 18.3, and 36.6 

cm of supplemental precipitation per month. Irrigation treatments were applied 

at half the 2005 rate during the 2006 and 2007 growing seasons—0.0, 0.7, 1.4, or 

2.8 l each day of treatment. The non-irrigated control received no supplemental 

water. Irrigation treatments were applied daily during the early morning. Drippers 

required 276 mb pressure for activation; the valve box and distribution lines were 

configured so that head pressure downstream of the valves did not exceed this 

value to avoid leakage between irrigation treatments. Standard meteorological 

conditions were monitored at an automated weather station located at the center of 

the plot, and we recorded hourly wind speed, wind direction, relative humidity, and 

precipitation. All data were recorded on a Campbell 23x data logger, which also 

was programmed to activate the irrigation solenoids. Power was supplied from 12 

V batteries charged by a solar panel.

Source of irrigation water used to fill the bulk tank in 2005 and 2006 was a 

sedimentation pond lower in the reclaimed watershed. White salt deposits were observed around some of the irrigated treatments in 2005 and 2006, particularly those 

trees receiving the high irrigation treatment, suggesting contaminated irrigation 

water. Only clean, potable water from a Hayden, Colordo, municipal hydrant was 

used to irrigate the trees during 2007. Root zone soil samples were analyzed by a 

soils testing laboratory for determination of saturated paste extract conductivity.

Circumstances allowed us to expand our observations beyond the original irrigation study to collect survival and growth data from additional aspen reproduction at 

the mine site. These included natural sprouts growing inside the fenced site where 

our irrigation plot was located, potted seedlings in another near-by fenced area, 

and unfenced sprouts. Un-watered aspen sprouts grew in the fenced area outside 

our irrigated plot, originating from aspen root segments that had been transported 

to the site in the two topsoil types. Natural aspen sprouts grew from root segments 

buried in un-irrigated areas of the roto-cleared and dozer-cleared soil adjacent to 

the irrigated blocks. Half of a selected number of these sprouts of uniform size 

were hand-hoed (hand weeded) and half were not, and the trees were measured 

for survival and growth. The natural sprouts grew at random spacing, and trees 

selected for measurement were thinned where necessary to no closer than 1.5 m 

spacing—the spacing of the irrigated plot trees. The potted and natural trees in all 

locations were from unknown genotypes, likely different from the irrigated study 

transplants. The natural sprouts growing on the roto-tilled soil were all probably 

from the same genotype since they came from the same area. Similarly, the natural 

sprouts on the dozer-cleared soil were likely from the same area and genotype. But 

since the two soil types were from different areas on the mine, the genotype from 

the dozer cleared and roto-cleared were likely different.
USDA Forest Service RMRS-RP-101. 2012. 5

Commercially grown potted aspen seedlings that were planted in a nearby unirrigated fenced area were also measured for survival and growth. Although the 

commercially grown potted aspen trees were planted on dozer-cleared soil, it was 

not determined if the seedling roots grew out of their potting mix or their augered 

hole into the dozer-cleared soil during the study. An un-mined and un-fenced area Figure 2. Experimental design for aspen reclamation project, Seneca Coal Mine, Hayden, Colorado. The inside 5 trees closest to the center of each row of 10 trees were weeded; the outer 5 trees of each row were left unweeded.
6 USDA Forest Service RMRS-RP-101. 2012.

on the nearby Yoast Mine and the source area for the aspen samplings used in the 

irrigation study were dozer-cleared of mature aspen. The sprouts growing on this 

site were monitored for ungulate impact on young aspen.

None of the experimental treatments were replicated, so this experiment is considered a case study for the survival, growth, and water status of trees in this specific 

study. Extrapolation of findings beyond the study site must be made with caution. 

Nevertheless, the findings of this study can provide valuable information useful for 

revegetation of reclaimed surface coal mines with aspen in similar settings.

Data collected

Survival and growth were recorded for each tree prior to budbreak and at the 

end of each growing season. Water status and tree growth were measured periodically throughout the experiment. Physical measures of growth were height (cm), 

basal caliper (mm), number of basal sprouts (count), length of the terminal leader 

(cm), and length of each of the next three branches on the upper portion of each 

tree (cm). Any disease and insect infestation on each tree was recorded at the end 

of each growing season.

During the 2007 growing season, water status (leaf water potential) of the plants 

was measured twice during the summer season as near to dawn as possible (0.5 hr 

predawn to 0.5 hr after sunup) to capture the minimum stress before rapid morning 

transpiration depleted leaf moisture. Treatment, ambient temperature, time of sampling, and exuding pressure level were recorded. Leaves were randomly collected 

from the different treatments to minimize time of sampling biases.

Leaf water potential increases as plant water stress increases when water is 

withheld from the plant. Water status measurements required removing one fully 

matured leaf from randomly selected trees in each treatment and measuring for 

water holding capacity using a Plant Water Status Console. The leaf was removed 

from the plant and immediately placed in a sealed chamber with the petiole extending through a sealing hole in the chamber. A fresh, slightly angled cut was made 

and nitrogen gas was delivered to the leaf under slowly increasing pressure until 

water exuded from the petiole surface. The pressure necessary for this to occur is 

an indication of the water potential or water holding capacity of the leaf, which is 

an indication of the water stress and thus physiological stress of the plant. Different 

plants from each treatment were selected at each testing to minimize leaf loss from 

sampling. Two to three total measurements were made from each treatment each 

day of measurement. Number of measurements depended on the time necessary 

for each measurement so that all measurements fell within the dawn-time window. 

Each day of measurements included leaves collected from all irrigation treatments.

Competing vegetation

 Invasive weeds, including tumbleweeds (Salsola spp.) and thistles (Cirsium

spp.), were common in the plantations from 2005-2007 as well as numerous native herbaceous species. Landscape fabric (about 1 m diameter) placed around 

potted trees during planting prevented weeds from growing next to those trees. 

We controlled competing vegetation in the irrigation and root-sprout treatments by 

repeatedly hand-hoeing and cutting all vegetation growing around half of the trees 

in each irrigation and soil treatment. The irrigation plots on each soil type were 

divided into two sections—one to be hand-hoed and one not to be hoed—with hoeing treatments superimposed on the existing study design. Growth, physiological 

parameters, and survival were compared as in other treatments. Soil samples were 

collected from each treatment for moisture content analysis.
USDA Forest Service RMRS-RP-101. 2012. 7

Differences in soils

Soil samples from the two soils types (roto-tilled/fresh-hauled and placed, and 

dozer-cleared/stored before placement) were collected and analyzed for organic 

matter and nutrient content, water holding capacity, and chemical and physical 

properties. Since the soils were mixed and soil horizons present in normal soils 

were missing, integrated samples were collected through the entire surface soil 

profile, approximately 0.75 to 1 m depth. Soils were analyzed for texture and fertility (organic matter, pH, nitrogen, phosphorus, potassium, and cation exchange 

capacity). Bulk soil samples were periodically collected and oven-dried for soil 

moisture determination.

It was important to quantify how the replaced soil differed from natural soils 

on the Seneca II-W Mine. Samples of undisturbed soil were collected under aspen 

stands in undisturbed areas of the mine and subjected to the same analysis previously described. In addition, differences in soil conditions between reclaimed soils 

in the study area and those under nearby undisturbed aspen clones were quantified 

by comparing physical and nutrient characteristics of soil samples from both the 

normal and augmented reclaimed soils to those of the natural soils. Sampling of 

the soils under nearby native, undisturbed aspen stands were extended to the same 

depth investigated in the reclaimed soils on the study plot.

Root growth

Aspen is a relatively short-lived tree that is susceptible to injury and disease 

and relies on periodic re-sprouting from lateral roots to maintain its presence on 

a site (Shepperd and others 2006). Therefore, the development and lateral extension of new roots is critical for the ultimate survival and re-establishment of any 

aspen planted on mined lands. We quantified new root development since planting 

by excavating randomly selected surviving trees at the end of the 2007 growing 

season and washing soil from the roots to quantify total root biomass and new root 

growth. Trees were chosen from each of the different irrigation, soil, and transplant treatments studied. Soil was carefully loosened and roots were exposed by 

washing soil away with a high pressure water jet. We then measured the spread of 

any lateral roots away from the tree base as distance of root spread down to 4 mm 

diameter size. Total below-ground biomass dry weight of roots was measured. It 

was particularly crucial to see if roots had extended beyond the planting hole for 

transplants or beyond the potting mix for potted aspen. Roots must reach a large 

enough size and be close enough to the surface for suckering to occur.

Results and Discussion

This study verified that aspen can be successfully established on reclaimed 

surface coal mine lands when certain conditions are met. The study was initially 

conducted to demonstrate the effectiveness of supplemental irrigation (four levels of watering) on survival and growth of transplanted saplings. We were also 

able to examine control of competing vegetation (hand-hoed or not hoed) in this 

experiment. Experimental conditions at this site allowed us to study soil type, rotocleared/fresh-hauled and dozer-cleared/stored. Other aspen plantings available at 

the mine site allowed examination of additional factors: plant source and stock 

type (transplanted rooted saplings, natural sprouts, and potted plants) and fencing 

(fenced or not fenced).
8 USDA Forest Service RMRS-RP-101. 2012.

Since not all treatment combinations existed, and none of the treatments were 

replicated, statistical analyses and inferences are limited. For example, differences 

in survival and growth between aspen at Yoast and II-W Mine soil sprouts, between dozer-cleared and roto-cleared, and among dozer-cleared seedlings, sprouts, 

and saplings may be due to differences in soil disturbance, genetic stock of aspen, 

transplant type, fencing, or microclimatic differences between sites. Although this 

was a case study, several observations were evident that are helpful for future aspen management and to identify areas for additional research. We present our data 

as box-plots, useful to compare different datasets for range, skewedness, and the 

presence of outliers.

Competing vegetation

Trees on weeded plots grew considerably better (Figure 3) and had higher rates 

of survival. Weeding was particularly important for survival of natural sprouts occurring from residual aspen roots in the replaced topsoil. Of 34 natural sprouted 

trees initially marked for study on the roto-tilled/fresh-hauled soil in year 1, half 

were weeded and half un-weeded in years 2 and 3. All of the weeded trees survived into year 3 while only four of the un-weeded trees survived the first three 

years of the experiment. Of the 21 natural sprouts selected for study on the dozercleared stored soil, 8 of the 11 weeded trees but only 2 of the 10 un-weeded trees 

Figure 3. Box-plots of 

average growth by 

irrigation, weeding, 

and soil type. Box-plots 

describe the distribution 

of the data, with the 

grey box showing the 

25th to 75th percentiles 

(the middle 50% of 

the data), dashed 

horizontal and vertical 

lines (H-line) indicate 

the maximum and 

minimum values (unless 

outliers are present 

where it extends to 

1.5 times the inner 

quartile range), solid 

circle is the median, 

and open circles values 

beyond the H-line are 

considered outliers.
USDA Forest Service RMRS-RP-101. 2012. 9

survived after 3 years. Most of the vegetative competition consisted of annual 

herbs, perennial grasses, and weed species. Weeds were primarily thistle species 

and tumbleweeds. Shading was not a factor since the trees were larger and growing 

above the competing vegetation canopy. Data from this study suggest that surface 

vegetation and trees may be competing for a limited amount of available water and 

nutrients.

Although soil moisture did not appear to be much higher in the weeded plots 

(Figures 4-6), weedy vegetation may have been a competitor for moisture and 

nutrients in the planted aspen plots. Soil moisture was lower near the surface than 

deeper (Figures 7-8).

Irrigation treatment

Rainfall was plentiful for the first two years during the study, and soil moisture was relatively high even in non-irrigated plots, as indicated by soil moisture 

matrix potential values and low leaf water potential data for all treatments. This 

prevented a good examination of the irrigation treatment effects in this experiment. Aspen survival and growth did not appear to be dependent on or, in some 

cases, consistent with irrigation treatment (Figure 9), suggesting that soil moisture 

from the frequent rain events was sufficient even in the non-irrigated plots. The 

supposition of adequate moisture available to all trees was further evident in that 

there appeared to be no relationship among irrigation treatment and average leaf 

area, total leader growth, terminal leader growth, stem diameter growth or caliper, 

or survival. Growth of second, third, and fourth lateral branches appeared to be 

Figure 4. Average soil moisture by 

soil type and weeding treatment, 

June 14, 2007.
10 USDA Forest Service RMRS-RP-101. 2012.

Figure 5. Average soil moisture by soil 

type and weeding treatment, July 

3, 2007.

Figure 6. Average soil moisture by 

soil type and weeding treatment, 

July 25, 2007.
USDA Forest Service RMRS-RP-101. 2012. 11

Figure 7. Average % soil moisture 

by depth for roto-cleared soil.

Figure 8. Average % soil moisture 

by depth for dozer-cleared 

soil.
12 USDA Forest Service RMRS-RP-101. 2012.

similar for all treatments but are reflected in total growth. Pre-dawn leaf water potential levels also indicate moisture stress was generally less than 5 bars (0.5 mPa) 

pressure and did not appear to be related to irrigation treatments during the years 

when these measurements were taken.

Salt deposits were observed around the base of trees with high irrigation in 2005 

and 2006, suggesting that salts were leached from the re-deposited topsoil or were 

present in the irrigation water. A soil chemistry salinity analysis confirmed that 

the soils with the highest rate of irrigation were indeed saline, likely the result of 

irrigation with saline water. An analysis of the water from the local pond used as a 

source for irrigation confirmed that the water was saline. Non-saline, potable water 

was used to irrigate the trees in 2007.

Saline water inhibited the growth of aspen on high and medium irrigation treatment plots the first and second year of the study. Remarkably, although these trees 

were still smaller in the third year, the annual growth of many of the trees had 

recovered to that of low and control irrigation treatments after discontinued use of 

local saline irrigation water (Figure 3). Growth of some of the low irrigation and 

no irrigation treatment trees was still higher than that for the high and medium irrigation treatments, suggesting that the reduced growth from the saline water used 

for irrigation in the first and second years of the experiment was still evident for 

those trees in the third year of treatment.

None of the trees that died in previous years re-sprouted from residual roots. 

Since growth of transplanted aspen saplings was good with the low and no irrigation 

treatments, there was sufficient natural rainfall during the three years of the study 

Figure 9. Average growth 

by irrigation treatment. 

Controls, naturals, and 

potted trees were not 

irrigated.
USDA Forest Service RMRS-RP-101. 2012. 13

for the trees to survive and become established without irrigation. Nevertheless, 

it is expected that growth under the high and medium irrigation treatments might 

have been higher than the lower irrigation treatments had non-saline water been 

used. Determining the benefit of irrigation of newly planted trees was limited in 

this experiment by the saline irrigation water and lack of low rainfall or drought 

conditions during the study.

Transplant type

The aspen saplings used in the irrigation study that were transplanted from the 

nearby Yoast Mine site exhibited considerably more injury and more disease infections than natural sprouts arising from buried root segments or potted plants (data 

not shown). Transplant shock was evident only the first year when leaf area growth; 

leader, stem, and diameter growth; and survival were considerably less with these 

plants than with natural sprouts or potted plants. However, the transplanted trees 

that survived the first year grew well the second and third year of the study and 

generally surpassed that of the natural sprouts and potted trees in year 3 (Figure 

9). After three years, survival was similar for all transplants and natural sprouted 

trees (50-57%) but was considerably higher for potted plants (80%). Survival and 

growth of the potted trees was excellent and appeared better than transplanted 

cuttings the first year of the study, but growth of the potted trees remained relatively stagnant after three years and these trees were considerably smaller than the 

transplanted trees. The transplanted trees were well established and growth was 

generally good by the third year of treatment. Growth of natural sprouts was less 

than that of transplants after three years.

Natural root sprouts had no lateral branches their first year. Leaves also appeared 

to be larger on these trees (data not shown). Nevertheless, these trees experienced 

somewhat greater pre-dawn water stress in July and September than trees in the 

irrigated treatments, including the irrigated controls with no water added. The data 

suggest that pre-dawn water stress levels as high as 14 bars and afternoon water 

stress levels as high as 20-25 bars were not of sufficiently high levels to cause 

enough stress to reduce survival or growth.

Soil type

Roto-cleared/fresh-hauled soil provided sufficient natural sprouting from residual aspen roots in the topsoil to provide an adequate stand of aspen trees, and 

these trees appeared to grow better and survival appeared higher than adjacent 

transplanted trees growing in the same soil in the first two years of the study. 

Dozer-cleared soil that had been temporarily stored had considerably lower numbers of natural sprouts than roto-cleared soil, and stocking was sparse (data not 

shown). Only a small number of sprouts occurred from the stored dozer-cleared 

soils, suggesting the lack of live roots for sprouting. Natural sprouts appeared to 

have greater total leaf area and stem diameter growth on roto-cleared soil than 

dozer-cleared soil, but terminal leader growth appeared similar on both soil types. 

While there appeared to be no difference in survival on the roto-cleared (53%) and 

dozer-cleared (52.5%) soils, average growth on dozer-cleared soils (18.9 cm) was 

only about two-thirds of that on the roto-cleared soils (29.4 cm). These data suggest that fresh hauling of soil from aspen stands to reclaimed land could allow for 

sufficient sprouting of aspen from residual roots without planting.

There were differences between the two soil types for many of the attributes 

measured in the experiment, particularly growth in the weeded plots (Figure 3). 
14 USDA Forest Service RMRS-RP-101. 2012.

Soil samples from the two soil types and undisturbed soils were collected and analyzed for organic matter and nutrient content, water holding capacity, and chemical 

and physical properties. Samples were taken from the entire topsoil depth placed 

at the site, about 1 m deep, because the soils were mixed during digging, storage, 

and placement, and no soil profile existed. Soil chemical properties were similar 

for the different soil sources, including undisturbed soils. Data indicate that neither 

the roto-cleared nor dozer-cleared soil type was toxic, except for high electric conductivity in high irrigation treatments on both soil types for 2006. Nutrient content 

such as nitrogen did not seem to be related to soil type and was at high enough levels to not be the limiting factor in tree growth. The difference in response of aspen 

tree growth between the two soils types appeared to be primarily due to original 

soil source or to storage rather than to method of tree removal. Stored soils were 

observed to be anaerobic.

Soil moisture

Dozer-cleared and roto-cleared soils were similar in soil moisture content 

(Figures 4-8). Visual observations suggested that the dozer-cleared soil was more 

compact and poorly drained as evidenced by water ponding in a soil pit at the 

site. Higher organic matter content of the roto-cleared soil may have made it less 

susceptible to compaction (Shepperd 1993). Soil moisture seemed to have no relationship to amount of irrigation applied (Figures 4-6) but had a higher range of 

variability in the weeded plots. Soil moisture was higher at 30-40 cm depth in the 

soil compared to the surface 10 cm (Figures 7-8).

Although soil moisture content on the roto-cleared soil was similar to that on 

dozer-cleared soils the moisture should have been less tightly held on the rotocleared soils since this soil was considerably less compact with better drainage. 

These conditions apparently favored growth of aspen trees and competing vegetation. The growth data suggest that roto-cleared soil could have provided other 

benefits such as improved aeration and soil structure or perhaps mycorhizae for 

tree growth.

Fencing

Fencing is necessary to obtain an adequate stand of aspen, regardless of the 

source of the trees. The unfenced Yoast Mine site had severe damage from ungulates, including stem breakage, browsing, and rubbing damage. Most trees at this 

site had some form of injury. Yet, survival and re-growth of these trees was good, 

suggesting that the presence of an extensive parent root system in undisturbed soil 

is ideal for aspen growth. Nevertheless, fencing of these trees is recommended in 

order to produce an adequate stand of mature aspen. Cattle were present in other 

areas of the mine, but aspen plantings were protected from browsing by fencing.

Physiological status

Initial analyses indicate that soil type and weed competition affected rate of 

physiological response, as reflected in plant growth. Highest rates of growth 

seemed to be in the weeded plots on roto-cleared soils, suggesting that these conditions are best for aspen survival and growth. Greater plant top and root growth of 

these plants seem to verify that finding. Plant water status measurements indicated 

that when these tests were conducted during the 2007 growing season (June 28 and 

August 1) the plants were not water stressed, with pre-dawn leaf water potential 
USDA Forest Service RMRS-RP-101. 2012. 15

pressures not exceeding 20 bars in June or 10 bars in August (Figures 10-11). Most 

often, stress was less than 5 bars. Irrigation was particularly helpful during June 

when stress approached 20 bars on some low and un-irrigated plots.

Plant moisture stress was less with high and medium irrigation treatments than 

with low or no irrigation during June (Figure 10), but trees were more stressed 

on roto-cleared soils in August (Figure 11). Larger trees showed the most stress. 

Maximum leaf water potentials measured one warm mid-afternoon found stress 

levels of about 25 bars or less; the levels appeared to be unrelated to treatment or 

to survival and growth.

Rainfall was frequent during the relatively wet 2005 and 2006 growing seasons, 

and monitoring of leaf water potential indicated that varying irrigation treatment 

did not affect leaf water stress condition of the plants. However, additional physiological conditions such as stomatal conductance, photosynthesis, and respiration 

of the plant that affect survival and growth were unknown. These physiological 

conditions may have shown response to drought prior to indication by plant water 

status; or they could at least indicate which trees are stressed and not likely to 

survive.

Root growth

Root growth of transplants was best in weeded plots on roto-cleared soil where 

lateral roots extended far from the base of the original tree (Figures 12). Roots 

were of sufficient size (4 mm or more) where suckering could begin, but many 

were too deep (15 cm or deeper), a result of the deep planting of the transplanted 

Figure 10. Average pre-dawn leaf 

water potential, June 28, 2007.
16 USDA Forest Service RMRS-RP-101. 2012.

trees. Effect of weeding on root growth of dozer-cleared soils was less evident, 

likely since weed competition was considerably less and growth was less on the 

dozer-cleared soils. Although root growth was best on roto-cleared soil plots, trees 

in other treatments are surviving and roots are extending outward. However, it 

will take additional years for most of these trees to obtain sufficient size at depths 

necessary for suckering. In any case, suckers are more likely to appear after injury 

or death of parent trees when apical dominance is inhibited.

Some of the transplants on dozer-cleared soil had roots mostly confined to the 

potting hole, perhaps a result of the high density and compaction of this dozercleared and stored soil. Similarly, roots growing from the potted trees, also planted 

on dozer-cleared soils, were likely confined to the potting hole. Excessive reaming 

of the soil planting holes with the power auger could have also attributed to difficulty of roots penetrating beyond the augered hole.

Depth of the root systems for the transplanted aspen ranged from about 15 to 40 

cm, with transplants in the roto-cleared soil planted somewhat deeper than those 

on the dozer cleared soil. These depths are too deep to allow effective suckering. 

Even though roto-cleared trees happened to have been planted somewhat deeper 

than dozer-cleared trees, growth was better on the roto-cleared trees. It is expected 

that trees planted deep will take longer to produce roots at a depth conducive to 

suckering, but those deep planted trees that survived are now producing shallower 

roots. Lateral root systems were already developing on most of the transplanted 

trees, and roots were observed near the surface several meters from the base of 

some trees, suggesting that these trees were becoming well established. Apical 

dominance of the rapidly growing transplanted trees likely prevented suckering 

Figure 11. Average pre-dawn leaf 

water potential, August 2007.
USDA Forest Service RMRS-RP-101. 2012. 17

of these lateral roots. It is expected that enough root system has developed that 

further irrigation of these trees is not necessary.

Conclusions

Although the results of this case study have limited application beyond the study 

area, several factors were important in survival and growth of the planted aspen. 

Insights gained have been used to design further studies to examine aspen survival 

under various weed control and irrigation conditions.

Fencing is necessary to protect small aspen trees from ungulate and cattle 

browsing. Aspen saplings in unfenced areas on the mines were badly damaged by 

browsing and by stem breakage from rubbing.

There was adequate natural rainfall during all three growing seasons, and survival and growth were not improved with supplemental irrigation treatment. Best 

survival and growth was with low or no irrigation, but salinity of irrigation water 

in the first two years of the experiment likely reduced growth of trees receiving 

high and medium amounts of irrigation. Reclaimed soils were not saline, but salinity levels were high enough in irrigation water from local ponds to reduce aspen 

growth. Care must be taken to provide low saline water when irrigating planted 

aspen trees on reclaimed lands. Survival and growth were similar with the low and 

no irrigation treatments, suggesting that enough rainfall and soil moisture occurred 

for the years of this experiment that irrigation was not necessary. It is expected that 

Figure 12. Average distance 

(cm) of root extension 

from base of tree to 4 mm 

root diameter.
18 USDA Forest Service RMRS-RP-101. 2012.

supplemental irrigation with clean water may have increased survival and growth 

above non-irrigated trees. All surviving trees now have developed enough root 

system after three years that further irrigation is not needed.

Best growth appeared to be the natural root segment sprouts on roto-tilled soils 

for the first year. But most of these natural suckers did not survive without weeding. Transplanted sprouts showed considerable transplant injury their first year, 

regardless of irrigation treatment. Survival and growth were lower and injury 

and diseases were higher in transplant cutting plots for the first year compared to 

natural sprouts and potted plants. The transplanted trees from local sources that 

survived the first year grew best in subsequent years. Potted aspen plants from 

nursery stock had a high rate of survival and grew well the first year, but after three 

years, growth was lower than for transplants and natural sprouts. Roots of potted 

aspens appeared to have stayed in the augured potting hole. This also occurred for 

a few of the transplanted trees in the more compact, stored dozer-cleared soil on 

the irrigation treatment plots, the same soil type where this occurred for the potted 

plants. This suggests that disking or other mechanical treatment to mitigate topsoil 

compaction may be beneficial after topsoil has been replaced. Care must be taken 

to avoid planting aspen saplings too deep. The upward growth of roots toward the 

soil surface that was observed indicates that care should be taken in future plantings to plant trees only to a depth of the original root collar.

Best survival and growth occurred on roto-cleared/fresh-hauled soil compared 

to dozer-cleared/stored soil. More natural sprouts from residual root segments 

were evident in roto-cleared soil. The higher number of natural sprouts and better 

growth was likely due to the shorter length of soil storage and the soil characteristics rather than the clearing method. The dozer-cleared soil appeared to be more 

compacted and was less well drained than the roto-cleared soil. It is expected that 

these physical characteristics and the storage effects contributed to the observed 

differences in response between soil types.

Weeded aspen trees showed the best growth. This was likely related to weeds 

competing with the trees for the limited water and nutrient supply and was particularly apparent on the roto-cleared soils where weed populations were high.

Overall recommendation

The best conditions for re-establishing aspen on reclaimed surface mined coal 

lands involved using transplanted saplings from local sources on freshly placed 

soil removed from aspen stands and controlling competing vegetation around individual trees (Figure 13). Care should be taken to avoid compaction of the replaced 

soil, and transplanted trees should be planted no deeper than the original root collar. Irrigation with non-saline water could enhance survival and growth in years 

with drought conditions. Follow-up observations through the 2010 growing season 

indicate that trees surviving the first three years of treatment are growing well, but 

treatment differences during 2005-2007 are still evident.

References

Baker, F.S. 1925. Aspen in the central Rocky Mountain region. Bulletin 1291. 

Washington, DC: U.S. Department of Agriculture. 47 p.

DesRochers, A.; Lieffers, V.J. 2001. The coarse-root system of mature Populus 

tremuloides in declining stands in Alberta, Canada. Journal of Vegetation 

Science 12: (3) 355-360.
USDA Forest Service RMRS-RP-101. 2012. 19

Figure 13. Weeded (left) 

and un-weeded (right) 

plots on roto-tilled 

soil, August 2006.

Hansen, E.A. 1988. Irrigating short rotation intensive culture hybrid poplars. 

Biomass 16: 237-350.

Jones, J.R. 1985. Distribution. In: DeByle, N.V.; Winokur, R.P., eds. Aspen: 

ecology and management in the western United States. Gen. Tech. Rep. GTRRM-119. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, 

Rocky Mountain Forest and Range Experiment Station: 9-10.

Lieffers, V.J.; Landhausser, S.M.; Hogg, E.H. 2001. Is the wide distribution of 

aspen a result of its stress tolerance? In: Shepperd, W.D.; Binkley, D.; Bartos, 

D.L.; Stohlgren, T.J.; Eskew, L.G., comps. Sustaining aspen in western 

landscapes: symposium proceedings; 2000 June 13-15; Grand Junction, CO. 

RMRS-P-18. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, 

Rocky Mountain Research Station: 311-323.

McDonough, W.T. 1979. Quaking aspen [Populus tremuloides]—seed germination 

and early seedling growth. INT-RP-234. Ogden, UT: U.S. Department of 

Agriculture, Forest Service, Intermountain Forest and Range Experiment 

Station. 13 p.

Pearson, L.C.; Lawrence, D.B. 1958. Photosynthesis in aspen bark. American 

Journal of Botany 45: (5) 383-387.

Preston, R.J. 1976. North American trees. Iowa State University Press, Ames. 

399 p.

Shepperd, W.D. 1993. The effect of commercial harvest activities on root 

compaction and suckering of aspen. Western Journal of Applied Forestry 8(2): 

62-66.

Shepperd, W.D.; Mata, S.A. 2005. Planting aspen to rehabilitate riparian areas: 

a pilot study. Res. Note RMRS-RN-26. Fort Collins, CO: U.S. Department of 

Agriculture, Forest Service, Rocky Mountain Research Station. 5 p.
20 USDA Forest Service RMRS-RP-101. 2012.

Shepperd, Wayne D.; Rogers, Paul C.; Burton, David; Bartos, Dale L. 2006. 

Ecology, biodiversity, management, and restoration of aspen in the Sierra 

Nevada. Gen. Tech. Rep. RMRS-GTR-178. Fort Collins, CO: U.S. Department 

of Agriculture, Forest Service, Rocky Mountain Research Station. 122 p.

Siemens, J.A.; Zwiazek, J.J. 2003. Effects of water deficit stress and recovery on 

the root water relations of trembling aspen (Populus tremuloides) seedlings. 

Plant Science 165: 113-120.

Strong, T.; Hansen, E.A. 1991. Response of three Populus species to drought. Res. 

Pap. NC-302. St. Paul, MN: U.S. Department of Agriculture, Forest Service, 

North Central Forest Experiment Station. 9 p.

van den Driessche, R.; Rude, W.; Martens, L. 2003. Effect of fertilization and 

irrigation on growth of aspen (Populus tremuloides Michx.) seedlings over 

three seasons. Forest Ecology and Management 186: 381-389.

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