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Tamm Review: Seedling-based ecology, management, and restoration in aspen (Populus tremuloides)

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Quaking or trembling aspen (Populus tremuloides Michx.) is a foundational tree species, which is native, common, and broadly distributed in North America. The ecology of aspen has been extensively studied throughout its range, but both research and forest management practices have focused primarily on its ability to regenerate asexually via root suckering. The seed-based reproductive ecology of aspen has received comparatively little attention, and information on the underlying processes, mechanisms, and requirements of seed regeneration tends to be scattered, somewhat anecdotal, or based only on localized research efforts. Here we review and explore some of the variables that influence the sexual reproduction and early establishment of aspen. We focus this review on western North America, where trembling aspen plays a dominant ecological role and may be disproportionately impacted by climate change. This synthesis presents existing information and identifies critical knowledge gaps in our understanding of seed-based aspen regeneration, in particular as it relates to flowering and seed production, as well as germination, first year growth, and survival of aspen seedlings. This information is discussed further in the context of aspen ecology and its application in both passive and active management approaches to aspen seedling regeneration and restoration.
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Forest Ecology and Management
journal homepage: www.elsevier.com/locate/foreco
Tamm Review: Seedling-based ecology, management, and restoration in
aspen (Populus tremuloides)
Simon M. Landhäusser
a,
, Bradley D. Pinno
a
, Karen E. Mock
b
a
Department of Renewable Resources, University of Alberta, Edmonton, Alberta T6G 2E3, Canada
b
Wildland Resources Department and Ecology Center, Utah State University, 5230 Old Main Hill, Logan, UT 84322, United States
ARTICLE INFO
Keywords:
Aspen regeneration ecology
Aspen sexual reproduction
Early seedling establishment
Nursery practices
Passive and active aspen restoration
Quaking aspen
Trembling aspen
ABSTRACT
Quaking or trembling aspen (Populus tremuloides Michx.) is a foundational tree species, which is native, common,
and broadly distributed in North America. The ecology of aspen has been extensively studied throughout its
range, but both research and forest management practices have focused primarily on its ability to regenerate
asexually via root suckering. The seed-based reproductive ecology of aspen has received comparatively little
attention, and information on the underlying processes, mechanisms, and requirements of seed regeneration
tends to be scattered, somewhat anecdotal, or based only on localized research efforts.
Here we review and explore some of the variables that influence the sexual reproduction and early estab-
lishment of aspen. We focus this review on western North America, where trembling aspen plays a dominant
ecological role and may be disproportionately impacted by climate change. This synthesis presents existing
information and identifies critical knowledge gaps in our understanding of seed –based aspen regeneration, in
particular as it relates to flowering and seed production, as well as germination, first year growth, and survival of
aspen seedlings. This information is discussed further in the context of aspen ecology and its application in both
passive and active management approaches to aspen seedling regeneration and restoration.
1. Introduction
Quaking or trembling aspen (Populus tremuloides Michx.) is the most
broadly distributed native tree species in North America, with a range
spanning 111° of longitude and 48° of latitude (Little, 1971), occurring
from sea level to 3505 m (Perala, 1990). Aspen is considered an early
successional species that can re-occupy a site quite readily after dis-
turbance through vegetative sucker regeneration from its large under-
ground root system. Aspen has also been considered a ‘foundational
species’ (Ellison et al., 2005), due to its importance in community
structuring and support of a disproportionately high level of plant, in-
sect, and vertebrate diversity (Mills et al., 2000; Rumble et al., 2000;
Simonson et al., 2001; Stohlgren et al., 1997a, 1997b). This is parti-
cularly true in the drier landscapes of western North America and in the
boreal mixedwood forests, where aspen is often the dominant decid-
uous forest tree species. Aspen can be an important source of mer-
chantable wood and fiber, especially in the boreal distribution of its
range (David et al., 2001; Peterson and Peterson, 1995), and it provides
other important services throughout its range such as wildlife and li-
vestock forage and shelter, and recreational/aesthetic value (DeByle
and Winokur, 1985; Peterson and Peterson, 1992 and references
therein).
Ecologically, aspen has traits typically associated with ruderal spe-
cies, including the production of large numbers of easily dispersed but
poorly provisioned seeds, rapid establishment following ground dis-
turbance events, a need for easily available resources, and poor com-
petitiveness with other species. Once established, however, aspen
clones are very persistent, and can provide long-term, stable ecosystems
over a wide range of soil, climatic, and disturbance conditions
(Mueggler, 1985). This combination of attributes, along with re-
markably broad ecological amplitude, makes aspen an excellent species
for ecological restoration (including reclamation, reforestation, and
afforestation) following fires, mining, bark beetle outbreaks, timber
harvesting and other disturbances. The need for increased restoration of
aspen is also anticipated due to the effects of changing climates, par-
ticularly in the warmer, drier portions of its range in both Canada and
the U.S. (Rehfeldt et al., 2009; Zoltai et al., 1990). Finally, because
aspen can serve as a fuel break in low- to moderate-intensity fires
(Fechner and Barrows, 1976; Fisher, 1986; Van Wagner, 1977), it can
potentially be a valuable species for strategically establishing barriers
and defensible spaces in fire-prone landscapes. However, the seed-
based reproductive ecology of aspen is incompletely understood and
https://doi.org/10.1016/j.foreco.2018.09.024
Received 27 July 2018; Received in revised form 13 September 2018; Accepted 15 September 2018
Corresponding author.
E-mail address: Simon.Landhausser@ualberta.ca (S.M. Landhäusser).
Forest Ecology and Management 432 (2019) 231–245
Available online 21 September 2018
0378-1127/ © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/BY/4.0/).
T
poorly synthesized, limiting the effectiveness of aspen establishment
and restoration practices.
This knowledge gap is largely due to the clonal nature of the species.
Aspen is known for its ability to regenerate asexually via root suckering
(Frey et al., 2003; Schier, 1973) and large, long-lived clones are
common (Barnes, 1966; Fetherolf, 1917). The suckering response in
aspen is suppressed by above-ground growth, and suckers are released
when stems are removed (Campbell Jr. and Bartos, 2000; Schier et al.,
1985; Shepperd, 1996; Wan et al., 2006). Vigorous suckering responses
(e.g. > 100,000 stems per ha; Baker, 1925; Bella, 1986; Navratil and
Bella, 1989; Schier and Smith, 1979) following clear felling have led
managers to rely on coppicing as the primary means of stand re-
generation, while research on seed-based regeneration strategies for
aspen has lagged behind that of other forest trees (Long and Mock,
2012). There are clear advantages to vegetative reproduction from the
root system. Under natural disturbance regimes (i.e. fire) the root
system is much more protected than the aboveground parts of the plant
and gives the species the ability to quickly reoccupy and capture the site
after disturbance due to its interconnected root system and reserves
storage within (Zasada et al., 1987). However, coppicing is obviously
restricted to existing aspen stands and clones; a different approach is
necessary for areas where aspen has been lost or, in the case of assisted
migration, where it currently does not exist (Aitken et al., 2008; Gray
et al., 2011; Schreiber et al., 2013).
Another serious disadvantage to complete reliance on vegetative
reproduction is its negative impact on genetic diversity, evolutionary
adaptation, and potentially deleterious mutation accumulation (Ally
et al., 2010). While coppicing can dramatically increase the number of
ramets in a stand, it can only maintain or reduce the numbers of genets
(genetically distinct clones which each originated from a single seed)
(Long and Mock, 2012). Genetic diversity should be a major goal of any
long-term conservation or restoration program. While favorable so-
matic mutations within genets may provide some level of genetic di-
versity and adaptive capacity (Otto and Orive, 1995), sexual re-
combination is necessary for the rapid adaptive evolution of multilocus
traits, particularly with the accelerating pace of environmental and
climate change. Because of extensive clonality in aspen, the number of
stems in a landscape can be a misleading indicator of the genetic di-
versity and adaptive potential or persistence in that landscape (Mock
et al., 2008). Further, reliance on vegetative reproduction alone does
not allow gene flow among populations or across landscapes (Eriksson,
1992), which limits the ability of the species to track climate change.
There is marked regional variation in the proportion of naturally
occurring seed-based recruitment in aspen (Kemperman and Barnes,
1976; Navratil and Bella, 1989). In Canada, Alaska, and the Eastern and
Upper Midwestern portions of the U.S., seedling recruitment appears to
be more common than in the southwestern U.S; however, there is in-
creasing evidence of aspen seedling establishment in recent post-burn
areas in the Intermountain West (Kay and Bartos, 2000; Turner et al.
2003; Fairweather et al. 2014; KM personal observation). This regional
difference in seed-based recruitment rates may be a reason for the
larger clone sizes (i.e. greater age) frequently observed in the south-
western U.S. compared to the much smaller clone sizes elsewhere
(Kemperman and Barnes, 1976). Further, differences are likely driven
by climates, substrates, herbivory levels, and competing vegetation that
reduce seedling survival (Kemperman and Barnes, 1976). There may
also be regional differences in seed production rates. In the western
U.S., seed production in aspen is less commonly observed than in other
parts of the species range (pers. observations SL & KM), possibly due to
regional differences in climate patterns and disturbance regimes (Dale
et al., 2001; Graham et al., 1990), phylogenetic differences (Callahan
et al., 2013), or rates of triploidy (Mock et al., 2012). Nevertheless, seed
production and seedling recruitment do occur at least episodically in
the southwestern U.S. environments (Fairweather et al., 2014; Kay,
1993; Mock et al., 2008; Romme et al., 1997). In Alaska and western
Canada, upslope range expansion in response to disturbance has been
shown to occur via seedling establishment (Landhäusser et al., 2010;
Zasada et al., 1983) suggesting that seedling-based restoration could be
an important, but underutilized, practice for aspen management.
The importance of seed reproduction in aspen is dramatically ap-
parent in the species’ biogeography. The majority of the species’ current
range since the last glacial maximum represents an expansion into
previously glaciated areas (Callahan et al., 2013; Delorme et al., 1977;
Little, 1971). Such rapid expansion following the glaciation would only
have been possible through wind-based seed dispersal. Wind-dispersed
seed is also likely responsible for the high levels of genetic diversity
within populations and the generally low levels of genetic structuring
among populations in northern distribution of aspen (Callahan et al.,
2013). Once established, aspen clones may persist for millennia (Ally
et al., 2010). Given the increasing incidence and current scale of dis-
turbances in forests (e.g. fire, beetle kill, fragmentation) and the in-
creasing rate of climate change, we anticipate an increasing need for
and reliance on seed-based reclamation, reforestation and afforestation
with aspen (Dale et al., 2001). Thus, the factors influencing seed pro-
duction and seedling recruitment in aspen are critical aspects in its
regeneration ecology and are crucial in the maintenance or restoration
of aspen on the landscape. Here we synthesize and explore the variables
influencing sexual reproduction and first-year seedling establishment in
aspen in western North America. We have focused our review on western
North America because of the ecological dominance and importance of
aspen in these landscapes; however, there are parallels to other regions
in the northern hemisphere where other closely related species such as
Populus tremula L. and P. grandidentata Michx. dominate. Specifically,
we synthesize existing information on (1) flowering and seed produc-
tion, (2) germination, and (3) first year growth and survival, which we
then discuss in the context of both passive and active aspen restoration
approaches.
1.1. Flowering and seed production
Aspen is generally dioecious, although instances of poly-
gamodioecy, monoecy, and intersexual trees have occasionally been
documented (Pauley and Mennel, 1957; Strothmann and Zasada, 1962;
Wyckoff and Zasada, 2008 and references therein). In most localities,
staminate: pistillate clonal ratios are 1:1, but male-biased ratios have
occasionally been noted (Baker, 1918; Pauley and Mennel, 1957;
Schreiner, 1974), and in one western U.S. study, males were found to
occupy higher elevations and harsher sites than females (Grant and
Mitton, 1979). The generality of these patterns is difficult to assess, as
large proportions of trees and clones may not flower in a given year,
and because clonal boundaries within stands are often cryptic
(Einspahr, 1960; Mock et al., 2008).
Flowering time is driven primarily by air temperature (Fechner and
Barrows, 1976; Moss, 1960), and has been shown to vary by up to
3 weeks from year to year in the same clone (Campbell Jr., 1984; Jones
and Schier, 1985; Perala, 1990). Early spring emergence of flowers can
lead to frost damage and ultimately catkin loss if early spring warming
is followed by a hard frost (Schier et al., 1985). We anticipate that
climate change will exacerbate this phenomenon through diminishing
spring snowpacks, resulting in early warm-up and reduced cold buf-
fering (Inouye, 2008). High rates of catkin damage or loss can also be
associated with fungal disease or insects (pers. observations SL & KM).
The formation of flower buds occurs during the late summer of the
year prior. Catkin emergence and fertilization generally occur in late
March through May before leaves flush (Fechner and Barrows, 1976;
Moss, 1960; Schreiner, 1974). Aspen flowers are small, relatively in-
conspicuous, unisexual, and are arranged in indeterminate pistillate or
staminate catkins (aments) (Fig. 1). This arrangement facilitates the
wind dispersal of both pollen and seeds (Peterson and Peterson, 1992).
The entire flowering process, from catkin appearance to seed matura-
tion, requires 4–6 weeks and is completed in the early spring prior to or
during leaf expansion (McDonough, 1979). Seeds ripen in May or June,
S.M. Landhäusser et al. Forest Ecology and Management 432 (2019) 231–245
232
although timing can vary with weather and geography (Einspahr and
Benson, 1964; Perala, 1990). The densely haired bracts subtending the
flowers make the catkins appear finely haired when they emerge from
the bud scales and before they expand (Fig. 1). As they begin to expand
and mature, bracts are more separated and the appearance of the cat-
kins becomes dominated by both the stamens and stigmas, which are
reddish in color. As they expand, both male and female catkins become
pendulous and extend to approximately 5–10 cm in length (Perala,
1990). Male catkins become quite flexible as they ripen, and can often
be distinguished from the more rigid female catkins at a distance in the
field. The two-chambered ovaries are green, flask-shaped, and 5–7 mm
long, and produce a capsule as a fruit (Fechner and Barrows, 1976).
Individual capsules can contain approximately 6–10 seeds, and catkins
contain approximately 70–100 capsules (Jones and DeByle, 1985b).
The proportion of developing capsules and viable seeds per capsule
vary by genet, within and between trees, and across years. Triploid
genets, which are common in the western U.S. (Mock et al., 2012), have
been observed to produce catkins (either male or female; KM un-
published data), although these are expected to be largely sterile.
Flowering can begin as early as two years after planting, but trees may
not produce heavy seed crops until 10–20 years of age (Maini and
Crayford, 1968; Schreiner, 1974; Stoeckeler, 1960).
Aspen can produce prolific seed crops. Maini and Crayford (1968)
estimated that a single tree can produce over 1.6 million seeds in a
season with about 700,000 to 1 million seeds per kilogram (Maini,
1968; Schreiner, 1974; United States Department of Agriculture, 2018).
Seed production can vary by year and by clone, but generally occurs
between mid-May and mid-June (Fechner and Barrows, 1976;
McDonough, 1985; Moss, 1938; Schreiner, 1965, 1974). Within a clone,
flower production can vary among ramets or branches within a ramet,
depending on ramet age and condition, but patterns have not been well
described over space and time. Stress-induced flowering has been ob-
served and stem girdling has been used to induce flowering (Pauley and
Mennel, 1957). Periodicity of aspen seed production (i.e. masting), at
both the tree and clone level, has been suggested but not well docu-
mented (Maini and Crayford, 1968; McDonough, 1979; Mitton and
Grant, 1996; Santamour, 1956; Schreiner, 1974; Strothmann and
Zasada, 1962).
Seeds are initially a translucent green, and when fully ripened, seeds
attain a light brown color. Individual aspen seeds are small (approxi-
mately 1–2 mm), and when dispersed from catkins they are attached
loosely to a network of long, white, silky hairs (pappus) which expand
from the capsule and can carry the seeds long distances (e.g. several
km) via wind dispersal. In heavy seed production years, accumulation
of pappus containing the seeds can cover the ground locally, settling
into low areas and against windbreaks (Fig. 2).
Fig. 1. Different stages of male (left) and female (right) catkin and flower development, detailed male and female flower, closed and opened capsule with protruding
pappus and a seed with the pappus attached (drawings by Sandy Long).
S.M. Landhäusser et al. Forest Ecology and Management 432 (2019) 231–245
233
1.2. Seed germination and initial root development
Aspen seeds have no dormancy and can germinate immediately
after ripening if provided with adequate moisture. Seed viability rates
are typically quite high (> 95%; Maini, 1960; Mitton and Grant, 1996;
Perala, 1995). However, seeds remain viable in the field for only a few
weeks even under most favorable natural conditions (low ambient
temperature and high humidity) (Fechner and Myers, 1981; Zasada
et al., 1983).
The germination stage is defined by breaking of the seed coat, and
appearance of hypocotyl and cotyledons, prior to development of the
apical meristem (∼2 days) (Fig. 3). Germination is triggered by
moisture, which is imbibed by the seed within a few hours (Wolken
et al., 2010), and can occur between 0 and 37 °C, with an optimum of
15–30 °C (Faust, 1936; Fechner and Myers, 1981; Jones and Cheliak,
1985). Germination can occur without light or even when seeds are
submerged in or floating on water (Faust, 1936; Perala, 1990). Thermal
tolerances and interactions with soil moisture and humidity during seed
germination can vary by genotype (Fechner and Myers, 1981;
McDonough, 1979). A brush of root hair (coronet) at the base of the
hypocotyl appears within 24 h (Fig. 3), and is a critical mechanism for
early water absorption and substrate attachment prior to radicle de-
velopment (Day, 1944; Yanchevsky, 1904; Young and Young, 1992),
but are easily damaged by inadequate moisture (Moss, 1938; Perala,
1990). Fechner and Myers (1981) suggested that the greatest suscept-
ibility to moisture stress in aspen germinants occurs subsequent to the
formation of the coronet, and likely during the expansion of the coty-
ledons and hypocotyl. The radicle extends to 20–40 mm within 2 weeks
(Wolken et al., 2010), and is also highly susceptible to drying (Maini,
1960). Green cotyledons (usually 2, occasionally 3; Maini, 1960) also
appear within 24–48 h (Fig. 3). The extreme sensitivity of aspen ger-
minants to even minor soil water deficits (Einspahr and Winton, 1976;
Faust, 1936; Fechner and Myers, 1981; McDonough, 1985, 1979, 1975,
1971; Moss, 1938) creates an important vulnerability for seedling
establishment, particularly in years where precipitation does not coin-
cide with the timing of seed production. Kemperman and Barnes (1976)
suggested that this vulnerability to drought was responsible for in-
frequent seedling establishment and the existence of the large clones in
the semi-arid Intermountain West of the US.
1.3. Early seedling development
Aspen seeds are poorly provisioned with little to no endosperm
(Nagaraj, 1952; Simak, 1980), so apart from water the immediate ac-
cess to light is also critical for continued development. This require-
ment for both consistent moisture shortly after ripening, along with full
to partial sun, severely limits seed establishment success rates in many
cases (DeByle and Winokur, 1985; Maini and Crayford, 1968;
McDonough, 1975; Strain, 1964). These conditions, when they occur,
tend to follow fires or other forms of soil surface disturbances at larger
spatial scales, providing appropriate seed beds and microsite conditions
(Barnes, 1966; Fairweather et al., 2014; Greene et al., 1999; Kay, 1993;
Krasnow and Stephens, 2015; Landhäusser et al., 2010; Quinn and Wu,
2000; Romme et al., 1997; Schott et al., 2014; Turner et al., 2003).
Seedling establishment events are particularly recognizable following
such large-scale disturbances, but these conditions may also occur more
frequently at very small spatial scales, potentially creating a mosaic of
clonal sizes and genetic diversity (Gill et al., 2017; Long and Mock,
2012; Mock et al., 2008). Thus, naturally occurring seedling recruit-
ment, particularly in more arid climates, tends to be highly episodic and
spatially varied, reflecting what Jelinski and Cheliak (1992) referred to
as “windows of opportunity” (Romme et al., 2005). We suggest in later
sections that these ‘windows’ can be extended (both spatially and
temporally) by targeted management and restoration practices, and
that these practices will be increasingly important for maintaining
aspen-dominated forests under rapidly changing climate regimes.
The development and maintenance of root resources is a core fea-
ture of aspen ecology, buffering against water stress, leaf damage, and
Fig. 2. Seed dispersal and ripened catkin (inset) of aspen (right panel), seed with pappus (top left panel), and accumulation of seed in depressions and sheltered areas
(bottom left panel; photograph by Darren McAvoy).
S.M. Landhäusser et al. Forest Ecology and Management 432 (2019) 231–245
234
defoliation (Barnes, 1966). Root growth is an especially critical com-
ponent of early establishment in aspen (Day, 1944; Martens et al., 2007;
Moss, 1938), and the positive effect of higher root carbohydrate re-
serves and root:shoot mass ratios on establishment success may become
more pronounced when nutrients or soil moisture are limited
(Landhäusser et al., 2012a, 2012b). During the first year, aspen
seedlings develop primarily fibrous lateral root systems with few tap-
roots (Jones and DeByle, 1985b; Maini and Crayford, 1968 and refer-
ences therein), but may also produce suckers (Brinkman and Roe,
1975). Root and shoot growth and their ratio are impacted by both
growing conditions and genetic source (Jelinski, 1993; Pregitzer and
Friend, 1996; Romme et al., 2005) (Fig. 4).
Fig. 3. Different stages of early aspen seedling development. A one-day-old germinant with the associated coronet and cotyledons, the two-leaf (top) and four-leaf
stage (bottom) of and the different shapes of the first true juvenile leaves. Drawing on the left shows the rapidly developing taproot after about 4–5 days (Drawing by
Sandy Long).
Fig. 4. One-year-old dormant aspen seedlings with high root:shoot mass established from seed in a natural soil. One-year-old aspen seedling established in a cutblock
(Alberta, Canada).
S.M. Landhäusser et al. Forest Ecology and Management 432 (2019) 231–245
235
Aspen seedlings have an indeterminate growth strategy, and typi-
cally attain a height of between 15 and 60 cm in the first growing
season when conditions are not limiting, with lateral roots extending
30–40 cm (Day, 1944; DeByle, 1964; Moss, 1938; Perala, 1990; Strain,
1964). Aspen seedling shoot growth corresponds closely with root
growth and function (Landhäusser et al., 2012b) and first year roots are
primarily limited to the upper 20 cm of soil (Pregitzer and Friend,
1996), so competition with other plants for water and nutrients in this
root zone (e.g. grasses) can be a serious constraint on seedling growth
and success (Bockstette et al., 2017; Landhäusser and Lieffers, 1998;
Powell and Bork, 2004; St. Clair et al., 2013).
1.4. Water
Continuous, accessible soil moisture is critical for initial establish-
ment of aspen seedlings (Fechner and Myers, 1981; Maini, 1960;
McDonough, 1979), and is thought to be a central driver in the land-
scape-scale distributions of clones and in continental-scale differences
in clonal sizes (Barnes, 1966; Jones and Cheliak, 1985; Perala, 1990).
At the landscape scale, moisture gradients are expected to be driven by
latitude, elevation, and aspect. In the southern extent of its range,
north-facing slopes or higher elevations will provide the best general
habitats for water retention into the growing season for aspen (Dixon,
1935; Larson, 1944). In northern portions and at higher elevations of its
range, aspen may be more limited by soil temperature than water,
growing best on south and southwest-facing slopes (Perala, 1990).
At the microsite scale, the natural drivers of moisture availability
during the growing season include soil texture, organic matter content,
microtopography (e.g. hillslopes, concavities), and seeps/springs. As
climates change, however, increasing aridity is projected to dramati-
cally decrease the amount of aspen habitat in western North America
(Rehfeldt et al., 2009), and recent large-scale declines of aspen stands in
these landscapes have been associated with drought stress (Anderegg
et al., 2012). Seedlings are particularly susceptible to drought condi-
tions because they lack the resources and resilience of a larger clonal
root mass, although they also suffer mortality in standing water
(Howard, 1996; Landhäusser et al., 2003).
1.5. Light
Aside from moisture, the most critical resource for early growth of
aspen seedlings is light (Kaelke et al., 2001), which can be at odds with
the need for continuous access to water. Aspen seedlings are considered
shade-intolerant (Landhäusser and Lieffers, 2001; McDonough, 1979;
Moss, 1938). Light restriction due to tree canopy cover (LePage et al.,
2000) limits aspen seedling establishment to stand edges and gaps,
where there may be additional competition for either light (e.g. deep-
rooted shrubs) or soil moisture (e.g. grasses). Compared with other co-
occurring tree species, aspen have more growth and higher rates of
photosynthesis in high to moderately high light levels, but are at risk for
lethal carbohydrate shortages in shaded understory conditions (Kaelke
et al., 2001; Landhäusser and Lieffers, 2001).
Given its wide geographic distribution, aspen experiences a wide
range of photoperiods and light qualities such as the amount of ultra-
violet light. Growth rates in aspen are known to be impacted by pho-
toperiod (Vaartaja, 1960), and photoperiod responses have been shown
to have a genetic basis (Ingvarsson et al., 2006), suggesting that local
adaptation to day-length may be an important consideration in seed
transfer zones or assisted migration practices across latitudes. Light
quality showed little effect on aspen growth (Kelly et al., 2015); how-
ever, excessive ultraviolet light can be damaging to leaves, particularly
at high altitudes, and some evidence suggests that the high levels of
tannins in aspen leaves may have at least a partial role as photodamage
inhibitors (Close and McArthur, 2002; Mellway and Constabel, 2009;
Stevens and Lindroth, 2005).
1.6. Soil
Aspen occupies a broad range of soil types across its range, but
achieves the best growth in well-drained, loamy soils high in organic
material (Campbell Jr. and Bartos, 2000; Jones and DeByle, 1985a;
Perala, 1990; Steneker, 1976). For germination and early seedling
growth, soil properties that influence water retention and availability
are critical (Wolken et al., 2010), but saturated soils prevent gas ex-
change and promote fungal pathogens, and may be problematic in
many sites (Fralish and Loucks, 1967; Kittredge Jr., 1938; Landhäusser
et al., 2003). The availability of bare mineral soils are often cited as a
requirement for successful germination and early seedling growth
(Fralish, 1972; McDonough, 1985), presumably due to the lack of
competition for light and water. However, in environments where
water is limiting, litter, coarse woody debris and moderate shade from
other plants may provide important facilitation for aspen seedlings (de
Chantal and Granström, 2007; Fairweather et al., 2014; Landhäusser
et al., 2010). Surface microtopography or sheltering structures may be
especially important in bare mineral soils remaining after fires, where
black surfaces may further increase risk of high temperatures and
evaporation (Maini, 1968). Aspen seedlings are sensitive to low soil
temperatures, which affect new root growth and soil water uptake
(Landhäusser and Lieffers, 1998; Peng and Dang, 2003; Wan et al.,
1999). However, soil temperature can be indirectly affected by other
variables such as shading (Landhäusser and Lieffers, 2001), competition
(Bockstette et al., 2017; Landhäusser and Lieffers, 1998) and site
characteristics such as flooding (Landhäusser et al., 2003). Little is
known about the nutrient requirements of aspen but aspen seedling
growth and survival has been shown to be responsive to soil nutrient
availability (DesRochers et al., 2003; Hobbie and Chapin III, 1998;
Kinney and Lindroth, 1997; Lu and Sucoff, 2001; Romme et al., 2005;
Van Cleve and Oliver, 1982; Yang, 1991), while overall, aspen is able to
grow over a large amplitude of nutrient conditions (Jones and DeByle,
1985a; Wolken et al., 2010).
1.7. Fungal associations
A broad range of ectomycorrhizal and (less frequently) arbuscular
mycorrhizal fungi are associated with aspen seedlings, which can im-
prove water and nutrient availability (Cripps and Miller Jr., 1993;
Cripps and Miller, 1995; Hankin et al., 2015; Hupperts et al., 2017).
Mycorrhizal fungi have been shown to stimulate growth in aspen in
controlled conditions (Ali, 1985; Cripps, 2000; Landhäusser et al.,
2002), but little is known about the relative importance of particular
mycorrhizal associations in aspen, or how climate, temperature, soil
conditions, stand development stage, or other plants in natural com-
munities may affect these associations (Clark and St. Clair, 2011).
2. Seed-based aspen restoration strategies
The ecological limitations and requirements for successful aspen
seedling establishment, reviewed above, provide an important frame-
work for the design of successful seed-based restoration strategies for
aspen. The importance of particular ecological factors varies with re-
gional climates and likely by genetic sources. The probability of success
and the acceptable costs vs. benefits of seed-based restoration also vary
by site. Thus, appropriate restoration strategies are dependent on spe-
cific management goals, which in turn are driven by societal values,
landowner and regulatory expectations, and site limitations. There are
two general approaches for seed-based restoration of aspen: passive
restoration, where seedling recruitment occurs passively from naturally
occurring seed rain but can be strategically enhanced by creating ap-
propriate microsites, and active restoration, where aspen seedlings are
nursery grown and then out-planted onto restoration sites. Seeding of
aspen could be considered an intermediate option between active and
passive aspen restoration; however, the information provided in this
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review will show that the collection of seeds and the direct seeding into
prepared sites, would be impractical and high risk because of the high
cost of seed collection, the difficulty of dispersing these small seeds, and
the extremely low survival rate of individual seeds when broadcast
across an area.
2.1. Passive restoration of aspen from seed
Past recommendations have presumed that forest managers can do
little to enhance natural aspen seedling establishment (McDonough,
1979), but we suggest that the current prevalence of human activity
throughout many forest lands provides exactly this opportunity. Suc-
cessful passive restoration of aspen from seed does require a naturally
occurring seed rain and exacting climatic conditions for dispersal,
germination, and early establishment, as well as the appropriate mi-
crosites that will provide the necessary resources and conditions for
successful establishment. A range of different microsites can be pro-
vided through surface soil disturbances created naturally or artificially.
For example, natural surface soil disturbances can result from fire,
windthrow, landslides, and floodplains while artificial disturbances can
result from forest harvesting and subsequent site preparation, agri-
culture, mining and energy development, and road construction. Nat-
ural seedling establishment has been observed following these events
and our observations can be instructive for the development of proto-
cols that may enhance passive seedling recruitment.
2.1.1. Fire
Aspen seedling establishment following fire has been documented
frequently, although there may be observational bias against other
kinds of seedling establishment events, as seedlings are particularly
easy to visualize against a blackened, otherwise unvegetated substrate
(Long and Mock, 2012). The 1988 Yellowstone fire is a well-known
example of post-fire sexual reproduction with recorded seedling den-
sities ranging from 31 to 46,000/ha (Kay, 1993; Romme et al., 2005).
As expected, differences in seedling establishment density in this event
depended on both proximity to seed source and site conditions, with
increased seedling density closer to mature aspen seed sources and on
suitable soils lacking intense vegetative competition (Turner et al.,
2003). Other documented examples of aspen seedling regeneration
after fire come from the Alaskan (Zasada et al., 1983) and Canadian
boreal forests (BP, SL personal observations) and the montane forests in
the Intermountain West (Fairweather et al., 2014; Krasnow and
Stephens, 2015) (Fig. 5). Most post-fire seedling regeneration occurs
within a few years following the fire, but can continue for longer per-
iods as long as a seed source and suitable microsites are still available
and have not yet become occupied or shaded by competing vegetation.
Fires do not always result in aspen seedling regeneration, however,
as there can be mismatches in the timing of fire, the quantity and timing
of seed rain, and the climatic conditions. For example, early growing
season fires, which are common in northern boreal aspen forests fol-
lowing dry years (Stocks et al., 2002), may create suitable seedbeds for
aspen establishment in early summer but if dry weather conditions
prohibit aspen seedling establishment, the narrow window of oppor-
tunity may be missed. Later growing season fires, which are relatively
more common in the southern parts of aspen’s range (Westerling et al.,
2003), may create seedbeds for the following year’s seed rain. However,
fire can also create surface conditions that are unsuitable for aspen
seedling establishment due to thick ash layers or increase soil surface
hydrophobicity (Robichaud, 2000).
An effective management strategy when fires in potential aspen
habitat are expected or planned might be to conduct post-fire surveys
for seedling establishment, and plan to protect those areas of higher
stocking with fencing, jackstraw barriers, and/or reduction of ungulate
herbivory pressure if needed. Herbivory from ungulates, livestock, and
insects is a well-known threat to vegetative regeneration in aspen (e.g.
Endress et al. 2012, Seager et al. 2013; Kay and Bartos, 2000 many
more), while much less studied, the impact of herbivory on individual
seedlings with much smaller root systems is expected to be even greater
(e.g. Hansen et al., 2016). Control of ungulate herbivory might include
increased hunting pressure for wild ungulates or temporary reduction
of livestock pressure. For example, successful European aspen (Populus
tremula) seedling regeneration after fire depended on protection from
ungulate browsing which occurred within aggregations of woody debris
(de Chantal and Granström, 2007). Additionally, to avoid reducing
available microsites for establishment and increasing competition, the
operational post-fire seeding of grasses should be avoided for areas
where natural aspen regeneration is expected (Turner et al., 2003),
provided that erosion is not a significant threat. In regions where nat-
ural aspen seedling establishment is more common (e.g. Canada and
eastern U.S.) or where herbivore populations are lower, protecting
seedlings is likely to be less important.
2.1.2. Forest harvesting and site preparation
Timber harvesting and site preparation activities can also create
microsites suitable for aspen seedling establishment. Harvesting re-
duces competition for light by removing the canopy, and causes surface
soil disturbance, either through the actual harvest or subsequent site
preparation such as mounding and trenching (Landhäusser, 2009).
These disturbances create suitable microsites, which have micro-
topographical features that can retain soil moisture and are free from
competition by grasses and shrubs, at least temporarily (Fig. 5). Har-
vesting of lodgepole pine in Alberta’s upper foothills followed by site
preparation done to promote pine regeneration was implicated in nat-
ural aspen seedling establishment at densities from 1500 to 10,000
seedlings per ha on sites with appropriate moisture conditions, i.e mesic
and subhygric moisture regimes, (Landhäusser et al., 2010) (Fig. 6).
The probability of natural seedling establishment after harvesting can
likely be enhanced through the use of appropriate harvesting and site
preparation techniques, which create microsites of bare soil to collect
seed and water or snow. The use of heavy equipment does, however,
pose a risk for soil compaction which should be avoided, localized, or
mitigated. In some environments, coarse woody debris can be left to
enhance microsite soil moisture, and/or piled to minimize herbivore
access (Fig. 6). Evidence of seedling establishment from more harvest
studies is lacking, potentially because of intense vegetation competition
after harvesting precludes aspen seedling establishment, or because
seedlings can be missed or misidentified in sucker-dominated aspen
regeneration (Frey et al., 2003; Long and Mock, 2012). As with post-fire
environments, seeding with competitors (e.g. grasses or other cover
crops) may reduce natural aspen recruitment, and post-harvest mon-
itoring for aspen seedlings could help guide spatially appropriate con-
servation efforts.
Another consideration to enhance natural aspen recruitment after
harvesting would be to leave potential seed trees in cutblocks (Long and
Mock, 2012). During harvesting operations, the aspen component in
stands is often not desirable, particularly for operators targeting con-
ifers. Leaving clusters of mature aspen could help to not only suppress
local suckering (distributing the risk of herbivory losses), but these trees
(if female) may become seed sources for a broad surrounding area. We
recommend that the sex of the trees/clones to be retained in a harvest
operation be determined ahead of time, to the extent possible, and that
mature aspen be left in clusters to retain the benefits of a shared root
system.
2.1.3. Other disturbances
Other disturbances through resource extraction operations such as
mining or road building activities can also create the necessary mi-
crosite conditions for aspen seedling establishment. For example,
naturally occurring aspen seedling densities of up to 20,000–100,000
per ha have been found on phosphate mine dumps in southeastern
Idaho (Williams and Johnston, 1984), on coal mines in Alberta (Schott
et al., 2014) and Pennsylvania (Brenner et al., 1984), and on oil sands
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mines in the boreal forest (Pinno and Errington, 2015). Reclamation
and forest restoration of mined land offers the opportunity to control
even more of the important factors influencing microsites for aspen
seedling establishment, including topography, soil type and competing
vegetation, than does timber harvesting. Mined land reclamation sites
are often characterized by exposed and reconstructed soil, with low
levels of vegetative competition relative to timber harvest or post-fire
sites. For example, at boreal forest restoration sites, increased aspen
density from natural seedling recruitment was positively associated
with peat-based soils that have high water holding capacity and lower
competition than other reclamation soil types and negatively associated
with fertilization, which resulted in increased competition (Pinno and
Errington, 2015). Surface roughness is also associated with increased
aspen regeneration, likely due to both trapping more seed relative to
flat and compacted microsites and providing a range of microsite types
(Melnik et al., 2018; Pinno and Errington, 2015; Schott et al., 2014).
There is clearly an opportunity to utilize mine reclamation sites across
the species range for aspen restoration.
Although natural aspen seedling regeneration is likely far more
common than previously thought (Long and Mock, 2012), it is still a
relatively rare event, and in many instances after natural or human
disturbances regeneration will not occur either because of a lack of seed
rain, limiting site conditions, or an interaction of the two. The lack of
appropriate microsite conditions for germination may be the primary
limiting factor for natural aspen seedling establishment, particularly in
drier environments including most of the western portion of aspen’s
range, so a more active approach may be required to establish aspen.
This can be accomplished through planting of seedlings, which can
mitigate some of these establishment limitations by growing seedlings
and getting them past the particularly vulnerable period of germination
and early establishment. For this reason, seedling-based restoration
using nursery-grown stock holds great promise for reforestation and
restoration efforts.
2.2. Active restoration using planted seedlings
The production of aspen planting stock can be accomplished using
either vegetative or seedling-based processes. Vegetative propagation of
aspen from root cuttings can be successful (Schier et al., 1985), but the
production of sufficient planting material has proven to be a significant
limitation (Schier et al., 1985; Snedden et al., 2010), and vegetative
propagation does not increase genetic diversity via sexual recombina-
tion. Therefore, the use of aspen seedlings is currently the preferred
method for producing aspen planting stock (Jacobs et al., 2015;
Macdonald et al., 2015). The collection, processing and storage of seeds
are the initial steps in the production of seedling stock, followed by
nursery production of the seedlings and their outplanting on site. Fol-
lowing the target seedling concept, the most limiting site factors are
Fig. 5. Aspen germinants establishing in a small depression (deer hoof print) two years after a fire in Alberta, Canada (top panel) and aspen germinants in the shelter
of a burned log observed one year following a fire in southern Utah, USA (bottom panel, photograph by Karen Mock).
S.M. Landhäusser et al. Forest Ecology and Management 432 (2019) 231–245
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used to determine ideal seedling stock types in regards to species- and
site-specific traits (Puttonen, 1996; Rose et al., 1990). However, while
seedling traits can influence afforestation success, identifying and po-
tentially mitigating some of the most limiting site conditions, such as
competing vegetation, is also crucial for the success of forest restoration
using seedlings.
Until recently, the knowledge of seedling production techniques for
aspen was based on information available for other commercially im-
portant tree species (mostly conifers in the northern regions)
(Macdonald et al., 2015; Oliet and Jacobs, 2012). However, recent
studies on aspen seedlings and their quality have shown promise in the
ability to manipulate physiological and morphological characteristics
during nursery culture, which has shown to impact early seedling
performance on stressful restoration sites (Landhäusser et al., 2012a,
2012b). Considering that the development of quality seedling stock of
conifers took decades to achieve, the development of quality growing
stock for less commercially important species such as aspen should be
considered to be in its infancy.
2.2.1. Seed collection, processing and storage
Since aspen seeds are wind-dispersed quickly after ripening, the
timing of seed collection needs to be anticipated. The window for col-
lection is generally between three to five days, but can be extended
during cool and damp weather or shortened to a single day if conditions
are warm, dry, and windy (Smreciu et al., 2013). Female trees should
be monitored regularly (almost daily) during the seed ripening phase,
and it is advisable that female trees (clones) are identified and pre-
emptively marked prior to leaf flushing, since leaves obscure catkin
visibility. Generally, catkins should be harvested when the enclosed
seeds are a light straw or darker tan color, but this can vary dramati-
cally among clones and collection areas. Another indicator of appro-
priate timing for catkin collection is the presence of capsules showing
the white pappus (Smreciu et al., 2013). Timing of collection is critical
since seeds collected prematurely will not germinate well if at all. Tall
aspen trees might need to be felled to collect catkins, but other means
such as pole pruners, slingshots, and guns (shotgun or small caliber
rifles) can be used. Care needs to be taken during collection, as fresh
catkins can heat up quickly from respiration activity when packed too
tightly, resulting in poor seed quality. If capsules are not quite ripe, cut
branches can also be kept for a short period of time (1–2 weeks) in
water (Moench, 1999). Ripe catkins are picked and dried slowly at
room temperature, and seeds can be collected using a steady airstream
to dislodge seeds from the pappus (eg. Smreciu et al., 2013).
Aspen seed can quickly lose viability (within a few weeks) when
stored at room temperature and high humidity, so seeds should be sown
or stored frozen immediately after cleaning. Although aspen seeds have
no natural dormancy, seeds refrigerated at -5°C have been shown to
have 90% viability after 48 weeks (McDonough, 1979; United States
Fig. 6. Passive aspen seedling establishment with shelter from coarse woody debris (top left (photograph by Karen Mock) and right panel) and in a concave microsite
position after mechanical site preparation (mounding) for pine regeneration (bottom panel).
S.M. Landhäusser et al. Forest Ecology and Management 432 (2019) 231–245
239
Department of Agriculture, 2018) and can maintain high levels of via-
bility when stored at -18 to -20 °C for up to 5–8 years (Pinno et al.,
2012; Young and Young, 1992).
2.2.2. Seedling production
Planted aspen seedlings have historically performed very poorly
after out-planting, with high mortality and slow growth (Johnson,
1996; Okafo and Hanover, 1978; Shepperd, 2000; Shepperd and Mata,
2005; Steneker, 1976). Slow initial growth is not always related to
competition (Einspahr and Benson, 1964) or resource limitations (van
den Driessche et al., 2003), suggesting that planting stock quality may
be a factor in poor establishment and growth of seedlings.
Recent work indicates that aspen seedling performance can be re-
lated to morphological and physiological characteristics of the seed-
lings, which can be manipulated during nursery production. This allows
seedlings to be tailored for specific site conditions (Fig. 7). For example,
on exposed upland forest reclamation sites, where evaporative demands
are high and soil moisture is often limiting, seedlings with high root:-
shoot ratios should have an advantage (Landhäusser et al., 2012a). As
another example, on nutrient-limited sites, nutrient-loaded aspen
seedlings performed much better than seedlings with lower tissue nu-
trient concentrations (Salifu et al., 2009; Schott et al., 2016). By con-
trast, on sites with vegetative competition, there will be trade-offs be-
tween physiological and morphological characteristics (Villar-Salvador
et al., 2012) depending on the type of competition (i.e. above vs. below-
ground competition). For example, on grassy sites with significant
above and below-ground competition, tall aspen seedlings experienced
little light competition; however, these tall seedlings performed more
poorly than a much shorter stock type that experienced light competi-
tion, but had higher root:shoot ratios and reserve concentrations
(Landhäusser et al., 2012b; Le, 2017). Seedling characteristics of aspen
will also play a role in their response to herbivory. Characteristics such
as nutrient and carbon reserve status have shown to affect the seedlings
allocation to chemical defense vs. growth, which influences not only the
damage to or the recovery of an affected seedling but also the fitness of
the herbivore (Osier and Lindroth, 2001; Lindroth and St. Clair, 2013;
Najar et al., 2014; Cole et al., 2016). Just as root:shoot ratios and re-
serve status can be manipulated in the nursery through growing con-
ditions and genotype selection, manipulation of defense chemistry in
aspen seedlings is likely possible in a nursery setting and represents an
exciting area for future research.
2.2.3. Nursery practices
Nursery practices for aspen were originally described for P. tremula
by Barth (1942) and later refined for P. tremuloides (Benson and
Einspahr, 1959; Einspahr, 1959; Wyckoff and Stewart, 1977). Although
bare-root stock production is possible, most information on seedling
quality and performance for aspen involves containerized stock. Gen-
erally aspen seedlings with high root:shoot ratios perform significantly
better in the field (Landhäusser et al., 2012a; Martens et al., 2007;
Schott et al., 2016), likely because a larger root system supporting a
relatively smaller shoot would have an increased capacity to supply
water and nutrients to the developing shoot after planting. Ad-
ditionally, seedlings with high root:shoot ratios tended to have high
non-structural carbohydrate reserve (NSC) (i.e. sugars and starch)
concentrations (Landhäusser et al., 2012a; 2012b; Martens et al., 2007).
We expect that the importance of high root:shoot ratios in seedlings
would be particularly important when outplanting into sites where
drought stress is anticipated.
Maximizing both root:shoot ratios and carbohydrate reserves could
potentially result in higher stress tolerance (Landhäusser et al., 2012b).
The positive relationship between NSC reserves and seedling survival
and performance is well established (Canham et al., 1999; Myers and
Kitajima, 2007; Ritchie, 1984), and NSC reserves are known to be cri-
tical for seedlings to survive stresses, such as drought, cold tempera-
tures, and physical and disease damage (Canham et al., 1999; Galvez
et al., 2011; Grossnickle, 2005; Lavender, 1984; Matson and Waring,
1984). High root NSC reserves (particularly soluble sugars) also in-
crease the osmotic potential of roots, providing seedlings with an ability
to extract more water from the soil matrix (Galvez et al., 2011). High
reserve status has also been found to increase the exudation of carbon
from roots (Karst et al., 2017) potentially attracting mycorrhizae that
Fig. 7. Range of aspen stock differences, using a common seed source, solely achieved by manipulating nursery conditions (Landhäusser et al. 2012a).
S.M. Landhäusser et al. Forest Ecology and Management 432 (2019) 231–245
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could alleviate drought stress (Landhäusser et al., 2002). Since aspen
seedlings invest heavily in root systems (Martens et al., 2007), and
rapid root growth after planting is considered a desirable characteristic
of planted trees (Grossnickle, 2005), sufficient NSC reserves could allow
for early root growth after planting (Eliasson, 1968), while stem NSC
reserves are needed to support the flush of new foliage in the spring
(Landhäusser, 2011).
During nursery production, seedling characteristics, including the
root:shoot ratio and NSC reserves, can be manipulated. Aspen has an
indeterminate growth strategy, which allows it to keep growing when
conditions are favorable. However, there is a direct trade-off between
photosynthate allocation to growth vs. NSC reserves in roots, with
faster growing seedlings allocating less NSC to reserves (Canham et al.,
1999; Chapin III et al., 1990). During periods of stress, aspen will slow
height growth and the production of new leaf area and in extreme cases
will set bud (Galvez et al., 2011; Landhäusser and Lieffers, 1998;
Martens et al., 2007), but will continue to photosynthesize. Martens
et al. (2007) suggested that a manipulation of bud set timing could be
used to increase NSC reserves and potentially lead to improvement of
out-planting performance of aspen seedlings. Further studies have
shown that restricting height growth in aspen while maintaining phy-
siological activity in the leaves and roots successfully diverted both
photosynthates and nutrients to storage rather than to growth (Schott
et al., 2016). On a cautionary note, there have been suggestions of in-
creasing root:shoot ratios in aspen seedlings by top pruning the shoot;
however, top pruning results in the reduction of viable buds, formation
of multi-stemmed seedlings, and change the root:shoot ratio without
improving NSC reserves, all of which could result in poor seedling
growth after outplanting.
There are several methods that can be used to successfully reduce or
terminate shoot growth in aspen, including reduced water availability
(Grossnickle, 2012), nutrition (Landhäusser et al., 2012b) and soil
temperature (Landhäusser and Lieffers, 1998), shortened day-length,
and use of growth inhibitors (Landhäusser et al., 2012b). Reducing
nutrition and exposing seedlings to controlled drought are considered
risky in a commercial nursery setting, as seedlings are not treated in-
dividually and therefore the risk of mortality or poor seedling quality is
high. Shortening day-length through blackout treatments has been used
for other species (Grossnickle, 2012) and has shown promise for aspen,
but seedling bud set was found to be highly variable and seedlings may
reflush (Landhäusser et al., 2012b), negating the objective to increase
storage in seedlings. Shoot growth of aspen can also be influenced
chemically by applying a shoot growth inhibitor such as Paclobutrazol.
This chemical compound, often used at low concentrations in the hor-
ticultural industry for slowing growth and has been found to be very
effective in the early termination of aspen shoot growth (Landhäusser
et al., 2012b). Shoot height, root:shoot ratio, and reserve concentra-
tions of the aspen seedlings can be controlled by the timing of its ap-
plication (Landhäusser et al., 2012b; Schott et al., 2013). While seed-
lings can be nutrient-loaded when conventionally grown by inducing
“luxury consumption” (Timmer, 1996; Timmer and Munson, 1991), this
fertilization approach has limited potential for aspen given its in-
determinate growth habit, because additional nutrients will induce
additional shoot growth, decreasing root:shoot ratios. However, if
lasting bud set can be induced, nutrient loading of tissues can occur
without additional shoot growth (Schott et al., 2013). Growing aspen
seedlings outside of a greenhouse has also shown some promise in
producing seedling with higher root:shoot ratios and higher NSC con-
centrations (Kelly et al., 2015). Currently the production of aspen
seedling stock occurs under controlled greenhouse conditions opti-
mizing growth. Under outside conditions, seedlings are predisposed to
periods of environmental stress due to the combined increases in light
intensity, wind and temperature (Niinemets et al., 1999). Non-lethal
stress has commonly been found to decrease plant growth more than
photosynthesis limitations (Chaves et al., 2009), potentially leading to a
greater accumulation of NSC and greater carbon allocation to roots
(Coutand et al., 2008; Galvez et al., 2011; Villar-Salvador et al., 1999).
While there is a greater pool of information available for containerized
aspen seedlings, information on the performance and seedling quality
of bare-root aspen seedlings is currently lacking. Bare-root stock can be
generated by direct seeding or by planting one-year old container
seedlings into bare root fields, where seedlings are grown for one
season. During the fall lift, the seedlings are often heavily root- and
shoot-pruned so they can be economically packaged and stored.
2.2.4. Seedling storage
The timing of planting (see below) will determine the date that a
seedling is lifted and whether overwinter storage is necessary. Time of
lift will affect the characteristics of the seedlings as they are in different
physiological and morphological stages. For example, Landhäusser
et al. (2012a) showed that summer-planted (lifted 3rd week of August)
seedlings had lower root volume, root dry mass, root to shoot ratio and
NSC reserves compared to fall-planted (lifted late September) or spring-
planted (lifted in November and stored frozen until spring) after first
growing season. If seedlings are to be planted in the spring, seedlings
generally are lifted in the late fall and then bagged and packed in waxed
boxes and stored frozen at a temperature of -3°C. We are not aware of
any study that explores the storage requirements of aspen seedlings;
however, a -3°C storage temperature is known to safely store most
conifer seedlings and does not result in frost damage to their root and
shoots (Stattin et al., 2012; Wang and Zwiazek, 2001). Anecdotal evi-
dence suggests that aspen can potentially tolerate much lower tem-
peratures, but there appears to be an important correlation between
frost tolerance and the NSC reserve status of aspen (Galvez et al., 2013).
2.2.5. Planting and establishment of seedlings
Apart from seedling quality (see above), factors such as handling,
planting procedures, timing of planting, site conditions, and microsite/
planting spot selection influence the successful establishment and
growth of tree seedlings on outplanting sites (Davis et al., 2010;
Franklin et al., 2012; Grossnickle, 2012; Löf et al., 2012). However,
there is little or no information for most of these factors and their re-
lationship to planting success (Landhäusser et al., 2012a). The timing of
planting can also affect outplanting performance of seedlings. Fall and
winter plantings of aspen are preferred since they use dormant seedling
stock. Spring- and fall- planted seedlings had 44% greater height
growth than summer planted seedlings (Landhäusser et al., 2012a).
A wide range of site preparation techniques has been developed for
reforestation using other species, but very few studies have addressed
this issue with aspen seedlings. Generally, site preparation can be used
to improve resource availability and manipulate abiotic and biotic
conditions such as shade, soil moisture and temperature, and particu-
larly competition that might limit early establishment (Jacobs et al.,
2015). Site conditions can be also altered by amending soils with ma-
terials that can provide a direct source of nutrients and/or water. The
effect of amendments on the early establishment on aspen seedlings has
received little attention with most work related to fertilizer type and
application (Schott et al., 2016; Sloan and Jacobs, 2013). Aspen gen-
erally shows a positive response to fertilization in the field (DesRochers
et al., 2003); however, some research indicates that if moisture is
limiting, fertilizer effects are only positive in conjunction with irriga-
tion (van den Driessche et al., 2003). Depending on the soil conditions,
the type and composition of fertilizer can have varied results in aspen
(Pinno et al., 2012). For example, in their study, a balanced N-P-K was
best for height growth, root to leaf mass ratio and bud set, while a P-K
and N fertilizer alone resulted in delayed bud set compared and NPK
which could lead to frost damage (Pinno et al., 2012). Overall though,
fertilizer may favor competing vegetation more than it favors aspen, so
it should be used cautiously.
Vegetation management might be required to provide newly
planted aspen seedlings with low competition conditions. Aspen is very
sensitive to competition, particularly grasses (Bockstette et al., 2017;
S.M. Landhäusser et al. Forest Ecology and Management 432 (2019) 231–245
241
Landhäusser and Lieffers, 1998; Schott et al., 2016), which compete
primarily via belowground processes (Bockstette et al., 2017; Le, 2017).
While herbicides can be useful tools in suppressing competing vegeta-
tion, they have limited use in aspen establishment, as herbicides are not
selective enough and aspen may be more sensitive to the herbicides
than the competing vegetation. As with passive aspen restoration
practices, fire may be used to reduce competition prior to planting and
protection from herbivory of the young establishing aspen seedlings
may be necessary in some areas.
3. Conclusions
Seedlings are unlikely to become the dominant form of aspen re-
generation of existing aspen forests across its range, given the prolific
suckering potential of many stands. However, aspen seedlings can play
a significant role in changing forests at the stand and landscape scale,
particularly in areas where the species is not currently found. Aspen
seedlings may be particularly valuable in climate change adaptation
and assisted migration projects. Given the exacting seedbed, microsite
and weather conditions that need to coincide for natural aspen seedling
to be successful, the impact of changing future climates on aspen
seedling establishment events is difficult to predict. For example,
weather events may become rarer for successful aspen establishment in
a potentially drier climate, but there may be more frequent seed pro-
duction with increased stress. The use of aspen nursery seedlings in
targeted reforestation is a potentially useful approach, but a great deal
of protocol development is necessary before seedling-based approaches
can become operational. Areas of investigation include optimization of
seedling stock quality, assessment of regional differences in stock re-
quirements, and determining the most appropriate traits and genetic
sources for successful establishment and growth. Some of these vari-
ables need also to be adapted to anticipated future climates.
For other management applications, the encouragement of aspen
seedling establishment after timber harvest or wildfire can be helpful in
the conversion of conifer-dominated forests to more deciduous-domi-
nated forests, reducing the threat of wildfire in surrounding commu-
nities. For that, forest management practices and site preparation
techniques can be altered to increase aspen seedling abundance through
microsite creation, strategic retention of mature aspen seed trees, and
the strategic planting of aspen seedlings across the landscape. However,
there is likely more work to be done in order to better predict seed crop
quality and abundance from both internal (e.g. tree age and size) and
external (e.g. weather) factors. Further, it may even be possible to in-
duce flowering through applying stressors and thereby increasing the
potential for seedling establishment following disturbance. The timing
of these inductions would be critical for maximizing success given the
relatively short time frame for establishment success.
Acknowledgements
We thank Jim Long and Caren Jones for their comments and edits
on this manuscript. Sandy Long created the line drawings and unless
acknowledged in the caption of figures, all photographs are provided by
SML. Work was supported by a discovery grant (2016-04686) to SML
and this collaboration with KEM was supported in part by the Utah
Agricultural Experiment Station, project UTA01291.
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... Conversely, wind-dispersed angiosperm seeds Trees do not have wings on themselves. Instead, they use winged fruit (samaras) or hair (pappi) for dispersal in the wind (Tan et al. 2018;Landhäusser et al. 2019). Since the winged seeds of gymnosperms and winged fruits of angiosperms are morphologically distinct, they can be distinguished using different terminologies. ...
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... However, sexual regeneration through aspen seedlings is more common in eastern NA including Canada than in the western United States (Peterson & Peterson, 1992), potentially because seed germination and seedling growth are extremely sensitive to soil moisture availability (Landhäusser et al., 2019;McDonough, 1979). ...
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Syntheses clearly show that global warming is affecting ecosystems and biodiversity around the world. New methods and measures are needed to predict the climate resilience of plant species critical to ecosystem stability, to improve ecological management and to support habitat restoration and human well‐being. Widespread keystone species such as aspen are important targets in the study of resilience to future climate conditions because they play a crucial role in maintaining various ecosystem functions and may contain genetic material with untapped adaptive potential. Here, we present a new framework in support of climate‐resilient revegetation based on comprehensively understood patterns of genetic variation in aspen. Elucidating species' genetic makeup and seed germination plasticity is essential to inform tree conservation efforts in the face of climate change. Populus tremuloides Michx. (aspen) occurs across diverse landscapes and reaches from Alaska to central Mexico, thus representing an early‐successional model for ecological genomics. Within drought‐affected regions, aspen shows ploidy changes and/or shifts from sexual to clonal reproduction, and reduced diversity and dieback have already been observed. We genotyped over 1000 individuals, covering aspen's entire range, for approximately 44,000 single‐nucleotide polymorphisms (SNPs) to assess large‐scale and fine‐scale genetic structure, variability in reproductive type (sexual/clonal), polyploidy and genomic regions under selection. We developed and implemented a rapid and reliable analysis pipeline (FastPloidy) to assess the presence of polyploidy. To gain insights into plastic responses, we contrasted seed germination from western US and eastern Canadian natural populations under elevated temperature and water stress. Four major genetic clusters were identified range wide; a preponderance of triploids and clonemates was found within western and southern North American regions, respectively. Genomic regions involving approximately 1000 SNPs under selection were identified with association to temperature and precipitation variation. Under drought stress, western US genotypes exhibited significantly lower germination rates compared with those from eastern North America, a finding that was unrelated to differences in mutation load (ploidy). This study provided new insights into the adaptive evolution of a key indicator tree that provisions crucial ecosystem services across North America, but whose presence is steadily declining within its western distribution. We uncovered untapped adaptive potential across the species' range which can form the basis for climate‐resilient revegetation. Todos los estudios muestran claramente que el calentamiento global afecta a los ecosistemas y a la biodiversidad de todo el planeta. Se necesitan nuevos métodos y medidas para i) predecir la resiliencia climática de las especies vegetales fundamentales para la estabilidad de los ecosistemas, ii) para mejorar la gestión ecológica y iii) para promover la restauración del hábitat y el bienestar humano. Las especies clave de amplia distribución, como el álamo temblón, son esenciales para estudiar la resiliencia a las condiciones climáticas futuras, ya que desempeñan un papel crucial en el mantenimiento de diversas funciones de los ecosistemas además de contener material genético con un potencial de adaptación sin explotar. Aquí presentamos una nueva estrategia para promover la revegetación resiliente al clima, basada en patrones bien estudiados de variación genética del álamo temblón. Les synthèses montrent clairement que le réchauffement climatique affecte les écosystèmes et la biodiversité du monde entier. La mise en place de nouvelles méthodes et de nouvelles mesures est alors nécessaire comme outils de prédiction de la résilience climatique des espèces végétales essentielles à la stabilité des écosystèmes. L'objectif est d'améliorer la gestion écologique ainsi que le bien‐être humain et soutenir la restauration des habitats naturels. Le peuplier faux‐tremble représente une espèce clef à large répartition géographique. C'est une cible importante dans l'étude de la résilience aux futures conditions climatiques car cette espèce est essentielle au maintien de diverses fonctions écosystémiques et elle est porteuse d'un matériel génétique avec un potentiel adaptatif encore inexploité. Nous présentons ici une nouvelle structure de soutien à la revégétalisation résiliente au climat, basé sur des patrons de variation génétique chez le peuplier faux‐tremble. Syntheses clearly show that global warming is affecting ecosystems and biodiversity around the world. New methods and measures are needed to predict the climate resilience of plant species critical to ecosystem stability, to improve ecological management and to support habitat restoration and human well‐being. Widespread keystone species such as aspen are important targets in the study of resilience to future climate conditions because they play a crucial role in maintaining various ecosystem functions and may contain genetic material with untapped adaptive potential. Here, we present a new framework in support of climate‐resilient revegetation based on comprehensively understood patterns of genetic variation in aspen.
... This study suggests that climate change and increases in burned area would favor deciduous species at the expense of coniferous species. This is because deciduous species (i.e., Populus davidiana) can better adapt to climate change and fire through asexual sprouting from roots or stumps and long-distance seed dispersal (Rehfeldt et al., 2009;Landhäusser et al., 2019). In our model simulation, the Populus davidiana seed dispersal distance was parameterized at 2400 m (Huang et al., 2018). ...
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Climate change could alter species composition, with feedback on fire disturbances by modifying fuel types and loads. However, the existing fire predictions were mainly based on climate-fire linkages that might overestimate the probability and size of fire disturbances due to simplifying or omitting vegetation feedback. We applied a model-coupling framework that combines forest succession, climate-fire linkages, and vegetation feedback to predict burned area, aboveground biomass, and species composition of boreal forests in Northeast China under climate change conditions. Results showed that climate change and fire would favor the recruitment of deciduous species, but these species need a long-time to replace the existing coniferous species. Burned area would increase with climate change. Climate change, historical and future fire disturbances affect aboveground biomass by altering tree mortality and regeneration. Further studies should address strategies for altering species composition through forest management practices to adaptation climate change and reduce carbon losses from fire.
... In contrast, the dispersal unit of angiosperms such as those from the genera Acer, Fraxinus, and Alnus is a winged fruit, generally called a samara [11], which carries the seeds. Additionally, for angiosperms with capsule-type fruits such as Populus, Salix, and Catalpa, seeds are released from dehisced fruits and dispersed with the aid of a pappus [12]. Morphologically, the dispersal of angiosperms should be distinguished from that of winged gymnosperm seeds. ...
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Natural regeneration in forest management, which relies on artificial planting, is considered a desirable alternative to reforestation. However, there are large uncertainties regarding the natural regeneration processes, such as seed production, seed dispersal, and seedling establishment. Among these processes, seed dispersal by wind must be modeled accurately to minimize the risks of natural regeneration. This study aimed to (1) review the main mechanisms of seed dispersal models, their characteristics, and their applications and (2) suggest prospects for seed dispersal models to increase the predictability of natural regeneration. With improving computing and observation systems, the modeling technique for seed dispersal by wind has continued to progress steadily from a simple empirical model to the Eulerian-Lagrangian model. Mechanistic modeling approaches with a dispersal kernel have been widely used and have attempted to be directly incorporated into spatial models. Despite the rapid development of various wind-dispersal models, only a few studies have considered their application in natural regeneration. We identified the potential attributes of seed dispersal modeling that cause high uncertainties and poor simulation results in natural regeneration scenarios: topography, pre-processing of wind data, and various inherent complexities in seed dispersal processes. We suggest that seed dispersal models can be further improved by incorporating (1) seed abscission mechanisms by wind, (2) spatiotemporally complex wind environments, (3) collisions with the canopy or ground during seed flight, and (4) secondary dispersal, long-distance dispersal, and seed predation. Interdisciplinary research linking climatology, biophysics, and forestry would help improve the prediction of seed dispersal and its impact on natural regeneration.
... Trembling aspen is a light demanding broadleaf tree species that can grow on many types of soil. Aspen can regenerate through root suckering, which allows it to re-establish quickly after large disturbances (Landhäusser et al., 2019). Within Cypress Hills Interprovincial Park, white spruce most often occurs near creeks or wet areas, lodgepole pine is found mostly on drier sites at higher elevations, and aspen are most commonly found at mid-slope positions on sites with intermediate soil moisture (Fig. 2) (Sauchyn and Sauchyn, 1991). ...
Article
Advances in unmanned aerial vehicle (UAV) technology have opened new opportunities for measuring canopy mortality processes through high-resolution aerial imagery. Unlike traditional plot-based sampling, UAVs can easily survey areas several hectares in size that encompass hundreds of canopy trees. The resulting imagery provides extensive data on the number and locations of dead trees (snags), which can be related to neighbourhood conditions and environmental factors that vary among stands. Here, we used a UAV to measure the relative proportions of live and dead canopy trees in stands varying in soil moisture across a mixed forest landscape. Crowns of live trees and snags were first segmented and classified to species from high-resolution canopy imagery. We then modeled whether crowns of each species were alive or dead as a function of tree height, neighbourhood competition, soil moisture, and the relative abundance of conspecifics. Short lodgepole pine (Pinus contorta var. latifolia) trees were more likely to be dead when surrounded by a tall canopy in sites with greater soil moisture, reflecting effects of competition on this shade-intolerant species. Trembling aspen (Populus tremuloides) was more likely to be alive when surrounded by tall neighbours, perhaps because this clonal species can benefit from facilitation through root connections with neighbouring aspen stems. There was a higher frequency of white spruce (Picea glauca) snags in sites where it had a high relative abundance, suggesting strong effects of intraspecific competition for this species. Soil moisture did not appear to have a direct effect on the snag frequency of any of these species, despite pronounced niche partitioning along an elevation-driven moisture gradient. Our models explained 39% and 33% of the variation in white spruce and lodgepole pine snag frequency , respectively, but did not have much predictive power for trembling aspen or total snag frequency. Our results reflect the important role of competition in determining tree mortality, but also indicate that stochastic or unexplained processes account for considerable variation in snag frequency among stands. As UAV technology becomes more widely used by ecologists, it may enable a better understanding of how biotic and abiotic processes produce local variation in canopy mortality.
... Aspen's primary mode of reproduction is asexual, via resprouting from its roots in response to canopy mortality; however, it also has small, wind-dispersed seeds and studies have documented aspen regeneration from seed after disturbance events (Fairweather et al., 2014;Kreider & Yocom, 2021;Landhäusser et al., 2010;Quinn & Wu, 2001;Romme et al., 1997). Aspen seedling establishment is associated with moderate temperatures, high-light environments, thin soil organic matter layers and microsites that capture moisture (Lafleur et al., 2015;Landhäusser et al., 2019;McDonough, 1979;Schott et al., 2014). Thus, seedlings are most often found in severely burned areas, where these seedbed requirements and nurse objects (in the form of snags and down logs) are more prevalent (Coop & Schoettle, 2009;Johnstone & Chapin, 2006;Turner et al., 2003). ...
Article
Aim Climate warming is expected to drive upward and poleward shifts at the leading edge of tree species ranges. Disturbance has the potential to accelerate these shifts by altering biotic and abiotic conditions, though this potential is likely to vary by disturbance type. In this study, we assessed whether recent wildfires and spruce beetle outbreaks promoted upward range expansion of trembling aspen. Location The San Juan Mountains of southern Colorado, USA (37°34′–37°50′N, 106°49′–107°21′W). Taxon Populus tremuloides. Methods We used aerial imagery to determine the upper elevational limit of adult aspen and conducted seedling surveys at and above this upper limit in burned and unburned areas, which had already incurred high canopy mortality due to spruce bark beetle (Dendroctonus rufipennis) outbreaks. We compared characteristics of burned versus unburned bark beetle-killed sites and assessed microsite conditions related to aspen seedling establishment using generalized linear models and interaction indices. Results Aspen seedling establishment occurred upslope of its previous range within burns, but not in unburned areas, despite severe beetle-driven canopy mortality across all sites before the fire. Aspen seedling establishment was associated more with the light and mineral soil created by fire than the presence of nearby seed sources. Aspen seedlings were associated with nurse objects such as logs and rocks at the highest elevations, where these objects may ameliorate a range of stressors associated with the high elevation range boundary. Main conclusions Not all disturbance types are equal in promoting tree species migrations at the leading edge. Range shifts can be highly localized, and microsites are important for driving local range expansions in transitional environments. The mosaic of future disturbances across the landscape will drive forest compositional shifts, depending on the disturbance types and the species they promote.
... Integration of climate-related risk to planning of stand and land management, e.g., [33] Use of risk assessment analyses, e.g., [132] Promotion of specific stand types, species mix and/or species for their suitability for future climate conditions and disturbance regimes, e.g., [26,133,134] Use of silvicultural treatments ensuring stand continuity with regeneration and biological legacies, while promoting structural complexity and species diversity (e.g., uneven-aged, irregular, and other even-aged systems creating high heterogeneity, variable density, or mixedspecies thinning), e.g., [133,135] Selective browsing can go against desired species mix Plantation of species with diverse functional traits, suitable for future climate conditions, e.g., [131] Browsing can reduce the growth and survival of planted seedlings Selective browsing can go against desired species mix Diversification of stand types, age and structure, and species composition to decrease vulnerability to threats related to climate change, and to reduce economic risk (diversify production), e.g., [57, 136] Use of silvicultural systems promoting diversity and complexity at all scales, e.g., [136,137] Browsing can increase or reduce diversity in regeneration Selective browsing can lead to regeneration failure Ungulates potential interactions Diversification of silvicultural systems and treatments at the landscape scale, e.g., [138] The diversification of approaches could change resource availability to ungulates with potential impact on their landscape use and population dynamics Ungulates can alter/nullify desired effect of approach on regeneration ...
... Propagation through seed is rarer, and typically requires consistent moisture availability during germination (Landhäusser et al., 2019). ...
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Questions Woody-plant encroachment and invasive plants are two critical factors that negatively impact grass-dominated ecosystems. While studies have extensively investigated these factors individually, research on how invasive species impact the susceptibility of grasslands to encroachment is less common. Using the aspen parkland, an endangered savannah-type ecosystem, we asked how the presence of a widespread invasive grass in North America, smooth brome (Bromus inermis), impacted the growth and survival of one-year old trembling aspen (Populus tremuloides) seedlings. We also asked if plant litter was a potential mechanism of inhibition (as has been proposed for brome litter) or co-existence (as has been hypothesized for aspen litter) in this system. Location Alberta, Canada. Methods We used a manipulative experimental approach to determine if the survival and subsequent growth of planted aspen seedlings was impacted by the presence of smooth brome and manipulation of litter amount and type. Results We found that the presence of smooth brome reduced aspen survival by 57% compared to un-invaded habitats, likely mediated by reduced soil moisture, while litter manipulation had no effect on survival. For surviving seedlings, local context had complex impacts on growth; the addition of aspen litter to brome-invaded communities increased seedling growth while aspen litter additions to native communities resulted in decreased growth. Conclusion These results suggest that invasion by smooth brome will alter the dynamics of aspen establishment in this system, potentially leading to significant changes to this already endangered landscape. Though smooth brome may serve as a barrier to aspen establishment, accumulation of aspen litter from nearby stands to brome patches could lead to faster growth of seedlings in invaded areas at the edge of existing aspen stands. Our results also suggest more generally that the impact of invasive plants on the establishment of native woody plants can be dependent on litter inputs.
... This information is now informing current management and restoration efforts (Brabec et al. 2017, Richardson andChaney 2018). Similar approaches may soon become possible for aspen, where seed sourcing is a key issue for restoration work (Landhausser et al. 2019). We hope that increasingly available genetic information will help to predict and support the response of this iconic species to environmental change. ...
Article
Species responses to climate change depend on environment, genetics, and interactions among these factors. Intraspecific cytotype (ploidy level) variation is a common type of genetic variation in many species. However, the importance of intraspecific cytotype variation in determining demography across environments is poorly known. We studied the tree species quaking aspen (Populus tremuloides), which occurs in diploid and triploid cytotypes. This widespread species is experiencing contractions in its western range, which could potentially be linked to cytotype‐ dependent drought tolerance. We found that interactions between cytotype and environment drive mortality and recruitment across 503 plots in Colorado. Triploids were more vulnerable to mortality relative to diploids and had reduced recruitment on more drought‐prone and disturbed plots relative to diploids. Furthermore, there was substantial genotype‐dependent variation in demography. Thus, cytotype and genotype variation are associated with decline in this foundation species. Future assessment of demographic responses under climate change will require knowledge of how genetic and environmental mosaics interact to determine species’ ecophysiology and demography.
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With advancing climate change, tree survival increasingly depends on mechanisms that facilitate coping with multiple environmental stressors. At the population level, genetic diversity is a key determinant of a tree species’ capacity to deal with stress. However, little is known about the relative relevance of the different components of genetic diversity for shaping tree stress responses. We compared how two components of genetic diversity, genotypic variation and ploidy level, shape growth, phytochemical, and physiological traits of Populus tremuloides, under environmental stress. In two field experiments we exposed eight diploid and eight triploid aspen genotypes to individual and interactive drought stress and defoliation treatments. We found that: 1) Genotypic differences were critical for explaining variation of most of functional traits and their responses to stress. 2) Ploidy levels generally played a subordinate role for shaping traits, as they were typically obscured by genotypic differences. 3) As an exception to the second finding, we found that triploid trees expressed higher levels of foliar defenses, photosynthesis, and rubisco activity under well-watered conditions, and displayed greater drought resilience than diploids. This research demonstrates that the simultaneous study of multiple sources of genetic diversity is important for understanding how trees will respond to environmental change.
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Restricted rooting space in response to soil compaction and below-ground competition with herbaceous plants are two main limiting factors for successful reforestation after surface mining. Fine-textured, nutrient rich soils with adequate soil moisture are particularly susceptible to both of these concerns and while there are recognized ways to manage competition, attempts to alleviate soil compaction through mechanical means have produced varying results. While roots of some herbaceous plants may penetrate compacted soil layers, possibly offering an alternative means to overcome physical restrictions, these potential benefits need to be weighed against negative effects from competition with planted trees. We examined the individual and combined impact of soil decompaction (deep tillage) and management of competing vegetation (herbicide) on soil properties, resource availability and above and below-ground growth of aspen (Populus tremuloides Michx.) seedlings on a reconstructed mine soil affected by severe...
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How carbon (C) flows through plants into soils is poorly understood. Carbon exuded comes from a pool of non-structural carbohydrates (NSC) in roots. Simple models of diffusion across concentration gradients indicate that the more C in roots, the more C should be exuded from roots. However, the mechanisms underlying the accumulation and loss of C from roots may differ depending on the stress experienced by plants. Thus, stress type may influence exudation independent of NSC. We tested this hypothesis by examining the relationship between NSC in fine roots and exudation of organic C in aspen (Populus tremuloides Michx.) seedlings after exposure to shade, cold soils and drought in a controlled environment. Fine root concentrations of NSC varied by treatment. Mass-specific C exudation increased with increasing fine root sugar concentration in all treatments, but stress type affected exudation independently of sugar concentration. Seedlings exposed to cold soils exuded the most C on a per mass basis. Through 13 C labeling, we also found that stressed seedlings allocated relatively more new C to exudates than roots compared with unstressed seedlings. Stress affects exudation of C via mechanisms other than changes in root carbohydrate availability.
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Environmental factors (such as light, moisture, nutrients, density, and temperature) and plant physiological factors (such as carbohydrate reserves, hormone levels, frost hardiness, and dormancy) interact to shape growth and survival of coniferous seedlings in nursery fields and after outplanting. Nursery managers can manipulate moisture, nutrients, and density to achieve desired seedling morphology and vigor. However, the annual growth cycle of perennial plants has evolved in response to environmental pressures. When the environment is modified, as with heavy irrigation in a nursery, to permit growth at a time when natural seedlings are dormant, the ensuing phases of the growth cycle will not be properly synchronized with their environments. Seedlings so cultivated lack vigor after outplanting. Nursery managers should aim at keying their cultivation schedules to both environmental conditions and endogenous seedling physiology toensure production of high-quality seedlings.
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Determining how ecological filters (e.g., climate, soils, biotic interactions) influence where species succeed in heterogeneous landscapes is challenging for long-lived species (e.g., trees), because filters can vary over space and change slowly through time. Stand-replacing wildfires create opportunities for establishment of tree-species cohorts and can catalyze rapid shifts in where species occur, facilitating unique opportunities for long-term study. We quantified effects of multiple ecological filters on a colonizing cohort of aspen (Populus tremuloides) that established from seed throughout burned lodgepole pine (Pinus contorta var. latifolia) forests after the 1988 fires in Yellowstone National Park (Wyoming, USA) to ask: (1) How have aspen presence, density, and size varied across the postfire landscape, and what filters explain these spatial and temporal patterns? (2) How does aspen above-and belowground biomass vary with postfire lodgepole-pine density? Aspen persisted to postfire year 25 in 58% of the plots in which aspen were present in postfire year 11 (n = 45), and mean stem density declined from 522 to 310 stems ha−1. Mean aspen height doubled (from 29 to 59 cm) over this period. Ecological filters related to climate, competition, herbivory, and soils all differentially affected aspen presence, persistence, and size. Growing season temperature, inter-specific competition, and herbivory also changed through time, altering their effects on the colonizing cohort, and shifting where on the landscape aspen persistence and growth were ultimately favored. Eleven years postfire, aspen were favored at warmer, low elevations; ungulate browsing strongly constrained aspen heights; and competition was unimportant. By 25-years postfire, temperatures warmed nearly 1 °C, and aspen were more likely to persist at cooler, high elevations. Browsing pressure declined, as ungulate populations decreased during this time, but aspen height and basal diameters were constrained by dense, rapidly growing postfire conifers. Landscape mosaics of ecological filters shift over space and time and can facilitate or constrain the persistence and growth of colonizing species. Long-term study of post-disturbance colonizing cohorts uniquely reveal how species are responding to real-time environmental change in heterogeneous landscapes, which will help us better anticipate 21st century species distributions and abundances.