REVIEWS REVIEWS REVIEWS
Understory vegetation as a forest ecosystem
driver: evidence from the northern Swedish
boreal forest
Marie-Charlotte Nilsson1* and David A Wardle1,2
Vegetation research in boreal forests has tended to focus on the tree component, while little attention has been
paid to understory components such as dwarf shrubs, mosses, and reindeer lichens. However, the productivity
of understory vegetation is probably comparable to that of the trees. We review recent research in the boreal
forest of northern Sweden to highlight the ecological importance of understory vegetation, both in the short
term by influencing tree seedling regeneration, and in the longer term by affecting belowground processes
such as decomposition, nutrient flow, and buildup of soil nutrients. Wildfire resulting from lightning strike is
a primary determinant of understory vegetation, and as such is a major driver of forest community and ecosystem properties. Forest management practices that alter the fire regime and the composition of understory vegetation may have long-term consequences for both conservation goals and commercial forest productivity.
Front Ecol Environ 2005; 3(8): 421–428
T
he forests of the boreal zone have a circumpolar distribution and dominate the landscape of much of
Scandinavia, northern Russia, Alaska, northern and eastern Canada, and the Great Lakes region of the contiguous United States. Collectively, they comprise 11% of the
Earth’s terrestrial surface (Bonan and Shugart 1989) and,
on a global basis, contain a greater amount of stored carbon (C) than any other terrestrial biome (Anderson
1991). They also have a vital global role in serving as a
sink or source of atmospheric CO2 (Goodale et al. 2002).
Boreal forests have a characteristic vegetation structure,
consisting of a tree layer and an understory of short
woody ericaceous shrubs (dwarf shrubs) and, frequently,
mosses and lichens. As such, they support a unique biota
of plant species, fauna, and microflora (Kuuluvainen
2002). Although there has been a long history of human
In a nutshell:
• Traditionally, ecologists have paid little attention to the understory component of the boreal forest
• Studies from northern Sweden show that key forest understory
components can drive forest regeneration, belowground properties, and long-term forest succession
• Wildfire is a major determinant of understory vegetation composition and therefore indirectly drives the ecological effects of
this vegetation
• Understanding understory vegetation ecology has important
implications for both conservation and production-oriented
forest management
1
Swedish University of Agricultural Sciences, Faculty of Forest
Sciences, Department of Forest Vegetation Ecology, SE-901 83
Umeå, Sweden *([email protected]); 2Landcare
Research, PO Box 69, Lincoln, New Zealand
© The Ecological Society of America
occupation of the boreal zone, this area has been increasingly affected by humans over the past 200 years; many of
these forests have been subjected to wildfire suppression
and heavily utilized for timber production. For example,
in Sweden and Finland, 96% of all boreal forests have
been harvested at least once for timber in the past two
centuries (Östlund et al. 1997).
Despite the global importance of boreal forests, they
have attracted less attention from ecologists than have
temperate and tropical forests, presumably because the
boreal forest supports a much lower human population
density. The vast majority of published ecological work
from the boreal zone has focused on larger organisms,
such as trees and vertebrates, while the understory dwarf
shrubs, mosses, and lichens that characterize these forests,
have usually been overlooked. Here we review recent
work carried out in the Swedish boreal forest, and present
this as a case study to show the importance of understory
vegetation as a driver of the functioning of the boreal forest. We discuss the key role of this vegetation in influencing community and ecosystem properties, both aboveground (eg forest tree regeneration and productivity) and
belowground (eg decomposer processes and nutrient
cycling), and use this information to demonstrate the
importance of understanding the ecology of understory
vegetation for forest management.
Swedish boreal understory vegetation
There are three main components of understory vegetation in northern Swedish boreal forests (particularly
those on well-drained humus): ericaceous dwarf shrubs,
feather mosses, and reindeer lichens. The three most
abundant dwarf shrub species are bilberry (Vaccinium
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421
Understory vegetation as a forest ecosystem driver
422
(a)
M-C Nilsson and DA Wardle
(c)
Figure 1. The three most abundant ericaceous dwarf shrub
species in the Swedish boreal forest: (a) bilberry (Vaccinium
myrtillus); (b) lingonberry (Vaccinium vitis-idaea); and (c)
black crowberry (Empetrum hermaphroditum).
(b)
increasing time since fire disturbance (or with increasing
fire-return interval), the dwarf shrub layer changes from
being dominated by lingonberry and bilberry to being
dominated by black crowberry (Sirén 1955; Wardle et al.
1997) and increasing biomass of feather mosses (DeLuca
et al. 2002a). In the absence of competition from dwarf
shrubs and feather mosses, it is common for the ground to
be covered by reindeer lichens; this often occurs in nutrient-poor conditions and sometimes in early succession. In
the prolonged absence of fire disturbance, replacement of
lingonberry and bilberry by black crowberry is accompanied by an ecosystem retrogression or decline phase
(Wardle et al. 2004) that also involves reductions in tree
biomass and productivity, and changes to several belowground properties (Table 1).
Table 1. Retrogressive successional trends in aboveground and belowground properties that occur in the
prolonged absence of fire in the Swedish boreal forest
Response variable
myrtillus), lingonberry (Vaccinium vitis-idaea), and black
crowberry (Empetrum hermaphroditum; Figure 1). The
moss layer is dominated by feather moss (Pleurozium
schreberi) and stair-step moss (Hylocomium splendens),
while the lichen component consists mostly of Cladina or
Cladonia. The dominant tree species are Norway spruce
(Picea abies), Scots pine (Pinus sylvestris), and birches
(Betula pubescens and Betula pendula). The relative abundance of all of these species is determined by a range of
abiotic and biotic factors that are in turn frequently driven by the successional stage of the ecosystem.
The main ecological determinant of successional stage
is wildfire induced by lightning strike, and the majority of
these forests are affected by recurrent fires (Zackrisson
1977; Niklasson and Granström 2000). Typically, with
www.frontiersinecology.org
Trend
Aboveground Abundance of bilberry and lingonberry
properties Abundance of black crowberry
Tree biomass and productivity
Ratio of ericaceous shrub biomass to
tree biomass
Feather moss abundance
Belowground Polyphenol concentration in soil
properties Decomposer microbial biomass
Litter decomposition rate
Soil carbon sequestration
N mineralization rate
Mineral N concentration
Ratio of mineral N to dissolved organic N
Symbiotic N fixation in mosses
I
I
I
I
I
I
`
`
I
I
I
I
I
Sources of information are Zackrisson et al. 1996, 2004; Wardle et al. 1997, 2003a; and
DeLuca et al. 2002a. Upward arrows indicate an increasing trend over time; downward
arrows indicate a decreasing trend.
© The Ecological Society of America
M-C Nilsson and DA Wardle
Relatively little research has been carried out on the
effects of understory dwarf shrubs and mosses on tree
seedlings in boreal forests, probably because these understory components represent a relatively small proportion
of the total forest plant biomass (Figure 2). Although
these components are unlikely to contribute much in
terms of mass effects, their biomass turns over much more
rapidly than does that of the trees with which they coexist; measurements of shrub productivity reveal that the
proportion of standing shrub biomass that is replaced each
year is around 62% for bilberry, 39% for lingonberry and
29% for black crowberry (Wardle and Zackrisson 2005).
As such, the aboveground net primary productivity of
these shrubs is over half that of the trees (Figure 2). In
northern Sweden, the standing biomass of the moss component is comparable to that of the shrubs (Wardle et al.
1997). Although no estimates have been made of their
net primary productivity (NPP) in northern Sweden,
turnover rates of moss segments in Scandinavian boreal
forests are rapid and probably comparable to that of the
shrubs (Økland and Økland 1996). Work in Alaskan
boreal forests also points to the high NPP of mosses relative to that of associated trees (Oechel and van Cleve
1986; Harden et al. 1997; Turetsky 2003). Therefore,
despite the relatively low contribution of understory vegetation to total standing biomass, their high turnover rate
suggests that they produce a substantial proportion of the
annual litterfall that is returned to the soil, and contribute
substantially to total annual ecosystem nutrient uptake.
As such, they have the potential to exert important interference effects against tree establishment and growth.
Of the understory components in northern Swedish
boreal forests, the one that arguably has the strongest
negative effect on tree seedling establishment and growth
is black crowberry. Experimental field studies have shown
that seed germination and seedling growth are vastly
reduced under black crowberry as compared to other
understory vegetation types (Zackrisson et al. 1995, 1997;
Nilsson et al. 2000), and increasing densities of black
crowberry across space and time are frequently associated
with reduced forest tree stand productivity (Zackrisson et
al. 1996; DeLuca et al. 2002a; Wardle et al. 2003a). This is
likely to be the result of allelopathic rather than competitive effects of black crowberry. This species produces
very high concentrations of a phenolic compound,
batatasin-III (Odén et al. 1992), that has been shown to
reduce germination and growth of seedlings when applied
at the concentrations in which it occurs in the soil
(Nilsson et al. 2000; Wallstedt et al. 2005).
While the issue of allelopathic effects in real ecosystems has generated controversy (Williamson 1990), it has
been shown that negative effects of black crowberry on
tree seedlings are largely mitigated in field plots when
activated C (which adsorbs polyphenolic compounds
through electrostatic charges) is added to them (Nilsson
© The Ecological Society of America
423
(a)
(b)
Biomass (kg m–2 )
organisms
10
5
0
(c)
Aboveground NPP (g m–2 yr–1)
Understory vegetation effects on associated
Understory vegetation as a forest ecosystem driver
200
trees shrubs mosses
Unproductive system
trees shrubs mosses
Productive system
trees
shrubs
Unproductive system
trees shrubs
Productive system
100
0
Figure 2. Productivity and biomass of understory components
relative to that of trees. (a) Trees are the dominant biomass
component of the Swedish boreal forest but shrubs and mosses
densely cover the ground surface; (b) and (c) biomass and
aboveground NPP of trees and understory components on
productive and unproductive lake islands in Lapland, Sweden.
Data from Wardle et al. 1997, 2003a.
and Zackrisson 1992; Nilsson 1994; Nilsson et al. 2000).
Charcoal produced during wildfires, which operates as a
form of activated C, may also minimize the effects of
batatasin-III on tree seedlings (Panel 1). Allelopathic
effects, similar to those described here for black crowberry, might also occur in other forests in which ericaceous shrubs form a major part of the understory, such as
has been shown for sheep laurel (Kalmia angustifolia) in
Canadian temperate and boreal forests (Mallik 2003;
Thiffault et al. 2004).
The interference effects of black crowberry are not just
restricted to tree seedlings. For example, ectomycorrhizal
fungi may also be impaired by black crowberry (Nilsson et
al. 1993), and humus under black crowberry supports considerably lower levels of decomposer microbes and fauna
than humus under other understory types (Wardle et al.
1998a). Furthermore, because of its water-soluble nature
and persistence (Wallstedt et al. 2005), batatasin-III
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Understory vegetation as a forest ecosystem driver
424
Panel 1. Ecological impacts of charcoal from wildfire
Activated carbon contains electrostatically charged surfaces that
adsorb compounds with polar molecules such as phenolics.
Addition of activated C to soil supporting plants that produce
these compounds has been shown to reduce their negative
effect on associated plant species (Nilsson and Zackrisson 1992;
Nilsson 1994; Callaway and Aschehoug 2000).Wildfire produces
charcoal, which is a form of activated C, and forest soils in
northern Sweden typically contain between 900 and 2100 kg ha-1
of charcoal (Zackrisson et al. 1996). Charcoal collected from
sites that have recently burnt has been shown to remove the
phenolic batatasin-III (produced by black crowberry) from aqueous solutions, and to promote emergence and growth of tree
seedlings treated with these solutions (Zackrisson et al. 1996).
Charcoal from older fires does not have the same effects
because there is increased physical obstruction over time of the
charcoal surface. Furthermore, addition of charcoal to the soil
surface at levels known to occur in the field has been shown to
strongly stimulate birch seedling productivity and nutrient acquisition for soil collected from under black crowberry (presumably because of its adsorption of batatasin-III), but not for soil
collected from under other ground-cover species (Wardle et al.
1998b).The surfaces of fresh charcoal also support high levels of
microbial biomass and activity (Zackrisson et al. 1996; Pietikäinen et al. 2000) and can support greater rates of ecosystem
level processes driven by components of the soil microflora,
such as decomposition and nitrification (Zackrisson et al. 1996;
Wardle et al. 1998b; Pietikäinen et al. 2000; DeLuca et al. 2002b).
It has been suggested that the increased dominance of black
crowberry, and associated diminished activity of charcoal with
increasing time since burning, contributes to diminished forest
tree productivity and biomass, as well as reducing the rates of
those soil processes that promote nutrient supplies available to
plants (Zackrisson et al. 1996).
leaches into nearby waterways during snowmelt, and concentrates in small streams and ponds (Brännäs et al.
2004). Short-term experimental studies have shown that
lethal effects on trout alevins and reduced mobility of
water fleas (Daphnia spp) were caused by batatasin-III but
not by a simpler phenolic compound (Brännäs et al.
2004), thus confirming the specific toxic effect of the
bibenzyl structure of batatasin-III (cf. Nilsson et al. 2000).
Other understory components in Swedish boreal forests
also influence tree seedling regeneration and growth,
although the effects are usually not as strong. For example, manipulative field experiments investigating exclusion of bilberry have shown this species to exert negative
effects on Norway spruce seedlings, although belowground resource competition rather than allelopathy was
probably the mechanism involved (Jäderlund et al. 1997).
Reindeer lichens do not show negative effects on tree
seedlings, and seedlings planted into ground dominated
by lichens show vastly superior growth to those planted
within other ground-layer vegetation (Steijlen et al.
1995; Zackrisson et al. 1995). Meanwhile, seedlings
planted into dense feather moss layers typically establish
and grow very poorly, despite the ability of mosses to
retain moisture (Steijlen et al. 1995). There is evidence
that this adverse effect of mosses is because they are very
effective in absorbing available nutrients (Oechel and
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M-C Nilsson and DA Wardle
van Cleve 1986; Zackrisson et al. 1999) and because mycorrhizal hyphae produced by ericaceous shrubs directly
take up nutrients from recently dead moss tissue, before
the tree seedlings are able to access them (Zackrisson et
al. 1997). This mechanism may also interfere to a lesser
extent with the growth of previously established, larger
seedlings and trees, because although their root systems
can access nutrients below the depth of mosses and shrub
roots, the moss–shrub layer is able to increasingly immobilize available nutrients over time (Wardle et al. 1997;
Zackrisson et al. 1999; DeLuca et al. 2002a). Note that the
above mechanisms differ from that described by Carleton
and Read (1991) for Canadian boreal forest species, in
which tree seedling mycorrhizae were able to directly
access nutrients from mosses.
Belowground effects of understory vegetation
The three dominant dwarf shrub species in the Swedish
boreal forest differ markedly in their ecophysiological
attributes. Bilberry has short-lived leaves, grows relatively
rapidly, and has poorly defended tissues (ie with low levels
of active secondary metabolites), while black crowberry
produces long-lived leaves, usually grows slowly, and has
well defended tissues. The attributes of lingonberry are
intermediate between these two. Consistent with these
species-specific differences, black crowberry produces poorquality litter compared to that produced by the other
species. Litterbag studies have shown that black crowberry
litter decomposes more slowly and releases less nitrogen (N)
during decomposition than co-existing ericaceous shrub
species (Wardle et al. 2003a,b) and most tree species
(Wardle et al. 2003a). Furthermore, a litter-mixing study, in
which litter from ten over- and understory boreal forest
species were mixed in all two-way combinations, generally
pointed to black crowberry litter as having the strongest
negative effects out of all species on litter decomposition
rates of associated litter (Wardle et al. 2003b). These belowground effects of black crowberry may impair nutrient supply rates from the soil and contribute to the adverse effects
of this species on seedling growth (Nilsson et al. 1999).
The likely ecosystem-level consequences of dwarf shrub
species are apparent from a study that has been operating
for the past 10 years on a series of forested lake islands in
northern Sweden (Wardle et al. 1997, 2003a; Wardle and
Zackrisson 2005). These islands represent a retrogressive
succession, with different islands representing different
periods of time since last burning. With increasing time
since the most recent fire disturbance, early successional
species such as bilberry and Scots pine are replaced by
later successional species such as black crowberry and
Norway spruce. Domination by black crowberry on these
islands is associated with high concentrations of polyphenolic compounds in the humus, and this in turn appears
to contribute to reduced soil microbial activity, lower
decomposition rates, reduced availability of soil N,
increased soil C sequestration and, ultimately, reduced
© The Ecological Society of America
Understory vegetation as a forest ecosystem driver
(b)
aboveground tree and shrub productivity
(a)
(Wardle et al. 1997, 2003a). Ongoing manipulative experiments, established in 1996, have
involved repeated experimental removals of all
possible components of the three dwarf shrub
species on plots across each of 30 islands, representing a wide gradient of time since burning
(Wardle and Zackrisson 2005). These show that
the bilberry and lingonberry have strong positive effects on litter decomposition, soil microbial activity, and depletion of soil mineral N,
while black crowberry does not. Bilberry and
lingonberry dominate the understory on early
successional rather than late successional
islands, and when they are experimentally
removed from plots, the relationship between
island successional stage and decomposer properties disappears. This suggests that the relationship between successional stage and decomposer activity is driven entirely by the types of
ericaceous dwarf shrubs that are present.
Feather mosses produce litter that decomposes slowly, and it has been shown that the
rate of mass loss and N release from this litter is
usually slower than that of the trees and dwarf
shrubs with which they co-exist (Wardle et al.
2003b). This often results in a thick layer of
moss litter forming below the live moss portions
and above the humus surface. This layer of litter
is important in retaining moisture and, unlike Figure 3. Micrograph of a section of moss leaf (x 200). (a) Under
the litter of black crowberry, it accelerates the ultraviolet-fluorescence micrograph with a green filter; and (b) under light
decomposition rates of associated litters from microscope. Coiled chains of the cyanobacterium Nostoc are hidden in the
vascular plant species (Wardle et al. 2003b). In leaf under light microscopy, but are readily observed as the red cells under
North American boreal forests, the understory ultraviolet-fluorescence microscopy. From DeLuca et al. (2002b), by permosses have also been found to have important mission of The Nature Publishing Group.
effects, buffering soils against temperature
changes by increasing surface insulation and reducing from field plots in a northern Finnish boreal forest were
incident sunlight, thereby potentially retarding litter found to impair decomposition of bilberry litter in litdecomposition rates (Oechel and Van Cleve 1986; terbags, presumably because the litter was subjected to a
Bonan 1991). It has also recently been shown that less favorable microclimate (Stark et al. 2000). The
feather mosses play a fundamental role in regulating adverse effect of grazing of reindeer lichens by reindeer on
ecosystem N input, because the live segments contain the soil microbial biomass and C flow in the soil is probahigh densities of cyanobacteria that fix atmospheric N bly attributable to alterations in soil microclimate and
into forms available to plants (DeLuca et al. 2002b; Figure reduced moisture availability caused by lichen removal
3). The quantities of N fixed are sufficient to account for (Väre et al. 1996; Stark et al. 2000).
Succession is characterized by predictable changes in
build up of organic N in the soil during succession
(DeLuca et al. 2002b), and this fixation is greater in late vegetation composition (Walker and del Moral 2003)
successional than in early successional boreal forests and, in the Swedish boreal forest, vegetation community
(Zackrisson et al. 2004). Although this mechanism is structure generally follows a predictable trajectory in the
undoubtedly important in maintaining the N capital of prolonged absence of fire (Table 1). As succession prothe ecosystem, how and when this N is transferred to co- ceeds through to a retrogressive phase, there is increasing
domination by black crowberry and feather mosses in the
existing dwarf shrubs and trees has yet to be elucidated.
Reindeer lichens produce litter that decomposes more understory, which creates an unsuitable environment for
slowly than that of most of the vascular plant species with tree seedling establishment. Species such as Scots pine
which they co-exist (Wardle et al. 2003b), and like the and birch, that rely on establishment from seeds, fail to
mosses they form a dense ground cover that impedes soil regenerate in sufficient numbers to maintain those
moisture loss. Experimental removals of reindeer lichens species, and Norway spruce becomes increasingly abun© The Ecological Society of America
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425
(a) Courtesy of P Lundgren. (b) Courtesy of U Rasmussen.
M-C Nilsson and DA Wardle
Understory vegetation as a forest ecosystem driver
426
M-C Nilsson and DA Wardle
conservation and restoration perspective have been debated in many
regions that have a natural fire ecology,
including Scandinavia (Niklasson and
Granström 2004). The recent work on
boreal understory vegetation effects
described above provides strong reasons for instituting or retaining forest
management practices that allow and
promote fire in the natural landscape.
Maintaining diversity in species and
ecosystem types at the landscape scale
is probably best encouraged by maintaining a mosaic of forest stands of
varying times since most recent fire.
Forestry is a primary export industry of
Sweden, and contributes 13% of its total
export earnings (Skogs-styrelsen 2005).
Evidence is emerging that understory
Figure 4. A regenerating forest stand in an area that was clearcut and subsequently vegetation is a major driver of forest
scarified. Ericaceous dwarf shrubs (eg black crowberry) have dominated the condition, both in the short term, by
understory and are likely contributors to the poor regeneration of this forest. affecting tree seedling regeneration, and
Successful regeneration and superior tree growth would require reinstatement of the in the long term, by driving soil
natural fire regime.
processes that regulate nutrient supply
for trees. It would therefore appear that
dant because it can regenerate vegetatively and is better the type of understory vegetation present has important
able to tolerate late succesional conditions. During this economic implications. Since fire is a major driver of underretrogressive succession, polyphenolic compounds story vegetation composition in the boreal forest it is likely
(largely produced by black crowberry) appear to accumu- that restoration of natural fire regimes in production
late in the soil, retarding microbial activity, decomposi- forestry would be commercially advantageous in the long
tion, and N mineralization (Wardle et al. 1998; DeLuca et term, as well as being beneficial for conservation. The
al. 2002a). This in turn reduces the rate of supply of plant debate about the benefits of fire for Swedish boreal forests
available N from the soil. Carbon accumulates in these has tended to focus on conservation issues rather than comsoils because decomposition rates decline faster than NPP mercial forest productivity, but there is an increasing awareduring retrogressive succession (Wardle et al. 2003a). ness of the probable long-term benefits of prescribed burnMeanwhile, N accumulates in the soil because of increas- ing for production (Niklasson and Granström 2004). Fire
ing N lock-up by polyphenols and greater rates of biolog- disturbance is likely to benefit production forestry simply
ical N fixation by cyanobacteria over time (DeLuca et al. because it prevents forest ecosystems from entering a long2002b; Zackrisson et al. 2004). This pattern of ecosystem term retrogressive succession. Scarification (or shallow culdecline can only be reversed through rejuvenation of the tivation) of the soil surface has often been the management
system by wildfire (Zackrisson et al. 1996).
tool of choice over prescribed fire in Scandinavian production forests, but this may not confer the same benefits to the
ecosystem as fire does. In particular, it is less successful than
Management implications
fire in preventing long-term dominance by those understory
Wildfire is the primary natural disturbance regime in species that suppress seedling regeneration and tree growth,
boreal forests, in both Scandinavia (Niklasson and often leading to reduced forest productivity (Figure 4).
Granström 2000) and elsewhere (Bonan and Shugart
1989). The work described above provides compelling Conclusions
evidence that this disturbance drives the functioning of
the ecosystem, mainly by determining the composition of Recent studies in the Swedish boreal forest have shown
the understory vegetation. In Scandinavia, the natural fire that understory components such as ericaceous dwarf
regime has been suppressed in recent times, and the prob- shrubs, mosses, and lichens are major community and
able long-term consequence of this is increasing domina- ecosystem drivers. In the short term, they operate as filters
tion of the understory by ericaceous dwarf shrubs and by helping to determine future forest tree species composifeather mosses, and corresponding declines in forest tree tion. In the longer term, they serve as major drivers of soil
regeneration and aboveground and belowground ecosys- fertility and thus influence nutrient availability and plant
tem process rates. The use and management of fire from a growth. Their net effect is to drive ecosystem succession,
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© The Ecological Society of America
M-C Nilsson and DA Wardle
including retrogressive succession. They also interact with
the fire regime to influence vegetation composition and
ecosystem functioning in the longer term. From an applied
perspective, production forestry and conservation are not
necessarily incompatible, and we suggest that management
for appropriate fire regimes may be beneficial for achieving
goals associated with both of these land uses.
Although relatively few studies have investigated the
ecological role of understory vegetation in other boreal
forests (but see Oechel and Van Cleve 1986; Carlton and
Read 1991; Mallik 2003), it is likely that the role that
this vegetation plays in Swedish boreal forests also operates elsewhere. Understanding these interactions in other
boreal forests could contribute to the debate over the role
of fire and its management in the boreal zone in general.
Nevertheless, much remains to be learned regarding the
ecological role of understory vegetation, both in northern
Sweden and elsewhere. For example, there is little understanding of the extent to which interactions involving
understory species at small spatial scales (characteristic of
most studies to date) are important at large spatial scales
(eg landscape scale or whole watersheds). Finally, little is
known about how major global change drivers such as
nitrogen deposition, global warming, and increases in
atmospheric CO2 may influence the composition of
understory vegetation, and how this will affect the functioning of the boreal forest ecosystem in the long term.
However, since several studies have shown potentially
large responses of ground layer vegetation to some of
these global change drivers in tundra ecosystems (Hobbie
et al. 1999; van Wijk et al. 2004), it is likely that important effects would also occur for similar understory vegetation types in boreal forests.
Acknowledgements
We thank our colleagues O Zackrisson, G Hörnberg, and
T DeLuca for commenting on an earlier version of this
manuscript. We also thank the funding agencies FORMAS, Vetenskapsrådet, and Carl Tryggers Stiftelse for
supporting the work of our colleagues and ourselves in
this field over the past 15 years.
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