Chapter 10
Rooting Patterns of Old-Growth Forests:
is Aboveground Structural and Functional
Diversity Mirrored Belowground?
Jurgen Bauhus
10.1 Introduction
When we think of old-growth forests, we generally imagine old forests with large
trees and possibly a highly diverse forest structure resulting from the death of
individual trees and the resulting gap-phase dynamics (Oliver and Larson 1996;
Franklin et al. 2002). This is also reflected in the definitions of old-growth forests
(see Chap. 2 by Wirth et al., this volume), which normally do not refer to
belowground structures and processes. This neglect of belowground aspects,
although intriguing, is understandable since so little information is available.
It is well known that the species richness, and often also the biomass, of inverte-
brates, fungi, and bacteria is much higher belowground than aboveground (e.g.
Torsvik et al. 1990), yet we know very little about how their diversity and
abundance belowground is related to forest age or forest structure. Carbon storage
is an important value of old-growth forests, and the carbon stored in soils, forest
floor, and belowground biomass often approximates the quantities stored in
aboveground biomass (e.g. Trofymow and Blackwell 1998; see Chap. 11 by
Gleixner et al., this volume). The turnover of fine roots is believed to be an
important driver of soil carbon accumulation, yet little is know about how this
process changes with forest age or forest developmental stages found in old-
growth forests.
Many of the attributes and values of old-growth forests are related to their
aboveground structural diversity (McElhinny et al. 2005). It is therefore interest-
ing to ask whether this structural diversity in old-growth forests is mirrored
belowground, and to what extent belowground structural diversity may contribute
to functional diversity. To approach these questions, I willaskfirstlywhich
attributes may comprise belowground structural diversity, whereby the term
belowground encompasses the substrates colonised by roots, the soil and forest
floor layers.
C. Wirth et al. (eds.), Old‐Growth Forests, Ecological Studies 207, 211
DOI: 10.1007/978‐3‐540‐92706‐8 10, #Springer‐Verlag Berlin Heidelberg 2009
10.2 What Comprises Belowground Structural Diversity?
Since belowground structural diversity has not been defined, I will first reflect on
the attributes that commonly are used to quantify structural diversity aboveground.
Stand structural diversity is a measure of the number of different structural attri-
butes present and the relative abundance of each of these attributes, as summarised
by McElhinny et al. (2005) for some forest types. These attributes are responsible
for variation in the vertical and horizontal structure of forest stands and are thought
to be indicative of biodiversity, i.e. related to the provision of faunal habitat. The
range of attributes comprises vertical layering of foliage; variation in canopy
density (e.g. caused by gaps); variation in the size and distribution of trees; the
height and spacing of trees, and their species diversity and biomass; the cover,
height and richness of the understorey including shrubs; and the number, volume
and range in decay stages of standing and fallen dead wood (McElhinny et al.
2005). The equivalent belowground attributes that may be important for below-
ground biodiversity and ecosystem functioning are listed in Table 10.1. In general
terms, these structural elements provide horizontal and vertical heterogeneity in
belowground structures. In the following, some of the analogies between below-
ground and aboveground structural elements or parameters will be pointed out and
discussed, focussing in particular on the question of whether the structural diversity
created by these elements increases with forest age.
Table 10.1 Potential attributes of belowground structural complexity
Contribution to: Attribute Quantified as:
Vertical structure Maximum rooting depth of
structural or fine roots
Depth in metres
Variation in maximum
rooting depth between species
Vertical distribution of fine
or coarse roots
No. of layers, evenness
of root distribution
in layers/horizons
over the profile
depth
Horizontal variation Size and distribution of roots systems Number, volume
Size and number of root gaps Area
Quantity and size of belowground
coarse woody detritus
(stumps, old root
channels, etc.)
Number, volume and
depth
Pit and mound topography Area covered by these
features
212 J. Bauhus
10.3 Root Gaps and Horizontal Variation in Rooting Density
in Old-Growth Forests
Old-growth forests are the result of the long-term absence of stand-replacing dis-
turbances (see Chap. 2 by Wirth et al., this volume). The old trees may comprise the
cohort that developed following the previous stand-replacing disturbance or they may
have developed subsequently in gaps created by the death of trees from the distur-
bance cohort. In any case, by the time an old-growth developmental phase is reached,
the disturbance dynamics, until then, will have been dominated by the formation of
gaps in most types of forests. Therefore, gaps are an important feature of old-growth
forests. They may occupy 5 15% of the stand area in temperate forests (e.g. Runkle
1982; Emborg et al. 2000). These gaps contribute greatly to the vertical structural
diversity of old-growth forests and also, through the successional processes trig-
gered within them (e.g. Busing and White 1997; Rebertus and Veblen 1993), to
species diversity. Some canopy species may not be able to regenerate in these gaps
and are replaced by more shade-tolerant species (Oliver and Larson 1996; Gilbert
1959). However, the prevalence of gap dynamics in old-growth forests leads to a
patchwork of developmental stages, creating horizontal variation in canopy height,
tree biomass and necromass and/or species composition (e.g. Emborg et al. 2000;
Korpel 1995; Franklin et al. 2002). Whether or not the aboveground structural
diversity of old-growth forests attributable to gap formation is mirrored below-
ground depends on whether root gaps are created in the process, and on whether
belowground structural elements vary with the developmental stage of the patches.
This would, for example, be the case if these gap phases have different levels of
fine- or coarse-root biomass, or roots with functional traits or mycorrhizal associa-
tions that differ from those found during other developmental stages.
First, I will explore the question of whether aboveground gaps create below-
ground gaps. Then I will ask to what extent belowground gaps contribute to
structural and functional diversity.
Aboveground gaps are created when one or more trees participating in the main
canopy layer are removed or killed. While the foliage of trees within the crown is
confined to a reasonably small projected ground area around the trees, the same is
not true for roots. These are far more wide-reaching than the branches (Stone and
Kalisz 1991), and so roots tend to overlap much more than crowns. For example,
Bu
¨ttner and Leuschner (1994) found complete spatial overlap of the root systems of
the co-occurring species Quercus petraea and Fagus sylvatica. Consequently, when
one tree or a small group of trees is removed or killed, the soil beneath them remains
partially occupied by the root system of neighbouring trees. Thus, in most cases,
‘‘root gaps’’ do not represent patches with no live roots, but may be characterised as
zones of reduced root competition.
Runkle (1982) and Brokaw (1982) have defined canopy gaps. These have a
certain minimum size, and extend from the top of the canopy through all vegetation
layers to a certain height above the ground. We can distinguish between the actual
canopy gap, between the edges of crowns, and the expanded gap between tree
10 Rooting Patterns of Old Growth Forests 213
stems. Since we usually have no zone without roots, it is very difficult and certainly
impractical to define a root gap in the field. Thus, attempts to identify root gaps have
focussed on the ground areas within the perimeter of aboveground canopy gaps.
Also, when root gaps have been studied, the focus was usually on fine roots, which
are responsible for belowground competition for soil resources.
The picture that emerges from the few available studies of root gaps is far from
clear. This is certainly due, in part, to the fact that the results are from different gap
sizes and that root measurements are from different soil depths and times since gap
creation. Many of these studies are from tropical forests, and, in most cases, only the
general gap area was analysed rather than the fine root distribution in relation to
distance to the gap perimeter. When fine root biomass between root gaps and intact
adjacent areas was compared, a reduction could be observed in most cases
(Fig. 10.1). This reduction in fine root biomass was usually not more than 60%
(mostly between 20 and 40%). The reduction in live fine roots following canopy
creation could be very fast, for example, a 40% reduction in a subtropical wet forest
system (Silver and Vogt 1993). However, when medium-term fine root growth
between these two areas was compared, root gaps more often showed an increase
than a decrease, possibly indicating a reasonably fast recovery of fine root biomass.
Unfortunately, very few studies provide information on the process of root gap
closure with time. Bauhus and Bartsch (1996) used in-growth cores to compare the
fine root growth of Fagus sylvatica at the centre and perimeter of 30 m diameter
gaps within an undisturbed ca. 160-year-old forest in the Solling area of Germany.
Fine root growth in the stand was 390 g m
–2
(0 30 cm soil depth) over a 12-month
period (Fig. 10.2).
0
1
2
3
4
5
150 140 120 100 80 60 40 >40
Fine-root biomass or growth in gaps relative to intact
forest (%)
Frequency
biomass
growth
Fig. 10.1 Reduction or increase in fine root (<2 mm diameter) biomass or growth observed in root
gaps associated with canopy gaps in different forest ecosystems, where fine root biomass was
compared to that of intact adjacent forest. Note that the time since gap creation differs between
studies (Bauhus and Bartsch 1996; Silver and Vogt 1993; Ostertag 1998; Denslow et al. 1998;
Wilczynski and Pickett 1993; Cavelier et al. 1996; Sanford 1990; Battles and Fahey 2000)
214 J. Bauhus
At a distance of 5 m from the edge trees into gaps fine root production over the
same period declined to 15 130 g m
–2
, whereas in the centre of gaps it was
negligible. Similarly, Mu
¨ller and Wagner (2003) found the greatest fine root growth
in gaps only 2.2 m from the edge of the gap in a 35-year-old spruce (Picea abies)
forest, while no live fine roots were found beyond 7.4 m from the gap edge. These
studies show that root gaps can persist for a substantial period of time, if gaps are
not, or only slowly, recolonised by other vegetation, as was the case in the two
studies cited above. However, fine root biomass may recover rapidly when gaps are
large enough to be recolonised by fast-growing understorey or shrub species
(Bauhus and Bartsch 1996). Jones et al. (2003) showed that belowground gaps in
Pinus palustris forests closed quickly because understorey vegetation compensated
for the absence of pine fine roots, in particular in gaps with higher soil moisture and
nitrate concentrations than in the surrounding forest. Campbell et al. (1998) also
found a rapid recovery of non-tree roots in small experimental gaps in mixed boreal
forests in Que
´bec. Whether the speed of recolonisation depends on the contrast in
soil nutrient and water availability between the root gaps and surrounding soil, such
that the root gaps represent rich patches, is not clear (e.g. Ostertag 1998). Higher
concentrations of nitrate and phosphate in soils of root gaps as compared to
undisturbed areas might facilitate colonisation by pioneer species (Denslow et al.
1998). Once saplings have established in gaps, fine root growth in them might be
higher than in the undisturbed surrounding forest (Battles and Fahey 2000).
The occurrence of fine root gaps is related to the horizontal distribution of fine root
mass of individual trees. Models of fine root distribution of single trees indicate that
the biomass over the entire soil profile is greatest near the stem and declines with
distance from the tree (Nielsen and Mackenthun 1991; Ammer and Wagner 2005).
However, other studies have indicated that the spatial distribution of roots around
stems does not follow such a symmetrical pattern but may be related more to soil
nutrient availability (Mou et al. 1995). Large-crowned trees have more fine root
0
100
200
300
400
500
600
0510
Distance from egde (m)
Fine-root ingrowth (g m–2)
Fig. 10.2 Fine root biomass production over a 16 months period determined by the ingrowth core
method in an undisturbed European beech forest (0 m) and at different positions (5 and 10 m from
the edge) within 30 m diameter gaps (0 30 cm soil depth) (after Bauhus and Bartsch 1996)
10 Rooting Patterns of Old Growth Forests 215