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Background: Severe canopy-removing disturbances are native to many temperate forests and radically alter stand structure, but biotic legacies (surviving elements or patterns) can lend continuity to ecosystem function after such events. Poorly understood is the degree to which the structural complexity of an old-growth forest carries over to the next stand. We asked how pre-disturbance spatial pattern acts as a legacy to influence post-disturbance stand structure, and how this legacy influences the structural diversity within the early-seral stand. Methods: Two stem-mapped one-hectare forest plots in the Czech Republic experienced a severe bark beetle outbreak, thus providing before-and-after data on spatial patterns in live and dead trees, crown projections, down logs, and herb cover. Results: Post-disturbance stands were dominated by an advanced regeneration layer present before the disturbance. Both major species, Norway spruce (Picea abies) and rowan (Sorbus aucuparia), were strongly self-aggregated and also clustered to former canopy trees, pre-disturbance snags, stumps and logs, suggesting positive overstory to understory neighbourhood effects. Thus, although the disturbance dramatically reduced the stand's height profile with ~100% mortality of the canopy layer, the spatial structure of post-disturbance stands still closely reflected the pre-disturbance structure. The former upper tree layer influenced advanced regeneration through microsite and light limitation. Under formerly dense canopies, regeneration density was high but relatively homogeneous in height; while in former small gaps with greater herb cover, regeneration density was lower but with greater heterogeneity in heights. Conclusion: These findings suggest that pre-disturbance spatial patterns of forests can persist through severe canopy-removing disturbance, and determine the spatial structure of the succeeding stand. Such patterns constitute a subtle but key legacy effect, promoting structural complexity in early-seral forests as well as variable successional pathways and rates. This influence suggests a continuity in spatial ecosystem structure that may well persist through multiple forest generations.
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RESEARCH ARTICLE
Legacy of Pre-Disturbance Spatial Pattern
Determines Early Structural Diversity
following Severe Disturbance in Montane
Spruce Forests
Radek Bače
1
*, Miroslav Svoboda
1
, Pavel Janda
1
, Robert C. Morrissey
1
, Jan Wild
2,3
,
Jennifer L. Clear
1
, Vojtěch Čada
1
, Daniel C. Donato
1,4
1Department of Forest Ecology, Faculty of Forestry and Wood science, Czech University of Life Sciences,
Prague, Czech Republic, 2Institute of Botany, The Czech Academy of Sciences, Průhonice, Czech
Republic, 3Department of Applied Geoinformatics and Spatial Planning, Faculty of Environmental Sciences,
Czech University of Life Sciences, Prague, Czech Republic, 4Washington State Department of Natural
Resources, Olympia, Washington, United States of America
*bace@fld.czu.cz
Abstract
Background
Severe canopy-removing disturbances are native to many temperate forests and radically
alter stand structure, but biotic legacies (surviving elements or patterns) can lend continuity
to ecosystem function after such events. Poorly understood is the degree to which the struc-
tural complexity of an old-growth forest carries over to the next stand. We asked how pre-
disturbance spatial pattern acts as a legacy to influence post-disturbance stand structure,
and how this legacy influences the structural diversity within the early-seral stand.
Methods
Two stem-mapped one-hectare forest plots in the Czech Republic experienced a severe
bark beetle outbreak, thus providing before-and-after data on spatial patterns in live and
dead trees, crown projections, down logs, and herb cover.
Results
Post-disturbance stands were dominated by an advanced regeneration layer present before
the disturbance. Both major species, Norway spruce (Picea abies) and rowan (Sorbus
aucuparia), were strongly self-aggregated and also clustered to former canopy trees, pre-
disturbance snags, stumps and logs, suggesting positive overstory to understory neigh-
bourhood effects. Thus, although the disturbance dramatically reduced the stands height
profile with ~100% mortality of the canopy layer, the spatial structure of post-disturbance
stands still closely reflected the pre-disturbance structure. The former upper tree layer influ-
enced advanced regeneration through microsite and light limitation. Under formerly dense
canopies, regeneration density was high but relatively homogeneous in height; while in
PLOS ONE | DOI:10.1371/journal.pone.0139214 September 30, 2015 1/18
OPEN ACCESS
Citation: Bače R, Svoboda M, Janda P, Morrissey
RC, Wild J, Clear JL, et al. (2015) Legacy of Pre-
Disturbance Spatial Pattern Determines Early
Structural Diversity following Severe Disturbance in
Montane Spruce Forests. PLoS ONE 10(9):
e0139214. doi:10.1371/journal.pone.0139214
Editor: Han Y.H. Chen, Lakehead University,
CANADA
Received: April 13, 2015
Accepted: September 9, 2015
Published: September 30, 2015
Copyright: © 2015 Bače et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any
medium, provided the original author and source are
credited.
Data Availability Statement: All relevant data are
within the paper and its Supporting Information files.
Funding: This work was supported by project Czech
Science Foundation (GACR) number P504/12/1218.
RB, PJ and RCM were supported by Ministry of
Finance of the Czech Republic, Norway grant
Frameworks and possibilities of forest adaptation
measures and strategies connected with climate
changeNo. EHP-CZ02- OV-1-019-2014. RB and MS
were supported also by ESF a MŠMT number
CZ.1.07/2.3.00/30.0040. JW was supported by long-
term research development project no.
former small gaps with greater herb cover, regeneration density was lower but with greater
heterogeneity in heights.
Conclusion
These findings suggest that pre-disturbance spatial patterns of forests can persist through
severe canopy-removing disturbance, and determine the spatial structure of the succeeding
stand. Such patterns constitute a subtle but key legacy effect, promoting structural complex-
ity in early-seral forests as well as variable successional pathways and rates. This influence
suggests a continuity in spatial ecosystem structure that may well persist through multiple
forest generations.
Introduction
Large, severe disturbances are native to many temperate forests, but are increasing in frequency
and extent in many forest landscapes, a trend predicted to continue with future climate change
[13]. These events profoundly influence ecosystem structure and function, generating wide
variability in ecological responses relative to smaller, less severe disturbances [4] and, thus,
greater uncertainty for stand development and management [5]. An emerging theme in forest
ecology is understanding the role of spatial heterogeneity relative to disturbances, biological
legacies, and other factors influencing stand development and biodiversity (e.g. [68]).
A key factor influencing the stability of ecosystem pattern through disturbance is the pres-
ence of biological legacies, defined as biologically-derived elements of the pre-disturbance eco-
system that carry over into the next stand [9]. The most commonly studied examples include
standing and down dead trees, pits and mounds from uprooted trees, soil organic matter, sexu-
ally mature live trees, understory seedling and sapling banks (advance regeneration), and vege-
tatively reproducing parts and seed banks [9]. The role of advance regeneration in driving
succession following canopy-removing disturbances (via abundance and composition of the
surviving cohort) has been well established across several forest types [1012]. Rarely studied,
however, is how pre-disturbance forest spatial patterns, including advance regeneration and
the former overstory, influence the structure of post-disturbance stands. Although disturbances
may be severe enough to erase pre-disturbance structural patterns [5,13], recent studies show
persistent horizontal patterns after disturbances [6,8], and that such fidelity of pattern can per-
sist for more than two centuries [1415]. However, the drivers underlying this continuity of
pattern and the consequences for early-seral stand structure and development are poorly
understood.
The legacy influence of advance regeneration, and the factors that structure its spatial pat-
tern, are especially important after disturbances that cause near complete overstory removal.
For example, Norway spruce (Picea abies (L.) Karst.) montane forests of Central Europe, where
wind storms and bark beetle outbreaks have recently caused total overstory dieback across
thousands of hectares [1619], regenerate primarily from an understory seedling and sapling
bank, and post-disturbance seedling germination is a relatively minor component of regenera-
tion [2021]. Patterns of advance regeneration in mature stands are influenced by overstory
distribution through seed distribution and light availability for understory trees [22], and
ground-layer vegetation which can act as a strong filter on succession of advance regeneration
both before and after disturbance [23]. Deadwood such as snags, stumps, down logs, and their
immediate vicinity, are also important to advance regeneration and survival by providing
Legacy of Pre-Disturbance Spatial Pattern in Spruce Forests
PLOS ONE | DOI:10.1371/journal.pone.0139214 September 30, 2015 2/18
RVO67985939. JLC and VC were supported by the
Czech University of Life Sciences, Prague (Project
CIGA No. 20154309). The funders had no role in
study design, data collection and analysis, decision to
publish, or preparation of the manuscript.
Competing Interests: The authors have declared
that no competing interests exist.
favourable germination sites and microenvironments for Norway spruce [24]. Thus, pre-dis-
turbance interactions between mature overstories, microsite availability, and understory distri-
bution and growth may effectively determine the stand structure following severe canopy-
removing disturbance.
Canopy-opening disturbances positively influence the diversity of organisms that peak in
newly-opened stands (e.g. fresh deadwood specialists) as well as those associated with more
developed early-seral vegetation for subsequent decades [2529]. However, open habitats (e.g.
sun-exposed large deadwood) are expected to diminish after tree canopy closure; thus, variable
post-disturbance pathways that include protracted early-seral stages and promote the survival
of early-seral dependent species can maintain greater biodiversity.
Identifying the influence of pre-disturbance spatial pattern on post-disturbance stand struc-
ture has been limited by a lack of spatially explicit data monitored through a disturbance event.
Most studies have used only post-disturbance data to reconstruct some components of the
prior stand [8,11,30]. Monitoring spatial pattern through time, even in a limited number of
plots, can yield a unique data set that gives more accurate insight into stand dynamics [31].
After two large research plots in a montane spruce forest with mapped stand structure data
were impacted by a major (100%) canopy-removing windstorm and bark beetle outbreak, we
re-measured and mapped the plots. We used these spatially explicit data to evaluate how the
structural pattern of regeneration relates to pre-existing microsites, overstory pattern, and
understory vegetation, factors known to influence stand development pathways. Tree species
composition, spatial distribution, height, and height heterogeneity were compared before and
after disturbance with the following objectives: (1) to determine how pre-disturbance spatial
pattern acts as a legacy to influence post-disturbance spatial structure and; (2) to evaluate how
the fidelity of horizontal pattern influences the structural diversity within the early-seral stand.
Methods
Study site
This study was conducted in a Norway spruce forest situated in the Bohemian Forest (Šumava
in Czech; 48°470N, 13°490E) located in the southwest of the Czech Republic. The Administra-
tion of the National Park and Protective Landscape Area of Šumava granted the permission for
data collection. The region was established as a nature reserve in 1933 before becoming part of
the Šumava National Park in 1991. Study plots are located between 12201270 m a.s.l., with a
north aspect and gentle slope (8°). Total annual precipitation is ca. 1400 mm and mean
annual temperature is approximately 4°C [32]. Snow cover usually persists from November to
early May and the maximum snow depth is about 2 m. The bedrock is coarse-grained granite.
Plant communities are classified as Athyrio alpestris-Piceetum [33] with abundant Alpine-lady
fern (Athyrium distentifolium Tausch ex Opiz) undergrowth.
Present day forest composition and structure is the legacy of the historical disturbance
regime, characterized by infrequent large-scale, moderate-severity (non-stand-replacing) dis-
turbances combined with frequent low-severity events during the last 300 years [3435]. Before
the most recent severe disturbance, the forest was dominated by spruce trees with an unbal-
anced bimodal height distribution [36]. The age distribution of trees was also bimodal; with
numerous trees older than 250 years, almost no recruitment during the first half of the 20th
century, then a more recently established advance regeneration layer [35]. Overstory canopy
cover was <30% on both study plots and regeneration was dominated by spruce and rowan
(Sorbus aucuparia L.). Sapling (50200 cm tall) density was 1095 and 253 individuals per hect-
are for spruce and rowan, respectively. Spruce seedling (<50 cm tall) density was 4 400 individ-
uals per hectare [36]. A bark beetle (Ips typographus L.) outbreak between 1996 and 1999
Legacy of Pre-Disturbance Spatial Pattern in Spruce Forests
PLOS ONE | DOI:10.1371/journal.pone.0139214 September 30, 2015 3/18
resulted in a mosaic of dead tree patches, each up to one hectare. This was followed in 2007 by
a winter storm (Kyrill) that caused severe and widespread uprooting of mature spruce trees. A
subsequent bark beetle outbreak resulted in mortality of all canopy spruce trees in 2008.
Data collection
In 2005, two permanent sampling plots (100×100 m) were established in the study area. Plots
were selected to minimize the inclusion of trees recently killed by the earlier bark beetle distur-
bance (between 1996 and 1999) and to avoid the occurrence of specific site conditions (e.g.
stream corridors, rock outcrops, or evidence of logging). Thus, sample plots were initially in
relatively undisturbed, unmanaged old growth forest.
Electronic and laser measuring devices (Field-Map
1
, Monitoring and Mapping Solutions,
Ltd.; www.fieldmap.cz) were used to map and field-tag all live individuals of spruce >50 cm
height and of rowan >30 cm height, all standing deadwood with diameter at ground level >10
cm, and down logs with large end diameter >10 cm and >2 m length. Species and height were
recorded for each tree. Down logs were classified into 4 categories according to side vegetation
cover (the vegetation growing along a log and/or rising over the log): (1) up to 5% of the log
covered; (2) 6 to 25% covered; (3) 26 to 50% covered and (4) >50% covered. The crown projec-
tion of trees taller than 4 m was recorded by a minimum of five radii extending to the edge of
the crown. Spruce individuals below 50 cm (seedlings) were sampled in 25 5×5 m subplots
located at every other intersection and end of a 10 m grid overlain on the plot. Seedlings were
measured and classified into one of five 10 cm height classes. The coverage of all vascular plants
was estimated (to the nearest 5%) in each 10×10 m grid cell and within each 5×5m subplot.
Following the complete overstory mortality generated by the windstorm (only ~10% of can-
opy trees were uprooted) and ensuing beetle outbreak (~90% canopy trees were killed), a post-
disturbance inventory of the stem-mapped plots was conducted in 2009. All individuals were
identified and recorded as; survived, damaged (breakage, bending, drying, uprooting) or dead.
The reason for tree mortality was subsequently identified as: bark beetle attack; wind uproot-
ing; falling caused by mechanical forcing (by another mature tree, snag or live uprooted tree);
uprooting from deadwood microsite; competition; ungulate damage; and other (e.g. snow). On
plot 2, all newly recruited (post-disturbance) saplings above 50 cm (spruce) and 30 cm (rowan)
were also spatially mapped.
Data analysis
Pre-disturbance. For the purpose of analysis, saplings were defined as individuals 2m
tall, and canopy trees were those >25 m in height. To identify differences in spatial pattern of
small versus large sapling regeneration, saplings were divided into two groups of approximately
equal abundance using a boundary value of 90 cm in height. Individuals between 2 and 25 m
tall were not included in the analysis regarding influence of canopy trees on regeneration. All
analyses were conducted using R statistical software (v.2.15.2; R Development Core Team
2009). Tree spatial patterns were analysed using pair correlation function (pcf), estimated by
smoothing with Epanechnikov kernel and Ripleys isotropic edge correction implemented in
spatstat R-package [3738]. Univariate pcf were compared to envelopes to assess spatial rela-
tions within the advance regeneration cohort. Envelopes were constructed by removing the
four highest and four lowest values of 199 random simulations of a complete spatial random-
ness (CSR) null model. The relations among different tree groups (i.e. among saplings of differ-
ent species, saplings and canopy trees, snags and stumps) were assessed using a bivariate form
of pcf [3738]. To test the independence of two spatial patterns we constructed null model by
Legacy of Pre-Disturbance Spatial Pattern in Spruce Forests
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random shifting of all position of one group, while keeping a fixed position of the second
group [3940]. The envelopes were than constructed similarly as by univariate pcf.
To examine the relationship between sapling density and potential overstory and understory
competition, as indicated by canopy cover and Alpine-lady fern cover, respectively, the rhohat
procedure [41] was implemented using spatstat R-package [38]. Sapling density was plotted as a
function of canopy cover and Alpine-lady fern cover in each 10×10 m grid cell. Canopy cover
within the grid cells was defined as 100% minus the area without crown projection. The cover of
Alpine-lady fern was used for analysis because it is the most abundant vascular plant in this local-
ity and reacts positively to an increase in light availability [42]. The rhohat procedure was also
used to evaluate the dependency of spruce sapling establishment on down logs, with distance
from the nearest long axis of a down log used as a covariate. Spruce sapling establishment associ-
ated with individual logs was then compared according to the amount of side vegetation cover.
Finally, we assessed how the occurrence of regeneration of different height classes and the het-
erogeneity versus homogeneity of sapling heights varied by patch type within the stands (high or
low canopy cover; high or low Alpine-lady fern cover) using a principal component analysis
(PCA). The PCA were based on a subplot-by-height class matrix, with the regeneration densities
occupying matrix cells. Spruce seedling and sapling densities were log-transformed and scaled in
analyses of both seedling (5×5 m) subplots and sapling (10×10 m) subplots. To describe the main
gradients, canopy and Alpine-lady fern cover variables (with r
2
>0.1 and p<0.001) were passively
projected into ordination space using the envfit function with 999 permutations [43]. This
method produces vectors that represent the most rapidly changing direction for a given variable
and have lengths proportional to the strength of the correlation between variables and the ordi-
nation. To display the position of regeneration characteristics in ordination space, the arrows
representing regeneration density (trees ha
-1
), height homogeneity (kurtosis of height distribu-
tion) and height heterogeneity (interquartile range of height distribution) were also overlain on
the ordination space. All independent variables are listed in S1 Table.
Post-disturbance. To determine whether disturbance-generated changes occur in a spa-
tially correlated way over pre-disturbance univariate patterns of trees, bivariate pcf between
killed and newly recruited individuals above 50 cm and survived individuals were used. To test
the spatial independence of survived and newly recruited spruce we constructed a null model
by random shifting of all position of survived trees, while keeping a fixed position of new
recruits [3940]. The spatial independence of demographic events like mortality and recruit-
ment within a fixed pattern approach were tested using random labelling of either killed and
survived or survived and newly recruited individuals for each simulation [3940]. The enve-
lopes for all pcf tests were constructed by removing the four highest and four lowest values of
199 simulations [38]. To evaluate changes in spruce-free patch size (gaps in the pattern), the
empty space distance F-functionwith Kaplan-Meier edge correction was used in the spatstat
package [38]. The empty space distance F is the distance from an arbitrary fixed location to the
nearest point of the pattern.
To test the difference in increase between spruce and rowan saplings, Pearson's Chi-squared
test with Yates' continuity correction was used. To compare the height increment among
spruce and rowan saplings, an ANCOVA and associated F-test was used with 2005 height mea-
surements as a covariate and species as an explanatory variable.
Results
Pre-disturbance
Spatial pattern of advance regeneration. When individually analysed (within species),
both spruce and rowan regeneration were spatially aggregated; strongly at distances of up to
Legacy of Pre-Disturbance Spatial Pattern in Spruce Forests
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23 m and slightly to a distance of 810 m (Fig 1A). The bivariate spatial pattern of spruce and
rowan regeneration indicates weak aggregation or randomness at most scales and this pattern
is variable between plots (Fig 1B). Rowan regeneration exhibited clustering with canopy spruce
Fig 1. Spatial patterns of regeneration. A) Spatial patterns of saplings (small: 5090 cm, large: 90200 cm); B) spatial relationship between spruce and
rowan saplings; C) spatial relationship between saplings and canopy living trees and D) spatial relationship between saplings and snags or stumps evaluated
with univariate and bivariate pair correlation function. Both strongly self-aggregated spruce and rowan saplings are clustered to canopy (>25 m) trees, snag
and stumps.
doi:10.1371/journal.pone.0139214.g001
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trees at distances between 13 m, particularly for smaller saplings (Fig 1C). Spruce regenera-
tion is more spatially independent of canopy trees, especially for tall saplings (Fig 1C). Both
rowan and especially spruce regeneration were clustered with pre-disturbance snags and
stumps at distances up to approximately 2 m (Fig 1D).
Structure of advance regeneration in relation to canopy and understory
cover
The density of both spruce and rowan advance regeneration exhibited a unimodal humpdis-
tribution with respect to both overstory canopy cover and Alpine-lady fern cover (S1 Fig).
Reduced sapling density is associated with highly closed tree canopies, and also under very
open canopies, where Alpine-lady fern was abundant. PCA also indicates that advance regener-
ation of spruce and rowan was strongly spatially limited by patches with dense herbaceous
cover formed by Alpine-lady fern (Fig 2). Height class trajectories and canopy cover exhibited
a negative relationship for spruce and rowan regeneration (tall saplings are associated with
more open conditions). Spruce saplings exhibited higher density values near logs with lower
levels of competition, particularly at distances <2m(S2 Fig). However, spruce sapling density
was low near logs with high vegetation cover, and they were often limited to microsites directly
on the deadwood (0 m).
Post disturbance
Species composition and height structure. After the 2007 windstorm, virtually all canopy
trees were killed; some by the initial windstorm but the majority by subsequent bark beetle out-
break. The height threshold for survival of spruce trees through the disturbance was approxi-
mately 24m(Fig 3). The disturbance effect on existing regeneration was relatively low (S2
Table). The only notable disturbance-related injury was damage and death of spruce saplings
by another large live tree falling during the windstorm, whether due to the whole trunk or just
branch fall. Rowan remained mostly unaffected; only 5 individuals died, and these were a result
of live tree fall (S2 Table). The number of newly recruited spruce saplings, above 50 cm, was
significantly higher than the number of newly recruited rowan saplings (Plot 2: χ
2
= 8.38,
p= 0.004), at 39% (535 new individuals) and at 26% (70 new individuals), respectively. Rowan
post-disturbance height increment (average [± standard deviation]: 43.0[±44.5] cm) was signif-
icantly higher than spruce (12.7[±39.6] cm).
Spatial pattern. The disturbance-related mortality among all spruce individuals above 50
cm (including canopy trees) was not random within fixed pre-disturbance pattern. The bivari-
ate pcf suggested partial segregation of killed individuals from survived individuals at finer
scales (up ~8 m) (Fig 4A). But disturbance-related mortality among saplings within fixed pre-
disturbance spatial pattern did not significantly differ from the random mortality null model
(Fig 4B). This indicates that canopy trees, which have the highest mortality rate (Fig 3), were
less aggregated to saplings than saplings were among themselves. New spruce trees were found
to establish close to survived individuals, more than predicted by the null model based on ran-
dom shifting of new recruits (S3 Fig). However, the process that distributed the label (survivor
or new recruit) within the fixed pattern was not random. Slight negative departures (to a dis-
tance of 3 m) from the random labelling null model show slight segregation between survivors
and new recruits (Fig 4C). Consequently, the post-disturbance spruce-free patches were in the
same locations (compare gaps on subplots Fig 4, Plot 2) and were slightly larger than spruce
free-patches formed before disturbance, despite the fact that the number of newly recruited
spruce above 50 cm was greater than the number of killed individuals (Fig 5A). The post-dis-
turbance living spruce spatial pattern remains strongly aggregated (Fig 5B).
Legacy of Pre-Disturbance Spatial Pattern in Spruce Forests
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Fig 2. Ordination biplots of rowan sapling, spruce seedling, and spruce sapling densities in different
height classes. Regeneration densities are represented by text (indicating height in cm) and arrows show
the direction from smaller to larger height classes of saplings. The passively projected environmental
variables (Alpine-lady fern and canopy cover) are represented by blue and brown arrow. The passively
projected spruce regeneration density, height homogeneity and height heterogeneity are overlain. Brown
isolines show the gradient in the canopy cover.
doi:10.1371/journal.pone.0139214.g002
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Discussion
Detailed analyses of horizontal and vertical forest structure pre- and post-disturbance allowed
us to reveal some of the drivers of regeneration pattern and revealed the potential precursors of
early-seral structural diversity. The spatial pattern of regeneration before disturbance was
driven mainly by canopy closure, the density of herb layer vegetation, and spatial aggregation
among various tree life stages. After the disturbance, all canopy trees were killed, but the distur-
bance had little impact on advance regeneration; variability of regeneration density and height
was similar pre- and post-disturbance. Overall horizontal post-disturbance spatial pattern was
maintained primarily by the strong spatial aggregation among surviving spruce and rowan
regeneration, killed canopy spruce trees, and pre-existing snags and stumps. The locations of
low-density and high-density tree patches remained largely the same through the disturbance.
Pre-disturbance spatial pattern influences post-disturbance stand
horizontal and vertical structure
The spatial pattern of living trees after severe bark beetle disturbance was almost exclusively
influenced by the pre-disturbance state. Pre-disturbance advance regeneration was resistant to
disturbance and had high survival, and newly recruited individuals established proximal to
existing individuals. Although the disturbance dramatically reduced the height structure
through total mortality of the canopy layer, post-disturbance vertical structure still partly
reflected the pre-disturbance structure. The memory of vertically structured stands is incorpo-
rated implicitly in to the localized severity of a disturbance of otherwise similar intensity; i.e.
larger trees are more susceptible to disturbance [4445]. Forests with more developed vertical
structure retain the pattern of the previous state to a greater degree, because the smaller trees
Fig 3. Histograms of individual tree heights categorized by species and plot before disturbance
(2005). Surviving and dead individuals after disturbance are marked in colour. The y-axes for spruce
histogram were square root-transformed.
doi:10.1371/journal.pone.0139214.g003
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survive the disturbance; homogeneous stand structures typical of managed forests have lower
vertical diversity, thus they tend to be less resistant and resilient [46]. Vertical heterogeneity
was further enhanced by the diversity of height growth responses of surviving regeneration
(coefficient of variation = 3.1) and the persistence of spruce-free patches, which are large
enough to allow the further establishment of younger individuals in the future and further
diversify the age and height structures. Suzuki et al. [6] attributed the observed long-term struc-
tural heterogeneity in part to the variability of individual tree growth associated with different
patterns of regeneration survival and distribution. We know of no other studies that have
examined the pre- and post-disturbance spatial patterning of regeneration, however, several
studies have examined post-disturbance tree distribution (e.g. [8,4749]).
The spatial aggregation among surviving spruce and rowan regeneration, killed canopy
spruce trees, and pre-existing snags and stumps (Fig 1) confirm the existence of a strong posi-
tive overstory to understory neighbourhood effect [8,50]. The positive neighbourhood effect of
overstory on spruce recovery is mediated by; (1) direct regeneration of spruce on deadwood or
in vicinity of stems [8,24] and (2) reducing competition with understory plants due to unfa-
vourable light conditions under the canopy [42]. Suitable microsites for young seedling estab-
lishment of both species occurred in patches with higher levels of canopy cover [42,5152]; in
Fig 4. Mortality and recruitment spatial pattern. Bivariate spatial pattern between disturbance-killed and survived individuals of; (A) all pre-disturbance
recruited spruce, (B) pre-disturbance recruited spruce saplings and (C) the bivariate pattern between all survived and newly recruited spruce above 50 cm.
Positions of spruces killed by disturbance (A, B) or newly recruited spruces (C) are marked in each subplots. Spruce-free patches (gaps) before disturbance
(A, B) and after disturbance (C) are stressed by colour spectrum that represents the distance to the nearest live individual in meters.
doi:10.1371/journal.pone.0139214.g004
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the presence of dense undergrowth vegetation, even the most suitable microsites (deadwood)
are only sparsely populated by spruce saplings (S2 Fig).
The relationship between regeneration density and undergrowth plant and canopy cover
suggests a disturbance-activated positive neighbourhood effect [50]. Although Norway spruce
evolved various adaptations for an advance regeneration systemincluding adventitious root
formation [53], the period of understory persistence is limited and probably does not exceed 50
years [24,54]. Spruce regeneration density tended to be low in areas of moderate canopy cover
(2055%). For seedlings, high competition pressure within the herb layer makes establishment
and survival difficult over long time periods [23,42,55]. Those that do survive to sapling size
have increasing light requirements with increasing size [22]. Without canopy disturbance, sap-
lings at this stage are increasingly subject to overwintering injuries [56], herbivore pressure
Fig 5. Distribution of empty space function (F(r)) from spruce point pattern. Point pattern is formed by living before and living after disturbance, killed by
disturbance and newly recruited spruces above 50 cm. The numbers of individuals are specified in brackets. The evaluation of post-disturbance living spruce
spatial pattern is shown in subplot using univariate pair correlation function and CSR null model.
doi:10.1371/journal.pone.0139214.g005
Legacy of Pre-Disturbance Spatial Pattern in Spruce Forests
PLOS ONE | DOI:10.1371/journal.pone.0139214 September 30, 2015 11 / 18
[57], and allelopathy and resource competition by herb layer [24,58]; thus, areas of moderate
canopy cover may have low or no spruce regeneration. A disturbance-activated positive neigh-
bourhood effect is evident with the release of residual seedlings, which improves height growth
through increased light availability.
Severe bark beetle disturbance preserved forest structural complexity
and composition
The severe bark beetle disturbance reduced vertical structure within these stands; the wind and
beetle disturbance strongly selected against overstory spruce, but the legacy of advance regener-
ation also displayed structural complexity. The heterogeneous structure imparts resistance and
resilience to these forests and decreases susceptibility to future similar disturbance types [1].
The large horizontal and vertical heterogeneity of surviving regeneration is a foundation of
high structural diversity within early-seral stands and is predicted to sustain such diversity
through several decades of early-seral stand development [29]. The importance of post-distur-
bance spatial pattern of regeneration for future development increases with decreasing density
of live residuals [5]. The structural heterogeneity evident in many primary Norway spruce for-
ests thus serves an ecological function supporting biodiversity not only before, but also after
disturbance.
The species composition of montane Norway spruce forests tends to remain unchanged
[21], regardless of prevailing disturbance typewindthrow or bark beetle outbreak [17,5960].
This differs from mixed forests in that wind or insect disturbances may be followed by spread-
ing of tree species resistant to the disturbance agent [12,30,61].
Maintenance of rowan, which contributes forage and biodiversity functions in these high-
elevation, spruce-dominated communities, is dependent on canopy-openings. Relatively few
rowan trees were recruited post-disturbance, but survival of advance regeneration rowan was
very high, and survivors exhibited higher, less variable height growth rates than spruce; this is
partly because the distribution of rowan seedling height was more unimodal compared to the
reverse-J distribution of spruce seedlings (Fig 3). Our results confirmed that renewal of rowan
is primarily dependent on the release of an advance regeneration bank [52]. The recruitment of
shade-tolerant rowan seedlings under mature canopies allows the formation of a seedling bank
[5152]. As the life span of rowan trees is shorter than the lifespan of spruce, rowan abundance
decreases in the long-term absence of canopy-open disturbance [62], and may also act as a
source of future canopy heterogeneity through death.
Pre-disturbance structure influences post-disturbance early-seral
structural development pathways
The Fig 6 illustrates how the structural complexity of an old-growth forest carries over to the
next stand, in spite of a severe canopy-removing bark beetle disturbance. Structurally simple
patches dominated by a dense overstory tend to be replaced by a young simple cohort (high
density, more evenly spatially distributed, shorter and more uniform heights). Structurally
complex patches that include gaps tend to be replaced by with a similarly complex young
cohort (lower density, larger patches of open space, greater maximum and wider variability in
heights). This difference in post-disturbance early-seral structural development pathways is
attributed to light conditions and microsite availability when the advance regeneration bank
was formed: Under densely closed canopies, there are suitable microsite conditions (sparse
herb layer) for seedling establishment but not enough light for sustainable height growth over a
certain threshold (~50 cm). Indeed, in a nearby stand with a more uniformly dense canopy,
advance regeneration density was an order of magnitude greater than in our dense patches,
Legacy of Pre-Disturbance Spatial Pattern in Spruce Forests
PLOS ONE | DOI:10.1371/journal.pone.0139214 September 30, 2015 12 / 18
Fig 6. The conceptual figure illustrates how the structural complexity of an old-growth forest carries
over to the next stand, in spite of a severe canopy-removing bark beetle disturbance. Structurally
simple patches dominated by a dense overstory tend to be replaced by a young simple cohort (high density,
more evenly spatially distributed, shorter and more uniform heights). Structurally complex patches that
include gaps tend to be replaced by with a similarly complex young cohort (lower density, larger patches of
open space, greater maximum and wider variability in heights). This difference in post-disturbance early-seral
structural development pathways is attributed to light conditions and microsite availability whenthe advance
regeneration bank was formed: Under densely closed canopies, there are suitable microsite conditions
(sparse herb layer) for seedling establishment but not enough light for sustainable height growth over a
certain threshold. In contrast, under moderate canopy cover, the establishment of new seedlings is inhibited
by a dense herb layer. Seedlings and saplings that do manage to establish in these patches are subject to
Legacy of Pre-Disturbance Spatial Pattern in Spruce Forests
PLOS ONE | DOI:10.1371/journal.pone.0139214 September 30, 2015 13 / 18
with an even lower maximum height [20]. In contrast, under moderate canopy cover, the estab-
lishment of new seedlings is inhibited by a dense herb layer. Seedlings and saplings that do
manage to establish in these patches are subject to mortality due to resource competition by
the herb layer [23,58], resulting in lower advance regeneration density, larger patches of open
space and wider height variability under moderate canopy cover.
Based on the above-described mechanism, the persistence of forest complexity within the
post-disturbance stand could also be possible in mature stands with developed old-growth
characteristics, such as a diversity of tree sizes and gaps in the forest canopy. As such, small- to
meso-scale disturbances that break up dense canopies are important to both creating and per-
petuating structural diversity across the forest sere.
Implications and Conclusions
Heterogeneous stands encourage the development of heterogeneous regeneration patterns,
thus, disturbances that minimize damage to the advance regeneration layer yields new stands
with still-heterogeneous structural patterns; this mechanism acts as a memory of pre-distur-
bance structural patterns [8]. The logical converse of this observation is that structurally simple
stands with a single dense canopy layer (e.g., managed second-growth in a stem-exclusion
stage; [9]) would yield structurally simple stands when subject to bark beetle disturbance; this
hypothesis could be verified in future research.
Increasingly, landscapes of Central Europe have become dominated by even-aged and even-
density stands. Two centuries of management designed to increase the homogeneity of spruce
forest structure have increased the susceptibility of forested landscapes to large-scale distur-
bance [1]. Forest stands with homogeneous canopies potentially diminish biodiversity by
reducing understory light gaps and microenvironments [27]. Structurally complex forests may
provide increased resistance and resilience to severe disturbances. Despite the perception that
early-seral forests related to severe disturbance are structurally simple, high-severity distur-
bances common in high-elevation Norway spruce forests can produce early-seral forests with
heterogeneous structure (sensu [26]). The wide spatial variability of regeneration associated
with pre-disturbance structures represents a crucial biological legacy, which, in turn, is related
to the previous disturbances that influenced stand structure. This memory of pre-disturbance
structural patterns may explain why spruce-free patches may persist for several decades
[23,55]. Prior canopy structure and persistence of microsite conditions (e.g. fern cover) can
provide relative stability of forest structure complexity and spatial pattern, which may persist
for several decades and even through multiple forest generations.
Supporting Information
S1 Fig. Sapling density as a function of A) canopy cover and B) Alpine-lady fern cover.
Grey areas represent the 95% confidence envelopes of the density functions. The rasters of can-
opy or Alpine-lady fern cover (colour range from light green representing the lowest cover to
violet representing the highest cover) are shown with the position of the saplings represented
by black dots.
(TIF)
S2 Fig. Relative distribution estimates of spruce sapling densities based on distance to the
nearest down log. Logs were classified according to side vegetation cover. The grey-shaded
mortality due to resource competition by the herb layer, resulting in lower advance regeneration density,
larger patches of open space and wider height variability under moderate canopy cover.
doi:10.1371/journal.pone.0139214.g006
Legacy of Pre-Disturbance Spatial Pattern in Spruce Forests
PLOS ONE | DOI:10.1371/journal.pone.0139214 September 30, 2015 14 / 18
areas indicate the envelope of uncertainty around the estimates of sapling density, as indicated
by the differentiated lines.
(TIF)
S3 Fig. Bivariate spatial pattern between survived and newly recruited spruces. The null
model was constructed using random shifting. Positions of survived and newly recruited
spruces (above 50 cm) are marked in subplot.
(TIFF)
S1 Table. List of independent and supplementary variables used in PCA.
(PDF)
S2 Table. Tree and regeneration density before and after disturbance. The number (total for
both plots) of survived undamaged, survived damaged, and dead spruce and rowan individuals
by height classes (50200 cm, >200 cm) four years after the wind and bark beetle disturbance.
Disturbance-related causes are in italics. The sign (-) means that χ2approximation of interspe-
cific differences would be incorrect because the number of expected values is too low.
(PDF)
Acknowledgments
We would like to thank Václav Pouska, Jitka Zenáhlíková, Lucie Vítková, Jan Rejzek, and Mar-
tin Starý for assistance with field data collection. We thank the Šumava National Park authori-
ties for administrative support.
Author Contributions
Conceived and designed the experiments: RB MS PJ JW. Performed the experiments: RB MS
PJ VC. Analyzed the data: RB PJ JW. Contributed reagents/materials/analysis tools: MS JW.
Wrote the paper: RB MS PJ RCM JW JLC VC DCD.
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Legacy of Pre-Disturbance Spatial Pattern in Spruce Forests
PLOS ONE | DOI:10.1371/journal.pone.0139214 September 30, 2015 18 / 18
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... Altered fire regimes pose a major challenge to the management of 'pyrodiverse' ecosystems, where high spatiotemporal variability in fire-regimes may promote biodiversitythe 'pyrodiversity begets biodiversity' hypothesis (Parr and Anderson 2006). The management of these ecosystems requires careful consideration of both the 'visible' mosaic of the most recent fire and the 'invisible' mosaic of prior fires at the landscape level; both these mosaics produce legacy effects that influence the recovery of ecosystems after fire (Bradstock et al. 2005, Bače et al. 2015, Bowd et al. 2021c. Moreover, understanding taxon-specific responses to different fire regimes is critical in facilitating ecologically-appropriate adaptive management (Lindenmayer et al. 2007, Taylor et al. 2012. ...
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Fire is one of the predominant drivers of the structural and functional dynamics of forest ecosystems. In recent years, novel fire regimes have posed a major challenge to the management of pyrodiverse forests. While previous research efforts have focused on quantifying the impacts of fire on above‐ground forest biodiversity, how microbial communities respond to fire is less understood, despite their functional significance. Here, we describe the effects of time since fire, fire frequency and their interaction on soil and leaf litter fungal and bacterial communities from the pyrodiverse, Eucalyptus pilularis forests of south‐eastern Australia. Using structural equation models, we also elucidate how fire can influence these communities both directly and indirectly through biotic‐abiotic interactions. Our results demonstrate that fire is a key driver of litter and soil bacterial and fungal communities, with effects most pronounced for soil fungal communities. Notably, recently burnt forest hosted lower abundances of symbiotic ectomycorrhizal fungi and Acidobacteria in the soil, and basidiomycetous fungi and Actinobacteriota in the litter. Compared with low fire frequencies, high fire frequency increased soil fungal plant pathogens, but reduced Actinobacteriota. The majority of fire effects on microbial communities were mediated by fire‐induced changes in litter and soil abiotic properties. For instance, recent and more frequent fire was associated with reduced soil sulphur, which led to an increase in soil fungal plant pathogens and saprotrophic fungi in these sites. Pathogenic fungi also increased in recently burnt forests that had a low fire frequency, mediated by a decline in litter carbon and an increase in soil pH in these sites. Synthesis. Our findings indicate that predicted increases in the frequency of fire may select for specific microbial communities directly and indirectly through ecological interactions, which may have functional implications for plants (increase in pathogens, decrease in symbionts), decomposition rates (declines in Actinobacteriota and Acidobacteriota) and carbon storage (decrease in ectomycorrhizal fungi). In the face of predicted shifts in wildfire regimes, which may exacerbate fire‐induced changes in microbial communities, adaptive fire‐management and monitoring is required to address the potential functional implications of fire‐altered microbial communities.
... Increasing climate extremes have also amplified the frequency and severity of disturbances (e.g., wind, fire, and insect outbreaks), which can cause extensive damage to the forest structure (Seidl et al., 2017). However, the impacts of climatic stress would have longerterm implications, because the natural recovery of forests lost under climatic conditions that are unsuitable for forest establishment would be more difficult than that after external physical disturbances, such as wind and fire (Bače et al., 2015;Everham and Brokaw, 1996;Peterson, 2000). Evaluations of the relative importance of SRIs and the geographic range where SRIs have a significant impact on vegetation establishment would improve our understanding of the responses of global forests to climatic stress. ...
Article
Climate change has the potential to cause forest range shifts at a broad scale and consequently can alter crucial forest functions, including carbon sequestration. However, global-scale projections of future forest range shifts remain challenging because our knowledge of the physiological responses of plants to climatic stress is limited to particular species and is insufficient for wide-range projections, in addition to the uncertainties in the impacts of non-climatic factors, such as wildfire, wind, and insect outbreaks. To evaluate the vulnerability and resilience of forests to climate change, we developed a new empirical approach using climatic indices reflecting physiological stressors on plants. We calculated the global distributions of seven indices based on primary climatic stressors (drought, solar radiation, and temperature) at high resolution. We then modeled the relationship between the seven indices and global forest extent. We found two key stressors driving climate-induced forest range shifts on a global scale: low temperature under high radiation and drought. At high latitudes of the Northern Hemisphere, forest establishment became difficult when the mean temperature was less than approximately 7.2 °C in the highest radiation quarter. Forest sensitivity to drought was more pronounced at mid-latitudes. In areas where the humidity index (ratio of precipitation to potential evapotranspiration) was below 0.45, shrubland and grassland became more dominant than forests. Our results also suggested that the impacts of climate change on global forest range shifts will be geographically biased depending on the areas affected by the key climatic stressors. Potential forest gain was remarkable in boreal regions due to increasing temperature. Potential forest loss was remarkable in current tropical grassland and temperate forest/grassland ecoregions due to increasing drought. Our approach using stress-reflecting indices could improve our ability to detect the roles of climatic stressors on climate-induced forest range shifts.
... The structure of disturbed landscapes, including different patterns of disturbance legacies (Franklin et al., 2000), is an important driver of forest vulnerability and resilience to future disturbances (Bace et al., 2015;Johnstone et al., 2016;Senf et al., 2019). Recovering forests are expected to be less vulnerable to future impacts because of reduced biomass levels, reduced proportion of vulnerable trees, and increased structural and compositional diversity (e.g., Adolf et al., 2020;Seidl et al., 2014). ...
Article
Present-day disturbances are transforming European forest landscapes, and their legacies determine the vulnerability and resilience of the emergent forest generation. To understand these legacy effects, we investigated the resilience of the aboveground forest biomass (Babg) to a sequence of disturbances affecting the forest in different recovery phases from the initial large-scale impact. We used the model iLand to simulate windthrow that affected 13–24% of the Babg in a Central European forest landscape. An additional wind event was simulated 20, 40, 60, or 80 years after the initial impact (i.e., sequences of two windthrows were defined). Each windthrow triggered an outbreak of bark beetles that interacted with the recovery processes. We evaluated the resistance of the Babg to and recovery after the impact. Random Forest models were used to identify factors influencing resilience. We found that Babg resistance was the lowest 20 years after the initial impact when the increased proportion of emergent wind-exposed forest edges prevailed the disturbance-dampening effect of reduced biomass levels and increased landscape heterogeneity. This forest had a remarkably high recovery rate and reached the pre-disturbance Babg within 28 years. The forest exhibited a higher resistance and a slower recovery rate in the more advanced recovery phases, reaching the pre-disturbance Babg within 60–80 years. The recovery was enhanced by higher levels of alpha and beta diversity. Under elevated air temperature, the bark beetle outbreak triggered by windthrow delayed the recovery. However, the positive effect of increased temperature on forest productivity caused the recovery rate to be higher under the warming scenario than under the reference climate. We conclude that resilience is not a static property, but its magnitude and drivers vary in time, depending on vegetation feedbacks, interactions between disturbances, and climate. Understanding these mechanisms is an essential step towards the operationalization of resilience-oriented stewardship.
... Undeniably, spatial pattern of trees is among the important structural characteristics, with the potential to influence a wide variety of forest processes (Pacala and Deutschmann, 1995;Turnbull et al., 2007;Hart and Marshall, 2009). Consequently, full understanding of forest function and dynamics will require knowledge of how major forest events and processes, such as disturbance, succession, and competition, influence spatial pattern (Bace et al., 2015;Carrer et al., 2018;Markham et al., 2019). Larson et al. (2015) provide terminology that helps to clarify discussion of this topic, distinguishing between the spatial pattern of mortality (i.e., the distribution of the trees that die, which could be aggregated, random or uniform at a range of distances or spatial scales), and the spatial outcome (i.e., the change that is revealed when comparing living pre-event or pre-interval stems to post-event or post-interval survivors). ...
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Growing appreciation for the importance of forest structural features to forest functioning has led to increased focus on tree spatial patterns. While the influence of disturbances on composition and diversity has been well-studied, understanding of how disturbances influence spatial pattern is still far from complete. In particular, very few studies have examined the change in spatial pattern as a result of wind disturbance. Here results are presented for change in spatial pattern after severe wind disturbance in four eastern North American forests. It is hypothesized that, as in other types of disturbances, severe wind will cause an increase in aggregation among survivors, and aggregated distribution of mortality. These trends were indeed observed; at very small distances (e.g., 5–10 m), all four sites showed significantly greater aggregation after wind disturbance. Mortality was also significantly aggregated at three of the four sites, but at different distances. Unexpectedly, at slightly larger distances (e.g., 30–50 m), all four sites showed significant decreases in aggregation; it is suggested that this second trend may generalize across disturbance types. A hypothesis proposed by Davis et al. (2005) is that intermediate disturbance severity produces the greatest aggregation; this hypothesis is tested in the four sites reported here, and the hypothesis is rejected. Two refinements to this hypothesis are proposed for further testing in future research.
Article
Acute or chronic drought stress caused by climate change can contribute to the weakening of forest ecosystems and lead to extensive bark beetle infestations. Siberian spruce (Picea obovata Ledeb.) forests of the Dvinsko-Pinegskiy, a natural reserve in the Arkhangelsk region, Russia, have been subject to unprecedented tree cover loss caused by the Eurasian spruce bark beetle (Ips typographus L.) in the last two decades. This is the first recorded case of such an extensive outbreak of Ips typographus occurring at higher latitudes. We used remote sensing and climate data to model and compute annual tree-loss change due to natural factors, with a focus on bark beetle outbreaks, over a 14-year period (2001-2014). Using linear regression models, we found a combination of average annual temperature and precipitation, temperature and precipitation in June, to be the most important drivers of annual tree-loss.
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Large, high‐severity wildfires are an important component of disturbance regimes around the world and can influence the structure and function of forest ecosystems. Climatic changes and anthropogenic disturbances have altered global disturbance patterns and increased the frequency of high‐severity wildfires worldwide. While the recovery of plant communities at different successional stages after fire is well known, the influence of prior disturbances and stand age is poorly understood. Despite this, high‐intensity wildfires can produce long‐lasting legacy effects, which can influence the resistance and resilience of ecosystems. Here, we quantified the influence of prior stand age and disturbance history on the recovery of plant communities in the Mountain Ash and Alpine Ash forests of south‐eastern Australia after high‐severity wildfire. Specifically, controlling for stand age, we compared the abundance (percent cover) of different plant life forms and reproductive strategies in forests that were, at the time of high‐severity wildfire in 2009, “young” (28–35 yr old and previously logged), “mixed” age (26, 70–83, >150 yr old), “mature” (70–83 yr old), and “old‐growth” (>150 yr old). We uncovered evidence that the legacy of prior disturbance and stand age at the time of high‐severity wildfire can influence the recovery of plant communities in early successional forests. Specifically, we found that “young” forests burnt in 2009 had a higher abundance of ruderal and graminoid species, but had a lower abundance of persistent, onsite seeders, including Acacia and eucalypt species, relative to “old‐growth” forests burnt in 2009. “Mature” aged forests burnt in 2009 also had a lower abundance of Acacia, eucalypt, and shrub species, relative to “old‐growth forests” burnt in 2009. Our findings provide evidence of advanced recovery in forests that were older when burnt by high‐severity wildfire, relative to younger forests burnt by the same wildfire. Further, we also demonstrate the influence of different environmental conditions on plant communities. In a period of rapid, global, environmental change, our study provides insights into the recovery of plant communities post‐wildfire with implications for forest management. Further, our findings suggest that predicted increases in the frequency of high‐severity wildfires may have consequences for forest regeneration.
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The tree natural regeneration and seed bank that survives a large-scale stand-replacing disturbance fully determines the future forest structure until new generation trees reach seed maturity and the supply of seeds is restored. We asked the following questions: (i) Does the advance regeneration stage prevail in the stand? (ii) Does the tree species composition changed after disturbance? (iii) Does the regeneration height structure change depend on elevation and characteristics of the previous stand? Mean seedlings (<130 cm) density was 4000 individuals per hectare 5 years after the disturbance; rowan covered 2% of the regeneration. The density of saplings (≥130 cm) was 252 individuals per hectare, of which 19% were rowan individuals. The advance regeneration consisted of 88% of all individuals. Microsites mostly preferred were soil covered by mosses (32% of individuals), the tree base of living mature trees and snags (28% of individuals), and dead wood (27% of individuals). The density of saplings significantly decreased with increasing elevation; for seedlings, this trend was not significant. Even an elevation gradient of 100 m can have a major impact on the growth rate of the regeneration. Although the variability of stand characteristics was high, no significant effect was recorded for any of the height categories of spruce and rowan with regard to the density of the regenerated stand. Our results provide the example of stands, where structure of regeneration after the disturbance is not explained by previous stand characteristic. Lower saplings density in higher elevation is probably caused by limiting growth conditions such a cold. High heterogeneity of regeneration structure must depend on many other factors.
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Extensive outbreaks of the mountain pine beetle ( Dendroctonus ponderosae Hopkins) will alter the structure of many stands that will likely be attacked again before experiencing a stand-replacing fire. We examined a stand of lodgepole pine ( Pinus contorta var. latifolia Engelm. ex S. Watson) in Grand Teton National Park currently experiencing a moderate-level outbreak and previously attacked by mountain pine beetle in the 1960s. Consistent with published studies, tree diameter was the main predictor of beetle attack on a given tree, large trees were preferentially attacked, and tree vigor, age, and cone production were unimportant variables for beetle attack at epidemic levels. Small trees killed in the stand were killed based mainly on their proximity to large trees and were likely spatially aggregated with large trees as a result of the previous outbreak. We concluded that the driving factors of beetle attack and their spatial patterns are consistent across outbreak severities but that stand structure altered by the previous outbreak had implications for the current outbreaks in the same location. This study should catalyze additional research that examines how beetle-altered stand structure affects future outbreaks — an important priority for predicting their impacts under climate change scenarios that project increases in outbreak frequency and extent.
Article
The study concerns the spatial relationships between rowan seedlings and variability of field-layer vegetation, density of spruce stand and distribution of adult rowan trees. A network of 0.05 ha circular plots spaced 40x40 m was established in the central part of a large, 27 ha rectangular tract in a subalpine spruce forest in the massif of Babia Góra (Polish West Carpathians). On each circular plot rowan seedlings were counted, the dbh of all spruces was measured, and the percentage of area covered with field-layer species was assessed. All adult rowans were measured and mapped over 27 ha and then their distances to 0.05 ha plots were measured. The recruitment of rowan seedlings starts before the destruction of the spruce stand and a seedling bank is formed. After gap creation rowan regeneration is hampered. Exuberant growth of the field-layer plants, especially the fern Athyrium distentifolium, seems to be the most important factor limiting rowan regeneration under sparse spruce canopy and in gaps. The regeneration is not enhanced under conspecific adults growing in old gaps. It is still limited by exuberant field-layer vegetation, mainly A. distentifolium, that exists under rowan trees. The size of seedling bank is related to the distance to fruiting trees. Beyond 60 m from the nearest fruiting tree the number of young rowans drops distinctly. The lack of abundant seedling bank at longer distances from mature trees suggests that the development of dense rowan thickets is possible only in gaps situated in the neighbourhood of fruiting individuals up to several dozen meters from them.
Chapter
This chapter first considers the influence of the understory filter on dispersal, germination, and survival of tree seeds and seedlings individually. It then explains the effects of the understory filter on tree seedling community attributes such as tree seedling distribution, species composition, and diversity; the influence of the understory filter on seedling growth; and the role of the understory in determining the size structure of tree seedling communities. The filtering activity of the understory is complex not only because of its activity at multiple life stages of a tree, but also because the understory itself is a complex and changing mosaic of understory plants. The chapter concludes by discussing the spatial and temporal distribution of understory plants and how the mosaic nature of the understory affects the complexity of the understory filter.
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Background: In forests subject to stand-replacing disturbances, conventional models of succession typically overlook early-seral stages as a simple re-organization/establishment period. These models treat structural development in essentially 'relay floristic' terms, with structural complexity (three-dimensional heterogeneity) developing primarily in old-growth stages, only after a closed-canopy 'self-thinning' phase and subsequent canopy gap formation. However, is it possible that early-successional forests can sometimes exhibit spatial complexity similar to that in old-growth forests - i.e. akin to an 'initial floristic' model of structural development? Hypothesis: Based on empirical observations, we present a hypothesis regarding an important alternative pathway in which protracted or sparse forest establishment and interspecific competition thin out tree densities early on - thereby precluding overstorey canopy closure or a traditionally defined self-thinning phase. Although historically viewed as an impediment to stand development, we suggest this process may actually advance certain forms of structural complexity. These young stands can exhibit qualities typically attributed only to old forests, including: (1) canopy gaps associated with clumped and widely spaced tree stems; (2) vertically heterogeneous canopies including under- and mid-stories, albeit lower stature; (3) co-existence of shade-tolerant and intolerant species; and (4) abundant dead wood. Moreover, some of these qualities may persist through succession, meaning that a significant portion of eventual old-growth spatial pattern may already be determined in this early stage. Implications: The relative frequency of this open-canopy pathway, and the degree to which precocious complexity supports functional complexity analogous to that of old forests, are largely unknown due to the paucity of naturally regenerating forests in many regions. Nevertheless, recognition of this potential is important for the understanding and management of early-successional forests.
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In the statistical analysis of spatial point patterns, it is often important to investigate whether the point pattern depends on spatial covariates. This paper describes nonparametric (kernel and local likelihood) methods for estimating the effect of spatial covariates on the point process intensity. Variance estimates and confidence intervals are provided in the case of a Poisson point process. Techniques are demonstrated with simulated examples and with applications to exploration geology and forest ecology.