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AGROFOR International Journal, Vol. 7, Issue No. 1, 2022
24
Original Scientific paper
10.7251/AGRENG2201024M
UDC 630:582.632
LONG-TERM EFFECTS OF OAK DECLINE ON SHRUB
INDIVIDUAL’S OCCURRENCES IN AN HUNGARIAN OAK
FOREST
Tamás MISIK*, Imre KÁRÁSZ
Department of Environmental Sciences and Landscape Ecology, Eszterházy Károly
University, Hungary
*Corresponding author: misiktom@gmail.com
ABSTRACT
Information about the occurrence of shrubby individual’s relation with oak decline
is fundamental to developing knowledge from forest stand. This paper focuses on
the following questions: (1) how are the shrub’s occurrences changed after oak
decline? (2) Which shrubs have the highest occurring in the subplots? The mixed
oak stand is located in the Bükk Mountains of Hungary. The monitoring plot (48 m
× 48 m) was subdivided into 144 permanent subplots; the measured parameter was
observed in the period 1972-2017. The shrub layer was divided into low (< 1.0 m
in height) and high (≥ 1.0 m) layer. High shrub specimens were randomly
distributed and had become more homogeneous by 2017; there was no subplot with
10 or more specimens in the last decade. In the subplots the policormon forming
shrubs were present with a higher occurrence. Correlation analysis showed that
occurrence of Acer tataricum,Cornus mas and Euonymus verrucosus in the high
and Cornus sanguinea and Ligustrum vulgare in the low shrub layer changed
significantly after the oak decline. High shrubs with the highest occurrence were E.
verrucosus and C. mas. The most occurrent low shrub species were E. verrucosus
and L. vulgare. Our results suggest that after the oak decline the most shrubs’
occurrence decreased considerably and the distribution was more homogeneous.
Keywords: Shrub community, oak decline, occurrence, subplots.
INTRODUCTION
Oak decline has been described as a widespread and complex phenomenon in many
countries (Tomiczek, 1993; Sonesson and Drobyshev, 2010). An increase in the
death of oak trees has been observed in many regions of Hungary since 1978
(Igmándy, 1987). In the Síkfőkút forest stand species composition of the canopy
was stable until 1979 and the healthy Quercus petraea Matt. L. (sessile oak) and
Quercus cerris L. (Turkey oak) also remained constant. Oak decline was first
reported in 1979–80 and by 2017, 62.9% of the oaks had died.
Relatively few studies deal with shrub layer dynamics after oak death and the
possible relation between trees and shrubs (Légaré et al., 2002). Understory and
AGROFOR International Journal, Vol. 7, Issue No. 1, 2022
25
overstory relationships are complex and mutual but are dominated by the canopy
structure and condition (Burrascano et al., 2011; Burton et al., 2011; Cutini et al.,
2015). Shrub layers of forest ecosystems change dynamically and respond
sensitively to the environmental changes (Chipman and Johnson, 2002; Rees and
Juday, 2002). They are strongly related to the composition and structure of the
overstory (Klinka et al., 1996; Palik and Engstrom, 1999). Shrub species play a
major role in the cycles of some essential nutrients, including the dynamics of
nitrogen, potassium and carbon (Gilliam, 2007). The shrub layers are directly
contributes to forest biodiversity (Kerns and Ohmann, 2004; Čermá et al., 2009),
including compositional and structural diversity, enhancing the aesthetics of forest
ecosystems and helping to protect watersheds from erosion (Alaback and Herman,
1988; Halpern and Spies, 1995; Muir et al., 2002). The distribution of shrubs is
strongly influenced by environmental conditions, such as climate (Pedley, 1979;
Westman, 1991; Kienast et al., 1998). Chemical and physical soil properties and
biotic interactions play a major role in influence the distribution of shrub species
(Pedley, 1979). Importance of shrub patch characteristics against other abiotic
factors driving the occurrence of shrub species is also poorly studied (Gavilán et
al., 2002).
Misik et al. (2013) described the possible responses of understory shrub layer’s
cover, basal area and diversity to the remarkable changes in stand density. Misik et
al. (2014) reported the dynamics behind the increase in the sizes of woody species
and the structure of the new subcanopy layer below the canopy. This paper focuses
on the following questions: (1) how are the shrub’s occurrences changed after oak
decline? (2) Which shrubs have the highest occurring in the subplots? (3) Finally,
is a strong relation between occurrences and densities of shrub species?
MATERIAL AND METHODS
Study area The reserve research site (Síkfõkút Project) was established in 1972 by
Jakucs (1985) and is located in the Bükk Mountains (47°552 N, 20°462 E) in the
north-eastern part of Hungary at an altitude of 320-340 m a.s.l. (Figure 1A). Mean
annual temperature is 9.9 °C and mean annual precipitation typically ranges from
500 to 600 mm. Descriptions of the geographic, climatic, soil conditions and
vegetation of the forest were reported in detail by Jakucs (1985, 1988). The
common forest association in this region is Quercetum petraeae-cerridis (Soó,
1963) (sessile oak-Turkey oak) with a dominant canopy of Q. petraea and Q.
cerris; the long-term dynamics of understory shrub layer dynamics are described
among others in works of Misik et al. (2013, 2014, 2017, 2020). The plots under
study were made up of evenly aged temperate, mixed species deciduous forest that
was at least 110 years old and had not been harvested for more than 55 years.
AGROFOR International Journal, Vol. 7, Issue No. 1, 2022
26
Figure 1.A. Location of the study area in Hungary. B. Study site location with
plots.
Sampling and data analysis The structural condition of the shrub layer was
monitored on an "A" plot at the research site, 48 m × 48 m in size; the plot was
subdivided into 144 4 m × 4 m permanent subplots (Figure 1B).
The subplots were established in 1972; the understory occurrences data collected at
subplots measured in the period of 1972-2017 vegetation season on site. Repeated
shrub inventories took place in every 4-5 years period. The shrub layer was divided
into low (< 1.0 m in height) and high (≥ 1.0 m) layer. Shrub specimens of the
vegetation lower than 1.0 m in height were categorized as low understory; higher
specimens were categorized as high understory. The occurrence of specimens of
each shrub species (occurrence % in subplots of the monitoring plot) was
determined as a number expressing in what percentage of the 144 subplots they
occurred. Presented frequency values in this paper represent the occurrence of the
species (in percentage) in the shrub layer.
Linear regression was used to analyse the relationship between shrub densities and
shrub occurrences. The experimental data were analysed by correlation analysis to
investigate the possible effects by the occurrences of shrubs on oak tree density
(SPSS Statistics 19, Tulsa, USA). Statistical analysis was performed using the
PAST statistical software and significant differences for all statistical tests were
evaluated at the level of *P <0.05; **P≤ 0.01; ***P≤ 0.001. There was no significant
correlation found between the test variables at n.s.P ≥ 0.05.
RESULTS AND DISCUSSION
Most species were less occurring in 2017 compared with 1972. The occurrence of
shrub species change between 0.7 - 98.6% in the period of 1972-2017. Mean values
of occurrence changed between 22.4% and 57.6% in the high and between 42.0%
A
A)
B
AGROFOR International Journal, Vol. 7, Issue No. 1, 2022
27
and 94.6% in the low shrub layer. High shrubs with the highest mean occurrence
were Acer campestre L. (field maple) (57.6%) and Cornus mas L. (European
cornel) (52.1%) in the last decades. The most occurring low shrub species with the
highest mean values were Euonymus verrucosus Scop. (spindle tree) (94.6%) and
Ligustrum vulgare L. (wild privet) (70.8%). The distribution of Q. petraea
seedlings and saplings occurrence fluctuated between 16.7% and 81.9% on the 144
subplots in the past 45 years. The most frequent species of the shrub community
was E. verrucosus; followed them Q. petraea and A. campestre (Table 1).
Table 1. Frequency of occurrence (%) of most common species in shrubs between
1972 and 2017.
species
high shrub layer
1972
1982
1988
1993
2002
2007
2012
2017
A. campestre
61.8
66.7
58.3
59.0
56.9
52.8
52.1
50.0
A. tataricum
31.9
27.8
26.4
22.2
24.3
19.4
14.6
12.5
C. mas
70.8
57.6
59.7
47.9
46.5
46.5
43.8
40.3
E. verrucosus
0.7
36.1
50.7
39.6
61.8
54.2
47.2
54.9
species
low shrub layer
1972
1982
1988
1993
2002
2007
2012
2017
C. sanguinea
93.1
67.4
72.2
25.7
13.9
37.5
18.1
16.0
E. verrucosus
95.1
94.4
96.5
89.6
91.7
98.6
91.7
95.8
L. vulgare
97.9
91.0
88.9
74.3
52.1
63.9
43.1
45.8
Q. petraea
98.6
16.7
63.2
19.4
41.0
41.7
81.9
78.5
Correlation analysis showed a significantly positive relationship between oak tree
density and the occurrence frequency of Acer tataricum L. (Tatar maple) (r =
0.82*), C. mas (r = 0.84**) and E. verrucosus (r = 0.91**) high shrub species and
the Cornus sanguinea L. (common dogwood) (r = 0.84*) and L. vulgare (r = 0.73*)
low shrub species in the subplots. There was a statistically non-significant
interaction (p > 0.05) between occurrence values on A. campestre high and E.
verrucosus, Q. petraea low shrub species and long-term trend of the canopy oaks
density (Table 2).
AGROFOR International Journal, Vol. 7, Issue No. 1, 2022
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Table 2. Long-term relationship between canopy oak trees density and occurrences
of dominant low and high shrub species on the Síkfőkút mixed oak forest (N=81).
high shrub
oak trees density ind. ha-1
correlation
layer
species
1972
1982
1988
1993
1997
2002
2007
2012
2017
p
F
A.
campestre
816
651
408
372
304
324
323
305
303
0.057n.s.
5.50
A. tataricum
0.012*
12.41
C. mas
0.009**
14.17
E.
verrucosus
0.002**
27.01
low shrub
oak trees density ind. ha-1
correlation
layer
species
p
F
C.
sanguinea
816
651
408
372
304
324
323
305
303
0.018*
12.16
E.
verrucosus
0.930n.s.
0.008
L. vulgare
0.039*
6.91
Q. petraea
0.244n.s.
1.74
Regression analysis resulted a highly significant relation (p< 0.001) between
density and frequency of occurrence by A. tataricum and C. mas high and C.
sanguinea and L. vulgare low shrub species. This association is lower (p< 0.05) by
A. campestre and E. verrucosus high shrub species. This relationship was not
significant (p≥ 0.05) in the cases of E. verrucosus and Q. petraea in the low shrub
layer (Table 3).
The effect of competition between trees was examined by Szwagrzyk (1990) in
terms of the spatial distribution of tree individuals. It was found that competition
did not affect the distribution of trees. Skov (2000) investigated the effect of
neighbouring individuals on tree distribution in forest communities. In his research,
he found that the main influencing factor of the distribution is the size and density
of open areas, open lanes along roads and the species diversity of neighbouring
stocks. Maestre and Cortina (2005) paper resulted than relative importance of shrub
size, species identity and abiotic factors as determinants of shrub species
occurrence.
AGROFOR International Journal, Vol. 7, Issue No. 1, 2022
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Table 3. Long-term relationship between densities and occurrences of dominant
shrub species on the Síkfőkút mixed oak forest (N=144) (SD=standard deviation).
high layer
min. density
ind. ha-1
max.
density
ind. ha-1
mean density
ind. ha-1±SD
linear regression
shrub species
r
p
t
A. campestre
543
1905
932±360
0.77
0.016*
3.16
A. tataricum
87
430
226±93
0.94
0.15-3***
7.40
C. mas
373
2335
857±453
0.95
8.89-5***
8.03
E. verrucosus
4
1263
663±314
0.87
0.002**
4.73
low layer
min. density
ind. ha-1
max.
density
ind. ha-1
mean density
ind. ha-1±SD
linear regression
shrub species
r
p
t
C. sanguinea
655
13673
3762±3561
0.92
4.38-4***
6.22
E. verrucosus
8098
22967
13471±2761
0.46
0.218n.s.
1.35
L. vulgare
1432
21059
8124±5353
0.93
2.62-4***
6.76
Q. petraea
417
47354
9791±10710
-0.18
0.651n.s.
-0.47
CONCLUSIONS
The consequences of serious oak decline cause notable changes in the light and
stand thermal conditions of forest community which led to structural changes of the
shrub layer (Chapman et al., 2006). Our results confirm that the decreasing canopy
density led to the occurrence condition changes of the shrubs. The most occurring
low shrub species were E. verrucosus and Q. petraea; the highest frequency of
occurrence values have got A. campestre and E. verrucosus shrub species in the
high shrub layer. The mean occurring values changed between 22.37% and 94.60%
in the shrub community on the basis of the 45-years long dataset. Our results from
1972 suggest that some dominant shrub species in the understory responded very
differently and counter to the oak decline; the occurrence of A. tataricum and C.
mas high shrubs decreased considerably after the oak decline. In parallel in this
layer the occurrence of E. verrucosus increased many times in a short time. We
found highly significant interaction between occurrence of C. mas and E.
verrucosus high shrubs and oak trees density. This relationship is lower or non-
significantly by other shrub species. Our results suggest that were a highly
significant impact of shrub density on shrub occurrence by A. tataricum,C. mas
high and C. sanguinea,L. vulgare low shrub species. A better understanding of
forest structure and shrub species occurrence in temperate ecosystems may provide
useful further information on the shrub layer dynamics of the community.
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