Sessile oak (
Modern techniques of forest regeneration in Central Europe aim at an optimal utilization of natural regeneration, both for economic reasons and for enhancing forest stand stability and species diversity. In the lower elevation areas of the Czech Republic (hereafter CR), sessile oak (
There are various silvicultural systems prescribing how to manage stands using vegetative or generative regeneration or both (
Knowledge of the potential and limits of oak generative regeneration in coppice-with-standards stands is essential for their maintenance and stability. However, the success of natural regeneration is conditioned by a number of biotic and abiotic factors (
Oak reaches fructification at the age of 40-50 years in open stands and 70-80 years in closed stands. Fruit production in open stands occurs almost every year, while in closed stands every 4-8 years (mast years). The mature stand production is 0.7-2.0 t acorns ha-1, a well-developed free-growing oak produces 40-100 kg acorns year-1. Mast years with production exceeding 50 acorns m-2 are rare, mainly due to late frost and insect pests (predominantly
Successful natural regeneration of oak stands is suppressed by ruminant ungulates, such as roe deer (
Light availability is another major limiting factor affecting the survival, growth and competitive ability of oak regeneration (particularly with respect to shade-tolerant species, such as beech, hornbeam or lime). Sessile oak is a shade-tolerant species in the earlier life-cycle stages, being able to survive at only 15% of the full solar radiation. However, at least 20% of relative radiation is required for its sustainable growth, with an optimum relative light intensity of 20-40% and 25-50% for one-year-old and two-year-old saplings, respectively (
The natural regeneration of oak stands is therefore affected by many factors. Tree density is related to seed production and distribution, and high density strongly limits sapling growth by reducing light intensity and water availability. Moreover, the success of regeneration is influenced by seed consumption and shoots browsing caused by mammals. The aim of the study was to evaluate the impact of wild boar, roe deer and rodents on acorn abundance and on the growth of oak regeneration under different light conditions in an oak over-aged coppice stand.
The study area was located in the southern part of the Czech Republic near the town of Moravský Krumlov (49° 2′ 42.849″ N, 16° 21′ 13.071″ E -
From a management perspective, the investigated oak stands are over-aged coppices and coppices-with-standards located in an area where the traditional coppice-with-standards was converted in 1955 to high forests with long rotation. Sessile oak was the dominant tree species. Afterward, a reverse conversion to coppice-with-standards was initiated 20 years ago, with the goal of enhancing biodiversity and the economic value of harvested standards.
Three oak-dominated forest stands with similar site conditions were chosen, representing loamy and fresh beech-oak forests (
Using the Field-Map system, a linear transect of length 150 m was established in the central, homogeneous part of each stand, far from neighboring stands, roads, etc. Each transect was then divided into 5-meter sections using pegs, obtaining 31 sampling points per transect (overall, 93 sampling points). The following stand characteristics were evaluated within each transect (see below): (i) stand structure and light conditions; (ii) acorn production and consumption; (iii) inventory of a natural regeneration. The experimental design adopted is skecthed in
Mammals which may potentially affect natural oak regeneration in the study area include wild boar (
All trees exceeding 50 mm diameter at breast height (1.3 m above the ground) were localized within 25 m at each side of the transect (including the endpoints), covering a total area 9463 m2 in each stand. We determined the species of each tree and measured its height and diameter at breast height. Hemispherical photographs were also taken (1.5 m above the ground, weather: overcast) at each sampling point along the transect (93 photographs in total) using a digital Nikon Coolpix 4500 camera with FC-E8 fisheye convertor (camera settings: Shooting-Mode: P; Programmed: Auto; Focus Mode: infinity; image format: TIFF). The indirect site factor ISF (
A circular plot of 1 m2 was established centered at each sampling point along the transects, and the species, density and height class of each sapling within the plot were determined. Sample oak saplings were also characterized by the height increments (cm) of the two preceding years (mean of both values), height (cm) and age (number of internodes). Based on the relationship between height and age (
To monitor seed fall and its consumption, three plots were placed in close proximity at each sampling point along the transect, each covering 0.25 m2 (15 sets in each stand, 45 in total). The first plot (seed trap) was used to determine the total seed fall, and consisted of a metal stand and a cloth-made seed trap which prevented animals from consuming the acorns. The second plot (fenced) was surrounded by a 1 m-high wire mesh with 10×10 cm openings which enabled unlimited access to small mammals, but prevented ungulates from consuming the acorns. The third plot (control) was only marked out and the acorns within it were freely accessible to animals. The consumption by small mammals was calculated as the difference between the total seed fall in the seed trap and the number of acorns left in the fenced plot. The consuption by ungulates was assessed based on the acorns left in the open plot. We checked the plots at one week intervals throughout the seed fall period (a total of 8 samplings from 10th September 2013 to 9th November 2013).
The intensity of acorn consumption during the seed fall determined from 15 sets of plots was used to estimate the total stand production. The counting of acorns was conducted after the seed fall (at the time of the last check of the sample plots) at 100 randomly selected plots covering 0.25 m2 in each stand,
To compare acorn production, acorn consumption and the density of oak saplings in the individual stands, one-way ANOVA (analysis of variance) and Tukey’s
A total of 722 trees were surveyed in the studied stands. At the time of establishing the study areas, the monitored stands were almost exclusively composed of oaks, except 1 individual of hornbeam and 1 individual of lime (
Oak dominated the composition of natural regeneration in all studied stands (
The relationship of the relative diffuse radiation with density (R2 = 0.14) and height increment (R2 = 0.58) of oak saplings was verified by a regression analysis (
Taking into consideration the acorn consumption in individual stands, we assessed a production of 731 000 (stand A), 350 000 (stand B) and 108 000 (stand C) acorns per hectare, with significant differences among stands (F[51, 180] = 2.1712, p < 0.001 -
Acorns represented a sought-after source of food for animals in the study area. By the end of the seed fall, approximately half of the fallen acorns had been consumed, on average. The highest proportion of consumed acorns was observed in stand C (67%), where the lowest seed fall was also registered. As for stand A, 44% acorns were consumed, while the lowest acorn consumption intensity was recorded in stand C (only 13% -
Acorn production and predation is the key factor influencing the process of oak natural regeneration. Our study demonstrated that acorn production in over-aged coppice stands (stand A: 73 seeds m-2; B: 35 seeds m-2; C: 11 seeds m-2) can be comparable to that of generative forests, where mast years with acorn production exceeding 50 seeds m-2 are rare (
Wild boar is the main ungulate species in our study area. The density of roe deer was low, as reported by local forest managers. Wild boar, which is considered the major acorn consumer in other parts of the world, is not present in Great Britain (
Consumption of acorns by large and small mammals differed according to the canopy openness. The dense stand A with a low shrub layer showed the highest impact of small mammals, while the stand C with a low density of mature trees and a rich undergrowth in the higher height/age classes (exceeding 50 cm) showed the highest impact by large animals. We assume that a shelter extends the duration of animal activities (
With regards to rodents, no increasing or decreasing trend with respect to the regeneration density or the stand characteristics was found in this study. However, acorn consumption was the highest in the stand A, characterized by a dense regeneration up to 20 cm. Rodents were responsible for a decrease of 292 000 acorns per hectare in the studied stand (40%). Nonetheless, the absence of acorns due to rodent activities can not be considered a direct consumption, because rodents build underground food storage (
With respect to the estimated total regeneration density (66 000-310 000 saplings ha-1) the observed quantity of acorns in spring (4600-22 000 acorns ha-1) is more than sufficient for the purposes of forest management. The high density of natural regeneration found in this study was probably due to the accumulation of saplings from previous mast years. The density of oak regeneration decreased with increasing canopy openness, while both height and age variability of saplings increased. This is probably due to intraspecific competition (autoreduction) of oak saplings and to different growth dynamics at different age. Nevertheless, oak survived (in our case up to 9 years with a height up to 50- 80 cm) even under full canopy (about 12% ISF). Generally, oak is able to survive several years at 15% relative radiation and oak sapling decrease in stocking of 0.8-0.9 after 6-7 years. We even recorded the occasional occurrence of oak saplings under extremely low radiation (as low as 5% ISF), according to several studies which describe oak as a shade-tolerant species in its early years (
We estimated also the impact of shoot browsing by roe deer. In the studied area the intensity of damaged shoots was less than 1% and had no effect on oak regeneration. Indeed, in this area characterized by mild winter and rich food supply, the roe deer prefer other food. Other biotic and abiotic factors, not only light and stand structure, influence oak saplings emergence and growth,
Our results proved that over-aged coppice stands are characterized by abundant fructification and successful generative regeneration under different light conditions and despite the strong animal impact. Indeed, more than half of acorns are consumed by the end of acorn fall and less than 10% of the total remain until spring. Nevertheless, a high stocks of acorns and saplings remained in the stands. Based on the number of different height/age classes of saplings observed in this study, the regeneration capacity of the studied stands was not threatened in the past years. Although the saplings may survive under unfavorable light conditions for several years, a basal areas of less than 16 m2 ha-1 of the parent stands is required to achieve a sustainable height growth. Therefore, generative natural oak regeneration in over-aged coppice stands can be relied upon in the conversion into a coppice-with-standards, which was carried out 20 years in the studied forest stands. The results obtained provide complementary information on the widely discussed issue of the optimal number of reserved trees and optimal basal area in a stand (in our case, max. 200 reserved trees ha-1 and 16 m2 ha-1, respectively) to secure a successful natural generative regeneration under the optimum relative diffuse radiation (min. 20% ISF).
This study was supported by the Ministry of Education, Youth and Sports of the Czech Republic, project No. CZ.1.07/2.3.00/ 20.0267 and by the Grant Agency of MENDELU, project No. 80/2013.
Location of the study area with indication of the three study stands.
Schematic representation of the experimental design adopted in this study.
Relationship between height and age of oak saplings in the three studied stands and the derived height/age classes (1-4) for saplings.
Relation between relative diffuse radiation (ISF) and oak regeneration density in the three investigated stands (A, B and C).
Relationship between relative diffuse radiation (ISF) and oak height increment in the three investigated stands (A, B and C).
Relationship between the stand basal area and the relative diffuse radiation (ISF) in the investigated oak stands (A. B. and C).
Characteristics of studied forest stands taken from the management plan. (QP): Sessile oak,
Stand | Area(ha) | Age(years) | Species composition | Standing volume (m3 ha-1) | Stand density(%) |
---|---|---|---|---|---|
A | 14.05 | 85 | QP 97%; CB 2%; TC 1% | 270 | 100 |
B | 9.91 | 101 | QP 95%; PS 3%; LD 1%; CB 1% | 145 | 50 |
C | 9.91 | 101 | QP 95%; PS 3%; LD 1%; CB 1% | 93 | 30 |
Altitude, stand composition, diameter at breast height (DBH), basal area and indirect site factor (ISF) of the transects. (QP):
Stand | Altitude(m a.s.l.) | Speciescomposition (%) | Trees(n ha-1) | DBH (cm)mean ± SD | Basal area(m2 ha-1) | ISF (%)mean ± SD (median) |
---|---|---|---|---|---|---|
A | 340-355 | QP 100 (PS+) | 401 | 22.4 ± 7.5 | 19.1 | 12.1 ± 2.0(11.7) |
B | 345-365 | QP 100 | 243 | 26.2 ± 6.0 | 14.1 | 25.5 ± 10.9(22.7) |
C | 370-380 | QP 100 (CB, TC+) | 120 | 30.8 ± 10.1 | 9.3 | 35.3 ± 14.3(31.8) |
Characteristics of the height/age classes (1-4) of oak saplings. The age of saplings in each height class (Expected Age) was derived from the relationship shown in
Height class(cm) | Expected Age(year) | Height/age class |
---|---|---|
≤ 20 | 1-5 | 1 |
20.1-50 | 6-9 | 2 |
50.1-80 | 6-9 | 2 |
80.1-130 | 10-11 | 3 |
130.1-200 | 10-11 | 3 |
200.1-300 | 12-13 | 4 |
300.1-400 | 12-13 | 4 |
Average density of the natural regeneration in the studied stands, in total and partitioned into height/age classes (1-4). Values in brackets are 95% confidence intervals. (QP):
Stand | Density - total(n ha-1) | SpeciesComposition(%) | Density - totaloak (n ha-1) | Decrease ofdensity oak % | Oak Density (n ha-1) | |||
---|---|---|---|---|---|---|---|---|
Class 1 | Class 2 | Class 3 | Class 4 | |||||
A | 310000.0(101136.3) | QP 100% | 310000.0(101136.3) | 0 | 298333.3(97688.3) | 11666.7(8091.3) | 0.0 | 0.0 |
B | 112758.6 (29880.2) | QP 99%, CB 1% | 111379.3(30118.4) | 64 | 86551.7(31926.6) | 16206.9(7360.0) | 8620.7(6326.2) | 344.8(706.4) |
C | 66129.0 (29959.5) | QP 100% | 66129.0(29959.5) | 79 | 14516.1(5902.3) | 27096.8(14462.6) | 24516.1(17612.6) | 1935.5(1473.1) |
Acorn production in the monitored stands and intensity of acorn consumption by small mammals and ungulates during the seed fall.
Stand | A | B | C |
---|---|---|---|
Production (thousands of acorns ha-1) | 731 | 350 | 108 |
Consumption during the seed fall (%) | 44 | 13 | 67 |
Small mammals (%) | 40 | 7 | 18 |
Ungulates (%) | 4 | 6 | 49 |
Remnants in spring (thousands of acorns ha-1) | 22 | 29 | 4.6 |