Competition and natural mortality in two mixed sessile oak (Quercus petraea (Matt.) Liebl.) dominated stands.
Sessile oak (Quercuspetraea (Matt.) Liebl.) is the dominant Quercus species in Romania, covering over 700,000 ha and inhabiting mostly the hills and lower mountain ranges, with maximum altitudes of (600) 700-800 (900) m (Nicolescu, 2010). Under such conditions, sessile oak grows either in pure or mixed stands along with other broadleaved tree species such as European beech (Fagus sylvatica L.), hornbeam (Carpinus betulus L.), sycamore (Acer pseudoplatanus L.), common ash (Fraxinus excelsior L.), linden (Tilia sp.) or other Quercus species such as pedunculate oak (Quercus robur L.), Hungarian oak (Quercus frainetto L.) or Turkey oak (Quercus cerris L.) (Haralamb, 1967; Stanescu et al., 1997; Sofletea and Curtu, 2007).
This rich mix of tree species consists of "genuine" light-demanding (Quercus spp.), moderate light-demanding or moderate shade-tolerant (e.g., common ash, sycamore, linden, hornbeam) and "genuine" shade-tolerant (European beech) tree species (Dracea, 1923; Poskin, 1926; Negulescu and Savulescu, 1957; 1965; Sevrin, 1997; Stanescu et al., 1997; Joyce et al., 1998; Balleux, 2005; Sofletea and Curtu, 2007; Lemaire, 2010). Such situation can create problems especially in young stands where moderate or "genuine " shade-tolerant, competitive, even very "invading" tree species such as hornbeam (Negulescu and Savulescu, 1957; 1965; Stanescu, 1979; Stanescu et al., 1979; Lanier, 1986; Bary-Lenger et al., 1988) or European beech (Sardin, 2008) can dominate and eliminate sessile oak trees.
In this context and taking into account the need for preserving high proportions (minimum 60-70%) of sessile oak trees in Romanian mixed stands, the paper aims at characterising the dynamics of the process of natural mortality in two sessile oak-dominated stands including tree species with different light requirements (either hornbeam as moderate shade-tolerant or Hungarian oak and Turkey oak as light-demanding tree species but definitely less than sessile or pedunculate oak Negulescu and Savulescu, 1957; 1965; Stanescu, 1979; Stanescu et al., 1997) as main competitors to the sessile oak trees. For reaching this aim, the paper performs the analysis of stocking (number of trees/ha) and basal area (sq.m/ha) as well as evolution of mean diameter of various tree species in the two stands between 2004 and 2011.
2. Material and methods
2.1. Description of the studied stands
The fieldworks were performed in two sessile-oak dominated forest stands as follows:
a. Sub-compartment (scpt) 87C, Working Unit IX Bunesti, Stejarul Private Forest District (46[grados]5'39.74" N, 25[grados]02'59.21" E and 635 m above sea level). Plateau with brown soil of high fertility for sessile oak. Naturally regenerated by uniform shelterwood cutting even-aged (25 years old) and highly productive (1st yield class) sessile oak-dominated stand. A control plot of 300 [m.sup.2] (20 x 15 m) was established in 2004, along with three other plots of the same size for silvicultural purposes. The initial number of living trees within the control plot was 376 (12,532 trees/ha), with a species composition by number of trees of 50% sessile oak (SO) and the remaining 50% hornbeam (HOR) and other broadleaved tree species such as sycamore (SYC) and wild pear, Pyrus communis L. (WP). The initial species composition by basal area were also 50% SO and 50% HOR (moreSYC and WP).
b. Sub-compartment 71E, Working Unit IV Rancaciov, Valea Mare Forest District (44[grados]0'42.91" N., 25[grados]21'02.36" E and 308 m above sea level). Plateau with brown soil of high fertility for sessile oak stands and, also for other oak species (Hungarian oak HO and Turkey oak TO), which are established naturally in sessile oak-dominated stands of the region. Natural vegetation consists of a highly productive (1st yield class) sessile oak-dominated stand, even-aged (25 years old) and naturally regenerated following uniform shelterwood cuttings. A control plot of 200 [m.sup.2] (20 x 10 m) was established in 2004, along with three other plots of 200 [m.sup.2] each for silvicultural purposes. The initial number of individuals within this plot was 128 (6,400 trees/ha) while the species composition by number of trees was 66% SO, 34% HO and other broadleaved tree species (Turkey oak TO and HOR). The initial species composition by basal area were 43% SO and 57% HO (more TO and HOR).
2.2. Sampling procedure
In both plots, the assessment of health state (living or dead) and measurement of diameter at breast height (dbh) was performed yearly for all individual trees between 2004 and 2011. Using all field data, the following outputs (for each year between 2004 and 2011) resulted during the office phase of research project: stocking (number of trees/ha), basal area ([m.sup.2]/ha), mean diameter, proportion and size of dead trees.
3. Results and discussion
3.1. Number of trees
In scpt 87C (Figure 1), between 2004 and 2011, the initial total number of trees (376 = 12,532 trees/ha) was reduced to 187 trees (6,233 trees/ha, i.e., a 50% of initial number). This reduction was much higher in case of sessile oak (from 188 to 51, i.e., 77%) than of hornbeam and other broadleaved tree species (from 188 to 136, i.e., 28%). Consequently, after seven years, the species composition by number of trees in this stand has switched from 50% SO and 50% HOR (more SYC and WP) to 27% SO and 73% HOR (more SYC and WP).
In scpt 71E (Figure 2), the initial total number of trees (128 = 6,400 trees/ha) was reduced to 56 (2,800 trees/ha, i.e., a 44% of initial density) in 2011. As in the previous stand, this reduction was much prominent in case of sessile oak (from 85 to 28, i.e., 67%) than of Hungarian oak (and other broadleaves) (from 43 to 28, i.e., 35%). As a result, the species composition by number of trees in scpt 71E has switched from 66% SO and 34% (HO, TO and HOR) to 50% SO and 50% (HO, TO and HOR).
As emphasized recently by various authors (Burrows, 1990; Kelty, 1992; Larson, 1992; Prentice et al., 1993; Hasenauer, 1997; Filipescu and Comeau, 2007a; 2007b; Tomaiuolo, 2009/2010; Contreras et al., 2011), the reduction of stand densities in both sub-compartments is a process based on competition (considered as "the primary pattern of interaction among tree species"--Oliver and Larson, 1996; Plauborg, 2004) for light and other resources (nutrients, water, gases) and subsequent natural mortality. As sessile oak is less shade tolerant and competitive than both hornbeam and Hungarian oak in young stands, its mortality was much higher and the species composition on both number of trees and basal area had changes in favor of the associated but less valuable tree species.
The high natural mortality of sessile oak trees in both stands owing to its lower shade tolerance and competitive potential in young stages of development is consistent with the results of Purcelean and Ciumac (1965) and Ciumac (1967).
3.2. Basal area
In scpt 87C (Figure 3), the total basal area had increased from 19.72 [m.sup.2]/ha in 2004 to 24.20 [m.sup.2]/ha in 2011, i.e., 4.48 [m.sup.2]/ha, an increase of 22.72%. The basal area of sessile oak trees grew from 9.86 [m.sup.2]/ha to 10.67 [m.sup.2]/ha, i.e., 0.81 [m.sup.2]/ha, an increase of 8.22%; while the increment of mixed broadleaves was much higher, 3.67 [m.sup.2]/ha (37.22%).
In scpt 71E (Figure 4), the total basal area had increased from 19.05 [m.sup.2]/ha to 22.50 [m.sup.2]/ha in 2011, i.e., 3.45 [m.sup.2]/ha, an increase of 18.11%. The increase of basal area of sessile oak was negative, in this case, -0.26 [m.sup.2]/ha (-3.2%); while the increase of basal area of Hungarian oak and other broadleaved species was 3.71 [m.sup.2]/ha (34.1%).
3.3. Mean diameter
The mean diameter of sessile oak in control plot of scpt 87C had increased from 4.06 cm in 2004 to 8.45 cm in 2011, meaning 4.39 cm (108.13%). The increase of mean diameter of hornbeam more other broadleaved tree species was much lower and reached only 1.51 cm (36.47%), i.e., from 4.14 cm to 5.65 cm (Figure 5).
This fact is due to the natural mortality of lower diameter trees in case of sessile oak and a less obvious mortality of such small trees in hornbeam more other broadleaved trees as shown in Figure 6.
The coefficient of variation (CV) of diameter has shown two opposite behaviors in the case of scpt 87C: i) a reduction of 11.91% (from 46.98% in 2004 to 35.07% in 2011) in sessile oak trees and, ii) an increase of 2.78% (from 41.00% to 43.78%) in hornbeam and other broadleaved species. The trees showing the highest diameter increment between 2004 and 2011 were initially the biggest ones while the majority of small-diameter individuals in 2004 had grown less than 1-1.5 cm during the same period as shown in case of sessile oak (Figure 7).
In the case of scpt 71E, the increase of mean diameter between 2004 and 2011 was similar in absolute terms but not relative ones (%). By sessile oak, 3.49 cm, from 4.64 to 8.13 cm, i.e., 75.33%, and by Hungarian oak more other broadleaved tree species, 3.39 cm (45.56%), from 7.44 to 10.83 cm (Figure 8). As in the previous case, the natural mortality in case of sessile oak trees occurred predominantly in the low diameter classes, this self-thinning being less obvious for Hungarian oak and other broadleaved species as shown in Figure 9. In this stand, the CV of diameter had small reductions in case of both species: 6.67%, from 36.80 to 30.13% in 2011, for sessile oak and 4.06%, from 41.24 to 37.18% in 2011, for Hungarian oak and other broadleaves.
As in scpt 87C, the sessile oak trees showing the highest diameter increment between 2004 and 2011 (over 3.5 cm) were initially the biggest ones while the majority of smallest individuals in 2004 had grown less than 1-1.5 cm during the same period (Figure 10).
As shown above, the mortality can be a very intensive and quick process especially when sessile oak, a light-demanding but not highly competitive tree species, grows in natural and mixed stands with other less light-demanding and more competitive tree species such as hornbeam or Hungarian oak.
Consequently, the species composition of these stands can be highly affected and the presence of sessile oak can diminish quite drastically. However, as the sessile oak trees most affected by natural mortality are those of small sizes (diameters and heights) and the biggest oak trees show the highest diameter increments this fact does not affect their mean diameter and its positive evolution (increase) in both absolute and relative terms. The same consequence was found by basal area, its increase being similar in the case of different stand compositions.
In addition, as the initial largest sessile oak trees had shown the highest diameter growth, it is obvious that only these trees should be used when selecting and marking potential final crop trees, based on the three well-known criteria (vigour-quality-spacing), during the future interventions with thinning.
As the hornbeam had shown its high competition potential but quite slow diameter growth when young, as also shown in the past by other authors (Poskin, 1926; Stanescu, 1979; Lanier, 1986; Bary-Lenger et al., 1988; Stanescu et al., 1997), its tallest and thickest trees should be kept under control by silvicultural interventions and its future role in mixed stands with sessile oak should be only for creating an understory to stimulate the natural pruning of sessile oak trees, avoid the occurrence of epicormic branches and also protect and improve the forest soils.
In case of Hungarian oak, it can be a very important competitor to sessile oak on fertile (but quite heavy) forest soils, more ecologically favourable to this tree species and where light competition is usually stronger than on sites with poorer soils (Pretzsch and Biber, 2010). Under such site conditions and without a proper application of silvicultural interventions (release cutting and cleaning-respacing), the lower light and soil requirements of Hungarian oak can make the species very dominant and subsequently oust the more valuable but less competitive sessile oak
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Received: 25 July 2012
Accepted: 19 October 2012
The authors wish to thank the 2004-2011 B.Sc class students of Faculty of Silviculture and Forest Engineering in Brasov for their help during the fieldwork phase of this research study.
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Petre, M. , Nicolescu, V.N. , Ghirda, B. 
 Dambovita County Branch, National Forest Administration-ROMSILVA and Faculty of Silviculture and Forest Engineering, University Transilvania of Brasov, Romania.
 Faculty of Silviculture and Forest Engineering, University Transilvania of Brasov, Romania.
 Stejarul Private Forest District, Rupea, Brasov County, Romania.
Corresponding author: firstname.lastname@example.org
Figure 1. Variation of total number and number of trees of sessile oak (SO) and hornbeam (HOR) and more other broadleaved tree species in control plot (sub-compartment 87C) between 2004 and 2011. no. of trees SO no. of trees HOR Total no. 188 188 376 149 183 332 101 153 254 92 153 245 80 148 228 67 143 210 59 139 198 51 136 187 Note: Table made from line graph. Figure 2. Variation of total number and number of trees of sessile oak (SO) and Hungarian oak (HO) and more other broadleaved tree species in control plot (sub-compartment 71E) between 2004 and 2011. no. of trees SO no. of trees HO Total no. 85 43 128 71 42 113 59 36 95 45 34 79 39 31 70 38 30 68 30 30 60 28 28 56 Note: Table made from line graph. Figure 3. Variation of basal area/ha of total control plot, sessile oak (SO) and hornbeam (HOR) and more other broadleaved tree species in control plot (sub-compartment 87C) between 2004 and 2011. Basal area SO Basal area HOR Total basal area 9.86 9.86 19.72 10.12 10.9 21.02 9.96 10.94 20.9 10.51 11.82 22.33 10.63 12.32 22.95 10.77 12.89 23.66 10.69 13.28 23.97 10.67 13.53 24.20 Note: Table made from line graph. Figure 4. Variation of basal area of total control plot, sessile oak (SO) and Hungarian oak (HO) and more other broadleaved tree species in control plot (sub-compartment 71E) between 2004 and 2011. Basal area SO Basal area HO Total basal area 8.16 10.89 19.05 7.84 11.6 19.44 8.36 12.82 21.18 7.85 13.1 20.95 7.9 13.25 21.15 8.11 13.81 21.92 7.93 14.45 22.38 7.9 14.6 22.5 Note: Table made from line graph. Figure 5. Variation of mean diameter of sessile oak (SO) and hornbeam (HOR) and more other broadleaved tree species in control plot (sub-compartment 87C), between 2004 and 2011. mean diameter SO mean diameter HOR 4.06 4.14 4.65 4.4 5.78 4.83 6.24 4.99 6.83 5.15 7.36 5.37 7.88 5.5 8.45 5.65 Note: Table made from line graph. Figure 8. Variation of mean diameter of sessile oak (SO) and Hungarian oak (HO) and more other broadleaved tree species in control plot (sub-compartment 71E) between 2004 and 2011. mean diameter SO mean diameter HO 4.64 7.44 4.95 7.77 5.61 8.95 6.28 9.35 6.79 9.84 6.95 10.18 7.88 10.38 8.13 10.83 Note: Table made from line graph.
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|Author:||Petre, M.; Nicolescu, V.N.; Ghirda, B.|
|Publication:||Spanish Journal of Rural Development|
|Date:||Oct 1, 2012|
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