Korean Journal of Agricultural and Forest Meteorology, Vol. 17, No. 1, (2015), pp. 58~68
ⓒ Author(s) 2015. CC Attribution 3.0 License.
(2014년 08월 13일 접수; 2015년 2월 17일 수정; 2015년 03월 04일 수락)
Silvicultural Implications, Daegwallyeong Korea
Korea Forest Research Institute, 57 Hoegiro, Dongdaemun-gu, Seoul 130-712, Korea
(Received August 13, 2014; Revised February 17, 2015; Accepted March 04, 2015)
Extreme wind such as storms and hurricanes causes extensive damage to trees in many parts of the world, but recurrent wind has more subtle effects on tree growth, form and ecology (Peltola et al., 2000). According to Ennos (1997) the effects of extreme wind need to be considered separately from the recurrent wind. Hurricanes and storms wreck extensive damages to climax and pioneer tree species and reverse the process of eco-logical succession while recurrent wind of a particular site has larger effect on forest ecology by causing trees to undergo acclimatory changes in both short and longterm. Recurrent wind also induces trees of a single species to exhibit genetic differences in their morphological traits, thereby becoming adapted to windy landscapes. Unlike extreme wind, recurrent wind damages can be effectively reduced with sound forest management practices (Stathers et al., 1994). To undertake, longterm wind risk assessment of trees, require the combination of knowledge on climate (including topographic influences), tree and stand characteristics and their interactions with features of the site (Peltola et al., 2000). In this context, models of tree stand stability can be used to describe the mechanism of damage, because they can define the causal links between factors affecting the risk. Wind stability of trees is often related to individual tree characteristics such as species, diameter at breast height, height, crown size, and root/stem ratio as well as site characteristics such as rooting depth, soil moisture, exposure and slope (Cremer et al., 1982; Ruel, 1995; Becquey and Riou-Nivert, 1987).
Further, recurrent wind damage at any given site depends on species and tree size at the stand scale; topographic, site and stand factors at the landscape scale (Wood et al., 2008) and; wind speed and precipitation at the regional scale (Xi et al., 2008). Tree species selection is an important criterion in plantation to minimize wind damages. Species and their stands differ in their vulnerability to wind damage due to differences in morphological characteristics, wood properties, rooting strategy and crown shape (Schelhaas, 2008). Early successional and shade intolerant, fast growing pioneer species that form over story dominants are more susceptible to wind throw than shade tolerant (Rich et al., 2007) slow growing species occurring as co-dominant, intermediate and suppressed canopy trees (Foster, 1988). Diameter and height growth and differentiation of trees in forest stands affect stability against wind and snow damages. H/d ratio of a tree is a relative measure of stand stability under wind and snow loads. Wonn and O’ Hara (2001) pointed out that trees with higher h/d ratios exceeding a threshold limit of 80 are prone to wind damage than trees with lower ratios. Trees having higher h/d ratios require less wind speed to be damaged (Peltola et al., 1999; Wilson and Oliver, 2000; Cameron, 2002). With increasing height, h/d ratios should be kept low to maintain the wind risk level (Cremer et al., 1982; Becquey and Riou-Nivert, 1987; Ruel, 1995). However, forest stands with trees of relatively high h/d ratios can still be stable, if stand density is adequate, because of mutual support and sheltering among the individual trees against wind (Schelhaas et al., 2007). Variation in h/d ratios results from spacing (Wonn and O’Hara, 2001) and can be lowered in plantation through reduced planting densities or early thinning. Thinning is known to reduce the stand instability for many years(Cremer et al., 1982; Ruel, 1995; Cameron, 2002)through lowering the average h/d ratios in the longterm. Stand instability can be addressed by maintaining reasonably low h/d ratios up to a stand height of 30 m (Wilson, 1998). Initial stand density can be easily adjusted by altering planting densities. In highly exposed sites, it is recommended to use a relatively sparse initial stand density and not to thin (Quine et al., 1995; Ruel, 1995) which accentuates the importance of stand density management for wind stability. Limited tree size variation in plantation make trees susceptible to developing higher h/d ratios making them more vulnerable to wind throw (Wilson, 1998). At the tree and stand scale, silvicultural regimes can be designed to promote tree and stand acclimation to wind and minimize damages (Mitchell, 2013). The Gini coefficient and coefficient of variation have been identified as the most useful descriptors of the size variation in forest stands (Weiner and Solbrig, 1984; Benjamin and Hardwick, 1986; Knox et al., 1989) which, in turn, is a useful predictor of a measure of stand stability (Wilson, 1998).
Reducing wind damage largely depends on forest managers and practitioners making appropriate silvicultural decisions. However, managers may not have the silvicultural knowledge needed to make informed decisions and understand the long-term implication of their actions. Little is known about the wind stability of the commercially important plantation tree species in the windy landscapes of Daegwallyeong, Korea. This study compares the wind stability of Larix kaempferi, Pinus koraiensis and Abies holophylla by measuring DBH and height and their size variation to understand and inform decisions on wind risk management of plantation trees at Daegwallyeong.
The study area is located at 37°40′ N latitude and 128°42′ E longitude Daegwallyeong-myeon in Pyeongchang-gun, Gangwon province in the Republic of Korea (Fig. 1).Fig. 1. Map of the study area (A = A. holophylla, P = P. koraiensis, L = L. kaempferi).
The total area is 221.63 km2 with a population of 6,085 persons in 2008 and; elevation ranged from 800-1,000 m above sea level (Pyeongchang County Office, 2008). The land uses are classified into 4 categories; plantation and recreation forest, agriculture, pastures and ecotourism.
At Daegwallyeong, the monthly temperature is maximum(22.8°C) in July/August and minimum (-12.6°C) in January with a total precipitation of 1898 mm/year(Koh, 2012). Annual precipitation concentrates from the month of July to September. Daegwallyeong receives an annual snowfall of 187 cm thickness during the winter months. The area is foggy and cloudy most times of the year. The average slope of the plantation area varies from 13-19° with soil pH of more than 5 (Koh, 2012). Soil type is brown forest soils classified into dry, slightly dry and moderately moist brown forest soils with distinct A and B horizons (Korea Forest Research Institute, 2005). The aspect is south west. According to KMA (2012), the average wind velocity is 4.3 m/s with a highest velocity in the month of December (5.8 m/s) and January (5.6 m/s) and lowest (2.7 m/s) in September (Fig. 2(a)). The wind flows in west and south west direction mostly (Fig. 2(b)).Fig. 2. Wind speed (m/s) by month (a) and direction (b).
Daegwallyeong is one of the high flat lands in the Baekdu – Daegan mountain landscapes that experiences recurrent wind. History revealed that in the past, residents used the area for agricultural crop cultivation. After mass population migration in 1968, the area was left in a state of devastation aggravated by recurrent wind and poor soil condition. In order to bring the area under managed land use and restore the degraded forest, wooden wind barriers of 3 m height, 20 m length at 50 m interval were installed prior to start of the plantation. Planting techniques such as windbreak net and weirs, wooden tripods and soil dressings were done to minimize wind and snow damages to trees and improve site condition (Koh, 2012). Protective wire mesh of 50 cm diameter and 70 cm height with its base at 30 cm soil depth were installed around seedlings. In the first year, seedling survival was reportedly poor due to desiccation from recurrent winds. From second year onwards, intensive measures were put in place to protect seedlings from wind desiccation. When seedlings attained saplings and pole size trees, wooden tripods buttressed individual trees to prevent lodging and allow rooting firmly in the soil. In between plantation stands, nitrogen fixing trees and shrubs were planted to fertilize and enrich soil and maintain understory cover, respectively. With an overarching objective of ecosystem restoration of Baekdu – Daegan Mountain System, 4 years-long special plantation program was implemented from 1999-2002. The main species planted were; A. holophylla, P. koraiensis and L. kaempferi. The management plan prescribes to carry out forest tending for improving stand productivity, developing appropriate silvicultural regimes and create a wind firm plantation for eventually developing into an ecologically sound forest for environmental conservation. Accordingly, thinning, weeding, replenishment planting, and fertilization were done. This study forms part of the current research project on ‘topography based knowledge for forest landscape restoration in the Baekdu – Daegan Mountain System’ (Kwon, 2014).
In this study, P. koraiensis (Korean white pine), A. holophylla (Manchurian fir) and L. kaempferi (Japanese larch) were selected due to their increasing commercial importance as domestic timber (Seo et al., 2014). In the Republic of Korea, plantation forest species composition have gradually changed from an exotic single species to mixed species with natives in response to the policy of sustainable forest management and enhancing the recreational value of forest to public (Korea Forest Service, 2013). P. koraiensis is a native conifer species of northeast Asia. In the Republic of Korea, P. koraiensis is planted mostly and occupies an area of 230,000 ha (Korea Forest Service, 2009). It also occurs as scattered natural stands in the mountains (Korea Forest Research Institute, 2010). It grows to a height of 20-30 m and attains 1 m diameter; bark is dark brown colour; leaves are 5 needles; and flowers are monoecious (Korea Forest Research Institute, 2011). A. holophylla is an evergreen tree that grows about 40 m height and 1.5 m diameter; grayish dark brown bark and twigs; linear leaves with sharp apex at the tip and; flowers are monoecious (Korea Forest Research Institute, 2011). L. kaempferi is native to Japan. It is a medium to large-size deciduous conifer tree reaching 20-40 m height and 1 m diameter. Leaves are needle-like, light glaucous green in colour, which turns bright yellow to orange before they fall in the autumn. In the Republic of Korea, it is planted in many places and occupies an area of 460, 000 ha (Korea Forest Research Institute, 2010).
Initial site and stand reconnaissance were done after consulting the Pyeongchang county forest office in January 2014. Our study focuses on the species and trees at stand scale. Temporary square plots of 20 m × 20 m (400 m2) were laid out. From the center of the plot, starting from the north and moving in a clockwise direction, DBH (diameter at breast height measured at 1.30 m above ground) and height (from the base to the top of the green crown using a digital hypsometer) of all trees for greater than 10 cm in DBH were measured by species. A total of 15 plots (0.6 ha) with 5 plots each in L. kaempferi, P. koraiensis and A. holophylla stands were sampled at random. In these sampled stands, low thinning was already done. Further details are not available from the Pyeongchang county forest office.
We used Statistical Package for Social Sciences(SPSS 16.0), 2004) for data analysis. H/d ratio was calculated from height divided by DBH (both in same unit) by species. The coefficient of variation and Gini coefficient measures tree size variation, which are useful indicators of stand stability against wind (Wilson, 1998). The coefficient of variation is a dimensionless ratio that allows comparison of variability in populations, where standard deviation changes with the mean (Benjamin and Hardwick, 1986). We used Gini coefficient (Weiner and Solbrig, 1984) to characterize DBH size inequality. Lorenz curve is the curve below the 45º line gained by plotting the cumulative frequency of tree population and cumulative frequency of basal area (Knox et al., 1989; Nilsson, 1994; Stöcker, 2002). In order to draw the curve, trees in the stands were sorted by increasing dbh order, then the cumulative tree number and basal area were calculated. If all trees have equal DBH, the Lorenz curve aligns with 45° line and thus, the area under the Lorenz curve has largest value of 0.5. The more the size of trees varies, the smaller is the area under Lorenz curve. Thus, Gini coefficient is defined as the proportion of the area between the 45° line and the Lorenz curve in percent of 0.5 as measure of variability of tree sizes in the stand. Data were tested for normality and homogeneity of variance before subjecting to parametric and non-parametric tests. Unless otherwise stated, all tests are two-tailed.Fig. 3. Height/diameter ratio versus DBH and height of L. kaempferi (a and b), P. koraiensis (c and d) and A. holophylla (e and f).
The h/d ratios of L. kaempferi, P. koraiensis and A. holophylla in relation to their DBH and height are given in the Fig. 3. The h/d ratio decreases as DBH increases. The mean h/d ratio of L. kaempferi was 69.3 and ranged from 53.4 to 95.2 (Fig. 3(a) and (b)). The pooled DBH of the trees was 25.5±3.6 cm and height 17.5±1.6 m and ranged from 16.1 to 36.5 cm and 11.4 to 21.6 m, respectively (Table 1). In proportionate terms, about 9% of the larch trees, had h/d ratios of 85.1 and ranged from 80.3 to 95.2, above the critical threshold limit of 80. These trees have an average DBH of 20.7±1.4 cm and height of 17.6±1.2 m and ranged from 18.7 to 23.1 cm and 15.6 to 20 m, respectively. The height of these trees were little more than the pooled height of all the trees. A large proportion (91%) of the trees has average h/d ratio of 67.7 and ranged from 53.4 to 79.1 falling below the threshold limit of 80. These trees have an average DBH of 26±3.4 cm and ranged from 16.1 to 36.5 cm and height was 17.5±1.6 m and ranged from 11.4 to 21.6 m. The average h/d ratio of P. koraiensis was 55.7 ranging from 42.3 to 78.8 (Fig. 3(c) and (d)). The pooled DBH of the trees was 23.2±3.8 cm and ranged from 13.2 to 34.3 cm while the height was 12.7±1.5 m and ranged from 8.1 to 16 m (Table 1). Most of trees have h/d ratios below the threshold limit of 80. Similar to P. koraiensis, A. holophylla has an average h/d ratio of 51.7 and ranged from 39.7 to 69.4 (Fig. 3(e) and (f)). The pooled DBH of trees was 26.2±4.2 cm and ranged from 16 to 37 cm and height was 13.4±1.7 m and ranged from 8.2 to 17.2 m (Table 1). Again, all trees have h/d ratios below the threshold limit of 80.
One –way analysis of variance result of h/d ratio and Gini coefficient of DBH sizes between species is given in the Table 2. There was significant difference in the h/d ratio between the species (p < 0.01). The Bonferroni’s post-hoc multiple comparison test detected significant mean difference in the h/d ratio between L. kaempferi and A. holophylla, (p < 0.01)
3.2. DBH/height variation and stand density
The Gini coefficient and coefficient of variation of DBH was significantly correlated in L. kaempferi (r2adj = 0.95, p < 0.01), P. koraiensis (r2adj = 0.97, p < 0.01 and A. holophylla (r2adj = 0.95, p < 0.01) indicating size variation (Fig. 4(a)). The coefficient of variation of height of trees was larger in A. holophylla (11%) compared to L. kaempferi (8%) and P. koraiensis (7%) but statistically not significant (Fig. 4(b)). There was no significant difference in the number of trees per hectare by species (χ2(2) = 3.248, p = 0.197 Kruskal-Wallis test). However, P. koraiensis has higher tree densities (800trees/ha) than L. kaempferi (730 trees/ha) and A. holophylla (640 trees/ha) (Fig. 4(c)). There was also no significant difference in the basal area between species(χ2(2) = 0.980, p = 0.613). L. kaempferi has higher basal area (38.12 m2/ha) than A. holophylla (35.28 m2/ha) and P. koraiensis (34.68 m2/ha).
There was significant difference in Gini coefficient of DBH sizes between stands (F2,14 = 4.95, p < 0.05) (Table 2). The Lorenz curve and Gini coefficient of DBH sizes of stands by species are given in the Figure 5. DBH variation was little in A. holophylla (16.4%) (Fig. 5(c)) closely fol-lowed by P. koraiensis (15.9%) (Fig. 5(b)) and L. kaempferi (14%) (Fig. 5(a)) stands. The Lorenz curves of the three species lie near the 45 line of equality.
Fig. 4. Comparison of coefficient of variation versus Gini coefficient, (a) average height versus coefficient of variation, (b) and trees per hectare versus height, (c) between species.
Fig. 5. Dbh distribution of L. kaempferi, (a) P. koraiensis, (b) and A. holophylla ©.
The h/d ratios were found to be higher in L. kaempferi compared to P. koraiensis and A. holophylla. The analysis of variance detected significant difference in the h/d ratios between the species. These results indicate that L. kaempferi is relatively vulnerable to wind damage than P. koraiensis and A. holophylla. The latter two species have comparatively lower h/d ratios falling below the critical threshold limit of 80 and considered stable. In our study, about 9% of the L. kaempferi trees, have h/d ratios above the critical threshold limit. These trees are vulnerable to wind damage and should be targeted for cutting in the next thinning regime. Wonn and O’ Hara (2001)found out that damaged western larch had extremely high h/d ratios due to its deciduous nature than ponderosa pine, lodgepole pine and Douglas fir in response to wind and snow damage. High h/d ratios of the 9% L. kaempferi trees may be attributed to increased height increment plausibly due to good site quality. In the European managed forest dominated by even-aged stands, storm damage typically increased with stand height and age (Mitchell, 2013). Our study results revealed that L. kaempferi trees were taller than P. koraiensis and A. holophylla. Generally, shade-intolerant tree species allocate more resources to height increment instead of overall structural strength (Givnish, 1995) making them less wind firm (Rich et al., 2007). Our results are consistent with the findings from longterm growth monitoring research, which pointed out that L. kaempferi height increment was higher than P. koraiensis but diameter increment was vice-versa (Seo et al., 2014). The lower h/d ratios in P. koraiensis and A. holophylla are attributed to thinning of the stands. The variation in h/d ratios can be largely attributed to thinning and spacing (Wonn and O’ Hara, 2001; Cremer et al., 1982). Lower tree densities but higher height increment of L. kaempferi trees makes them vulnerable to wind damage due to little mutual support and sheltering among the individual trees (Schelhaas et al., 2007). A. holophylla in particular has relatively lower tree densities and lower height increment. Lower tree densities, however, result from wider spacing created by thinning, reducing tree densities as in the case of A. holophylla. Wonn and O’ Hara (2001) pointed out that heavy thinning lowers h/d ratios indicating the potential of thinning to spacing and consequently improving stand stability. Post thinning damages from snow and wind were minimum (1-20%) of the basal area compared to unthinned stands (Valinger and Pettersson, 1996; Valinger et al., 1994). The lower h/d ratios in A. holophylla and P. koraiensis could also be attributed to morphological differences between species. A. holophylla and P. koraiensis are shade tolerant native species (Park and Jeon, 2010) with lower stand height and dense canopies similar to wind resistant black pine (P. thunbergii) (Bitog et al., 2011; Kwon et al., 2004; Kim, 2010) than shade intolerant exotic larch. Everham and Brokaw (1996) concluded that differences in tree morphology, wood properties and disease resistance result differences in species vulnerability within and between stands, however, these differences can be obscured by differences in site, stand and silvicultural characters. Studies in the European forests revealed that Douglas fir has better stem form and anchorage particularly in permeable soil condition than larch and pines (Schütz et al., 2006; Bosshard, 1967). However, we observed a few A. holophylla and P. koraiensis trees top snapped, which may be due to stem structural weakness (Stathers et al., 1994).
In general, there was less DBH variation of trees between species and stands. Among the species, A. holophylla and P. koraiensis has relatively large DBH variation than L. kaempferi stands. Less tree size variation of L. kaempferi stands may have developed high h/d ratios. Wilson and Oliver (2000) pointed out that limited tree size variation in coastal Douglas fir plantation make them susceptible to developing high h/d ratios in dominant trees. Relatively large DBH sizes of A. holophylla and P. koraiensis stands may be attributed to acclimatory responses developed over longterm due to recurrent wind effects. Ennos (1997) pointed out that trees reduces height increment but increases diameter increment particularly at the base of the trunk and branches, acting to reduce the peak mechanical wind stress and overturning moment in trees. Diameter increment relative to height increment in P. koraiensis trees have been reported after thinning (Bae et al., 2010) and similar aged A. holophylla trees had diverse DBH values (Park and Jeon, 2010). Further thinning, in these species and stands, should aim to increase diameter sizes to improve wind stability.
It can be concluded that A. holophylla and P. koraiensis have relatively lower h/d ratios and large DBH variation than L. kaempferi. This result clearly indicates that the former two species are more wind firm than the latter species. Generally, there was limited DBH size variation between the stands. To increase wind firmness, low thinning or thinning from below should concentrate to remove trees with high h/d ratios above 80 coinciding with the early or at the time of stand distinction phase. This will benefit remaining trees to quickly utilize growing space and increase diameter increment relative to height increment (Schmidt and Seidel, 1988; Cameron, 2002). Thinning done after the stand distinc66 tion phase may result in stand instability (Cameron, 2002; Wilson and Oliver, 2000). Forest managers and practitioners should measure, calculate and maintain h/d ratios of trees below the critical threshold limit of 80 through stand density management. Trees with higher h/d ratios than 80 should be cut in the successive thinning regimes. Variable density thinning approach that deliberately leaves trees on variable spacing, so that some residual trees are nearly in open-grown conditions, whereas others are in heavy competition (OHara and Waring, 2005) resulting in a variety of tree sizes in an even-aged stands (Thornburgh et al., 2000)
본 연구는 대관령 특수조림지에 식재된 주요 경제수종인 잣나무, 전나무, 일본잎갈나무의 내풍 안정성을 비교 분석하여 조림지의 풍해관리에 대한 이해와 인식을 높이고자 수행되었다. 각 수종별로 5개씩 총 15개의 임시 방형구(20m × 20 m)를 설치하였으며, 흉고직경 10cm 이상의 수목에 대하여 수고 및 흉고직경을 측정하였다. 수종별 수고/흉고직경 비율(h/d 비율)을 분석한 결과 잣나무와 전나무가 일본잎갈나무에 비해 비교적 낮은 h/d 비율을 나타내어 내풍성이 상대적으로 높은 것으로 보인다. 약 9%의 일본잎갈나무가 내풍 임계치(80) 이상의 h/d 비율을 나타낸 것으로 조사되었으며 이들 수목들은 풍해에 매우 취약하여 다음 간벌 기간 동안 제거되어야 할 것으로 판단된다. 분산 분석 결과 수종별 h/d 비율과 흉고직경의 지니계수에서 각각 유의한 차이가 나타났다. 수종별 흉고직경의 지니계수는 전나무 16.4%, 잣나무 14%, 일본잎갈나무 14%로 나타났다. 낮은 h/d 비율은 수종별 형태학적 차이와 간벌 시업에 기인한 것으로 판단된다. 수목의 내풍성을 향상시키기 위해서는 h/d 비율이 80 이상인 수목에 대한 하층간벌이 초기 혹은 임분 분화기(stand distinction phase)에 집중되어야 한다. 산림관리자와 시업자는 수목의 h/d 비율을 측정하고 임분 밀도를 관리하여 비율을 내풍 임계치인 80 이하 수준으로 유지하여야 한다. 따라서 동령림에서 수목의 흉고직경을 증가시키기 위해서는 h/d 비율이 높은 수목에 대한 택벌이 수행되어야 할 것으로 판단된다.
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