ORIGINAL ARTICLE

Youngkeun Song • Youngryel Ryu

Seasonal changes in vertical canopy structure in a temperatebroadleaved forest in Korea

Received: 26 December 2014 / Accepted: 13 May 2015 / Published online: 21 May 2015� The Ecological Society of Japan 2015

Abstract Field measurements of vertical canopy struc-ture have been challenging for decades and still impor-tant for understanding forest ecosystems. We measuredthe vertical canopy structure and its seasonal changes ina temperate deciduous forest canopy in Gwangneung,Korea. Time-series measurements of leaf area index(LAI) were collected in 2013 from a five story (4-mvertical intervals) tower. We evaluated crown depth andspecies composition by height from a vegetation survey.The vertical distribution of leaf and woody area densitywas described from measurements taken during the leaf-on and leaf-off seasons, and averaged 0.18 and0.04 m2 m�3, respectively. Three strata were character-ized: (1) the dense upper crowns with large trees (>16-m) of Quercus serrata, in which 29.3 % of the plantmaterials were distributed; (2) abundant foliagedominated by Carpinus laxiflora at about 16-m(40.8 %); and (3) a diverse and well-developed under-story vegetation at about 4-m (15.5 %), consisting of C.laxiflora, Carnipus cordata, and Styrax japonica com-munities. Per-layer phenology of each species was suc-cessfully illustrated by the drastic increase in LAI duringthe leaf-out season [days of the year (DOY) 110–140],the full-leaved stage LAI of 3.4 ± 0.9 m2 m�2

(mean ±1 standard deviation), and a decrease duringthe leaf-fall season (DOY 280–320). The seasonal var-iation in gap fractions reflected different light conditionsvarying with canopy height. This type of vertical profile

archive is valuable not only for comparing the structureof various forests but also for monitoring changes in thisecosystem in the future.

Keywords Foliage-height profile Æ Gwangneung Æ Leafarea density Æ Leaf area index Æ Phenology

Introduction

Canopy structure, defined as the spatial arrangement ofthe above-ground organs of plants (Campbell andNorman 1989), provides key information to understandthe microclimate, ecological processes, succession, andevolutionary history of a forest ecosystem (Lowman andRinker 2004). In particular, a number of studies havehighlighted the influence of canopy structure on forestturbulence characteristics (Pereira and Shaw 1980, 1982;Queck and Bernhofer 2010), forest hydrological cycles(Crockford and Richardson 2000; Link et al. 2004; Songet al. 2014), or radiation regimes and photosynthesis incanopies (Ross 1981; Ellsworth and Reich 1993; Bal-docchi et al. 2002). Thus, canopy structure should besurveyed in the field, particularly to describe the non-uniform spatial distribution of plant materials, such asleaves, stems, twigs, and branches. However, the diffi-culties of quantitatively describing canopy features andtheir heterogeneity at various spatial and temporal scales(Norman and Campbell 1989) remain challenging. Ad-ditionally, the limited number of data measurementmethods (Ishii et al. 2004) and the inaccessibility to theheight of upper canopies are further difficulties (Parkeret al. 1992).

Vertical canopy structure was formerly measured bydestructive sampling of plant materials based on thestratified clip technique (Monsi and Saeki 1953, 2005).This method quantifies the vertical distribution of thetotal plant area by harvesting all plant materials in ap-propriate horizontal layers and separately measuring theplant area in each layer. Although this required atremendous amount of fieldwork and could not be used

Y. Song Æ Y. RyuBrain Korea 21 Plus Team, Seoul National University,Seoul, Republic of Korea

Y. Ryu (&)Department of Landscape Architecture and Rural SystemsEngineering, Seoul National University, Seoul 151-921,Republic of KoreaE-mail: ryuyr77@gmail.comTel.: 82-2-880-4871

Y. RyuInterdisciplinary Program in Agricultural and Forest Meteorology,Seoul National University, Seoul, Republic of Korea

Ecol Res (2015) 30: 821–831DOI 10.1007/s11284-015-1281-3

to monitor temporal changes in the canopy, this methodwas useful to describe less-studied forest ecosystems,such as savanna and tropical forests of Thailand (Ogawaet al. 1961), a mature dipterocarp forest in Borneo(Yamakura et al. 1986), a Japanese larch plantation(Satoo 1970), and representative forest types across Ja-pan (Kira et al. 1969). Hutchison et al. (1986) measuredthe vertical distribution of plant area and leaf inclinationangles directly within sampled volumes of a forest ca-nopy instead of complete harvesting; however, thismethod still required a large amount of detailed field-work on a hydraulic lift with a 20-m vertical extension.In contrast, the point-quadrat method (Wilson 1960,1965) was proposed to estimate foliage-height profiles.This method involves non-destructive sampling becausethe canopy structure is retrieved from a number of line-intercept samples (the position of leaf contact) takenthrough the canopy in several predetermined directions.This method has been used to vertically profile aspenand oak stands in North America (Miller 1969) and achestnut coppice woodland in England (Ford andNewbould 1971). However, this method is laborious andinvolves the danger of working at heights, particularlywhen conducted in mature stands in taller canopy layers.

Based on theories from these direct sampling meth-ods, researchers have developed indirect and more effi-cient methods for estimating vertical canopy structure,e.g., by measuring leaf-contact heights using a camerawith a telephoto lens fixed on a tripod (MacArthur andHorn 1969; Aber 1979; Parker et al. 1989), or a laserrange finder (Radtke and Bolstad 2001), by measuringcanopy gap fractions using quantum sensors (Normanand Jarvis 1975), or light-emitting diode (LED) sensors(Ryu et al. 2010a) mounted at multiple canopy depths(Ryu et al. 2014). All of these efforts attempted toquantify vertical canopy profile, and focused on how todescribe canopy stratification (Parker and Brown 2000).More technically, it was how to estimate leaf area den-sity (LAD) or leaf area index (LAI) (Weiss et al. 2004) atdifferent canopy heights. A number of indirect methodsof estimating LAI have been developed, as reviewed inJonckheere et al. (2004) and evaluated [e.g. Ryu et al.(2010c)] for widely used instruments such as hemisphericphotographs using a fisheye lens (Bonhomme andChartier 1972; Neumann and Den Hartog 1989; Zhanget al. 2005) or a Plant Canopy Analyzer (LICOR Inc.,Lincoln, NE, USA) (Chason et al. 1991; Welles andNorman 1991; Cutini et al. 1998). However, the use ofthese to estimate vertical structure has not been inves-tigated extensively. Light detection and ranging (Li-DAR) remote sensing has been developed as an efficientmeans of estimating the spatial distribution of LAD(Lovell et al. 2003; Hosoi and Omasa 2006, Hosoi andOmasa 2009; Song et al. 2011). However, the method iscontroversial due to its inconsistent results (Bater et al.2011), which depend on the sensor configuration, sitecharacteristics, and the LiDAR data processing method.

Accessibility to the upper forest canopy is a criticalissue for surveying canopy structure. Traditional forest

scientists used a rope system (Perry 1978; Perry andWilliams 1981), ladder and aerial walkways (Muul andLiat 1970), hydraulic lifts (Hutchison et al. 1986), ca-nopy rafts (Sterck et al. 1992), or cranes (Parker et al.1992) to approach higher canopy layers. Mi-crometeorological observation towers established by theFLUXNET community (Baldocchi et al. 2001) haveenabled direct access to the entire vertical structure fromthe forest floor to the above-canopy area and allow theobservation of forest phenology at multiple canopylayers (Ryu et al. 2014). Currently, more than 630 fluxtowers have been established within representativeecosystems worldwide (http://fluxnet.ornl.gov/), andmay provide the opportunity to extend our observationsof those diverse ecosystems.

The Gwangneung experiment forest, an old naturalforest located in the west-central part of the KoreanPeninsula (Fig. 1), is a valuable ecosystem with a typicalclimax community of local deciduous-broadleaved spe-cies dominated by Quercus spp. and Carpinus spp. (Limet al. 2003; Kang et al. 2009). This site has been desig-nated as a national conservation area for research since1929 and as a UNESCO Biosphere Reserve since 2010,and continues to provide valuable study areas for re-searchers as a long-term intensive monitoring site.About 300 studies on this ecosystem have been pub-lished in Korea, and a number of biometeorologicalstudies (Hong et al. 2008; Ryu et al. 2008; Kwon et al.2010) using the tall flux towers (20 and 40 m heights) inthe forest have been reported. Previous studies at-tempted to estimate seasonal changes in on-ground LAIof the deciduous forest district at this site using upwardhemispherical photographs (Lim et al. 2003) or PlantCanopy Analyzer instrument (Kwon et al. 2010). How-ever, these have limitations in terms of quantifying thevertical structure of this forest. Ryu et al. (2014) suc-cessfully conducted tower-based observations of themulti-canopy-layer phenology during the green-up sea-son. However, further understanding would be achievedby conducting observations of the vertical species com-position throughout the year.

This study aimed to identify vertical canopy structureand its seasonal changes in the Gwangneung temperatedeciduous forest in Korea, based on seasonal measure-ments of LAD (and LAI) at multiple canopy heights.Specifically, we present the vertical structure in terms ofthe (1) distribution of leafy and woody materials, (2) thenumber of individuals and species composition, and (3)seasonal variations in the total LAI, species-specificLAD, and the light environment with height.

Materials and methods

Study site

The study site is located in a complex, hilly deciduousforest catchment (�220 ha) of Gwangneung, Pocheon-

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si, Gyeonggi-do, Republic of Korea (37.748717�N,27.148176�E, elevation: 260 m; Fig. 1). The mean an-nual air temperature at the site is 11.5 �C, and the meanannual precipitation is 1332 mm. The local climate is atypical temperate climate, with hot, wet summers underthe effect of the East Asian monsoon and a dry winterwith snow. A walkable 20-m-high tower was built in theforest to measure ecological variables in the canopylayers. We studied vertical canopy structure using thistower. The topography around this tower slopes down-ward at about 10� in an easterly direction. More detailedinformation about this site has been published previ-ously (Lim et al. 2003; Kim et al. 2006; Kang et al. 2009).

LAI measurements and data processing

LAI at a particular canopy height was determined fromthe gap fractions measured using an LAI-2200 PlantCanopy Analyzer. This optical sensor records hemi-spheric radiation at five rings (centered at 7�, 23�, 38�,53�, and 68�) for light below 490 nm; these wavelengthswere used to reduce errors from light transmitted andreflected by the canopy. Below-canopy light intensitywas measured in five tower layers (every 4 m), towardsthe north, west, and south directions per layer and threetimes (right-side, center, and left-side) per direction. Weused a 90� viewing cap on the LAI-2200 lens for thedirectional measurements. We excluded the east-directedobservations because of large canopy openness induced

by the topography and fallen trees. Cho (1992) reportedthat a large gap size >200 m2 is abnormal (<8 % of allgaps) in Gwangneung Forest and that most of the gapsin this forest are small and made by only a few fallentrees. Therefore, our survey did not include this ‘‘ab-normal’’ quarter in the easterly direction of the circularplot (Fig. 1). Above-canopy light intensity was mea-sured at the top of the 20-m tower before and after thebelow-canopy measurements. We also collected pairedsunlit and shaded measurements with a diffuser cap tocompute the beam proportion of incoming light to re-move scattered light caused by the canopy, which causesunderestimation of the LAI; we used a 270� viewing capto compute directional variation in the sky radiation,according to Kobayashi et al. (2013), which enabled usto use this instrument during daytime. Measurementsbased on this protocol were conducted through days ofthe year (DOY) 105 (April 15) to 317 (November 13),which was sufficient to observe the phenology of thisforest.

The measured time-series LAI data were correctedfor the pattern of seasonal change in the gap fractions.The ideal shape of a seasonal change in the gap fractionwould be close to a u-shaped curve because it decreasesas the canopy closes during leaf-out and increases as thecanopy opens during leaf-fall. We occasionally encoun-tered unexpected outliers in a series of measurements.Plant materials that were not viewed in other observa-tions seemed to be included on some specific dates. Wemay have had some inconsistent measurement positions,

Fig. 1 Location of study site and sampling design

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although we attempted to measure at the same positionsas much as possible. We replaced these erroneous mea-surements with values linearly interpolated from thecorresponding gap fractions measured before and afterthe current observations. This correction process wasapplied to all observations at the same positions andrings of the LAI-2200. We averaged the logarithm of gapfractions instead of the logarithm over the mean of gapfractions to account for foliar clumping effects in com-puting LAI (Ryu et al. 2010b).

Evaluation

LAI measurements were evaluated using the results ofRyu et al. (2014), who estimated seasonal changes inLAI at the same study site using upward-pointing digitalcameras and LED sensors. The LED sensors, whichwere used as spectrally-selective light detectors (Ryuet al. 2010a), enabled us to estimate LAI through Beer’sLaw by measuring incoming irradiance at different ca-nopy depths (Ryu et al. 2014). Our measurements werecompared with the average LAI for 5 days (i.e., 2 daysbefore and after the corresponding day), so we couldmatch the LAIs even though the observations of Ryuet al. (2014) may not have been performed on the sameday. Seasonal measurements of the whole canopy LAI inthis study (i.e., LAIs measured in the bottom layer inFig. 1) were compared to the observations in Ryu et al.(2014), using upward digital-cameras installed on theground at breast height and LED sensors mounted at2 m height of the tower in Ryu et al. (2014). LAI valuesof the overstory (12–20 m) and understory (2–12 m)canopy layers in Ryu et al. (2014) corresponded to thesum of the per-layer LAIs at the top to the second andthe third to the bottom in Fig. 1, respectively.

Vegetation survey

A tree survey was conducted within a 20-m radius of thetower to identify species structural characteristics (Fig. 1).A quarter in the east direction of our circular plot wasexcluded from this tree survey, because the LAI measure-ments did not cover that area (see ‘‘LAI measurements anddata processing’’). Tree species, diameter at breast height(DBH), crown top height, and crown base height weremeasured for all trees in the plot with DBH >3 cm.

Analysis

The vertical distribution of leafy and woody parts in theforest was quantified using the LAI measurements dur-ing the leaf-on and leaf-off seasons, respectively. All LAImeasurements were converted to LAD by taking thedifference in the LAI values of the neighboring two ca-nopy layers and dividing this difference by the verticalinterval of observations (i.e., 4 m in this study). The

maximum LAD value at each measured height duringthe full-leaf season [from DOY 138 (May 20) to DOY249 (September 6)] was regarded as the total magnitudeof plant materials, including both leafy and woodyparts. The woody parts were quantified using the woodyarea density (WAD), which was measured during theleaf-off season [DOY 105 (April 15)] as the LAD.

We derived a crown depth diagram from the verticaldistribution of tree heights collected during the fieldsurvey. According to Ogawa et al. (1961), cumulativecurves for crown top and base heights could be deter-mined based on the relative frequency of the number oftrees. Then, crown depth was calculated from the dif-ference between the two curves. A crown depth diagramwas obtained for each dominant species and was used todiscuss species composition at different canopy heights.

The phenology of each layer was described based onthe time-series LAI measurements at different canopyheights, and specified into the phases of selected domi-nant species. The contribution of selected species to theLAD and WAD values in each layer was determined bythe ratio of the number of individuals of the selectedspecies to the total number of individuals in the layer.We also used the weighted average values of gap frac-tions in the five rings of LAI-2200 to show the lightcondition inside the canopy. The weighting factor foreach ring was determined by multiplying the sine of themean zenith angle by the corresponding ring-width (ra-dians), and normalized to sum to 1.0, according to LI-COR (2009).

Results

Evaluation of LAI measurements

The LAI measurements agreed well with the LAI esti-mates of Ryu et al. (2014) (Fig. 2). LAI values measuredon the forest floor (i.e., total LAI estimates in all canopylayers) throughout the year using the LAI-2200 instru-ment in this study presented a linear relationship withthe digital camera [r2 = 0.91, root mean square error(RMSE) = 0.39, bias = �0.01] and LED sensor esti-mates (r2 = 0.90, RMSE = 0.33, bias = �0.10) ofRyu et al. (2014) (Fig. 2a). Seasonal measurements ofthe canopy stories (Fig. 2b) also exhibited good agree-ment between our study and that of Ryu et al. (2014),showing r2 = 0.91 (RMSE = 0.23, bias = �0.05) forthe over-story and r2 = 0.56 (RMSE = 0.25, bi-as = �0.06) for the understory. These measurementssuggested that the subsequent analysis based on our LAImeasurements was acceptable.

Vertical structure of leafy and woody parts

Figure 3 describes vertical canopy structure by the LADprofile. Vertically averaged LAD during the full-leaf

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season was 0.18 ± 0.13 m2 m�3 (mean ± 1 SD). LADduring the leaf-off season (i.e., WAD) less varied verti-cally than LAD of the full-leaf season, with0.04 ± 0.02 m2 m�3 at the vertical average.

The structural details were assessed by comparing theWAD with the LAD at the same height. The canopy atheights of 12–16-m (0.37 m2 m�3 in LAD) had the mostabundant foliage in vertical distribution, even more than

the uppermost canopy layer (0.27 m2 m�3 in LAD), al-though the amount of woody materials were almostidentical to each other (0.06 m2 m�3 in WAD of bothlayers). Therefore, seasonality in terms of leaf-out andleaf-fall was most remarkable in this layer. On the otherhand, little seasonal change was observed in the layers atheights of 4–8 and 8–12 m because of the small amountof leaves present at those heights; those LAD recordsmostly resulted from woody materials, not from leaves,because there was little difference between the values ofLAD and WAD (0.02 and 0.05 m2 m�3, respectively).

Species composition at different heights

Ninety-six individuals of 14 species (DBH >3 cm)were identified in the study area. The species composi-tion was characterized by large-sized Quercus serrata(50.6 ± 11.7-cm DBH) and C. laxiflora (29.9 ± 11.6-cm DBH), and large numbers of Carpinus cordata and S.japonica (Table 1).

The cumulative curves in Fig. 4 show the verticaldistribution of the number of trees at the study site. Theslight increases in the cumulative crown top and baseheight curves at heights >16 and >12 m, respectively,reflect the small number of trees in the over-story(Fig. 4a). A drastic increase in both curves, particularlyfrom 8 to 4 m, represents the large number of under-story trees at those heights. The crown depth curve,which was derived from the difference between bothcurves, effectively described the vertical profile of leafyparts and its stratified structure, based on the number ofindividuals. According to this crown depth curve, ca-nopy structure was stratified with the following threelayers: (1) top layer >16 m, with a small number oftrees; (2) intermediate layer from 16 to 8 m, with a smallnumber of trees; and (3) bottom layer from 8 to 2 m,with a large number of trees.

The crown depth curves for selected species (Fig. 4b)illustrate the species composition according to height.The top layer was occupied only by large oak trees (Q.serrata). The upper crown of the intermediate layer wasdominated by Carpinus laxiflora, but the species com-position of the lower part of this layer was diverse. Theunder-story trees in the bottom layer consisted of C.laxiflora, C. cordata, and S. japonica communities.

Phenology with height

Figure 5 clearly shows the phenological characteristicsof the deciduous forest; the drastic increase in LAIduring the leaf-out season (DOY 110–140), a relativelystable phase during full-leaf period (DOY 140–280), anda decrease during the leaf-fall season (DOY 280–320).The spatial variation in LAI of all canopy layers duringthe full-leaf season was 3.36 ± 0.88 (mean ± 1 SD;range 1.81–4.90). LAI values for DOY 317 were greater

Fig. 2 Cross validation of the leaf area index (LAI) measurementsin this study with those of Ryu et al. (2014), depending on a theinstruments used and b the heights measured

Fig. 3 Vertical distribution of the leafy (right-hand section) andwoody (left-hand section) parts of trees. Open dots with error barsare plotted from the gap fraction statistics at the measured heightand represent the average gap fraction value and range, respectively

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than those for DOY 105 because some senescent leavesremained attached to branches.

Species-specific phenology in vertical structure (Fig. 6)reveals that how the dominant species are changing theamount of leaves at each layer through the seasons.Seasonal change of LAD values inQ. serrata ranged from0.07 to 0.27 m2 m�3 over 16-m height throughout theyear, while the other species had no foliage at this height.The 12–16 m layer was the most competitive heights forseasonal development of foliage, and was dominated byC. laxiflora (0.04 to 0.20 m2 m�3 of LAD), followed byQ. serrata (0.02 to 0.08 m2 m�3) and the other species(0.02 to 0.10 m2 m�3). Carpinus cordata and S. japonicashowed similar seasonal variations of LAD at heightsunder 12-m. The variation in seasonal LAD values waslarger at lower heights because more trees in these specieswere found at the height closer to the forest floor.

Variation in relative light intensity at different heights

Figure 7 shows the spatial and temporal variations ingap fractions in terms of measured height. Gap fractionsin the highest canopy layer were 20–100 % (Fig. 7a; seevertical error bars), particularly during the full-leafseason of DOY 224–302. In contrast, the light environ-ment on the ground (Fig. 7e) was more homogenous, asthe range of variation in gap fractions (i.e., vertical errorbars) was <20 % throughout the year. These changes invertical light conditions occurred gradually in the in-termediate canopy layers (Fig. 7b–d); a larger variationin the upper canopy and a smaller variation in the lowercanopy were observed in the year-round measurements.

In addition, this tendency in light conditions to varywith height depended on the vertical distribution ofplant materials. The dots in Fig. 3 indicate that the re-

Table 1 Trees within the footprint of the leaf area index (LAI) measurements

Species No. of trees Stem density (trees ha�1) DBH (cm) Crown topheight (m)

Crown baseheight (m)

Mean SD Mean SD Mean SD

Quercus serrata 15 159.2 50.6 11.7 20.5 7.3 10.6 3.9Carpinus laxiflora 9 95.5 29.9 11.6 13.3 3.1 6.9 2.2Carpinus cordata 17 180.5 10.1 4.2 7.4 1.8 2.2 0.6Styrax japonica 16 169.9 6.7 2.1 6.2 1.6 2.9 1.5Acer pseudosieboldianum 9 95.5 5.2 2.0 4.8 1.3 2.4 0.5Rhus trichocarpa 5 53.1 4.3 0.5 5.3 0.9 3.4 0.6Sorbus alnifolia 5 53.1 4.2 0.8 5.2 2.2 1.8 0.4Fraxinus rhynchophylla 3 31.8 7.9 6.2 4.2 2.4 2.3 1.2Cornus kousa 3 31.8 7.2 4.7 7.1 3.4 2.9 1.0The others 6 63.7 6.0 4.2 5.5 3.9 2.1 1.3

Only individuals with a diameter at breast height (DBH) >3 cm were surveyed

Fig. 4 Crown depth diagram based on thedistribution of crown heights and thenumber of individuals. The crown depthcurve was derived from the difference incumulative curves between the crown topand base heights (a), and was partitioned byspecies (b)

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lative light intensity (i.e., gap fraction) on the forest flooraveraged 11.5 % under a full-leaf canopy (the right-sideblock) but 54.5 % without foliage (left block). The range

in the gap fractions was larger during the full-leaf seasonthan that during the leaf-off season, particularly in theupper canopy.

Discussion

Quantification of vertical structure using leaf areadensity

Canopy structure was well-illustrated by using the ver-tical distribution of LAD (Fig. 3) and it was better thanusing a single LAI value. As LAI, by definition, de-scribes only a vertically ‘‘stacked’’ structure, limitationsremain for retrieving the vertical structure. Kira et al.(1969) clarified the difference in vertical structures be-tween two Castanopsis cuspidata stands, despite theirsimilar LAI values. Accordingly, the vertical profile be-yond the LAI value was determined only by measuringLAD at different heights. Using the LAD values, thestructural characteristics could be compared to those ofother forests. Vertically averaged full-leaved LAD (see

Fig. 5 Seasonal changes in cumulative leaf area index (LAI) valuesmeasured at different canopy heights

Fig. 6 Seasonal changes in the vertical distribution of leaf (bars in green color) and woody (bars in brown color) area density depending onthe selected dominant species

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‘‘Vertical structure of leafy and woody parts’’) was si-milar to that determined for other temperate deciduousforests (0.1–0.3) (Tadaki 1966; Parker 1995). A Faguscrenata community, a typical climax deciduous-broad-leaved-forest in Japan, showed a similar vertical struc-ture in terms of LAD values at corresponding heightsand strata (Kira et al. 1969). Compared to an oak forestin Tennessee, USA (Hutchison et al. 1986), Gwangne-ung had a less closed canopy (i.e., lower LAD) in theupper part, but a denser understory (i.e., higher LAD)near the forest floor, and more noticeable multiple layerswith height.

Integrated interpretation of vertical leaf area densityand species distribution

The vertical canopy structure was clearly understoodbased on information regarding the amount of plantmaterials distributed (Fig. 3) and the number of indi-viduals or species diversity at different heights (Fig. 4).The upper crown (>16-m), in which 29.3 % of thevertical LAD was distributed (Fig. 3), consisted of a fewlarge-sized Q. serrata (Fig. 4). Similarly, a small numberof large-sized C. laxiflora dominated the canopy from 16to 12 m, which was the most-dense layer, comprising40.8 % of the vertical LAD. However, large numbers ofunderstory trees of diverse species at about 4-m height

(Fig. 4) provided only a small canopy cover, which was5.3 % at 8 to 4-m and 15.5 % at <4-m height.

Although this forest was dominated by old Q. serrata(80–200 years old, Kang et al. 2009), a number of othershade-tolerant tree species were present under the ca-nopy besides Q. serrata saplings. Quercus spp. is lesscompetitive on the forest floor because it is a shade-intolerant species. Even in the improved light environ-ment in gaps, Quercus spp. was suppressed because itgrows slowly; most of the gaps in this forest are smalland will close rapidly (Cho 1992). In contrast, Carpinusspp. (C. laxiflora and C. cordata) was present at differentheights from the forest floor to the sub-canopy layerbecause they are shade-tolerant and grow rapidly. Long-lived Q. serrata is expected to dominate this forest, but adrastic change in species composition may occur if alarge number of Quercus spp. die suddenly, e.g., due tooak wilt disease, which is at present spreading in Korea.

Implication of phenology observations

The intervals among the phenology curves at differentcanopy heights (Fig. 5) indicate per-layer seasonalchanges fromWAD to LAD (Fig. 3), which has not beenreported previously. Ryu et al. (2014) quantified leaf-outdates for over- and under-story canopy layers at thecurrent study site. Bi-weekly measurements by LAI-2200

Fig. 7 Time-series variations in the gapfractions measured at canopy heights of0–4 m (a), 4–8 m (b), 8–12 m (c), 12–16 m(d), and >16 m (e). In these box andwhisker plots, the ends of the whisker are setat 1.5 · IQR (interquartile range) above thethird quartile (Q3) and 1.5 · IQR below thefirst quartile (Q1), where the IQR is Q3–Q1

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at different canopy heights were useful in describingvertical structure, but inadequate for estimating the leaf-out date because of low observation frequency. The LEDsensors and upward-pointing digital cameras yielded lessuncertainty in leaf-out estimates because those sensorsprovided daily LAI values. However, the LED sensorsand upward-pointing digital cameras did not monitorcanopy structure in different directions per layer. Thus,integrating the LAI-2200 and LED sensor/digital cameracould facilitate spatially and temporally continuous ob-servations of multiple layers of the canopy structure.

Seasonal variation of total LAI (Fig. 5) was largelydriven by the two species, Q. serrate and C. laxiflora,which formed the overstory canopy layer (Fig. 6). Sea-sonal variation of LAD in the other species appearedrelatively marginal, presumably because of the limitationof the light environment. Our measurements can detectthis kind of unique phases of niche occupation and thedynamics of forest vertical structure, depending on theseasons, species, and canopy heights. Therefore, long-term and periodic campaigns to obtain these measure-ments could aid in gaining a better understanding of therelationship among forest species composition and thestructure in relation to changing climatic factors.

The characteristics of the vertical light conditions(Fig. 7) seemed closely related to sunfleck, which drivesa highly dynamic light environment with brief and oftenunpredictable periods of direct solar irradiance (Chaz-don and Pearcy 1991). The sunfleck in this forest moreappeared during the leaf-on season and in the uppercanopy layers. The sunfleck was ‘‘blurred’’ on the forestfloor because incoming solar irradiance was often in-tercepted or scattered by upper-layer plant materials.

Conclusion

We quantitatively evaluated the vertical canopy struc-ture of a deciduous forest in Gwangneung, Korea usinga time-series of LAI measurements taken from multiplestories of a tower located in the forest and a vegetationsurvey. A small number of large and old Q. serrata treescovered the uppermost canopy with dense leafy andwoody materials, and �30 % of the plant materialswere vertically distributed. The sub-canopy layer wasoccupied mostly (�40 %) by foliage rather than woodymaterials, as it included large C. laxiflora. Despite thefew trees in the intermediate layer, diverse shade-toler-ant tree species occurred near the forest floor (20 %).

Per-layer phenology of each species was describedsuccessfully. However, more frequent measurementswould be required to identify days on which the char-acteristics of different canopy heights would be evident.Measurements during the leaf-on and leaf-off periodsfacilitated separate quantification of the spatial distri-bution of leafy and woody parts. The integrated inter-pretation of the vertical LAD distribution with crowndepth and species composition deepened our under-

standing of canopy strata characteristics. The gap frac-tion records used here for the estimation of LAI have thepotential to allow evaluation of light conditions insidethe canopy. These canopy structural data are valuableand should be extended to diverse global ecosystems.

Acknowledgments This work was funded by the Korea Meteoro-logical Administration Research and Development Program underthe Grant Weather Information Service Engine (WISE) project(KMA-2012-0001-A). The authors were supported by BK21 PlusProject in 2014 (Seoul National University Interdisciplinary Pro-gram in Landscape Architecture, Global Leadership Program To-wards Innovative Green Infrastructure). English editing wassupported by the Research Institute for Agriculture and Life Sci-ences, Seoul National University.

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