Gap Light Index Research Articles

Height growth of planted black spruce seedlings in response to interspecific vegetation competition: a comparison of four competition measures at two measuring positions

Four simple measures of interspecific competition (percent cover visually estimated in the field, percent cover derived from hemispherical photographs, percent full sunlight measured by a ceptometer, and gap light index derived from hemispherical photographs) obtained at two reference positions (the top and the middle of crop seedlings) were evaluated in relation to two growth variables (relative height growth rates in 1998 and during 1996 to 1998) of black spruce (Picea mariana (Mill.) BSP) seedlings planted on boreal mixedwood sites in southeastern Manitoba. The four competition measures assessed at the two measuring positions explained 57.2-68.0% of the total variation in black spruce height growth rate. Significant relationships were found among the four measures, and between the two measuring positions for each measure. The measuring position was not critical for all competition measures except the percent full sunlight measured by the ceptometer, for which the middle position was much better. When assessed at their preferred positions, the four competition measures ranked as follows: (i) percent cover derived from hemispherical photographs or percent full sunlight measured by the ceptometer; (ii) gap light index derived from hemispherical photographs; and (iii) visually estimated percent cover of vegetation.

Height growth of planted black spruce seedlings in response to interspecific vegetation competition: a comparison of four competition measures at two measuring positions

Four simple measures of interspecific competition (percent cover visually estimated in the field, percent cover derived from hemispherical photographs, percent full sunlight measured by a ceptometer, and gap light index derived from hemispherical photographs) obtained at two reference positions (the top and the middle of crop seedlings) were evaluated in relation to two growth variables (relative height growth rates in 1998 and during 1996 to 1998) of black spruce (Picea mariana (Mill.) BSP) seedlings planted on boreal mixedwood sites in southeastern Manitoba. The four competition measures assessed at the two measuring positions explained 57.2-68.0% of the total variation in black spruce height growth rate. Significant relationships were found among the four measures, and between the two measuring positions for each measure. The measuring position was not critical for all competition measures except the percent full sunlight measured by the ceptometer, for which the middle position was much better. When a...

A comparison of several methods for estimating light under a paper birch mixedwood stand

In 1996 we initiated a study to evaluate several techniques for measuring light under broadleaf canopies. Hourly average photosynthetic photon flux density and percent transmittance were measured 1 m above the ground at four points in each of three canopy densities created by a spacing experiment in a 35-year-old paper birch (Betula papyrifera Marsh.) dominated stand located near Prince George, B.C. At each point, fisheye photographs were taken and LAI-2000 plant canopy analyzer (LAI-2000), spherical densiometer, and competition index (Lorimer's index) measurements were made. Percent transmittance measurements on an overcast day (1-h average), transmittance measured over periods of 3 h or longer on a clear day, LAI-2000 diffuse noninterceptance measurements, and gap light index determined from fisheye photographs were strongly correlated with growing season percent transmittance (r2>= 0.96) as was competition index (r2 = 0.928). Concave spherical densiometer measurements and midday percent transmittance measurements on clear days were also well correlated with measured percent transmittance (r2>= 0.89). Estimates of understory light by the LITE model were strongly correlated with growing season percent transmittance. Correlations improved with increasing length of the period simulated (r2 = 0.755 for a point measurement on a clear day; r2 = 0.936 for an entire sunny day; and, r2 = 0.953 for the entire growing season). However, this version of the model underestimated percent transmittance in these spaced birch stands by 34-90%.

An empirical model for predicting the gap light index in an even-aged oak stand

An empirical model for predicting light levels under a canopy gap in an even-aged oak stand is presented. The effects of gap size, distance from the gap, and directional position (north–south) within the gap on gap light index (GLI) [sensu Canham, C.D. 1988. An index for understory light levels in and around canopy gaps. Ecology 69, 1634–1638] are quantified by multiple regression analysis. Data were obtained by the `erasing method' (Yanagisawa et al., in preparation 1 In order to estimate the light levels after the cutting, hemispherical photographs taken before the actual cutting were computerized and all the crowns to be cut (identified by field observations) were digitally erased (named `erasing method'). Light levels predicted by using the erasing method were highly correlated with that calculated by the ordinary photograph-analyzing method after the actual cutting ( R 2=0.911, P<0.001). 1 ) which predicts the GLI from computerized images of hemispherical photographs. Forty-five `virtual canopy gaps' were assumed, and at 945 points, light levels were calculated using HEMIPHOT [ter Steege, H. 1993. HEMIPHOT, A Programme to Analyze Vegetation Indices, Light and Light Quality from Hemispherical Photographs. The Tropenbos Foundation, Wageningen, The Netherlands]. The model presented in this study has the advantage that the indices needed for the prediction can be measured easily in field survey. The applicability of the model is discussed.

Effects of potential PAR on shoot extension in juveniles of the main tree species in a Japanese temperate forest

We studied the relationship between potential photosynthetically active radiation (PAR), estimated from hemispherical photographs, and shoot extension rates of juveniles of 12 seral tree species. We distinguished between direct light site factor (DIR), diffuse light site factor (DIF), and gap light index (GLI). We used a log‐linear model to relate growth to DIR, DIF and GLI. Potential PAR explained shoot growth rates significantly in 33 cases out of 36 (12 species × 3 PAR). DIF explained the shoot extension rates better than DIR and GLI for 10 of the 12 species. The mean values of maximum shoot extension rates of the tolerant, the intermediate and the intolerant species were 52.4 cm year–1, 64.3 cm year–1 and 87.7 cm year–1, respectively. The maximum shoot extension rates of the tolerant and the intolerant species were recorded in about 50% DIF and more than 70% DIF, respectively. The minimum light level for seedling establishment of the intermediate and intolerant species was more than 20% DIF. We explained distribution characteristics and shoot extension of the species in relation to light in terms of shade tolerance. Predictions based on light‐growth curves and maximum growth curves were similar to field observations made elsewhere, suggesting that these models may be useful to predict extension growth of juvenile trees in mixed species stands.

Comparison of various methods for estimating the mean growing season percent photosynthetic photon flux density in forests

Five methods for estimating the mean growing season percent photosynthetic photon flux density (PPFD) were compared to continuous measurements of PPFD throughout the growing season within a young bigleaf maple stand on Vancouver Island (Canada). Measured PPFD was recorded continuously as 10-min averages over the growing season (May 18–October 14, 1996) using 52 gallium arsenide phosphide photodiodes in the understory and a LI-COR quantum sensor (LI-190SA) in the open. Photodiodes were randomly located on a systematic grid of points and represented a wide range of above canopy openings which were classified into three different types of light environments: closed canopy, gaps of various sizes, and open canopy. Objectives of this study were to compare different methods for estimating the growing season %PPFD and to determine the efficiency of these methods in the three light environments. At each photodiode location, instantaneous light measurements using a Ceptometer on sunny days around noon and a LAI-2000 Plant Canopy Analyzer were made and hemispherical canopy photographs were taken. 10-min averages recorded by the photodiodes during completely overcast sky conditions were used as surrogate values for a method that uses instantaneous measurements on overcast days. Finally, a new light model (LITE) developed to estimate growing season %PPFD in a deciduous canopy was tested. All these five methods provided estimates of growing season %PPFD and are much less time consuming than continuous measurements of %PPFD using photodiodes. The three most accurate ( r 2>0.89) methods to estimate the growing season %PPFD were the 10 min averages on overcast days, the diffuse non-interceptance calculated using the LAI-2000, and the gap light index (GLI) calculated from the hemispherical canopy photographs. These three methods performed similarly in each type of light environment. Although the relationship between the LITE model and the growing season %PPFD was good ( r 2=0.79), the model systematically underestimated light transmission. The instantaneous sunny days around noon method was the least efficient method ( r 2=0.68) for estimating the growing season %PPFD, although replacing instantaneous measures with the mean of two 10-min averages improved r 2 to 0.84. Estimates on sunny days tended to be low in low light and high in high light. Practical considerations such as equipment availability, cost, sampling and processing time, sky conditions, and the number of microsites to be sampled should be taken into account in the selection of the suitable method for a particular study.

A comparison of several methods for estimating light under a paper birch mixedwood stand

In 1996 we initiated a study to evaluate several techniques for measuring light under broadleaf canopies. Hourly average photosynthetic photon flux density and percent transmittance were measured 1 m above the ground at four points in each of three canopy densities created by a spacing experiment in a 35-year-old paper birch (Betula papyrifera Marsh.) dominated stand located near Prince George, B.C. At each point, fisheye photographs were taken and LAI-2000 plant canopy analyzer (LAI-2000), spherical densiometer, and competition index (Lorimer's index) measurements were made. Percent transmittance measurements on an overcast day (1-h average), transmittance measured over periods of 3 h or longer on a clear day, LAI-2000 diffuse noninterceptance measurements, and gap light index determined from fisheye photographs were strongly correlated with growing season percent transmittance (r2>= 0.96) as was competition index (r2 = 0.928). Concave spherical densiometer measurements and midday percent transmittance measurements on clear days were also well correlated with measured percent transmittance (r2>= 0.89). Estimates of understory light by the LITE model were strongly correlated with growing season percent transmittance. Correlations improved with increasing length of the period simulated (r2 = 0.755 for a point measurement on a clear day; r2 = 0.936 for an entire sunny day; and, r2 = 0.953 for the entire growing season). However, this version of the model underestimated percent transmittance in these spaced birch stands by 34-90%.

Influence of light conditions in canopy gaps on forest regeneration: a new gap light index and its application in a boreal forest in east-central Sweden

Forest light conditions change dramatically after canopy gap formation; other micro-environmental changes such as air and soil temperatures, and soil moisture can be related to changes in radiation. Light conditions therefore play a key role in gap-phase regeneration of forests. In this study, a new gap light index, GAPLI (GAP Light Index) indicating the light conditions at any point in any gap was developed. It is a function of several parameters, including site (latitude and longitude, coordinates of the point in the gap), vegetation-structural (mean height and cover of the canopy layer of the forest), geometrical (long and short axes and orientation of the gap) and climatic (mean cloudiness of the region). The model was tested in 30 gaps with different sizes and orientations in a Picea abies forest in Fiby Urskog, Uppsala, east-central Sweden. The results showed that the mean annual growth (in the last 5 years) of spruce seedlings was highly significantly correlated with GAPLI (adjusted R2 = 0.83, P = 0.0001 in 145 quadrats in 29 gaps; adjusted R2 = 0.47, P < 0.0001 in 104 quadrats in a single gap). The mean annual growth of P. abies seedlings coincides with a range of GAPLI values from 0.9 to 1.7, centred at GAPLI 1.3; that of Betula pubescens appeared in a narrower range (1.3–1.5) but also centred at 1.3, while the mean growth of Pinus sylvestris seedlings was related to light conditions with GAPLI higher than 1.5. Moreover, most spruce seedlings appeared in the shaded parts of canopy gaps (i.e. those with low GAPLI values). It is concluded that GAPLI can be used to predict the growth and distribution of seedlings in forests; with further testing, it may be useful in other types of forest and in other locations.A computer program ‘Light’ was written for the calculation not only of GAPLI, but also sun height and direction (azimuth angle) at any time of the year and at any location on earth. The program can be obtained from the author.