Hemispherical Canopy Photographs Research Articles

Micropropagation of Mespilus germanica L.

The data presented in this paper is supporting the research article “Variable seed behavior increases recruitment success of a hardwood tree, Zelkova serrata, in spatially heterogeneous forest environments” (Oyama et al., 2018) [1]. We provided the data of several ecological properties of the two types of the seeds (i.e. shoot seeds vs. single seeds) with distinctly different dispersal behaviors. We provide data of terminal velocity, which was measured by releasing 50 replicates of each seed type from a height of 5.0 m in dead air space in a gymnasium. We also show the data of germination cue [i.e. red:far-red (R:FR) ratios], which was examined in plant growth chambers that received three distinct R:FR ratios (0.1, 0.4, and 1.0; 16 h photoperiod) or no illumination. Further, we show the data of the rates of multi-locus outcrossing rates and biparental inbreeding in each of single- and shoot seeds. The mating system parameters were estimated by assaying a total of 80 shoot seeds and 70 single seeds that were randomly collected from Parent 1 and Parent 2 for five microsatellite loci. Finally, we show the data of hemispherical canopy photographs, which were taken at different distance from the adults using a digital camera equipped with a fisheye lens.

Solarcalc 7.0: An enhanced version of a program for the analysis of hemispherical canopy photographs

This paper describes solarcalc 7.0, a new implementation of a computer program designed for the analysis of hemispherical photographs. Our aim was to recode, optimize, enhance and recompile the previous version of solarcalc program using Realbasic, a modern, cross-platform BASIC language programming environment. The main improvements of the new version are described as follows: Programming and layout – a revision and enhancement of the original source code in a new, redesigned and user-friendly interface; Calculations – the possibility of allowing calculations for a growing season length, the possibility of defining the number of zenith and azimuth angles used for the calculations, the calculations of direct, indirect and total site factors, the inclusion of two versions of the Standard Overcast (SOC) sky weighting formula, the ability to compute leaf area index using two different methods and to compute the sky view factor, the ability to compute the total daily extraterrestrial radiation, the capacity of handling grayscale and color images and to manually or automatically threshold the images. The performance of the new version of solarcalc was assessed using a series of hemispherical images taken along a light gradient. Comparisons of calculated canopy light and structural indices between the new version of the program and gla (Gap Light Analyser) show highly significant positive correlations (R2=1.00), indicating that solarcalc performs similarly to gla across a variety of forest canopy openings. The incorporated modifications in the new version of the program allows for more detailed and more versatile analyses of hemispherical photographs and as such, represent a significant improvement in comparison with the previous version of the program. Finally, one of the most salient feature of solarcalc compared to other similar software is the possibility of using compiled versions of the program compatible with Macintosh, Windows and Linux/Unix operating systems, localised in English, French and Spanish languages.

Evaluation of irradiance metrics with respect to predicting sapling growth

Accurate and efficient measurement of photosynthetically active radiation (PAR) is critical in ecological studies. We evaluated 25 metrics of PAR with respect to predicting growth of sugar maple ( Acer saccharum Marsh.) and white ash ( Fraxinus americana L.) saplings from understory to large gap conditions. PAR metrics were derived from gallium arsenide photodiodes, hemispherical canopy photographs (film and digital), and a LI-COR LAI-2000 plant canopy analyzer. In general, percent canopy openness, estimated with film photographs or LAI-2000, best predicted growth. Mean daily photosynthetic photon flux density (PPFD) from photodiodes ranked intermediate; direct beam radiation (from digital and film photographs) was among the poorest growth predictors. Metrics that integrate direct radiation may be relatively poor predictors of growth, because sunflecks were above PPFD levels at which photosynthesis saturates but fully contributed to the calculation of mean daily PPFD. Mean daily PPFD based on truncated sunflecks (to PPFD levels at which photosynthesis saturates) improved predictions of white ash radial growth. Film canopy photographs and the LAI-2000 had relatively low measurement error (indicated by repeatability). High contrast in film photographs, compared to digital, reduced ambiguity in manual thresholding. From a plant-centered perspective, percent canopy openness measured with either the LAI-2000 or film hemispherical photographs provided the best growth predictions.

Cultivation of Non‐timber Forest Products Alters Understory Light Availability in a Humid Tropical Forest in Mexico1

ABSTRACTThe planting of non‐timber forest products (NTFPs) in the understory of tropical forests is promoted in many regions as a strategy to conserve forested lands and meet the economic needs of rural communities. While the forest canopy is left intact in most understory plantations, much of the midstory and understory vegetation is removed in order to increase light availability for cultivated species. We assessed the extent to which the removal of vegetation in understory plantations of Chamaedorea hooperiana Hodel (Arecaceae) alters understory light conditions. We also examined how any changes in light availability may be reflected by changes in the composition of canopy tree seedlings regenerating in understory plantations. We employed a blocked design consisting of four C. hooperiana plantation sites; each site was paired with an adjacent, unmanaged forest site. Hemispherical canopy photographs were taken and canopy tree seedlings were identified and measured within 12 3 × 2 m randomly placed plots in each site for a total of 96 plots (4 blocks × 2 sites × 12 plots). Plantation management did not affect canopy openness or direct light availability but understory plantations had a higher frequency of plots with greater total and diffuse light availability than unmanaged forest. Comparisons of canopy tree seedling composition between understory plantations and unmanaged forest sites were less conclusive but suggest that management practices have the potential to increase the proportion of shade‐intolerant species of tree seedlings establishing in plantations. Given the importance of advanced regeneration in gap‐phase forest dynamics, these changes may have implications for future patterns of succession in the areas of forest where NTFPs are cultivated.

The angular distribution of diffuse photosynthetically active radiation under different sky conditions in the open and within deciduous and conifer forest stands of Quebec and British Columbia, Canada

The angular distribution of diffuse photosynthetically active radiation (PAR) was characterised in the open and beneath deciduous and conifer forests in Quebec and British Columbia, Canada, under overcast and clear sky conditions, using a restricted field of view light sensor and hemispherical canopy photographs. The angular distribution of PAR was described by the relative light reading (RLR). In the open on overcast days, light was best characterized using the standard overcast sky distribution with the light intensity at the zenith set to four to five times greater than the light intensity at the horizon. RLR under forest stands was found to decrease with decreasing elevation angles under both overcast and clear sky conditions. Aspen (Populus tremuloides Michx.) and Jack pine (Pinus banksiana Lamb.) stands transmitted more light from a relatively wider angle around the zenith than the spruce (Picea glauca (Moench) Voss and Picea mariana (Mill.) BSP) stands, which transmitted light mainly from the zenith. RLR estimated with the hemispherical canopy photographs (RLR hc_corr) generally provided a comparable prediction of the effect of the canopy composition on the angular distribution of PAR.

Canopy closure and amphibian diversity in forested wetlands

The crowns of trees and shrubs often overtop temporary wetlands in forested regions. By shading pond basins, canopy can dramatically change the conditions experienced by residents such as amphibians. In this study, we estimated the presence of 8 amphibian species across 17 temporary wetlands at the Yale–Myers Forest in northeastern Connecticut, USA. In addition, we quantified the light environment using a grid of hemispherical canopy photographs to calculate Global Site Factor (GSF) within each wetland. Amphibian richness was low in most wetlands, and most wetlands were relatively shaded. Amphibian richness increased in lighter wetlands. This result was not confounded by relationships with wetland size. Most amphibian species tended to be absent from heavily shaded wetlands (‘open canopy specialists’). However, three species were often found in the shadiest wetlands (‘canopy generalists’). Field transplant experiments using one canopy generalist and one open canopy specialist showed that development of the generalist was less affected by wetland light levels compared with performance of the specialist. These findings suggest that canopy may be an important determinant of amphibian diversity patterns across wetlands. Further, conservation strategies dependent on universally applied, inviolate shoreline vegetation buffers may inadvertently contribute to species loss. Because species differ in their sensitivity to changes in canopy, these losses may be predictable.

Using airborne and ground-based ranging lidar to measure canopy structure in Australian forests

Airborne and ground-based lidars are useful tools to probe the structure of forest canopies. Such information is not readily available from other remote sensing methods but is essential for modern forest inventory in which growth models and ecological assessment are becoming increasingly important. This study was undertaken to investigate the capacity of current airborne and ground-based ranging systems to provide data from which useful forest inventory parameters can be derived. Additional data collected included standard forest inventory, hemispherical photography, and optical point-quadrat sampling. Four contrasting study sites were established within an existing study area in the Bago and Maragle State Forests, New South Wales, Australia. A simple and standard set of models was fitted to the data to establish consistency between methods and current practice. Methods to reduce the bias induced by interaction of the size of the airborne laser scanner (ALS) footprint and thresholding used in ranging systems are demonstrated by the use of first and last returns and the intensity of the returns. A measure analogous to predominant height was calculated from an average of a number of the highest ALS returns within an area. This estimate agreed with field measured predominant heights within the uncertainty of the measurements. Data from a ground-based scanning rangefinder system were used to model leaf area index (LAI). These LAI estimates coincided with those from hemispherical canopy photographs. The validation work presented in this paper justifies further development of the instrumentation and analyses to combine results from multi-angular systems with data from airborne systems to alleviate some of the problems associated with the vertical view. Current laser ranging systems can be used to derive canopy structural parameters such as height, cover, and foliage profile provided information based on multiple returns or the intensity of returns is used to minimize the bias induced by the size of the footprint and the detection threshold.

Effects of a wide gradient of retained tree structure on understory light in coastal Douglas-fir forests

We characterize understory light of seven stands that varied along a gradient of tree retention. Using hemispherical canopy photographs and digital image, we estimated gap light or solar radiation reaching the understory through the canopy. Using nonlinear regressions, we related gap light to several structural attributes in the examined silvicultural treatments. The silvicultural treatments affected both the median and range of gap light in the understory. As overstory removal increased from uncut second growth to green-tree retention, the median value of light increased from 8 to 68% full sun, while the range of light increased from 3–22% to 26–88% full sun. We found strong, significant, and negative nonlinear relationships between gap light at a particular microsite (0.04 ha) in the understory and the height, diameter at breast height, density, and volume of surrounding retained trees (ra2 = 0.77–0.94). These relationships can aid planning of treatments that retain forest structure, such as variable retention, by allowing predictions of understory light from commonly used field data. These predictions allow forest managers to understand some of the ecological consequences and tradeoffs associated with retaining structure during harvesting.

A study of the effect of logging on the understorey light environment in riparian buffer strips in a south-east Australian forest

Commercial timber harvesting is carried out across much of the wet sclerophyll forests of south-east Australia. Undisturbed forest buffers are retained along streamlines to maintain aquatic and riparian habitat values, but the consequences of this strategy for many habitat values are not well understood. Light penetration plays a major role in the ecology of the riparian forest and protection of the light environment is seen as an important function of buffers that aim to protect riparian habitat values. We used hemispherical canopy photographs, taken at 1, 3.4 and 6.8 m at 10 m intervals along 100 m transects to describe the spatial variation in the forest understorey light environment along a gradient from streamside vegetation to the upslope eucalypt-dominated forest. Post-logging photographs taken at the same points were used to model the light penetration edge effects following clearfelling. We also examined the relationship between light penetration and physical and vegetation characteristics. The natural understorey light environment was influenced by proximity to the streamline to about 50 m upslope, light penetration increasing at a relative rate of about 9% for every 10 m from the streamline. Light penetration was also influenced by topography and vegetation characteristics. There was a high level of spatial heterogeneity, but the light environment was relatively unchanged over a 9-month period, indicating temporal stability. Creation of a sharp edge by logging of the upslope forest resulted in major changes in light penetration. The zone close to the edge experienced increases in light penetration in excess of 100%, with reducing but detectable changes penetrating to 70–100 m.

THE ROLE OF VEGETATION DYNAMICS IN CHOCTAWHATCHEE SAND PINE INVASION OF LONGLEAF PINE FORESTS

The expansion of Choctawhatchee sand pine (Pinus clausa) into longleaf pine forests (Pinus palustris) has been widespread on the Florida Panhandle since the early 1900s. This study examines the population-level processes of this sand pine invasion. Expansion of sand pine started by 1920, but rapidly increased in the 1940s. Population establishment and growth in invasive stands has been rapid, with establishment of many sand pine pioneer individuals followed by rapid infilling as these individuals reached reproductive maturity. Both noninvasive and invasive stands had numerous sand pine seedlings and saplings. Sand pine will likely persist in these communities in the near future. Analyses of hemispherical canopy photographs and regeneration patterns illustrate the ability of sand pine to establish under a variety of light conditions. Radial growth analysis of incremental tree cores highlights the importance of episodic wind disturbances in maintaining these sand pine populations. [Key words: sand pine, longleaf pine, Pinus clausa, Florida, pine invasion, Pinus palustris.]

Variation in stand structure, light and seedling abundance across a tropical moist forest chronosequence, Panama

Abstract. We asked whether forest structure and understory light environments across a tropical moist forest chronosequence followed predictions of a 4‐phase model of secondary succession (establishment, thinning, transition and steady‐state) and whether seedling density and diversity were functions of light availability as predicted by this model. Using aerial photographs, we identified eight second‐growth stands (two each aged ca. 20, 40, 70, and 100 yr) and two old‐growth stands within Barro Colorado Nature Monument, Panama. Trees and seedlings were sampled in nested, contiguous quadrats in 2 160‐m transects in each stand. Light was measured as percent transmittance of diffuse photosynthetically active radiation (TPAR) at each seedling quadrat and by estimation of percent total incident radiation during the growing season from hemispherical canopy photographs. Basal area, tree density, and canopy height followed predictions of the 4‐phase model. Percent total radiation, but not TPAR, declined with stand age as did seedling density. While seedlings were more likely to occur in quadrats at higher light levels, much variation in seedling density was not related to light availability. Seedling patch sizes were small irrespective of light patches, estimated as semivariance ranges. Seedling species richness was a function of seedling density; estimates of species diversity unbiased by density did not vary systematically as a function of stand age. Proximate seed sources, efficient dispersal mechanisms, and appropriate establishment conditions can promote establishment of species‐rich communities early in successions of heterogeneous tropical moist forest.

Forest fire smoke and a test of hemispherical photography for predicting understorey light in Bornean tropical rain forest

Hemispherical canopy photographs are now widely used to estimate the proportion of above canopy PPFD (photosynthetic photon flux density) that is transmitted to the forest understorey. Some studies have obtained good results, while others have found that predictions are not always independent of forest type or condition, and that errors are sometimes large beneath very dense canopies. Lack of repeatability can make it difficult to compare sites, studies, or photographs taken at different times. In this study the method for calibrating predictions from photographs was tested in logged and undisturbed tropical broadleaf forest in Borneo, during both the wet and dry seasons. During the dry season a regional smoke ‘haze’ caused by widespread forest fires provided very uniform diffuse conditions and thus a unique test of the calibration. Total transmission (the ‘global transmission factor’, T t) was modelled as the weighted average of direct and diffuse transmission, with the weighting on diffuse transmission ( f d′) determined as the value which maximised the correlation between T t and measured PPFD. f d′ is an indirect measure of f d, the actual proportion of above canopy PPFD that is diffuse. f d′ should increase with cloudiness or smoke haze. Accurate determination of f d′ is important for prediction of transmission over short time-scales, such as days or weeks. f d′ was modelled for each day and was correlated ( r 2 = 0.65) with daily shadow band measurements of the above canopy diffuse component during the wet season. During the dry season, f d′ initially decreased with the expected increase in sunshine hours, then the smoke haze halved above canopy PPFD and f d′ increased from less than 0.5 to 1.0, thus validating this method of calibration. After calibration, predictions of T t from photographs in logged forest were repeatable, accurate ( r 2 > 0.96) and linear for sites with PPFDs ranging from less than 5% of above canopy PPFD to sites with more than 50% of above canopy PPFD. f d′ modelled from monthly or longer averages of transmission factors and PPFD was within 0.12 of f d. Results were more variable ( r 2 = 0.68) if calibration was based on data from undisturbed forest only. Overall accuracy and repeatability between sites were attributed to increased digitised image resolution, the use of a spot meter with a narrow field of view to set photograph exposure times, and other improvements in methodology.

Predicting and managing light in the understory of boreal forests

This paper reviews current information relating to the dynamics of light in northern and boreal forests and discusses factors affecting overstory light transmission, seasonality of light, sunflecks, canopy gaps, and understory development, particularly with regard to tree regeneration. Techniques for measurement of light in forests such as radiometers, photosensitive paper or chemicals, hemispherical canopy photographs, the plant canopy analyzer, or visual estimators of canopy density are each discussed in terms of their accuracy, costs, ease of use, and conditions required during measurement. Predictive models of light transmission based on canopy architecture are also described in terms of their assumptions, accuracy, and input data costs. Lastly the paper discusses the relationship among overstory and understory densities, ground-level light, and "windows of opportunity" for regeneration of trees in the understory following management interventions.

Ecology of Aglaia mackiana (Meliaceae) Seedlings in a New Guinea Rain Forest1

The large seeds of Aglaia mackiana (Meliaceae) germinate and produce vigorous seedlings under closed canopies or in large gaps. To assess seedling ecology after germination, we measured growth, herbivore damage, and survivorship of seedlings over one year. The sample included shaded seedlings from dispersed seeds, undispersed seeds under parent trees, and seedlings transplanted to gaps. We quantified the light environment using hemispherical canopy photographs taken above seedlings at the beginning and end of the one–year study. Seedlings transplanted to gaps grew faster and had more leaves, larger total leaf surface area, longer secondary roots, and greater root mass than shaded seedlings. Seedlings in gaps did not differ from shaded seedlings in survivorship or amount of herbivore– and pathogen–caused leaf damage. The canopy photographs taken one year apart suggest there is a rough equilibrium in closed canopies with slight changes occurring around an average light level. Sites with 0.06 ISF tended to close; few gaps grew larger. Seedlings under parenr trees and seedlings away from parent trees had similar amounts of leaf damage and virtually identical survivorship after 18 months, but seedlings under parent trees had slower growth rates and smaller total leaf surface areas. Dispersal did not strongly benefit seeds via escaping high levels of mortality or comperition around the parent.

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.

Understory Light Regime, Shrub Layer and Sand Pine (Pinus clausa) Regeneration in Four Scrub Stands

-Estimates of direct and indirect light regime were derived from hemispherical canopy photographs for four Florida sand pine (Pinus clausa) scrub stands. Shrub foliageheight density and sand pine regeneration were tallied in 4 m2 cells around photo points. We found negative correlations between shrub foliage-height density and understory light regime in all four stands. Sand pine regeneration was spatially associated with higher surface light levels and reduced shrub foliage-height density within only one of our four stands, a recently burned (early 1970s) peninsular site. We found more consistent evidence of shade intolerance in our two Ocala sand pine stands. Sand pine regeneration was abundant in high light patches within an open, recently burned site; however, in an adjacent 60-yr-old, closedcanopy stand, regeneration was lacking despite a local seed source. Our two Choctawhatchee sand pine stands were characterized by higher surface light regime values than the two peninsular stands and sand pine seedling growth rates were lower than in the one peninsular stand that had regeneration. The closed-canopy stanid had a sparse shrub layer; regeneration was ample in this stand, but did not covary with light regime or shrub foliage-height density. The open-canopy stand was in a coastal dune setting with stunted canopy trees exceeding 100 yr of age. Sand pine regeneration in this stand was not sufficiently abundant to test for spatial association with light regime or shrub layer.

A three-dimensional crown architecture model for assessment of light capture and carbon gain by understory plants

A model simulating the three-demensional crown architecture of a plant was developed with the objective of assessing the light capture and whole-plant carbon gain consequences of leaf display in understory plants. This model uses geometrical measurements taken in the field to reconstruct the projected image of a plant so that light absorption from any direction can be assessed. The photon flux density (PFD) from a given direction was estimated from the canopy openness derived from hemispherical canopy photographs and equations simulating the daily course of direct and diffuse PFD. For diffuse PFD, the directional fluxes and absorbed PFD were integrated over 160 different directions representing 8 azimuth classes and 20 elevation angle classes. Direct PFD absorption was determined for the time that a solar track on a given day intersected a canopy gap. Assimilation rate was simulated for the sunlit and shaded parts of leaves separately and then summed to give the whole-plant carbon gain. Comparisons of simulations for a tropical forest edge species, Clidemia octona, and an understory species, Conostegia cinnamomea, illustrate the operation of the model and show that the edge species is more efficient at capturing side light while the understory species is slightly more efficient at capturing light from directly above, the predominant light direction in this environment. Self-shading within Conostegia crown and steep leaf angles in the Clidemia crown reduced light capture efficiencies for light from directly above. Whole-plant daily carbon gain was much higher in the forest edge site, mostly because of the additional PFD available in this site. However, simulations for both species in the understory light environment show that the higher light capture efficiencies of the understory species in this environment conferred a 27% advantage in carbon gain in this environment.

The Effects of Gap Size and Age on the Understorey Herb Community of a Tropical Mexican Rain Forest

1. The effects of gap age and size on understorey herb communities and their light environments (indirect skylight and direct sunlight estimated from hemispherical canopy photographs) were investigated in a tropical rain forest in Los Tuxtlas, Veracruz, Mexico. Thirteen gaps, ranging in size from 70 to 700 m 2 and ranging in age from young (1-2 years old) to old (>>5 years old), were studied. 2. The mean amounts of relative indirect skylight and relative direct sunlight were 0.083 and 0.116, respectively

Growth and Reproductive Allocation of Adenocaulon Bicolor Following Experimental Removal of Sunflecks

We used experimental and multiple—regression approaches to test the hypothesis that the availability of direct light (sunflecks) is important for the growth and reproduction of Adenocaulon bicolor, a perennial herb occurring in the understory of a coastal redwood forest. Plants were shaded from direct—beam light (PFD = photosynthetic photon flux density) with shadow bands that still allow receipt of most of the diffuse light. After two growing seasons the shaded plants receiving only diffuse light were significantly smaller than control plants that received both direct and diffuse light. In general, shaded plants decreased in size whereas those receiving sunflecks remaining at the same size. Reproduction was reduced more than vegetative size in the shaded plants. Despite the dramatic effect of removing sunflecks from the light energy available to individual plants, we found no significant relationship between plant performance and the amount of direct PFD available in a microsite, as predicted from hemispherical canopy photographs. Instead, the size and reproductive output of plants growing under natural light regimes was correlated with the average daily diffuse light availability. Although additional direct PFD could be expected to increase carbon gain, the lack of a correlation between the amount of PFD and plant performance may be due to other factors such as water or nutrient stress that increase in microsites with increased direct light.

Understorey light environments in a north-east Australian rain forest before and after a tropical cyclone

ABSTRACTThis paper describes estimates of canopy openness and associated light availability in the understorey of a north-east Australian rain forest before and immediately after a tropical cyclone. On the basis of 20 hemispherical (fisheye) canopy photographs it was shown that direct, diffuse and total site factors increased significantly as a result of the slight-to-moderate canopy disturbance caused by the cyclone. In the understorey, median total site factors ranged from 2.5–3.4% before the cyclone and from 6.0–8.6% after the cyclone, representing a 2- to 3-fold increase in potential light availability. Following the cyclone, mean relative gap frequencies increased substantially at all altitudes but particularly at canopy positions more than 70° above the horizon. Cyclone-induced canopy disturbance not only reduced the complexity of the understorey light regime but may have also increased the seasonal variability of light within the understorey of the forest during the interval of canopy recovery. The implications of these results for the ecophysiology of understorey tree seedlings and saplings at several temporal scales are discussed.