Understory Light Research Articles

Resiliency and response to hurricane disturbance in a tropical shrub, Ardisia escallonioides (Myrsinaceae), in south Florida

The resiliency to hurricane disturbance and the response toposthurricane environmental conditions were examined in the tropicalshrub Ardisia escallonioides (Myrsinaceae). Hurricane Andrewstruck three of four study sites in subtropical hardwood forests insouth Florida on 24 August 1992. Posthurricane understory light levelsin the disturbed sites ranged from 21 to 53% of full light in1993 and remained high in 1994; in contrast, light levels averaged only9% in the undisturbed forest. Significant differences inmortality, damage, and defoliation were observed among the threehurricane-damaged populations. Mortality was low, but adults sufferedhigh levels of damage and defoliation. Following the hurricane,populations in the most severely disturbed forests had more growth thanpopulations in lightly damaged or undamaged forests. Seed germinationand seedling growth were not associated with light availability. Nolong-term seed dormancy was observed. The observed response toposthurricane environmental conditions is consistent with understoryspecies that show release following canopy opening, but are able topersist under periods of canopy closure. The local dominance of thisspecies in many coastal forests in south Florida may be due to the highfrequency of hurricanedisturbance.

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 overstory and understory vegetation on the understory light environment in mixed boreal forests

Abstract. The percentage of above‐canopy Photosynthetic Photon Flux Density (%PPFD) was measured at 0, 50 and 100 cm above the forest floor and above the main understory vegetation in stands of (1) pure Betula papyrifera (White birch), (2) pure Populus tremuloides (Trembling aspen), (3) mixed broad‐leaf‐conifer, (4) shade‐tolerant conifer and (5) pure Pinus banksiana (Jack pine) occurring on both clay and till soil types. %PPFD was measured instantaneously under overcast sky conditions (nine locations within each of 29 stands) and continuously for a full day under clear sky conditions (five locations within each of eight stands). The percentage cover of the understory layer was estimated at the same locations as light measurements.Mean %PPFD varied from 2% at the forest floor under Populus forests to 15% above the understory vegetation cover under Betula forests. Percent PPFD above the understory vegetation cover was significantly higher under shade intolerant tree species such as Populus, Betula and Pinus than under shade tolerant conifers. No significant differences were found in %PPFD above the understory vegetation cover under similar tree species between clay and till soil types. The coefficient of variation in %PPFD measured in the nine locations within each stand was significantly lower under deciduous dominated forests (mean of 19%) than under coniferous dominated forests (mean of 40%). %PPFD measured at the forest floor was positively correlated with %PPFD measured above the understory vegetation and negatively correlated with cumulative total percent cover of the understory vegetation (R2 = 0.852). The proportion of sunflecks above 250 and 500 mmol m–2 s–1 was much lower and %PPFD in shade much higher under Populus and Betula forests than under the other forests. Differences in the mean, variability and nature of the light environment found among forest and soil types are discussed in relation to their possible influences on tree succession.

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%.

Overstory Density Affects Field Performance of Underplanted Red Oak (Quercus rubra L.) in Ontario

Abstract Red oak seedlings were underplanted in a closed-canopy mature northern hardwood stand and an adjacent shelterwood in central Ontario. Overstory density effects on seedling survival and growth were assessed 2 yr after planting. After 2 yr, seedling survival was 90% in the uncut stand and over 99% in the shelterwood. Seedlings in the uncut stand experienced negligible or negative annual increments in stem diameter and height. Seedlings in the shelterwood were about 2 mm larger in diameter and nearly twice as tall as those in the uncut stand after 2 yr. Second-year stem diameter and height distributions of planted oak were significantly different in the shelterwood and the uncut stand. Root volume, area, and dry mass were substantially larger for seedlings grown in the shelterwood than those in the uncut stand. Loss of vigor, growth declines, and increased mortality of planted oak were largely a result of extremely low understory light levels in the uncut stand. Results of this study suggest that red oak underplanted in an uncut stand 2 or more years before a shelterwood harvest will be at a competitive disadvantage once they are released. Establishment and performance of underplanted red oak is promoted by shelterwoods that provide adequate light at the forest floor and should exhibit a strong positive growth response following final overstory removal. North. J. Appl. For. 14(3):120-125

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.

Hurricane disturbance and the regeneration of Lysiloma latisiliquum (Fabaceae): a tropical tree in south Florida

Large-scale catastrophic disturbances such as hurricanes may be critical events for the regeneration of late secondary canopy tree species. The impact of Hurricane Andrew, a severe Category 4 hurricane, on the tropical tree Lysiloma latisiliquum was examined in three south Florida subtropical forests along a gradient of hurricane disturbance (high, moderate, none). The population closest to the northern eye wall of the hurricane had the highest mortality and most severe structural damage while the population near the southern eye wall of the hurricane experienced less mortality and damage. Posthurricane reproduction was least in the disturbed sites while the undisturbed site had extensive reproduction in both 1993 and 1994. Average seed production was low due to seed predation by a bruchid beetle but some populations experienced spatial and temporal escape from this seed predator. Germination from a dormant seed bank occurred only at the most severely disturbed site in 1992. Seed germination was associated with both tip-up pits and high understory light levels. In experimental trials, fluctuating heat treatments for 5 days increased seed germination. Severe hurricane disturbance is believed to have triggered germination from a dormant seed bank through strong diurnal soil temperature fluctuations resulting from extensive canopy removal. In south Florida, the temporal and spatial dynamics of both hurricane and fire disturbance are predicted jointly to influence the population dynamics of this canopy tree species.

Leaf δ13C in Pinus resinosa trees and understory plants: variation associated with light and CO2 gradients.

Our objective was to evaluate the relative importance of gradients in light intensity and the isotopic composition of atmospheric CO2 for variation in leaf carbon isotope ratios within a Pinus resinosa forest. In addition, we measured photosynthetic gas exchange and leaf carbon isotope ratios on four understory species (Dryopteris carthusiana, Epipactus helleborine, Hieracium floribundum, Rhamnus frangula), in order to estimate the consequence of the variation in the understory light microclimate for carbon gain in these plants. During midday, CO2 concentration was relatively constant at vertical positions ranging from 15 m to 3 m above ground. Only at positions below 3 m was CO2 concentration significantly elevated above that measured at 15 m. Based on the strong linear relationship between changes in CO2 concentration and δ13C values for air samples collected during a diurnal cycle, we calculated the expected vertical profile for the carbon isotope ratio of atmospheric CO2 within the forest. These calculations indicated that leaves at 3 m height and above were exposed to CO2 of approximately the same isotopic composition during daylight periods. There was no significant difference between the daily mean δ13C values at 15 m (-7.77‰) and 3 m (-7.89‰), but atmospheric CO2 was significantly depleted in 13C closer to the ground surface, with daily average δ13C values of -8.85‰ at 5 cm above ground. The light intensity gradient in the forest was substantial, with average photosynthetically active radiation (PAR) on the forest floor approximately 6% of that received at the top of the canopy. In contrast, there were only minor changes in air temperature, and so it is likely that the leaf-air vapour pressure difference was relatively constant from the top of the canopy to the forest floor. For red pine and elm tree samples, there was a significant correlation between leaf δ13C value and the height at which the leaf sample was collected. Leaf tissue sampled near the forest floor, on average, had lower δ13C values than samples collected near the top of the canopy. We suggest that the average light intensity gradient through the canopy was the major factor influencing vertical changes in tree leaf δ13C values. In addition, there was a wide range of variation (greater than 4‰) among the four understory plant species for average leaf δ13C values. Measurements of leaf gas exchange, under natural light conditions and with supplemental light, were used to estimate the influence of the light microclimate on the observed variation in leaf carbon isotope ratios in the understory plants. Our data suggest that one species, Epipactus helleborine, gained a substantial fraction of carbon during sunflecks.

Factors Influencing the Distribution of Pteridophytes in a Southeastern Ohio Hardwood Forest

GREER, G. K., R. M. LLOYD, AND B. C. MCCARTHY (Department of Environmental and Plant Biology, Ohio University, Athens, Ohio 45701). Factors influencing the distribution of pteridophytes in a southeastern Ohio hardwood forest. J. Torrey Bot. Soc 124: 11-21. 1997.-Phytosociological and environmental analyses of the pteridophyte community of a mixed-mesophytic forest in southeastern Ohio were used to evaluate the influences of major environmental gradients on species distribution and community structure. The community was dominated by Adiantum pedatim, Asplenium platyneuron, Botr-ychiunm dissectum, B. vit-ginianum, Deparia acr-ostichoides, Diplazium pycnocar-pon, Polystichum acrostichoides, and Thelypteris hexagonoptera. The first two axes of a detrended corresporndence analysis (DCA) captured <30% of the total variation. Correlations between species DCA scores and environmental parameters indicate that species, and associations identified using unweighted paired-group means analysis (UPGMA), segregate along an environmental plexus dominated by moisture and soil nitrate; both gradients correlated well with the first ordination axis. Non-significant correlations between canopy cover and the first two ordination axes indicate that the highly fluctuating nature of understory light environments may have little influence on the distribution of long-lived understory perennials such as pteridophytes. Direct gradient analyses and correlations between species presence/abundance and environmental values were used to generate more detailed information regarding species-specific distributional patterns. The data presented here emphasize the complexity of factors influencing pteridophyte communities and the need for studies to integrate demographic and environmental analyses of both gametophytes and sporophytes when studying pteridophyte communities.

First-Year Results of Partial Overstory Removal and Direct Seeding of Mahogany (Swietenia macrophylla) in Quintana Roo, Mexico

Mahogany (Swietenia macrophylla King) is the single most important commercial tree species in the forests of the state of Quintana Roo, Mexico. Mahogany regeneration has been associated with natural disturbances that increase understory light levels. A study to analyze the effect of overstory removal on seed germination and early seedling survival and growth of mahogany was conducted. Twenty-five 0.2 ha plots were treated to create different levels of overstory removal and then seeded. Two seeding methods were used, dibbling and dropping. Dropping was used to simulate the way that seeding occurs in nature. Understory light levels were measured after the overstory removal was done. A year after applying the treatments, seed germination and seedling survival were not affected by the understory light levels created by the overstory removal. It is expected that over the next several years seedling survival and growth will be more affected by understory light levels. Seedling establishment by dibbling and dropping gave similar results. There was a high correlation (r = 0.95) between the percent of overstory removal and understory light levels.

Landscape-scale evaluation of understory light and canopy structures: methods and application in a neotropical lowland rain forest

Light is a key resource controlling tree regeneration in the understory of closed-canopy old-growth forests. To evaluate the distribution of understory light environments at a landscape scale, we used stratified random sampling in a 500-ha stand of Costa Rican tropical rain forest. Fifteen 100 m long transects were placed using random coordinates within two soil–geomorphology units (flat alluvial terraces and dissected ridge-slope-swale terrain). At 2.5-m intervals we measured canopy height and slope angle, classified topographic position, and took canopy photographs with a fish-eye lens at 1 and 3 m above the ground (and at 0.6 and 5 m height at five stations per transect). Photographs were analyzed for global site factor (GSF), which is analogous to the percentage of full sun radiation reaching a point. Canopy height and GSF at 1 and 3 m above the ground were significantly autocorrelated (Moran's I) at 2.5-m intervals. The autocorrelation rapidly declined at greater intervals, reaching nonsignificance at ca. 20 m. Both canopy height and GSF at 3 m height had a weak tendency for negative autocorrelations at intervals of 25–50 m. Median canopy height (615 stations) was 23 m (range 0–37). Gaps (canopy height ≤2 m, Brokaw 1982) were only 1.5% of sample points. Gaps were more frequent on steep slopes than on terraces, ridgetops, swales, and gentle slopes. Canopy height varied significantly across this topographic gradient. At all four heights (0.6, 1, 3, and 5 m) median GSF was ≤2.4%. GSF values &gt;8% accounted for only 3% of the total sample (N = 1380). GSF was only weakly negatively correlated with canopy height and the relation was not monotonie. Under canopies 13-19 m tall, nearly all GSF values were ≤5%. Higher GSFs were more frequent under both shorter and taller canopies. Given the observed variance in GSF and canopy height, 100–200 points separated by intervals of ≥20 m are necessary to measure the forest-wide means of these variables to ±10%. We discuss implications of these results for current approaches to modeling understory light based on canopy characteristics. We compared the random background of light environments from 1–3 m above the ground at La Selva with those occupied by saplings of pioneer and nonpioneer tree species. The two pioneers (Cecropia spp.) occurred in microsites significantly brighter than random sites, while sapling microsites of all five nonpioneer species were significantly darker than random. Comparing the landscape-scale distribution of key resources with species' actual distributions at similar scales offers a quantitative method for assessing plant life histories within and among forests.

Effects of elevated CO2 and increased nitrogen deposition on photosynthesis and growth of understory plants in spruce model ecosystems.

We studied the effects of atmospheric CO2 enrichment (280, 420 and 560 μl CO2 l-1) and increased N deposition (0,30 and 90 kg ha-1 year-1) on the spruce-forest understory species Oxalis acetosella, Homogyne alpina and Rubus hirtus. Clones of these species formed the ground cover in nine 0.7 m2 model ecosystems with 5-year-old Picea abies trees (leaf area index of approx 2.2). Communities grew on natural forest soil in a simulated montane climate. Independently of N deposition, the rate of light-saturated net photosynthesis of leaves grown and measured at 420 μl CO2 l-1 was higher in Oxalis and in Homogyne, but was not significantly different in Rubus compared to leaves grown and measured at the pre-industrial CO2 concentration of 280 μl l-1. Remarkably, further CO2 enrichment to 560 μl l-1 caused no additional increase of CO2 uptake. With increasing CO2 supply concentrations of non-structural carbohydrates in leaves increased and N concentrations decreased in all species, whereas N deposition had no significant effect on these traits. Above-ground biomass and leaf area production were not significantly affected by elevated CO2 in the more vigorously growing species O. acetosella and R. hirtus, but the "slow growing" H. alpina produced almost twice as much biomass and 50% more leaf area per plant under 420 μl CO2 l-1 compared to 280 μl l-1 (again no further stimulation at 560 μl l-1). In contrast, increased N addition stimulated growth in Oxalis and Rubus but had no effect on Homogyne. In Oxalis (only) biomass per plant was positively correlated with microhabitat quantum flux density at low CO2, but not at high CO2 indicating carbon saturation. On the other hand, the less shade-tolerant Homogyne profited from CO2 enrichment at all understory light levels facilitating its spread into more shady micro-habitats under elevated CO2. These species-specific responses to CO2 and N deposition will affect community structure. The non-linear responses to elevated CO2 of several of the traits studied here suggest that the largest responses to rising atmospheric CO2 are under way now or have already occurred and possible future responses to further increases in CO2 concentration are likely to be much smaller in these understory species.

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.

Early Woody Invasion Under Tree Plantations in Costa Rica: Implications for Forest Restoration

AbstractThe role of tree plantations as facilitators of tropical forest restoration in degraded lands has been explored recently, but there are few data on the effect of different tree species on invasion of the plant understory. We evaluated early patterns of understory composition in three‐year‐old native tree plantations in lowland Costa Rica using two pure‐species treatment (Jacaranda copaia and Vochysia guatemalensis) and one mixed‐species treatment (J. copaia, V. guatemalensis, Stryphnodendron microstachyum, and Calophyllum brasiliense). We also monitored woody invasion in unplanted control areas dominated by grasses. The understory of the different plantation treatments differed in light environment, woody‐plant growth and recruitment, and quantity and quality of woody regeneration. Forest tree invasion appeared to be enhanced under Vochysia, while shrubs were more abundant under the Jacaranda and mixed‐species treatments. Woody plant growth, herbaceous cover, and understory light availability were highest under Jacaranda, intermediate under mixed species, and lowest under Vochysia. Soil‐stored seeds seemed an important source for woody plant recruitment in Jacaranda and mixed species and of minimal importance under Vochysia, probably due to light suppression. It appears that competition from grasses is a major factor influencing early woody invasion in our study area. We found no woody recruitment after one year in the unplanted controls. We suggest that to promote the use of plantations as tools of forest restoration, there is a need to gather basic ecological information on how different tree species may influence patterns of plant understory colonization.

Woodland canopy structure and the light response of juvenile Quercus lobata (Fagaceae)

I investigated competition for light between canopy plants and juvenile valley oaks (Quercus lobata Nee) in a mixed‐broadleaf woodland of California's northern Coast Ranges. Canopy effects on understory light supply were separated among the overlying adult valley oak, the adult's woody understory, and neighboring trees and shrubs through a series of light sampling surveys and measurements of the number, size, and spatial distribution of neighboring plants. Light supply in the understory was primarily influenced by neighboring plants, with no detectable effect of the overlying adult valley oak. Light supply in the understory averaged 25% full sun due to a high frequency of canopy gaps and a typically open understory. Seedling response to understory light supply was investigated in an experimental sunfleck gradient (10%, 19%, and 100% full sun). Between 10% and 100% full sun, seedling growth increased by 90% and the shoot:∗∗∗root ratio changed from 1.561 to 0.607. Shade seedlings were also taller and produced fewer, larger, and thinner leaves than seedlings grown in full sun. A field survey of the spatial distribution and crown morphology of saplings and young adults found 1) the distance between young valley oaks and neighboring overstory trees to increase with neighbor size, and 2) crowns of the young oaks to be skewed away from neighbors. Although shading by the canopy was only moderate, canopy effects on understory light supply may restrict juvenile recruitment of valley oak in this woodland.

Effect of leaf flutter on the light environment of poplars.

The dynamics of the canopy light environment for two poplar species (Populus tremuloides Michx., and P. fremontii Wats.) were characterized with an array of photocells in fixed positions within the canopy or attached directly to leaves and using a data logger that recorded photon flux density (PFD) at frequencies from 1 to 20 Hz. The majority of sunflecks were short in duration (<1 s) with a similar short interval between sunflecks. Sunflecks contribute as much as 90% of the total daily PFD in the lower canopy. Leaf flutter may cause high frequency (3 to 5 Hz) variations of PFD in poplar canopies. The amount of light intercepted by a fluttering leaf at the top of the canopy decreased with increasing flutter, whereas a fluttering lower canopy leaf showed no such trend. When leaves fluttered at the top of the canopy the understory light environment showed an increased number of shorter sunflecks. Leaf flutter may increase mean PFD for understory leaves. It also creates a canopy light environment that is more dynamic temporally and more evenly distributed spatially. The potential benefits of these changes in light dynamics are discussed.

Photosynthetic gas exchange response of poplars to steady-state and dynamic light environments.

The steady-state and dynamic photosynthetic response of two poplar species (Populus tremuloides and P. fremontii) to variations in photon flux density (PFD) were observed with a field portable gas exchange system. These poplars were shown to be very shade intolerant with high light saturation (800 to 1300 μmol photons m-2 s-1) and light compensation (70 to 100 μmol m-2 s-1) points. Understory poplar leaves showed no physiological acclimation to understory light environments. These plants become photosynthetically induced quickly (10 min). Activation of Rubisco was the primary limitation for induction, with stomatal opening playing only a minor role. Leaves maintained high stomatal conductances and stomata were unresponsive to variations in PFD. Leaves were very efficient at utilizing rapidly fluctuating light environments similar to those naturally occurring in canopies. Post-illumination CO2 fixation contributed proportionally more to the carbon gain of leaves during short frequent lightflecks than longer less frequent ones. The benefits of a more dynamic understory light environment for the carbon economy of these species are discussed.

Some measurements of the microclimate within a Sri Lankan tropical rainforest

This study characterizes the microclimate and soilwater conditions of three gaps within mixed-dipterocarp rainforest of southwest Sri Lanka. The gaps were each of a different size and within a particular part of the forest topography. The largest gap was in a valley and the smallest on a ridge. Soil water, light quantity, and temperature were recorded within demarcated zones across each gap. Although trends between gaps and within gaps are relatively easily explained, the numerical values reported give a good impression of differences and amplitudes concerned as well as of the actual values in the rainforest understory. Surface soilwater measurements revealed that the ridge gap was more prone to water stress than the valley and midslope gaps. Gap centers of all three disturbances appeared to have lower amounts of surface soil water than adjacent forest understories. Light quantities on a sunny day in the forest understory were between 0.26 and 0.92 mols, of which sunflecks contributed between 24 and 62%. These understory light levels are approximately 1% of full sun exposure. The central zone of the valley gap had the longest duration of full sun (17–54% of the daily photosynthetic photon flux density (PPFD) of full sun). The center of the midslope gap had between 12 and 22% and the ridge had between 7 and 16% of daily PPFD of full sun. Diurnal temperature ranges in gap centers are wider than in adjacent forest understories. Sunny days had the greatest ranges of difference with a maximum of 34°C on the soil surface of the valley gap center, while surface temperatures in the understory did not exceed 25°C.

Light regimes beneath closed canopies and tree-fall gaps in temperate and tropical forests

Light regimes beneath closed canopies and tree-fall gaps are compared for five temperate and tropical forests using fish-eye photography of intact forest canopies and a model for calculating light penetration through idealized gaps. Beneath intact canopies, analyses of canopy photographs indicate that sunflecks potentially contribute 37–68% of seasonal total photosynthetically active radiation. In all of the forests, potential sunfleck duration is brief (4–6 min), but the frequency distributions of potential sunfleck duration vary because of differences in canopy geometry and recent disturbance history. Analysis of the photographs reveals that incidence angles for photosynthetically active radiation beneath closed canopies are not generally vertical for any of the forests, but there was considerable variation both among and within sites in the contribution of overhead versus low-angle lighting. Calculations of light penetration through idealized single-tree gaps in old growth Douglas-fir – hemlock forests indicate that such gaps have little effect on understory light regimes because of the high ratio of canopy height to gap diameter. However, single-tree gaps in the other four forest types produce significant overall increases in understory light levels. There is also significant spatial variation in seasonal total radiation in and around single-tree gaps. Our results demonstrate that there can be significant penetration of light into the understory adjacent to a gap, particularly at high latitudes. As gap size increases, both the mean and the range of light levels within the gap increases, but even in large gaps (ca. 1000 m2) the potential duration of direct sunlight is generally brief (&lt;4 h). The major differences in gap light regimes of the five forests are largely a function of canopy height and latitude. The effects of latitude should also result in differences in gap light regimes across the geographic range of individual forest types.

Understory light levels in mature hardwood stands after partial oversotry removal

Understory light levels have significant effects on the type and amount of advance regeneration in natural hardwood stands. This study was initiated to look at the effects of partial overstory removal on understory light levels and to develop regression equations to predict understory light levels from stand characteristics and felling levels. Understory light levels were measured in five types of relatively mature, unmanaged, bottomland hardwood stands in southeastern Louisiana. Study plots were selected within each stand type to encompass a broad range of initial basal areas. Each stand type was then subjected to six levels of overstory removal. Percent basal area removed, and percent crown cover, had higher correlations with relative understory light levels than eight other stand characteristics, accounting for 71% of the variation in relative understory light level across all stand types. Regression equations presented provide a means by which relative understory light levels can be replicated even when initial stand basal areas very among treatment plots. This has important implications for further study of natural hardwood regeneration by encouragement of advance regeneration.