Understory Light Regimes Research Articles

Simulating environmentally‐sensitive tree recruitment in vegetation demographic models

Vegetation demographic models (VDMs) endeavor to predict how global forests will respond to climate change. This requires simulating which trees, if any, are able to recruit under changing environmental conditions. We present a new recruitment scheme for VDMs in which functional-type-specific recruitment rates are sensitive to light, soil moisture and the productivity of reproductive trees. We evaluate the scheme by predicting tree recruitment for four tropical tree functional types under varying meteorology and canopy structure at Barro Colorado Island, Panama. We compare predictions to those of a current VDM, quantitative observations and ecological expectations. We find that the scheme improves the magnitude and rank order of recruitment rates among functional types and captures recruitment limitations in response to variable understory light, soil moisture and precipitation regimes. Our results indicate that adopting this framework will improve VDM capacity to predict functional-type-specific tree recruitment in response to climate change, thereby improving predictions of future forest distribution, composition and function.

Compositional dynamics and light regimes in a midwestern old-growth forest preserve1

Forest structure, composition, and understory light patterns were measured at Allee Memorial Woods (AMW) near the forest–prairie ecoregional boundary in west-central Indiana to determine (1) compositional trends over five decades, (2) how different disturbance histories may have affected the dynamics, (3) current understory light regimes and their relationship to sapling densities, and (4) implications for the future of the forest preserve. Permanent sampling grids were re-established during 2011–12 in an uncut stand and two others that were selectively or heavily cut during 1890–1920. All trees > 1 m tall were censused, and those > 7.5 cm in diameter were mapped and measured, to compare structure (density, mean diameter, basal area) and species composition (richness, diversity, similarity) within and across size classes. Photosynthetic photon flux (PPF) was measured in three ways and correlated with sapling densities. Substantial compositional changes occurred in both the uncut and selectively cut stands during 1959–60 to 2011. They have declined in species richness and diversity, especially in smaller size classes, and the relative densities of Acer saccharum L., Fagus grandifolia Ehrh., or both have increased in sapling (1 m tall to 7.5 cm diameter) and mid-sized (7.6–50.7 cm) strata while the overstory (> 50.7 cm) has become more heavily dominated by Quercus spp. and Liriodendron tulipifera L. Light levels under intact canopy are very low in general and sapling densities are correlated with light, especially with longer-term direct PPF measurements at 0.5 m. All three stands show patterns consistent with widespread understory shade tolerance shifts in eastern North America, and the uncut and selectively cut stands have converged in overall size class structure and composition through different pathways related to their disturbance histories. Increased dominance of the overstory by Acer and Fagus, further declines in tree species diversity, and inhibition of regeneration by less shade-tolerant species are predicted for AMW unless there are substantial changes in climate and/or disturbance.

Disponibilidad de luz bajo dosel en rodales de Abies religiosa

La disponibilidad de luz en los bosques es uno de los principales factores que influyen en el establecimiento y desarrollo de especies vegetales bajo el dosel forestal. A fin de caracterizar el ambiente lumínico bajo el dosel de Abies religiosa (Kunth) Schltdl. et Cham., así como la estructura del dosel y su relación con la diversidad vegetal a nivel del sotobosque, se establecieron sitios de muestreo en los cuales se colectó información para evaluar atributos del rodal como área basal, densidad y diámetro cuadrático. Además, se determinaron, mediante fotografías digitales hemisféricas, los parámetros estructurales del dosel, índice de área foliar, fracción de apertura del dosel e índice de transmisión de luz; los parámetros lumínicos bajo el dosel: radiación directa y difusa, frecuencia, distribución de haces de luz, parches de luz, claros de luz y claros; duración media acumulada diaria de haces de luz y duración promedio de un haz de luz. Se determinó la relación existente entre las variables de estudio mediante regresión lineal simple y de asociación de estas a partir del coeficiente de correlación de Pearson. Los resultaron mostraron relaciones significativas entre parámetros estructurales del dosel (p < 0.001) y parámetros lumínicos; mientras que las relaciones de parámetros de estructura del rodal y parámetros lumínicos (p < 0.05) tuvieron una menor correlación (r ≈ 0.50). La fracción de apertura del dosel fue el mejor predictor para el índice de área foliar, radiación directa y difusa disponible en el sotobosque, así como para el índice de transmisión de luz del dosel. La variabilidad del régimen de luz bajo dosel fue mejor explicada por los parámetros de la estructura que por las características estructurales del rodal.

Effects of intermediate-severity disturbance on composition and structure in mixed Pinus-hardwood stands

Increasingly, forest managers intend to create or maintain mixed Pinus-hardwood stands. This stand assemblage may be driven by a variety of objectives but is often motivated by the desire to enhance native forest diversity and promote resilience to perturbations. Documenting the effects of natural disturbances on species composition and stand structure, and thus successional and developmental pathways, in stands with these mixtures is essential to achieve these goals. The specific objectives of this study were to quantify and compare the impacts of an intermediate-severity canopy disturbance on woody species composition, canopy structure, understory light regimes, and species diversity in mixed Pinus (Pinus taeda and Pinus virginiana)-hardwood stands on the Cumberland Plateau in Alabama. The natural intermediate-severity disturbance disproportionately removed large Pinus stems, promoted hardwood dominance, and effectively accelerated succession. The resultant stand structure did not resemble one of the widely recognized stages of stand development and was best characterized by the mixed-stage of development. The canopy disturbance did not significantly alter canopy-layer species diversity, but seedling- and sapling-layer diversity was significantly greater in disturbed neighborhoods. Results from this study may be used as guidelines by managers of mixed Pinus-hardwood systems. To maintain a Pinus component in stands that are succeeding to hardwood dominance, canopy disturbance alone is insufficient and must be planned in conjunction with competition reduction measures in the regeneration layer, such as fire or herbicide application. Conversely, if managers wish to promote a hardwood component in pure or near pure Pinus stands, creation of variably sized canopy openings throughout the stand may recruit hardwood reproduction to larger size classes, as the intermediate-severity disturbance documented here accelerated succession toward hardwood dominance.

Determining light requirements of groundcover plants from subtropical natural forest using hemispherical photography

In order to determine light requirements of indigenous groundcover plants for potential use in urban landscaping, we conducted a plant census in Yinpingshan Nature Reserve, Dongguan, China, and measured canopy structure and understory light regimes using hemispherical photography. We found that canopy openness, transmitted direct solar radiation, and transmitted diffuse solar radiation exhibited highly significant spatial heterogeneity. Species composition and diversity of groundcover plants were highly dependent on canopy structure and understory light condition. Greater diversity and more stems of groundcover plants were associated with greater canopy openness and understory radiation in most cases. Highly significant differences in species composition were detected along canopy openness, transmitted direct solar radiation, and transmitted diffuse solar radiation gradients, respectively. We also detected indicator species for specific understory light regimes, which will provide useful information when applying such species in urban greening under various light environments.

Seasonal Variation in Understory Light Near a Gap Edge and Its Association with Conifer Seedling Survival in a Southern New England Forest

Forest edge-generated gradients in understory light are known to influence the survival of tree seedlings in North American deciduous forests, but the seasonality of that influence has not been investigated for the forests of southern New England. We examined the winter versus summer difference in photosynthetically active radiation (PAR) transmitted through a southern New England forest canopy in the vicinity of a large experimental canopy gap and investigated if this difference influenced the survival of Pinus strobus L. (White Pine) and Tsuga canadensis (L.) Carr. (Eastern Hemlock) seedlings along a gap—forest gradient using Kaplan-Meier survival curves and Cox proportional hazards models. The mean and the spatial distribution of winter understory PAR transmittance (%T) across the gap—forest gradient were significantly different from those of summer %T. Distance from gap edge into the forest and variability in %T during the winter months were associated with the survival of White Pine seedlings, but not with that of Eastern Hemlock seedlings, which had a constant survival probability throughout the gap—forest gradient. Summer %T was not associated with the survival of seedlings of either species. Experimentally, we found that digital fisheye canopy photos did not accurately capture the range in understory light level along a gap—forest gradient. Thus they are a poor proxy for direct PAR transmittance measurements and should be used with caution in southern New England forests, which are highly fragmented and contain many edges. Forest management actions for the conservation of White Pine should take into account winter understory light regimes rather than summer regimes.

Effects of intermediate-scale wind disturbance on composition, structure, and succession in Quercus stands: Implications for natural disturbance-based silviculture

Forest disturbances are discrete events in space and time that disrupt the biophysical environment and impart lasting legacies on forest composition and structure. Disturbances are often classified along a gradient of spatial extent and magnitude that ranges from catastrophic events where most of the overstory is removed to gap-scale events that modify local environmental conditions only. Without question, a paucity of data is available on disturbance events of the intermediate scale (i.e. those events too localized to be classed as catastrophic and too widespread to be considered gap scale). The specific objectives of this study were to quantify and compare canopy structure, understory light regimes, woody species composition, and tree species diversity along a gradient of canopy disturbance caused by an EF1 tornado and to analyze the influence of intermediate-scale disturbance on the successional trajectory of an upland Quercus forest. Statistically significant differences in diversity measures between control (no storm damage), light, or moderate damage class plots were only found in the sapling layer. We documented significant differences (P<0.01) in percent of intercepted PAR between the control and moderate damage classes and between moderate and light classes. Three growing seasons post-disturbance, the understory light regime had largely returned to pre-disturbance conditions. The disturbance event acted primarily as a release mechanism for advanced reproduction in the understory and for stems in the midstory. Our results provide quantitative information on disturbances of this extent and magnitude and can be used to guide silvicultural systems designed to emulate natural disturbance processes, which is an increasingly popular management approach especially on public lands.

Influence of light conditions on the allometry and growth of the understory palm Geonoma undata subsp. edulis (Arecaceae) of neotropical cloud forests

Knowledge on the growth responses of understory palms to changing light conditions within neotropical cloud forests is limited. The low light regime of these environments, in addition to persistent cloudiness, low ambient temperatures, and slow nutrient cycles, imposes significant constraints on biomass accumulation. Here, we evaluate how changes in the understory light conditions influenced the allometry and growth of G. undata subsp. edulis in two cloud forests in Costa Rica. We examined the structural relationships between stem diameter, stem height, and crown area in reproductive and nonreproductive individuals. We related the variation in stem growth and crown area with allometry, leaf production and longevity, and light conditions that we measured using hemispherical photographs over 1 year. The allometric and growth pattern of G. undata subsp. edulis was characterized by its investment in crown area, which was strongly and positively related to increments in palm height and reproduction. Growth, measured as the increase in crown area and stem height, was not explained by the variation in the light regime spanning 1 year. However, reproductive individuals were generally taller, more slender, and had larger leaf areas than nonreproductive individuals. Our results demonstrated that stem growth responses were mostly controlled by initial crown size rather than by temporal differences in the understory light regimes of cloud forests. These results suggest that cloud forest understory palms have a limited capacity to respond to light changes and rely mostly on an opportunistic strategy for biomass accumulation and reproduction.

Effects of reproduction methods and overstory species composition on understory light availability in longleaf pine–slash pine ecosystems

The restoration of longleaf pine (Pinus palustris Mill.) ecosystem types ranging from xeric uplands to hydric flatwoods is the goal of significant management efforts in the southeastern United States. Overstory species composition across ecosystem types varies from pure longleaf to mixed species stands, with slash pine (Pinus elliottii Engelm.) becoming more predominant on hydric soils, though species rich understories are prevalent throughout the landscape. Understory light regime has been determined to be one of many important environmental factors affecting regeneration and understory diversity; however it is not clear how different management regimes across ecosystem types affect light levels during restoration of degraded sites. In this study, we used Digital Hemispherical Photography (DHP) at multiple sites in northwest and north-central Florida to examine understory light availability in longleaf/slash pine forests treated with shelterwood and uneven-aged systems relative to uncut control plots. Basal area in these stands ranged from approximately 5.0m2ha−1 to 40m2ha−1 and species composition ranged from pure longleaf pine to pure slash pine. As expected, these management systems led to significant decreases in leaf area index (LAI), cover fraction, direct fraction of Absorbed Photosynthetically Active Radiation (fAPAR) and diffuse fAPAR, and increase in visible sky. These changes indicated increased light availability in shelterwood and uneven-aged stands compared to uncut control stands. Mean LAI ranged between 1.7 and 1.8 for control plots and from 0.3 to 0.9 for the various management systems. Shelterwood systems generally had the highest amount of understory light availability, while the greatest variability was observed in the group selection system. The overstory species composition also affected understory light availability. For a given basal area, longleaf pine showed greater understory light availability than slash pine. Light availability in mixed species stands differed significantly from pure longleaf pine stands only when the proportion of slash pine basal area was 70% or higher. Our observations suggest advantages of group selection management over the other management systems when understory restoration is a primary objective, but long-term monitoring of the understory will be needed for confirmation.

Understory light regimes following silvicultural treatments in central hardwood forests in Kentucky, USA

Manipulation of the light regime is a primary goal of many silvicultural treatments, but the specific light conditions created remain poorly documented for many forest types and geographic locations. To help quantify effects of silvicultural treatments on light conditions, measurements of basal area, canopy cover, and photosynthetically active radiation (PAR), measured both instantaneously and across time, were collected in central hardwood forests in Kentucky, USA following silvicultural treatments. These measurements were used to: (1) Investigate the magnitudes of differences in understory percent ambient PAR following implementation of shelterwood with reserves and thinning treatments; (2) document the spatial and temporal distribution and variability of understory percent ambient PAR in shelterwood with reserves treatments (mean residual basal area=5.2m2/ha), thinning treatments (18.5m2/ha), and untreated controls (27.1m2/ha); and (3) examine relationships between: basal area and canopy cover; basal area and measured percent ambient PAR; and canopy cover and measured percent ambient PAR.Mean light levels from instantaneous measurements were 78% of ambient in the shelterwood with reserves, 33% of ambient in the thinning and 9% of ambient in the control. Similarly, only 1.3% of the approximately 140h of PAR measurements in the controls indicated high light conditions (>60% of ambient), 15.9% in the thinning treatment and 65.4% in the shelterwood with reserves treatment. There were only 32 periods of high light found across all plots in the control, 176 periods of high light in the thinning treatment, and 441 periods of high light in the shelterwood with reserves treatment. Indexes of variability in light across time and among sampling locations within a stand did not differ statistically between the shelterwood with reserves and thinning treatments but both treatments were statistically more spatially and temporally variable than the uncut control. Simple linear regression relationships were observed between stand basal area and mean relative PAR (r2=0.8784 for instantaneous measurements, r2=0.9697 for continuous measurements), and basal area and canopy cover (r2=0.8479). Such relationships provide a means for including light management in forest planning and application of silvicultural treatments. The results for the distribution of light also suggest, however, that treated stands may have similar mean light levels, but differ substantially in the spatial and temporal distribution of light.

Influence of foliar phenology and shoot inclination on annual photosynthetic gain in individual beech saplings: A functional–structural modeling approach

We developed a functional–structural plant model for Fagus crenata saplings and calculated annual photosynthetic gains to determine the influences of foliar phenology and shoot inclination on the carbon economy of saplings. The model regenerated the three-dimensional shoot structure and spatial and temporal display of leaves; we calculated the hourly light interception of each leaf with a detailed light model that allowed us to estimate hourly leaf photosynthetic gain taking leaf age into account. To evaluate the importance of simultaneous foliar phenology and slanting shoots in beech saplings, we calculated the photosynthetic budgets for saplings with contrasting foliar phenologies and shoot inclinations. In our simulations, we distinguished between simultaneous and successive foliar phenologies, upright and slanting shoot inclinations, and environments with and without a vertical gradient in light intensity. Other model parameters (including photosynthesis vs. light curve, leaf size, and leaf shape) were obtained directly from live beech saplings. With no vertical gradient in light intensity, modeled saplings with simultaneous foliar phenology and slanting shoots (as in live beech) had larger annual photosynthetic gains than saplings with other combinations of traits. Hence, simultaneous foliar phenology and slanting shoots are efficient ways to display leaves in the shaded forest understory light regime where beech saplings thrive. In the presence of vertical light gradients, which can occur in canopy gaps, saplings with upright shoots had larger annual photosynthetic gains than counterparts with slanting shoots. Although mean daily photosynthetic gains of saplings with successive foliar phenology were elevated by exposing leaves to strong light when young and productive, the annual photosynthetic budget of these saplings was reduced (compared to saplings with simultaneous foliar phenology) by their relatively short leaf lifespan. Overall, our results suggest that slanting shoots with simultaneous foliar phenology are particularly successful in shaded environments, where beech often dominates, because they appear to maximize the annual carbon budget by avoiding self-shading and extending leaf lifespans.

Changes in understory light regime in a beech–maple forest after a severe ice storm

We assessed canopy openness (%) in an old-growth beech–maple forest immediately before and in the 3 years following a severe ice storm. We estimated canopy openness using hemispherical photographs taken at a height of 0.6 m above the soil surface in 101 permanent plots. Mean canopy openness increased from a prestorm value of 7.7% to 16.6% in the summer immediately following the storm. However, the mean canopy openness returned to prestorm levels within 3 years. The changes in canopy openness immediately after the storm were significantly influenced by canopy openness prior to the storm and also by species composition; plots with lower canopy openness prior to the storm and plots that consisted of more shade-tolerant species had greater canopy damage. While canopy gaps are often considered to promote the establishment of shade-intolerant species in the deciduous forests of eastern North America, gaps created by ice storms at our study site may not persist long enough to promote the establishment of these species.

Light attenuation following selection harvesting in northern hardwood forests

Canopy disturbance exerts a profound influence on regeneration by altering understory light regimes. Although many studies have quantified canopy disturbance rates and the attendant increases in light availability, few studies have examined the spatio-temporal dynamics of light following canopy disturbance. In this study, we used harvested stands spanning a 20-year chronosequence (0–19 years since harvest) to estimate the rate of light attenuation following selection harvesting in northern hardwood forests of central Ontario. Light availability (% of full sun) was measured along vertical transects (0.8–6.8 m in height) using hemispherical photographs taken in the center of 10 m radius plots that were inventoried to quantify the amount of basal area cut (BA cut). The rate of light attenuation was estimated as a function of height, time since harvest, and local harvest intensity (BA cut). The initial light level at 0.8 m after high (BA cut = 30 m 2/ha) and medium (BA cut = 15 m 2/ha) intensity harvests was 27.5% and 17.5%, respectively, compared to 7.5% after low intensity harvest (BA cut = 0 m 2/ha). However, the rate of attenuation near the forest floor was significantly faster in heavily cut plots. As a result, the light level at 0.8 m fell below 5% after 10 years in heavily cut plots, lower than predicted in lightly cut plots (6%). The steepness of the vertical light profiles varied with harvest intensity as well: the difference in light between the bottom and top of the light profiles was much greater in heavily cut plots (except immediately after harvest). The shape of the vertical light profiles varied with harvest intensity and time since harvest. Light increased exponentially with increasing height in plots with little or no harvesting, but the exponential curvature decreased as the time since harvest increased. In contrast, the light profile in heavily cut plots was approximately linear immediately after harvest, and became increasingly exponential as time since harvest increased.

Understory light at the fushan experimental forest in Northeastern Taiwan: Watershed and landscape perspectives

Understory light regimes have a major impact on seedling growth and patterns of forest regeneration. Many researchers have reported understory light regimes for relatively small areas, and then extrapolated their results to the landscape level with little knowledge of landscape-level heterogeneity. We used hemispherical photography to characterize understory light regimes at both the watershed and landscape scales within the Fushan Experimental Forest in northeastern Taiwan. We established 20 transects, 7 of which were located in Experimental Watershed #1 and the other 13 were scattered across an area of 150 ha at the Fushan Experimental Forest. Understory light was not normally distributed across the Fushan Experimental Forest, with some microsites having yew high light levels due to typhoon-induced canopy damage. Median understory light levels exceeded 15% of the incident sunlight at both the watershed and landscape scales. These light levels are much higher than those reported for many mature tropical and temperate forests in other parts of the world, where understory light levels of ＜5% of incident sunlight are common. Aspect, rather than spatial scale, had the largest impact on undercanopy light. In forests with rough topography, utilizing transects that run from the ridge to the valley is more likely to adequately characterize spatial heterogeneity than plots or a few longer transects.

Photosynthetically active radiation transmittance of forest plantation canopies in the Ethiopian highlands

Hemispherical photographs and a quantum sensor were used to study the canopy photosynthetic photon flux density (PPFD) transmittance of five forest plantation species in Ethiopia. Red/far-red (R/FR) ratios and temperatures beneath the forest plantations were also measured. The species studied were: Eucalyptus globulus, Pinus patula, Cupressus lusitanica, Grevillea robusta, and Juniperus procera. The canopies of E. globulus, P. patula and G. robusta transmitted about three times as much PPFD as J. procera or C. lusitanica plantations. Furthermore, plantations of G. robusta and E. globulus had the highest, while J. procera had the lowest R/FR ratio. In contrast to J. procera and C. lusitanica, E. globulus and G. robusta had relatively open crowns, higher crown-bases and lower leaf area indices, and as a result, their canopies had higher PPFD transmittance percentages as well as higher below-canopy R/FR ratios and temperatures. From the perspective of understory light regime, plantations of E. globulus and G. robusta are best suited for promoting the natural regeneration of native species. In general, opening of gaps in plantations of heavy-shading tree species may increase the herbaceous layer ground cover, enhance the colonisation and growth of native woody species and consequently may also increase the floristic diversity of mono-specific plantations.

Influence of typhoon disturbances on the understory light regime and stand dynamics of a subtropical rain forest in northeastern Taiwan

In 1996 and 1997 we examined the effects of typhoon disturbance on the understory light environment and sapling dynamics of the Fu-shan Experimental Forest, a subtropical rain forest in northeastern Taiwan. Mean understory light levels were approximately 30% of those in the open immediately following the two 1996 typhoons which affected the forest, yet in 1997, following two more typhoons, mean understory light levels were 10%–20% of those in the open. The decline of understory light levels to those present prior to the typhoons was more rapid in 1996 than in 1997, even though the two typhoons in 1996 were more intense than those in 1997. This difference might be the result of the timing of the typhoons. In 1996 the typhoons occurred earlier in the growing season, before August 1 rather than the middle and end of August in 1997. The regular defoliation caused by the frequent typhoons that impact Fu-shan (average of 1.4 per year) and the low stature of the forest (mean canopy height of 10.6m), results in much higher light levels beneath the canopy (9%–30% of levels in the open) than those found in most tropical and temperate forests. As a result, understory light levels are not limiting the distribution of canopy tree saplings within the forest and there is no evidence that canopy gaps play an important role in canopy tree regeneration within the Fu-shan Experiment Forest. This is in contrast with the pattern reported for some tropical forests. With frequent typhoons impacting northeastern Taiwan, the forests of this region are perpetually recovering from wind disturbances.

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.

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.

Biomass allocation in Aster acuminatus: variation within and among populations over 5 years

Patterns of biomass allocation were analyzed over a 5-year period for 15 patches of the forest herb, Aster acuminatus Michx., on Kent Island, New Brunswick. The patches were distributed over a wide range of understory light regimes but otherwise occupied similar environments. In every year, light level was positively correlated with mean ramet size, ramet density, and plot standing crop (grams per square metre). This suggests that the patches were at or near their light-determined carrying capacities. There was significant variation in patterns of biomass allocation among plots in every year. This variation was always correlated with plot standing crop. In general, as standing crop increased, mean relative allocation to stems, flowers, and clonal growth increased, while allocation to leaves and rhizomes decreased. Patterns of allocation varied significantly between years for those plots sampled more than once, yet among-plot relationships with standing crop were maintained. Both thinning and shading of natural patches had pronounced effects on growth and allocation. Thinning resulted in larger plants and greater relative allocation to flowers and clonal growth, compared to controls. Shading resulted in generally opposite patterns. This study documents the important role of light availability, working directly and through intraspecific competition, in determining patterns of growth and biomass allocation in A. acuminatus.