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.Résumé : Cet article passe en revue les connaissances courantes sur la dynamique de la lumière dans les forêts nordiques et boréales. Les auteurs y discutent des facteurs qui affectent la transmission de la lumière dans le couvert, les variations saisonnières de la lumière, les trouées de lumière, les trouées du couvert et le développement du sousétage, particulièrement en relation avec la régénération des arbres. Les diverses techniques de mesure de la lumière, telles que l'utilisation de radiomètres, de produits chimiques ou de papiers photosensibles, de photographies panoramiques du couvert, d'analyseurs de la composition du couvert ou d'estimateurs visuels de la densité du couvert, sont analysées sur la base de leur précision, de leur coût, de leur facilité d'utilisation et des conditions requises pour effectuer les mesures. Les modèles de prédiction de la transmission de la lumière qui reposent sur l'architecture du couvert sont aussi décrits en fonction des hypothèses sur lesquelles ils reposent, de leur précision et du coût d'acquisition des données. Finalement, les auteurs discutent des relations entre la densité de l'étage dominant et celle du sous-étage, de la lumière au niveau du sol et des circonstances opportunes pour la régénération des arbres en sousétage suite à des interventions d'aménagement.[Traduit par la Rédaction] Review / Synthèse 811

Five methods for estimating the mean growing season percent photosynthetic photon¯ux 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 in the open. Photodiodes were randomly located on a systematic grid of points and represented a wide range of above canopy openings which were classi®ed 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 ef®ciency 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 ®ve 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 ef®cient 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. #

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%.

We examined variation in species composition in a low-diversity, anthropogenic grassland in response to 11 years of nitrogen (N) manipulation and disturbance. The speciespoor grassland (2-3 species/0.5 m 2 ) represents a wide spread vegetation type (>10 million ha in North America) dominated by the introduced perennial grasses Bromus inermis and Agropyron cristatum. Four levels of N and three of soil disturbance were applied in all combinations to plots (5 · 15 m, N = 120) in a completely randomized design each year. Seeds or transplants of 47 species were added to ensure that dispersal was not a barrier to changes in species composition. After 11 years of treatment, all but the most disturbed plots continued to be dominated by B. inermis. The cover of the second-most abundant species, A. cristatum, decreased with disturbance but did not vary significantly with N. Despite the lack of changes in the identity of the dominant species, our environmental manipulations strongly influenced ecosystem characteristics. Added N increased soil available N, and decreased the cover of bare ground and light availability. Soil disturbance decreased aboveground biomass, and increased the cover of bare ground and light availability. Sawdust application, designed to decrease N availability, significantly reduced community biomass, and increased light availability and the cover of bare ground, but did not alter nutrient availability or species composition. The results highlight the difficulty of restoring diversity in species-poor, anthropogenic communities dominated by introduced species, and thus the importance of conserving remnants of diverse natural grasslands.

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