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 (r2>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 (r2=0.79), the model systematically underestimated light transmission. The instantaneous sunny days around noon method was the least efficient method (r2=0.68) for estimating the growing season %PPFD, although replacing instantaneous measures with the mean of two 10-min averages improved r2 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

Temporal variations in the understory photosynthetic photon flux density of deciduous stand: the effects of canopy development, solar elevation, and sky conditions

The effects of canopy development, solar elevation, and sky conditions on temporal variation in photosynthetic photon flux density (PPFD) were examined within a 9-year-old bigleaf maple stand on Vancouver Island (Canada). PPFD was measured every second and stored as 10-min averages from 18 May to 14 October 1996, at 52 microsites characterised according to their growing season %PPFD (GSP). PPFD and %PPFD variability was examined at three different temporal levels. Specific days in May, July, and September with clear and overcast sky conditions were selected to separate the effects of canopy development and solar elevation on diurnal and seasonal light variability. \ud Diurnal light variability expressed as the mean of the difference between two consecutive 10-min averages of PPFD and %PPFD decreased with increasing GSP on clear days in May. For clear days in July and September, variability was characterised by arc-shaped relationships with high variability for microsites receiving between 20 and 80% GSP and lower variability for microsites below 20 and above 80% GSP. On overcast days, diurnal variability in PPFD increased with increasing GSP while diurnal variability in %PPFD showed an arc-shape relationship. The coefficient of variation of PPFD and %PPFD decreased with increasing GSP on clear days and sunflecks decreased with decreasing GSP and from May to September. \ud Day-to-day light variability expressed as the mean difference between consecutive daily PPFD increased with increasing GSP while the mean difference between consecutive daily %PPFD was higher for microsites receiving between 20 and 80% GSP. The coefficient of variation for the daily PPFD and %PPFD was higher for microsites receiving <20% GSP compared to other microsites. Seasonal light variability showed that microsites with <50% GSP received up to eight times more light in May than in July on both clear and overcast sky conditions because of canopy development. From July to September in clear sky conditions, decrease in light was variable for microsites receiving <40% GSP; probably because of the position of microsites in relation to the solar track. On overcast days, mean daily PPFD above canopy and in the understorey was 2 to 3 times higher in July than in September while mean daily %PPFD remained stable. The possible effects of the types of diurnal and day-to-day light variability on physiological and morphological responses of understorey plants are discussed

Growth and crown morphological responses of boreal conifer seedlings and saplings with contrasting shade tolerace to a gradient of light and height

The effects of gradients in light levels and tree height on growth and crown attributes of six conifer species were studied in eastern and western Canada. Three conifers were studied in British Columbia (Abies lasiocarpa (Hook.) Nutt., Picea glauca (Moench) Voss x Picea engelmannii Parry ex Engelm., and Pinus contorta Dougl. ex Loud. var. latifolia Engelm.), and three in Quebec (Abies balsamea (L.) Mill., Picea glauca, and Pinus banksiana Lamb.). For several growth and morphological parameters, conifers reacted strongly to both an increase in light and tree height. Significant or nearly significant interactions between light classes and height were found for height and diameter growth of most species as well as for many crown attributes for both Abies and Picea. These interactions usually indicated that growth or morphological changes occurred with increasing height from a certain light level. Within a single genus, both eastern and western tree species showed the same overall acclimation to light and height. As generally reported, Pinus species showed less variation in growth and morphological responses to light than Abies and Picea species

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

Early above- and below-ground responses of subboreal conifer seedlings to various levels of deciduous canopy removal

We examined the growth of understory conifers, following partial or complete deciduous canopy removal, in a field study established in two regions in Canada. In central British Columbia, we studied the responses of three species (Pseudotsuga menziesii var. glauca (Beissn.) Franco, Picea glauca (Moench) Voss x Picea engelmannii Parry ex Engelm., and Abies lasiocarpa (Hook.) Nutt.), and in northwestern Quebec, we studied one species (Abies balsamea (L.) Mill.). Stem and root diameter and height growth were measured 5 years before and 3 years after harvesting. Both root and stem diameter growth increased sharply following release but seedlings showed greater root growth, suggesting that in the short term, improvement in soil resource capture and transport, and presumably stability, may be more important than an increase in stem diameter and height growth. Response was strongly size dependent, which appears to reflect greater demand for soil resources as well as higher light levels and greater tree vigour before release for taller individuals. Growth ratios could not explain the faster response generally attributed to true fir species or the unusual swift response of spruces. Good prerelease vigour of spruces, presumably favoured by deciduous canopies, could explain their rapid response to release

Chondrostereum purpureum as a biological control agent in forest vegetation management. II. Efficacy on Sitka alder and aspen in western Canada

A national research program was established to test the field efficacy of the native fungal pathogen Chondrostereum purpureum (Pers. ex Fr.) Pouzar) for control of hardwood vegetation. During 1995, two fungal isolates were applied as a biocontrol agent to the cut stumps of Populus tremuloides Michx. (trembling aspen) and Alnus viridis ssp. sinuata (Regel) Á. Löve & D. Löve (Sitka alder). Treatments consisted of two formulations (BC, ON) in combination with two fungal isolates (2139, JAM6), blank formulations, cutting only, triclopyr herbicide application, and an uncut control. Analysis of Sitka alder clump mortality indicated isolates JAM6 and 2139 in combination with the BC formulation caused clump mortality of 90 and 88%, respectively. The blank formulation treatments caused the lowest clump mortality and appeared to promote sprouting and growth of Sitka alder when compared with cutting alone. At the aspen installation, the BC formulation with isolate 2139 was found to be the most effective fungal treatment resulting in 84% aspen stem mortality. The results from both installations suggest that C. purpureum efficacy appears to be dependent on the virulence of the isolate and the formulation