LIGHT RELATIONS OF TERRESTRIAL PLANT COMMUNITIES AND THEIR MEASUREMENT
Authors: ANDERSON MARGARET C. 
Published: January 2008

Summary1. ‘Light’ covers a variety of related measures of radiant energy: the particular usage must be defined for each investigation.2. Light in the open varies in intensity and spectral composition, both with time and over space. These variations can usually be formulated mathematically and used to predict conditions in plant communities.3. Light may be measured directly with radiometric instruments, responding equally to all wavelengths within a certain waveband, or with photometric instruments, whose response approximates to that of the human eye. The use of the former is desirable whenever possible, as it simplifies subsequent interpretation of the results.4. Photographs covering an entire hemisphere can be used to estimate the contribution of diffuse and direct light separately under many circumstances. They are a great help with the interpretation of many forms of measurement.5. If the percentage of light transmitted by a plant community is to be treated as constant, diffuse and direct light must be treated separately. Even so, it is only the average transmission over considerable periods that is reasonably constant. The pattern of diffuse light distribution in the community is very different on clear and on cloudy days.6. In general, most diffuse light penetrates the plant community near the zenith. However, on clear days the aureole of light near the sun shining through a low‐altitude canopy gap may entirely alter this relationship.7. The distribution of direct light can be analysed from tracing the path of the sun across the canopy. The sun shines unobstructed through gaps of an angular width greater than 1/2.8. In some herbaceous communities the logarithm of relative intensity of diffuse light inside and outside the community is approximately proportional to the cumulative leaf area per unit area of ground above the level investigated. The relation varies with the angle of incidence of direct sunlight. The relation is not valid in all communities, and the errors involved in the assumption of its validity require further investigation.9. The leafy plant community reflects and transmits light selectively. It is important to allow for the interaction of changed spectral composition on the spectral sensitivity of the instrument when making estimates of albedo or percentage light transmission.10. Although the spectral characteristics of reflexion and transmission of individual leaves are nearly identical, this is not so for plant communities with a highly inhomo‐geneous structure.11. The percentage of total light transmitted usually increases with increasing cloud cover of the sky. The absolute intensity of diffuse light in the stand is usually higher when the sky is partly overcast than when the sky is entirely covered or clear. This is due to the increase of total light in the open with decrease of cloud cover, while the proportion of diffuse light in the total decreases with decrease in cloud cover. Provided the stand transmits more diffuse than direct light, the percentage transmission of the total will therefore fall as the total increases.12. Daily variation of diffuse light generally follows that in the open.13. In deciduous woods a light phase before leaf expansion and a shade phase after it can be distinguished. Each phase is characterized by a fairly constant percentage of diffuse light: that of direct light depends on daily and seasonal variation of solar altitude, increasing with increasing altitude.14. Light measurements given as percentages of that in the open may disguise important features of the variation of absolute amounts of light. Wherever possible these absolute quantities should be calculated. Where no direct meteorological data exist, it is often possible to make reasonable assumption from other data. The precise time, place and cloud conditions should be given for each measurement,15. Because of the great variety of instruments and techniques used, often inadequately described, comparison of results is nearly impossible. A thorough examination of the magnitude of the errors of each technique is badly needed.