Hemispherical Photography for Silviculture and Forest Inventory

Canopy parameters are measured during forest inventories and to make silviculture recommendations. Despite the rise of satellite imagery and aerial photos, field estimation of trees and stands remain popular. Recently improved field-based methods, like hemispherical photography, can be powerful additions to the forester’s toolkit. So, how is hemispherical photography helpful in the forests?

How Hemispherical Photography Can Help in Forest Inventory

Often, conventional instruments and techniques, such as sensors, spherical densiometers, moosehorn crown-closure estimators, and line-intercept methods, are used on the field.

There are newer techniques available, of which hemispherical photography is the most promising as it is fast and accurate.

Hemispherical Photography consists of taking photos of the canopy from the ground with a camera that has a wide-angle or fisheye lens. Image segmentation by software analysis shows areas of visible sky and those portions that are blocked by tree foliage to create canopy cover. It can also measure the amount of solar radiation that manages to filter through the upper story, or light extinction, and important canopy parameters, such as the Leaf Area Index.

Forest inventories are massive operations that take time, planning, and complicated logistics. They also have to be repeated often and are expensive. Data collection follows strict protocols and includes stand level and tree level parameters.

Stand Level

Stand level data collected are ecosystem type, tree density, basal area, canopy cover, light transmittance, and leaf area index (LAI)

Tree density is the number of trees per plot, while the basal area is the total cross-sectional area of all trees at breast height (1.3 m). There are conventional instruments for measuring both of these parameters.

However, light transmittance measurements of  hemispherical photography can be used to directly calculate basal area without complicated and time-consuming calculations. Light transmittance is inversely related to the basal area. That is, as stem areas increase, there is less light that penetrates the canopy. However, the exact relationship must be established for each forest type and is less reliable for very dense forests.

Canopy cover is commonly used in forest ecology and forestry management practices. The presence of an open or closed canopy and the percentage of openness can be crucial for tree regeneration, as each species need different light conditions to germinate and establish. The threshold canopy cover for specific species regeneration is unknown and could be established quickly with the help of  hemispherical photography.

There are also attempts to relate stand canopy cover to stand basal area.

Though researchers recommend transmittance and leaf area index (LAI) measurements, the use of these measurements on the field is just beginning.

Tree Level

Tree level data are crown dimensions, tree height, and tree diameter. These are used to calculate wood volume.

In forest simulation models, the crown area is a substitute to quantify foliage and photosynthesis. Earlier, predefined crown shapes and simple measurements gave inaccurate values of the crown area, especially in the case of broadleaved trees that have irregular and asymmetrical crown shapes.  Hemispherical photography, which gives precise measurements, is a vast improvement. 

Fisheye hemispherical photography estimates are accurate only when the trees are leafless and the distance between the camera’s optical axis and tree height is known. For trees with leaves, there is a simple solution. Adding an inexpensive screw-on-mirror filter blocks visual light and uses only near-infrared light for images. The filter works by differentiating the stem from the foliage. The method has been extended to also calculate tree volume.

Variations among species and growth over time can also be recorded by hemispherical photography, as can changes in the Gap Fraction (or percentage of sky visible through the crown). Since many  hemispherical photography instruments, like the Plant Canopy Imager CI-110 produced by CID BioScience Inc., have a GPS, repeat measurements are not a problem.

Use of  Hemispherical Photography in Silviculture

Silviculturists manage the forest to maintain its current use and simultaneously promote tree regeneration and future growth. Foresters use canopy cover for this purpose. They manipulate canopy cover to balance regeneration and maintain the current forest stand. Therefore,  hemispherical photography, which is useful in measuring canopy cover, has found application in forest management.

Regeneration:  Hemispherical photography estimates of total basal area are combined with a percent canopy cover and visual estimates of canopy cover to predict regeneration conditions in the understory. Also, the amount of photosynthetically active radiation (PAR) measured by  hemispherical photography can be used to check seedling survival in tropical forests, where primary species need darker conditions to survive. Conversely, in the Mediterranean, scientists have shown light transmission of at least 40% was necessary for understory seedling survival and growth of holm oaks.

However, light is not the only important factor, other parameters will also influence seedling survival in forests.

Thinning: Forest managers thin seedlings through spacing or partial cutting to create optimum conditions for seedling survival. Here, the relationship between total basal area and gaps is used to check how many trees or saplings must be cut. For the northern UK, a Gap Fraction of 0.1 was found to be ideal for natural forest regeneration.

It is not just the basal area, but how the trees are distributed that will also influence light transmission. In some cases, the thinning has to be patchy, as is the case with longleaf pine (Pinus palustris) seedling regeneration. The light the seedlings needed for regeneration could be achieved not by uniform thinning but by allowing clumps of trees to produce forest patches.

Light requirements also change as seedlings grow. For instance, initially, coastal black pine (Pinus thunbergii) seedlings need only 30% canopy openness, but as the saplings age, they require 40% canopy openness for optimum growth.

Since hemispherical photography measures canopy structure and light transmission, it is more useful while managing forests for regeneration. Other tools that measure only light transmission or LAI must be combined with other processes to be useful.

Hemispherical Photography in Ecological and Forest Research

With increasing awareness of the importance of forests, especially biodiversity hotspots like rainforests, there is a growing emphasis on reforestation projects. In many areas like the Philippines, there is very little area under natural forests. Therefore, besides providing wood, monoculture and mixed-species plantations are becoming important to conserve natural forest species.

Research is focusing on the size, type of plantations, management practices, and species mix to find where and what the ideal growing conditions are in reforestation projects.  
Focusing on the different understory conditions of different forest/plantation types allows scientists to make important recommendations for supporting natural species regeneration in tropical forestry.

For example, in one such tropical forestry study, a mixed-species plantation, a regenerating selectively logged forest, and a monoculture were compared. The soil nitrogen and phosphorus, forest type, and LAI measurements were correlated with understory species and traits.

The number of understory species was the lowest in monocultures and the highest in the selectively logged plantation. It was forests with higher canopy cover and LAI that had the most species regenerating. The mixed-species plantations have the second-largest seedling diversity. Therefore, it is not just the number of species in the upper story, but also a well-managed canopy cover, through cutting and thinning, that was necessary for providing conditions that allowed more species to grow in the understory.

Advantages and Disadvantages of  Hemispherical Photography

Though the  hemispherical photography technique has been around for 50 years, it is not as widely used as the other methods. To spread its use in modern tools, it is important to understand its strengths and historical weaknesses.

Advantages of  Hemispherical Photography

Hemispherical photography’s advantages for foresters and silviculture are as follows:

1. Photographs create a permanent record of the canopy that can be compared over time.
2. Many parameters can be calculated by fisheye images, such as percent canopy cover, length and duration of sun flecks, and leaf area index.
3. It is faster than other methods when multiple sites must be measured.

Historical Disadvantages of Hemispherical Photography

Over the years, hemispherical photography devices have had their issues:

1. In the past, hemispherical photography has only been possible when the sun is low in the sky (in the morning or evening) or when the sky is overcast.
2. Unsuitable light conditions have also affected the contrast of foliage and sky, and hemispherical photography performance has been affected when light transmittance is under 10%.
3. Without filtering, image segmentation and differentiation of pixels belonging to foliage and sky have been a problem.

Not all Hemispherical Photography Devices are Created Equal

The CI-110 Plant Canopy Imager has been designed to do away with many of the disadvantages. It uses a high resolution digital camera that is adjustable for varying canopy heights, and an array of filters. The digital color filters help in differentiating sky and foliage across varying light and weather conditions, and included digital masking covers the unwanted portion in the image. The 7″ touch screen display allows for instant review of captured images and data.

Hemispherical photography can also be used in other science and practical applications, such as the following:

• Forest Ecology
• Horticulture and Agriculture
• Forest meteorology
• Stream hydrology
• Riparian ecology

Hemispherical Photography Today

The CI-110 Plant Canopy Imager is a useful tool that can be recommended for wide use in forest inventory and silviculture. It is useful for planning forest regeneration, woodcutting, and estimating wood volume. Many canopy variables can be extracted from a single image using the advanced algorithms deployed by the instrument, and with post-processing, even greater depth can be gained. These cover the important ecological and forestry-related measures needed in various stages of forest management.

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Vijayalaxmi Kinhal
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture

Feature Photo courtesy of Sean Bernstein.

Sources

Mailly D. (2017) Hemispherical Photography in Support of Forest Inventory and Silviculture. In: Fournier R., Hall R. (eds) Hemispherical Photography in Forest Science: Theory, Methods, Applications. Managing Forest Ecosystems, vol 28. Springer, Dordrecht. https://doi.org/10.1007/978-94-024-1098-3_8

Wills, J., Maranguit Moreno, M. O., & Avela, M. S. (2016). Next generation tropical forests: Recruitment of species and functional diversity underneath reforestation types in a human dominated landscape. UQ ESpace. doi:10.14264/uql.2016.234
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