How is Human Activity Altering Forest Canopy Dynamics?

• Forest disturbance alters microclimate, including light quality and quantity, that can have a far-reaching influence on forest structure and composition.
• Species need different wavelengths or quality of light for germination.
• Modulation of light by the canopy influences understory growth.
• Edge effects favor early succession species.

Forests nowadays face unprecedented levels of disturbance from human activity. These anthropogenic disturbances like elective logging, clearcutting, deforestation for agriculture, and grazing, affect forests differently from natural causes like storms and tree falls. Disturbance changes the structure and composition of forests, altering the microenvironment.

Germination, growth, survival, and reproduction all depend on microenvironmental factors like light, temperature, soil water and oxygen, and relative humidity. This article examines light changes that occur due to disturbance of the forest canopy, including Photosynthetically Active Radiation (PAR) or the range between 400 to 700 nm, and how it holistically impacts forests.

Forest Canopy & Seed Germination

Forest canopy openness decides how light will percolate through to the ground. It also changes the quality of light, specifically, the red to far-red ratio of solar radiation. Unfiltered light has nearly identical amounts of red to far-red light (R:FR ~1.2). Foliage absorbs red and reflects the far-red component, so under an undisturbed canopy, the R:FR can fall to 0.2 to 0.3 and 0.1 under leaf litter where seeds exist. There is fine-scale variation in the R:FR ratio depending on a forest canopy’s characteristics that will decide what species germinate.

Many primary forest species require darkness or light in the far-red range to germinate. These conditions exist when the canopy overhead is intact and little light filters down.

A gap in the vegetation due to disturbance increases the red light that reaches the soil, see Figure 1. As the gap size increases, more red is allowed through. The soil seed bank holds a mixture of seeds from various species. Plants have phytochromes, using which they can detect changes in red and far-red light. So, with changing light quality, the species that germinate in a place will start to differ.

With changes in the forest canopy, primary species often no longer have the light quality they need to germinate. Secondary, light-demanding species will germinate at higher rates and grow in the open when there is direct radiation with more red light.

The size of seeds will also play a role in a plant’s response to light. Larger seeds don’t use light as a cue to germinate. Instead, they use temperature differences. Smaller seeds are more reliant on light than larger seeds to germinate in both tropic and temperate climates. This is true whether the seeds are from herbs or trees. Smaller seeds also need higher R:FR or more red light available in open spaces in the temperate region. In contrast, small tropical seeds can germinate under a broader range of R:FR ratio in light and shade, and water status is more important.

The light wavelengths that species require can thus be different even within a single forest. So any changes to the forest canopy will alter germination patterns and, therefore, the forest’s composition.

Figure 1: “Direct sunlight is rich in red light, whereas light reflected from neighboring vegetation is depleted in red and rich in far-red. (A) The world as we see it. (B) The world according to phytochrome. A representation of the R:FR for the same scene, produced by digitally subtracting the pixel values from a photograph taken using a camera with an infrared filter from those of a copy taken with a red filter. Darker colors show areas of low R:FR; bright colors are areas of high R:FR,” Devlin, 2016. (Image credits: https://doi.org/10.1073/pnas.1608237113)

Forest Canopy Influences Understory Growth

Light will continue to influence plants’ relative growth rate (RGR) depending on their species. The same species that germinate in red-rich PAR of open spaces will also continue to benefit and establish themselves. Therefore, the light-demanding secondary species have a better RGR in open areas than in the shade.

Small-seeded plants do better in the open, while larger seedlings produced from large seeds can tolerate shade and drought due to the energy reserves the seeds provide them and can survive in a broader range of conditions.

All seedlings need light to grow. Light drives photosynthesis, and when there is more light, plants accumulate more biomass and grow larger. This sets off a positive feedback cycle, where more leaves lead to more photosynthesis and more photosynthesis leads to more growth.

In a forest with a closed canopy, seedlings that manage to germinate will invest more energy in survival and grow taller to access more light. In these plants, less energy is left for other forms of plant growth, leading to fewer leaves. This strategy is called shade avoidance. A tree fall that opens up a small gap in the forest canopy gives these suppressed saplings a chance to reach maturity by providing the necessary light and space. The tree-gap phase is well documented in tropical forests’ natural regeneration and is an essential part of forest dynamics. However, large canopy gaps due to logging and deforestation encourage germination and the establishment of secondary species at the expense of native species.

In temperate forests, the growth of seedlings and saplings is also influenced by light, especially in light-demanding species. Temperate forests differ, however, from tropical forests because diffuse light can also support tree growth of shade-tolerant species like beech. The amount of light reaching the understory can also influence its species richness. In gaps 20-30 meters wide, PAR is found to be 2-3 times higher than in undisturbed temperate forests. So, while selection logging doesn’t greatly impact the number of species in the understory, clearcut forests will have far fewer understory species.

In both temperate and tropical forests, large-scale disturbances change species composition, structure, and forest dynamics due to both faster growth rates or higher numbers of secondary pioneer species.

Forest Canopy & the Edge Effect

Besides the effect on forest interiors, fragmenting forests into smaller portions separated by open lands creates the “edge effect.” Forest edges in tropical, temperate, and boreal forests are increasing as deforestation increases for agriculture and large-scale clear-cuts. The environment in the forest edge differs significantly from the interior and is influenced by the extreme microclimate of the altered open habitat.

There is a steep gradient in the microenvironment at the edges. Edges experience more light, wind, evapotranspiration, nutrient cycling, and decomposition rates due to an increase in light, tree growth, productivity, and reproduction.

Access to other species also increases in these areas. Secondary pioneer species adapted to more extreme climates will establish at the edges, increasing the species’ number and sapling density. However, more wind leads to downed trees and reduced canopy cover.

The conditions experienced at the edge will depend on the contrast that the adjacent open area presents and the region’s climate. Edge effects are milder in cold boreal and sub-boreal areas than in tropic and temperate regions, where the open spaces will have significantly more light and higher temperatures.

Geographical orientation also determines how far into the forest the edge influence occurs. Edge influence on microclimate is deeper and greater on the south and south-western side in the northern hemisphere. Edge structure, whether it is open or covered with vegetation, will also determine the magnitude of the edge effect.

As time passes, the abiotic and biotic gradient edge diminishes, but the changes in structure and composition of forests will persist.

How to Measure PAR

Scientists can measure PAR directly; or indirectly by estimating canopy openness through hemispherical canopy images. Many scientists prefer direct measurements of PAR as canopy can differ among forest types. For example, light percolation will vary for 90% canopy cover in the needle and broad-leaved forests. The  CI-110 Plant Canopy Imager measures both canopy openness and provides direct PAR measurements. The tool measures canopy openness with a 150° fisheye image of the canopy using the GAP fraction method and has sensors on its long handle to detect PAR values. It is light and portable, can be easily carried to the field, and doesn’t require a reference reading. Tools like the CI-110 help us understand the central role of PAR in forest dynamics more easily than ever before.

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

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