Does Forest Canopy Structure Affect Net Primary Production?

• Yes, as forest canopy structure complexity increases, the site’s net primary productivity (NPP) rises.
• The impact of canopy structure is higher than species diversity on NPP.
• Factors altering forest canopy structure complexity, such as forest diversity, density, age, annual stability, disturbance, forest type, and seasons, affect NPP.
• Forest canopy structure can be a reliable indicator of NPP for integration into modeling for carbon sequestration.

The effect of tree canopy structure in space and time on net primary production was poorly understood. Recent research has therefore focused on this crucial theme due to its implications for carbon sequestration and storage. Some questions addressed are the relationship trends between canopy structure and net primary production in different ecosystems due to disturbance or age of forests. This article covers some of the recent findings on the topic.

Canopy Structure’s Importance

A forest canopy structure is the arrangement of branches, leaves, and space. It is determined by forest species diversity, density, age, forest disturbance, and nature of origin (planted vs natural).

Forest canopy structure is essential as it affects the ecology and functioning of ecosystems. For example,

• Canopy structure changes the physiology and morphology of leaves.
• Canopy gaps and leaf clumping increase the amount of light penetration to the lower vegetation stories and forest floor, raising soil respiration and evaporation.
• Leaf arrangement variations affect the microclimate- light, shade, and moisture content of air and soil beneath the canopy.
• Canopy denseness controls wind penetration and turbulence, thus, gas diffusion rate in and out of the tree tops.
• Vertical and horizontal tree canopy structure is critical in determining the habitat type.
• Canopy gap size determines the species regenerating in the lower forest story and seed dispersal distance, affecting future species recruitment.

Various canopy traits describe canopy structural complexity or rugosity, such as height and width, Gap fraction, vertical leaf density profiles, leaf area, and leaf area index (LAI). Each forest’s canopy structure differs based on the combination of these traits.

Table 1: “Average annual GPP and NPP for different forest types in Italy according to Corine Land Cover 2000 classification,” Nole et al. 2015. (Image credits: DOI:10.1007/978-3-642-32424-6_5)

Canopy Structure and NPP

The canopy structure influences many tree functions, chiefly photosynthesis, by determining leaf area index and controlling light interception, which is the amount of carbon trees capture. Canopy structure also determines soil evaporation, influencing tree transpiration and leaf stomatal conductance, influencing photosynthetic and respiration rates. These processes affect the plant’s net productivity.

The site net primary production (NNP) is the organic material that plants store after using the fixed carbon for respiration in the measurement period. The increase in NPP partly reflects the growth of the carbon sink.

Several studies show that as canopy structure complexity increases, net primary production also increases. Compared to other variables, canopy complexity or rugosity is the main factor influencing NPP, even more than species diversity. Canopy complexity can explain 83% of variations in a site NPP.

Structurally complex forests have higher vegetation-area indices; these forests are more diverse and absorb and use more light for biomass production. In multi-story canopies with more structural variability, leaves occur in various light conditions on plants in various niches. So, more sunlight percolating into the forest is used than in a forest with less structural diversity. Table 1 shows the difference in NPP because of different ecosystems and their canopy structure on NPP in Italy.

Canopy structural complexity effects are broad and robust enough for use as NPP indicators in future modeling to predict and manage forest carbon sequestration.

However, the factors influencing forest canopy structure will moderate its effects on NPP.

Factors Affecting Canopy Structure Contribution to NPP

The main factors influencing canopy structure and NPP are forest species diversity, age, tree density, interactions, forest disturbance, and successional stages, as discussed below.

Species diversity

Taxonomic diversity increases canopy complexity. Plots with more species and significant variation in stem diameters had higher canopy complexity. The positive relationship between species diversity and NPP is nearly universal. As explained earlier, ecosystems with diverse plant communities tend to exploit complementary light niches and optimize light use on the ecosystem scale, leading to higher NPP. The annual NPP can be higher in forests with greater canopy complexity and crucial for carbon fixation.

Forest age and density

As forests age, their canopy structural complexity increases, as do NPP and carbon sequestration. The increasing canopy structural complexity is the main factor that explains forests’ ability to capture more carbon as they age. For example, century-old deciduous forests fix more than expected carbon compared to new growths in Michigan.

Also, as tree density increases, the canopies become denser, capturing more light and increasing NPP. Moreover, stands with higher densities have less inter-annual variations in canopy complexity, leading to stable productivity rates.

Tree interactions

Canopy structure can change due to competition between trees. Smaller tree size allows trees to take advantage of small gaps in canopies, and shade tolerance helps many species grow lateral branches. Meanwhile, shade-intolerant species will reduce branches in neighbors’ shade and grow more in well-lit areas away from them. Damage due to branch and twig breaks in the wind increases with height, as in the case of thin trees. Hence, many interactions, influenced by species mixtures, will shape the canopy structure in a plot.

Resource allocation

A stable canopy structure will give higher NPP. In contrast, a dynamic canopy uses more resources to build new structures for canopy space exploration, and there is less for storage as biomass.

More carbon fixation through photosynthesis from more canopy areas will not lead to higher NPP if the resources are invested in growing and maintaining the canopy. The carbon gain must be higher than the carbon spent for NPP to increase. Hence, annual variability in canopy structure is not conducive to forest productivity.

Lower annual canopy variations result in more height growth and build canopy complexity over the years to increase NPP.

Forest disturbance

Forest disturbance reduces NPP by dramatically changing the canopy structure. Moderate disturbance can lead to species-specific and patchy mortality or large areas of species’ functional groups dying. In the case of disturbance, it was found that it was not changes to the vegetation index that affected NPP but the change in canopy structure.

Forest canopy structures act as a material legacy, providing structural stability. The effect of disturbance on the legacy can be substantial. Species and forests with high structural resistance could continue to function after moderate or slow-acting but severe disturbances and continue contributing to NPP. Moreover, the survival of an under-story canopy structure or material legacy was essential for growth and compensation for disturbance losses. The under-story could also contain younger ontogeny of canopy species to rebuild future forest canopy structures.

Figure 1. “Planted 25-year-old (a) and naturally regenerated 20-year-old (b) red pine (Pinus resinosa) stands in north Michigan, USA. Planted stands are more structurally uniform, with relatively even stem spacing and heights, along with high stem densities. Naturally regenerated stands are uneven-aged, more sparsely vegetated, and contain trees varying in height,” Hickey et al. 2019. (Image credits: https://www.mdpi.com/1999-4907/10/7/566)

Artificial vs natural successional forest

Another factor that can introduce variability in canopy structure is the origin of a forest stand. Planted forests have trees of the same age and usually are mono-species. The trees are evenly spaced at high densities. Therefore, the forest canopy structure is not complex. A naturally regenerating forest will have many species, less density, and trees of varying ages and heights, distributed randomly in open spaces, see Figure 1. These naturally regenerating forests with multilayers have more canopy structure complexity and better light use efficiency, even if they are mono-species. Some studies found that naturally regenerating forests had higher NPP, but other studies show that the differences between planted forests and those of natural forests are insignificant. So, in the short term, both kinds of forests, for example, 45 years, can form reliable carbon sinks.

Nowadays, many forests are a mix of planted and naturally regenerated stands of various ages and structures. Planted forests are growing at 2% in Asia and North America. It is essential to know their contribution to NPP and carbon sinks to address deforestation.

Seasonal variations in ecosystems

Seasonal variations will exist depending on the forest type. The temperate deciduous forests have a denser canopy in warmer seasons, while the trees in savannas could lose leaves in the dry season. Forests like tropical rainforests and boreal have fewer variations. Forests with seasonal variations have intra-annual differences in photosynthesis and lower NPP.

Measuring Forest Canopy Structure

An index has recently been introduced to measure canopy structure complexity or rugosity. Tree height, Gap fraction, and LAI are other canopy traits valuable for measuring structure complexity. The CI-110 Plant Canopy Imager produced by CID Bio-Science Inc. can non-destructively and accurately estimate canopy cover parameters like gap fraction, LAI, and the photosynthetic active radiations (PAR) in real-time on the site. PAR estimation reaching the under-canopy is crucial for studying rugosity, as the difference in light use is the main impact of forest canopy structure.

Find out more about the tools and services provided by CID-Bio-Science Inc. that can help in your forest canopy structure research.

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