How Does Climate Change Affect Forest Canopy?

• Drought, carbon dioxide levels, and warm temperatures are the factors that impact forest canopy the most.
• Climate change weakens trees, reduces leaf area, and leads to tree mortality.
• Increasing wildfires and insect attacks due to drought will alter the canopy completely.

Tree canopy degradation can occur due to climate change effects on various levels of trees and ecosystems. These impacts are uneven, and depend on specific local climate change effects, the secondary outcome it triggers, and the resilience of trees. It is worth learning about current and anticipated alterations driven by climate change since forest canopies are vital for determining forest structure, ecosystem productivity, carbon sequestration, and landscape hydrology.

Climate Change Factors

Climate change factors affecting forest canopy are temperature rises, increasing drought and extreme weather and storms, high carbon dioxide, increased precipitation in some places, insect outbreaks, wildfires, and expansion of invasive species.

• Warm temperatures can increase growing season length, and variations can be seasonal. For example, northern Europe gets evenly warm throughout the year, but west and southern Europe only gets warm in summer.
• Patterns of precipitation are changing. Dry regions are getting drier and wet areas are getting more precipitation. Also, precipitation occurs in shorter and heavier bursts with more extended periods of dryness. The number of extreme events, including windstorms, is increasing worldwide. Higher temperatures alter snowmelt timing and change the seasonal availability of water.
• Drought incidence and severity also increase due to climate, especially in the warm tropics. Drought is also increasing the risks of wildfires that burn down forests.
• Carbon dioxide increase in the atmosphere enhances photosynthesis with enough precipitation and soil nutrients. Carbon fertilization can change tree distribution, but its occurrence is not even.
• Pest outbreaks increase because of climate change for different reasons. Lack of water reduces sap production by trees, making them vulnerable to insect pests like pine beetles. Milder winters in temperate regions also increase insect numbers.

The impact of climate change on forest canopy will vary, and broad results across ecosystems are discussed below.

Forest Canopy Changes

Figure 1: “Expected cascading effects (direct and indirect) of climate change on forest ecosystems from physiological to geographical processes (a). Being slow-migrating, sessile organisms, extant trees will have to acclimate to climate change in situ or, in some extreme cases, face mortality and ultimately local extinction resulting in ecosystem shifts” Kramer et al., 2020. (Image credits illustration by Hikaru Ishii, https://doi.org/10.1111/1440-1703.12127)

Forest canopy is considered to be in decline if half its components, branches, twigs, and leaves are missing or dead. The loss in the canopy can occur from top to bottom and outer to inner parts of a tree crown. Changes in forest canopy affect forest structure and functioning and depend on the severity of canopy loss.

Climate change causes canopy decline by affecting cellular, tree, and ecosystem processes, with global repercussions, as shown in Figure 1.

Cellular Level

Physiological processes like photosynthesis, respiration, and transpiration are affected at the cellular level by higher temperatures and CO2 levels.

More photosynthesis can occur due to CO2 fertilization if there is enough water and nutrient availability, in which case the canopy size can increase. Higher CO2 levels also reduce stomatal conductance so that water use efficiency will increase. Respiration also decreases under warmer conditions. But during prolonged drought, closed stomata will ultimately reduce photosynthesis. Drought-tolerant species, which keep stomata open for carbon fixation, risk losing too much water content leading to tree mortality.

At higher temperatures, the ability of leaves to cool is reduced and increases photorespiration. Leaves are already at temperatures over the ambient temperature. If temperatures rise further, leaves cannot cool, causing them to suffer heat stress and the ability to fix carbon dioxide.

In many cases, a reduction in plant productivity due to drought is more than an increase in photosynthesis due to CO2 fertilization.

Individual Tree Level

CO2 fixation, transpiration, and respiration changes will affect plant/tree-level processes like growth phenology and mortality.

Increasing photosynthesis due to CO2 fertilization will increase Leaf Area Index (LAI) and make the forest canopy denser. However, rainfall irregularities and drought can increase leaf mortality or canopy degradation, see Figure 2.

Tree mortality can occur due to drought, wildfire, or insects, leading to canopy dieback.

Canopy LAI is affected by water availability in dry regions, cloud cover variability, and lack of incident solar light in areas with more precipitation. There is no difference in drought-induced mortality between angiosperms and gymnosperms or between deciduous and evergreen species. However, the mortality of trees with lower wood density and high specific leaf area due to more potential for leaf water loss is more likely to occur in species with lower specific leaf area.

When a tree is weakened, its foliage quantity and quality are affected- leaf size and biochemical content will be altered. Tree crown degradation is the main characteristic during forest diebacks.

Ecosystem Level

A tree’s growth and health will determine its productivity and maintenance of the forest’s carbon sequestration ability.

Trees’ survival ability will determine if a species can persist in an ecosystem. A species’ absence from a forest will change the future forest structure and function. Moreover, it will affect the geographical distribution of species.

Trees in arid, tropical, and temperate forests are affected the most due to water shortages and increased variability in climate. Further changes will impact their ability to recover from climate change effects. Boreal forests’ benefits from CO2 fertilization outweigh climate change’s adverse effects.

Global level

A species’ persistence or local extinction due to range shift is also a result of direct and indirect climate effects.

Warm temperatures increase growing season length and make problematic areas ideal for particular species. As a result, a geographical shift in tree species ranges is occurring. Conversely, extinction of species due to range changes is also occurring.

The poleward movement of warmer climate zones is estimated to be 0.42 km/year. However, previous and competing species in the newly expanded range slow actual species migration.

However, deciduous forests are turning into savannas in arid regions as forest dieback. Also, temperate deciduous species are growing in coniferous boreal forest regions.

Figure 2. “Oaks (Quercus petraea) exhibiting moderate (A) and pronounced (B) symptoms of decline with a reduced foliage density and an accumulation of dead branches,” Adapted from Salle et al. (2021). (Photo credits: Aurélien Sallé (A,B) and Sébastien Damoiseau (C,D),  https://doi.org/10.3389/ffgc.2021.710854)

Forest Canopy Change Patterns

Scientific investigations are trying to find factors that can explain global trends in the forest canopy. However, there isn’t a single factor. Climate change effects depend on existing climate, precipitation patterns, and species characteristics.

Damages in Temperate Forests

Forest canopy in temperate regions and Mediterranean areas changes due to drought, windstorms, wildfires, and insect outbreaks.

Drought and windstorms break branches and twigs and cause canopy degradation and tree mortality. Indirectly the two factors can also result in boosting opportunistic species.

Deciduous forests are experiencing more droughts, affecting the health of beech and birch in the western USA, northern China, and southern Europe. In the Mediterranean, trees cannot photosynthesize due to extreme heat.

Drought-weakened trees are more susceptible to wildfires and insect attacks. China, Europe, and West Europe are also experiencing more drought-driven wildfires destroying entire forests and wiping away forest canopy. Fires encourage the growth of invasive tree species and insect pests.

Due to mild winter and drought, insect pests are killing trees over large areas in northern Europe and the southwest USA. For example, pine beetles damaged over 650,000 acres of forest in Colorado, and in western Canada and southern Alaska, spruce beetles damaged over 3.7 million acres by 2007.

Changes in Tropical Forest Canopy

Forest canopy structures are adapted and shaped by long-term climate and historical water deficits. Initially, tropical forests in areas with high water stress have less leaf area and coverage. Any additional and short-term drought stress can push trees outside their typical water or hydraulic strategies, increase tree mortality, and affect forest canopies. The maximum water deficit in the arid months can predict canopy structure.

Alternatively, moist forests experience dieback due to drought as they are not adapted to cope with a lack of water availability. For example, forest diebacks causing canopy gaps in the Amazons are attributed to prolonged droughts. Drought also causes mortality and forest canopy degradation in Ugandan tropical moist forests, Zimbabwean mountain forests, and moist forests in Indonesian Borneo and Malaysia.

Forests will differ in their sensitivity to drought. As drought frequency and intensity increase, forest species must adapt or make way for species with a more suitable hydraulic strategy for drought.

Droughts interact with human activity like logging and forest fragmentation to modify forest canopy structures. As temperatures fall and forests are protected from anthropogenic disturbance, canopy structure improves.

Measuring Forest Canopy

Forest canopy studies in the northern hemisphere boreal and temperate ecosystems are more common than in tropical forests. During these studies, forest canopy attributes are measured directly or indirectly.

Direct techniques include measuring leaves through destructive planimetric or gravimetric methods and upscaling to stand level. Non-destructive leaf area measurements are also possible with instruments like the CI-202 Portable Laser Leaf Area Meter and CI-203 Handheld Laser Leaf Area Meter, manufactured by CID Bio-Science.

Indirect optical techniques can measure individual and stand-level canopy structures.

• One rapid non-destructive technique is Hemispherical photography with the CI-110 Plant Canopy Imager.
  • It can measure the canopy through the Gap Fraction method.
  • LAI is the one-sided leaf area divided by the total ground area.
  • Another method is to measure the fraction of absorbed photosynthetically active radiation (fAPAR).

Future Research

Field observations and long-term experiments will be necessary for a better understanding of climate change effects on the forest canopy, especially on a large scale. However, the experiments can be challenging and need funding. Nonetheless, considering that forest canopy performs several ecosystem services, it will be crucial to understand how all forest types can be managed to minimize climate change effects.

Source

EPA. (n.d.) Climate Impacts on Forests. Retrieved from https://19january2017snapshot.epa.gov/climate-impacts/climate-impacts-forests_.html

European Commission DG Environment News Alert Service. (2023, Jan 19). Most forests are less able to cope with hazards under climate change. Retrieved from https://environment.ec.europa.eu/news/most-forests-are-less-able-cope-hazards-under-climate-change-2023-01-19_en

Harris, A. (n.d.). How Does Climate Change Affect the Temperature in a Deciduous Forest? Retrieved from https://education.seattlepi.com/climate-change-affect-temperature-deciduous-forest-6755.html

Kramer, R. D., Ishii, H. R., Carter, K. R., et al. (2020a). Predicting effects of climate change on productivity and persistence of forest trees. Ecological Research, 35(4), 562–574. https://doi.org/10.1111/1440-1703.12127

INN. (2022, Sept 13). Climate change could have devastating effects on forests’ carbon uptake. Retrieved from https://www.innovationnewsnetwork.com/climate-change-could-have-devastating-effects-on-forests-carbon-uptake/25408/

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Pfeifer, M., Gonsamo, A., Woodgate, W. et al. (2018). Tropical forest canopies and their relationships with climate and disturbance: results from a global dataset of consistent field-based measurements. For. Ecosyst. 5, 7. https://doi.org/10.1186/s40663-017-0118-7

Sallé, A., Cours, J., Le Souchu, E., et al. (2021). Climate change alters temperate forest canopies and indirectly reshapes arthropod communities. Frontiers in Forests and Global Change, 4. https://doi.org/10.3389/ffgc.2021.710854

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