Drought Effects on Forest Productivity

• The effects of increasingly frequent and severe droughts are reported globally.
• Forest productivity decreases due to reduced growth, canopy dieback, and tree mortality.
• Drought effects are not even and are influenced by internal and external factors.
• Some adaptation measures to drought-proof forests are suggested based on available information.

Drought intensity, frequency, duration, and extent are increasing due to climate change. It is impacting forest productivity across scales and biomes. Lower productivity reduces the ability of forests to sequester carbon and meet biomass demands. Therefore, understanding drought impacts on forests will be necessary to plan adaptation measures in forest management. In this article, you will find how drought affects forest productivity by influencing tree processes.

Drought Effects on Tree Processes

Climate change-driven drought severity and frequency are reducing forest productivity globally. For example, in tropical dry evergreen forests in Costa Rica, gross productivity fell by 13% and 42% in the drought of 2014 and 2015, respectively.

Though most forest productivity decreases are the usual trend due to drought, in some instances, as in Northeast China, a mild or short-term drought with warmer temperatures increased and produced lag effects on net primary productivity.

Drought leads to three primary changes that impact productivity- crown dieback, tree growth, and tree mortality. Drought causes secondary changes such as increased risks of wildfires, wind throw, and pest and disease attacks that can increase tree damage and mortality to lower productivity.

Drought affects tree productivity due to changes in physiological processes and morphological and anatomical features. These include:

• Hydraulic failure: Increased water stress in plants due to lower soil moisture availability and higher vapor pressure deficit leads to hydraulic failure, impaired long-distance water transport, and xylem embolism.
• Carbon starvation: Lower canopy cover, leaf area, stomatal conductance to avoid water vapor loss, photosynthetic rate, and light use efficiency due to drought, decrease carbon assimilation rates, causing carbon starvation.

Reduced Growth

Drought effects on tree growth trends must be monitored due to their role in global water, carbon cycles, and forest productivity.

As the weather becomes drier, trees change allocation to various organs. One of the changes is less resource allocation to increase tree height and more for root and radial growth. The drivers are anatomical in nature. Taller trees require vessels and tracheids of larger lumen diameters at the stem base to sustain hydraulic conductivity. However, larger vessels make trees more susceptible to drought, so trees reduce lumen diameter during water stress. Thus, the trees’ stature remains small.

Globally, drier climates have been linked to smaller trees. This phenomenon is moderated by stem or radial diameter. A tree close to its maximum size gains less height than a shorter tree of the same diameter, which attempts to reach full size.

Figure 1: “Extensive canopy dieback at: (a) Mt Duval, (b) Billywillinga, (c) Mt Ainslie, (d) Munghorn Gap National Park, and (e) Eugowra Nature Reserve. Photos were taken between November 2019 and February 2020,” Losso et al. 2022. (Image credits: https://doi.org/10.1038/s41598-022-24833-y)

Canopy Dieback

Drought events are also associated with minimum NDVI (Normalized Difference Vegetation Index) or plant greenness due to changes in phenological cycles in some forests. More severe cases of drought cause leaf drying. This browning leads to loss of leaf area. The loss of leaves accompanied by incomplete recovery and lagged growth lays bare branches and canopy (wholly or partially). The cause is a hydraulic failure, which can even prevent the re-sprouting of new leaves.

Canopy disturbance can result in tree dieback; see Figure 1. Trees experiencing complete canopy dieback may not be able to recover even in a succeeding period of favorable precipitation, which may lead to forest decline.

Mortality

Severe cases of drought hamper trees’ ability to recover, leading to mortality. Increased temperatures along with water stress have been indicated as problems in several studies. Extreme weather and even excessive rainfall in the two years after drought can lead to mortality in moist forests. Hydraulic failure (>80%) and carbon starvation are the main drivers of mortality, even though several abiotic and biotic factors are involved.

The drought effects are not felt evenly by all trees in a forest:

• Some species, like evergreens, softwoods, and pioneers, are more vulnerable to mortality due to lower resilience to drought.
• Similarly, short-lived species have higher mortality rates.
• Drought causes thinning, especially in young stands and dense forests. The higher density increases the water stress trees suffer in any area.

Tree mortality can change forest dynamics and ecosystem services, such as:

• Forest structure alteration due to tree mortality negatively affects forest functioning and water, carbon, and energy fluxes, reducing productivity.
• Mortality changes species composition and shifts in forest dominance towards shrubs.

Legacy Effects

A growing body of research findings indicates that drought impacts forest productivity as a result of current situations as well as legacy effects from previous years and seasons.

A study that looked into drought effects at scales ranging from individual trees to the global scale for a century found that subsequent drought effects are worse than initial droughts. However, the legacy effects differ and depend on forests and ecosystems. Gymnosperms and conifer-dominated habitats are more vulnerable to multiple droughts.

Legacy also effects differ according to the measurement scale:

• At the scale of individual trees and stands, legacy effects are caused by vulnerabilities to hydraulic failure, carbon starvation, delay in leaf regrowth, pest outbreaks, and mortality.
• At more extensive forest and ecosystem scales, species and age structure are significant factors, but their effects are poorly understood.

Figure 2: “Conceptual model for the reasons for different sensitivities of natural forests (NFs) and planted forests (PFs) to drought,” Zhong et al. 2021. (Image credits: https://doi.org/10.1029/2021JG006306)

Factors Shaping Drought Effects

Several intrinsic and external factors influence the risk of drought and the extent of impacts on productivity.

External Factors

The external factors shaping drought effects on forest productivity are timing, location, soil type, and management practices.

• Timing: The time that drought occurs can influence its effects, especially at higher latitudes. Drought in spring will impact trees in the current season, while late summer drought impacts the next season’s growth. An increase in frequency, with repeated droughts, will have a cumulative effect, reducing growth for several years after that.
• Location: Climate change effects of temperature and precipitation frequency and intensity differ globally. Some regions, like northern Europe, will get wetter and drier in southern Europe, where droughts are severe. Another location effect is species shifts of trees growing at the limits of their distribution. Temperate trees in the drier Mediterranean regions are at greater risk of drought mortality than the same species in the northern limits, leading to a subsequent change in species composition in the locality.
• Soil type: Since soil moisture levels are a significant driver of drought effects, the soil type is crucial. Trees growing on soils that can retain more water, like loam, are at less risk due to less rainfall, while sandy soils with little water retention capacity are prone to water stress and drought effects. Also, trees growing on soils with poor drainage will suffer from waterlogging in rainy seasons and soil dryness in summer.
• Management practices: Forest density is higher in planted forests and areas where traditional management practices are no longer followed. This higher density increases water stress as competition increases among trees, leading to dieback and mortality.

Tree and Stand Attributes

The intrinsic tree features that determine drought effects are as follows:

• Tree species: Drought resilience differs among species. Birch, beech, and sycamore are more drought-sensitive than native oak species in temperate forests. Western hemlock and Douglas fir are less susceptible to stem cracking than Sitka spruce. Moreover, differences in drought resilience can also occur between local populations.
• Root depth: Species and plant habits with smaller root systems, like shrubs, are more drought-prone than deeper-rooted tree species that can explore extensively for water. Therefore, younger and newly planted trees with shallow roots are vulnerable, especially on open sites.
• Forest size: Forests have moderate temperatures and microsite conditions. Therefore, fragmented and smaller forest patches more exposed to open environments are warmer and drier and have different species composition, especially at the edges.
• Tree age: Newly planted and ancient trees are more drought-prone. A combination of species-specific drought sensitivity and age can alter forests’ species composition and age structure.
• Stand health: Forest recovery from droughts can be influenced by stand health before the drought event. Due to pest attacks, weaker stands are more likely to suffer from drought effects. Conversely, drought makes trees more susceptible to pests and diseases as well.
• Forest type: Natural forests are more drought-resilient than planted forests for many reasons, see Figure 2. They have more species, deeper roots, and older trees than planted forests. However, planted forests manage to recover faster from drought. Older natural forests use less water and sequester more carbon than planted forests. However, drought effect differences get narrow as drought intensity and frequency increase.
• Biodiversity: The more species a forest has, the more stable its productivity. Species have asynchronous dynamics and drought-tolerance traits, which can benefit productivity. Differences in drought response and performance can balance annual differences in productivity in species-rich forests than monocultures. See Figure 3.

Figure 3: “Graphical illustration of asynchronous species responses in mixed-species tree communities to contrasting climatic conditions: The tree community experiences a “normal” (year 1), an arid (year 2), and an exceptionally wet (year 3) year, which result in distinctly different productivity responses of the participating species but the same community productivity due to compensatory dynamics,” Schnabel, et al. 2021. (Image credits: DOI: 10.1126/sciadv.abk1643).

Adaptation Measures

Site- and ecosystem-specific actions are the best ways to maximize productivity. However, based on the available information from global research, it is possible to suggest some general measures to reduce drought risks to forests and their productivity. Some of these suggestions are as follows.

• Maintain and increase tree species diversity in forests.
• Use mixed species stands in planted forests instead of monocultures.
• Choose species with a diverse forest structure to benefit from species with varying root sizes and depths.
• Integrate natural regeneration, as the native species are better adapted to the local soils and weather.
• Make management drought-proof by maintaining optimum density by thinning stands to help forests cope with water stress. Also assist young forests to tide over drought.
• Have contingency plans for tackling drought effects in place. For example, to prevent forest fires after dieback.

Measuring Drought Impact

It is possible to measure the extent of drought impact on growth, canopy dieback, and mortality using canopy imagers. The CI-110 Plant Canopy Imager can provide real-time canopy cover and Leaf Area Index (LAI) readings of individual trees and forests. A GPS helps establish spatial documentation and repeat measures to monitor the effects of climate change over the years. CID Bio-Science Inc., a trusted source of forest and environmental research tools, produces the imager.

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