What is microclimate and why do we care about it?#

This page provides a high-level overview of what microclimate is, what it affects at ecosystem-scale and why we care about it.

Factors affecting microclimate#

Microclimates are mediated by macroclimate, topography, vegetation type and structure, and soil. Many of these factors can be affected by human cativities, for example through deforestation and other land use changes.

Macroclimate#

  • Solar radiation: The latitude and associated seasonal changes of the sun angle determine the amount of solar energy received at a location. Local features like tree cover can create microclimatic variations in sunlight exposure.

  • Baseline temperature: Macroclimate establishes the general temperature range for a region. Local features can cause variations within this range (e.g., heat islands, shaded areas).

  • Precipitation patterns: The overall amount and distribution of precipitation are dictated by macroclimate. Local factors such as topography can modify precipitation (e.g., rain shadows, increased moisture in valleys).

  • Wind patterns: Large-scale atmospheric circulation influences regional wind patterns. Local topography and vegetation structures can alter wind flow, leading to microclimatic differences.

  • Humidity levels: The macroclimate sets the general humidity level of an area. Proximity to water bodies, vegetation density, and soil moisture can create localized variations.

  • Seasonal variations: Seasonal changes in temperature, precipitation, and solar radiation are governed by the macroclimate.

Topography#

  • Elevation: The height above sea level affects temperature, pressure, and precipitation. Higher areas tend to be cooler, have a lower pressure, and have a higher chance of receiving precipitation as snow.

  • Slope and aspect: The angle and direction of a slope influence sunlight exposure, wind exposure, and water runoff. This can affect microclimate directly (e.g. temperature, moisture availability) and indirectly (e.g. soil erosion changes surface properties).

Vegetation#

  • Leaf Area Index (LAI): The leaf area per unit ground area influences light penetration, temperature, and humidity. LAI is an important factor in determining the productivity and energy balance of an ecosystem.

  • Canopy cover: The proportion of the ground covered by the vertical projection of tree crowns affects light availability, wind patterns, and temperature.

  • Plant height: The height of vegetation can influence wind patterns, shading, and temperature.

Soil#

  • Soil albedo: The reflectivity of the soil determines how much incoming solar radiation is reflected to the sky.

  • Soil type: The soil type affects the thermal and hydraulic properties of the soil which determine how well heat is stored and conducted and how easily water infiltrates, evaporates, and percolates through the soil.

  • Soil moisture: Soil moisture is a key factor in partitioning turbulent fluxes at the surface. Evaporative cooling and the associated buffering effect of vegetation of maximum temperatures.

Balancing energy, water, and carbon#

The dynamics of microclimate in a terrestrial ecosystem in primarily driven by five key components: radiation balance, energy balance, water balance, carbon balance, and turbulent transfer (see Fig. 2). These components are connected through the exchange of energy, water, and carbon and can be described with the general energy balance equation:

\[\begin{split} \begin{align} \frac{dQ}{dt} & = (1 - \alpha) S_\downarrow + L_\downarrow - \epsilon \sigma (T_{sfc} + 273.15)^4 \\ & = H + L_{v}E + G + PP \end{align} \end{split}\]

where \(\frac{dQ}{dt}\) is the change in heat storage, \(\alpha\) is the surface albedo, \(S_\downarrow\) and \(L_\downarrow\) are downwelling shortwave and longwave radiation, respectively. \(\epsilon\) is the emissivity of the surface, \(\sigma\) is the Stefan-Boltzmann constant, and \(T_sfc\) is the surface temperature in Celsius. \(H\) is the sensible heat flux, \(\lambda E\) is the latent heat flux, \(G\) is the ground heat flux, and \(PP\) stands for primary productivity.

  • Radiation balance: The radiation balance refers to the equilibrium between incoming solar radiation and outgoing terrestrial radiation within an ecosystem. How much radiation is reflected, scattered and absorbed depends on the albedo and structure of the surface and vegetation.

  • Energy balance: The energy balance describes the equilibrium of absorbed and released energy at a surface, for example the soil surface or the canopy. This balance is closely coupled to the radiation balance through net radiation, which is partitioned into turbulent fluxes (latent, sensible, and ground heat flux), used for photosynthesis, and changes in heat storage.

  • Water balance: The water balance refers to the equilibrium of absorbed and released water by different (here abiotic) ecosystem components. This balance is linked to the energy balance via evapotranspiration and latent heat flux from the soil surface. Beyond the local water balance, we also consider hydrology at the catchment scale.

  • Carbon balance: The carbon balance is linked to the radiation, energy and water balance by net primary productivity: the conversion of light, atmospheric carbon, water, (and nutrients) into biomass minus respiration. The carbon cycle continues as plant biomass is either eaten by herbivores or falls to the ground where it is decomposed. If not respired by animals or plants, carbon enters the soil where it is and eventually recycled to the atmosphere.

  • Turbulent transfer: The turbulent transfer and wind mix all the atmospheric properties vertically and horizontally, leading to ecosystem characteristic patterns and profiles of microclimatic variables.

Key microclimatic state variables#

Solar Radiation#

  • Direct solar radiation: Sunlight that reaches the surface directly from the sun affects temperatures, photosynthesis in plants, and the energy balance of an area.

  • Diffuse solar radiation: Sunlight scattered by molecules and particles in the atmosphere is essential for plant growth in shaded or cloudy conditions.

  • Photosynthetically Active Radiation (PAR): PAR is the portion of sunlight (400-700 nm wavelength) that plants use for photosynthesis.

Temperature#

  • Air temperature: Air temperature at different vertical levels within and below vegetation cover is probably the most important microclimate variable because it determines metabolic rates, growth, and survival of organisms.

  • Soil temperature: The temperature of the soil at various depths affects plant growth, root activity, and microbial activity. Soil temperature can significantly differ from air temperature due to soil composition and moisture content.

  • Surface temperature: The temperature of the ground or other surfaces can be significantly different from the air temperature above; often it is much hotter.

Humidity#

  • Relative humidity: The percentage of moisture in the air relative to the maximum amount the air can hold at that temperature. It impacts endotherm comfort, plant transpiration, and microbial activity.

  • Vapour pressure deficit (VPD): The difference between the amount of moisture in the air and how much moisture the air can hold when it is saturated. VPD is a crucial factor for plant transpiration and water stress; higher VPD indicates a greater potential for evaporation and transpiration.

  • Soil moisture: The amount of water contained within soil affects evaporation rates, soil microbial activity, and the availability of water for plants.

  • Soil matric potential: Soil matric potential refers to the energy status of water in soil which represents the force per unit area that the soil matrix exerts on water due to capillary and adsorptive forces. It influences the movement and availability of water to plants, with lower values indicating drier conditions and higher values indicating wetter conditions.

Wind#

  • Wind Speed: The rate at which air is moving horizontally influences temperature regulation, evaporation rates, and the dispersion of atmospheric gasses.

  • Wind Direction: The direction from which the wind is blowing affects microclimate by influencing temperature distribution, humidity, and the transport of seeds and pollen.

  • Turbulence: Turbulence refers to the irregular, chaotic movement of air that affects the mixing of atmospheric elements. Turbulence influences heat exchange, moisture distribution, and the dispersion of aerosols.

Precipitation#

  • Rainfall: The amount of rain that falls in a given area over a specific period. Rainfall is vital for replenishing water sources and maintaining soil moisture.

  • Snowfall: The amount of snow that falls, which can be measured as snow depth or water equivalent. Snowfall influences temperature regulation, soil moisture, and water supply.

  • Dew and Frost: The formation of water droplets or ice crystals on surfaces, which can affect plant health and soil conditions. Dew and frost are critical in areas where precipitation is low.

Atmospheric Pressure#

Barometric pressure describes the pressure exerted by the atmosphere at a given point. Pressure decreases with height and influences weather patterns; changes in atmospheric pressure can indicate upcoming weather changes, such as storms.

Atmospheric \(\ce{CO_{2}}\)#

Although atmospheric \(\ce{CO_{2}}\) is not a microclimatic variable, it is closely tied in the balance of energy through its critical role in photosynthesis. It should therefore be considered when studying the dynamics on ecosystems.

Methods for microclimate science#

This section gives a broad overview over common methods and models in microclimate research. Recent advances in data acquisition for microclimate research and microclimate modelling are provided in a comprehensive review by Kemppinen et al. (2024).

Microclimate data collection#

Effective microclimate modelling relies on accurate data collection, ideally from a combination of sources:

  • Meteorological stations: Meteorological stations provide detailed, real-time data on temperature, humidity, wind speed, and solar radiation for locations across an area.

  • Remote sensing: Satellites, planes, drones, and other technologies offer extensive spatial coverage, capturing data on land surface temperatures and vegetation health.

  • In-situ measurements: Ground-based sensors complement provide high-resolution data on specific local conditions.

  • Reanalysis: Reanalysis data sets such as ERA5 represent a combination of observations and modelling which can complement measurements by filling gaps in spatial cover and timeseries.

Microclimate modelling#

Microclimate models use various computational approaches to simulate the interactions between atmospheric, terrestrial, and hydrological processes:

  • Energy balance models calculate the exchange of energy between the surface and the atmosphere, accounting for factors like radiation, convection, and conduction.

  • Statistical models utilize historical data to identify patterns and predict future conditions.

  • Process-based models, which incorporate physical laws and biological processes, offer a detailed simulation of microclimate dynamics over time and space, providing insights into how changes in one factor may impact the entire system.

Challenges and limitations#

  • Data availability and quality: Microclimate models require high-resolution, long-term meteorological data for calibration and validation. Most ecosystems, especially in remote areas, lack sufficient weather stations and ground observations to offer the data required to model microclimate accurately.

  • Computational resources: High-resolution microclimate models can be computationally intensive, requiring significant processing power and time.

  • Model complexity: Accurately simulating microclimate processes involves complex interactions between various atmospheric, terrestrial, and hydrological factors. Not all of these interactions are well understood. Simplifications and assumptions necessary for computational feasibility can reduce model accuracy.

  • Parameter sensitivity: Microclimate models are highly sensitive to input parameters such as soil moisture, vegetation cover, and land surface properties. Small errors in these parameters can lead to significant deviations in model outputs.

  • Representation of extreme events: Accurately modelling extreme weather events with high spatial and temporal precision (e.g., heatwaves, storms) is difficult due to their complex and unpredictable nature. However, these events can have disproportionate impacts on ecosystem dynamics and in particular mortality rates.

  • Integration with other models: Combining microclimate models with broader ecological or hydrological models involves addressing compatibility and consistency issues. This includes for example interconnected processes happening at different time scales.

  • Human impacts: Incorporating the effects of human activities (e.g. urbanization, land use changes, conservation actions) into microclimate models adds another layer of complexity.