Review of Microclimate and Water Balance Modification through Tree Integration in Temperate Cropping Systems

Introduction

Climate change is causing increased droughts, heatwaves, and extreme rainfall in Central Europe.
Alley cropping, the integration of trees and shrubs in agricultural land, is proposed to enhance the resilience of cropping systems by positively modifying the microclimate and water balance of croplands.
The consecutive summer droughts in Central Europe in 2018 and 2019 led to severe water mass deficits, with yield reductions of up to 50% reported for the growing season of 2018.
Under the RCP8.5 climate change scenario, such intense droughts could become a common occurrence if society does not reduce greenhouse gas emissions.
There is an additional concern regarding the coincidence of dry spells and high temperatures, referred to as compound extreme events, which are expected to occur more frequently across central Europe.
Past compound events have negatively impacted the yields of major European crops, including wheat, maize, barley, and potatoes.
Current cropping systems face significant challenges in adapting to changing environmental and climatic conditions, resulting in increasing crop failure in regions like Germany.
More resilient, diversified cropping systems are required to better cope with droughts and increased temperatures and/or regulate the microclimate to reduce the risks related to climate change.
Systems are considered resilient when they can maintain their system functions despite exposure to complex and changing social, environmental, and institutional pressures, such as climatic changes and extreme weather events.
Agroforestry, the integration of trees and shrubs in the agricultural landscape, can play a major role in creating resilient agro-ecosystems in temperate Europe by modifying the microclimate through shading and wind protection.
Modern alley cropping systems, where annual crops are grown between tree lines or hedges, have shown the potential to achieve higher system yields than sole cropping systems and can provide diverse ecosystem services, including water regulation, soil protection, and biodiversity.
Trees and hedges contribute to climate change mitigation through enhanced carbon sequestration in aboveground and belowground biomass, as well as soil organic carbon.
Approximately 43% of agricultural land worldwide had a tree cover of more than 10% in 2010, with most agricultural land in Central and Western Europe having a moderate tree cover of 10–30%.
Agroforestry systems have historical relevance in Europe, such as hedgerows and Streuobstwiesen.
Alley cropping systems are the most common form of silvoarable agroforestry systems in temperate regions, closely integrating trees and shrubs in cropland through the establishment of tree rows within the agricultural field.
The planting of trees in rows allows for the continuation of field management with large machinery while diversifying the crop rotation by integrating perennial cultures.
This combination maintains the functionality of the former cropping system, adds diversification, and creates various economic and ecological benefits for farmers and society.
Decisive factors in the choice of tree species and crops are productivity and economic profitability, and machinery width can determine the distance between tree rows.

Agroforestry systems

Alley cropping systems with a single tree species, such as poplar ( $Populus$ spp.) or walnut ( $Juglans$ spp.), are most common, with biomass production by the trees being the main purpose (82% of cases).
However, there is large variability in the design of alley cropping systems, such as the choice of tree species and crops or crop rotations, spatial arrangement of the trees, width of crop alleys, orientation of tree rows, age of the system, and the purpose and management of trees and crops.
Tree rows in short rotation coppice alley cropping systems are generally characterized by higher tree density than alley cropping systems for fruit and timber trees.
Regular biomass harvesting in short rotation coppice systems causes dynamic changes in aboveground biomass throughout the harvesting cycle, while fruit and timber tree-based systems develop more gradually over time.
These differences in design and management influence how the microclimate is altered within an agroforestry system and which components of the water balance, such as evapotranspiration, plant water use and rainfall interception, are affected.
Differences in site characteristics, including topography, soil type, climate, and surrounding land use, might also influence how including trees in agricultural systems affects the microclimate and water balance, but this has not been investigated to date.
To make informed decisions on whether to apply agroforestry practices, it is essential to understand how the inclusion of tree rows influences the microclimate and water balance of the cropping system at the field scale as well as the landscape scale.
This knowledge is a prerequisite to design appropriate alley cropping systems that enhance the resilience of agriculture to increasing climate-related risks in a given landscape and climatic context.
Such knowledge is specifically lacking for temperate alley cropping systems, as these have been less researched compared to their tropical counterparts.
Differences in the design, types of trees and crops, and climate conditions limit the transferability of results from tropical to temperate systems.
This review aims to provide an overview of the existing knowledge about the effect of temperate alley cropping systems on the microclimate and the water balance.
Research from all temperate regions has been included in this review due to the limited number of studies on Central European agroforestry systems.

Methods

Factors determining the components of the microclimate and the water balance were analyzed based on current peer-reviewed and selected grey literature on temperate alley cropping systems.
A literature search was conducted in Web of Science and Google Scholar using keyword combinations such as (agroforest* OR "alley cropping") AND microclimat* AND (temperate OR Europe* OR USA OR China OR “North America” OR Canad), whereby microclimat* was replaced by relevant terms for specific microclimate and water balance components.
Scientific evidence from 36 field-scale studies, 3 landscape-scale studies and 8 literature reviews was combined with expert knowledge to develop a conceptual understanding of the influence of tree rows on the microclimate and water balance components in alley cropping systems.
Effects were distinguished between at the local scale, i. e. small-scale changes in and between tree rows of an agroforestry system, and at the landscape scale, i.e. beyond the boundaries of the agroforestry system.
Where possible, effects were separated by type of alley cropping system (short rotation coppice alley cropping systems, young (< 5 years) alley cropping systems with fruit and/or timber trees and mature alley cropping systems with fruit and/or timber trees).
The level of agreement among the available studies was quantified as the proportion of all studies for a given parameter that reported the same response.
Based on this synthesis, open questions for future research were identified.

Local scale effects and gradients

The inclusion of trees in cropland creates cropping systems with a higher structural diversity compared to sole cropping systems.
The tree rows will generate gradients in microclimate and water balance components within the alley cropping system, which are driven by the distance from the tree row.

Microclimate

Light intensity

Availability of sunlight for photosynthesis is a key factor in crop productivity, competition for light due to the inclusion of trees on cropland is a major concern when considering the adoption of agroforestry.
The spatial extent of shading is highly variable from daily to annual time scales, depending on the position of the sun relative to the tree rows , whether the trees are foliated and on the actual weather (sunny vs. cloudy days).
Gillespie et al. (2000) reported a decrease in photosynthetically active radiation of up to 42% close to the tree row (planted in N-S orientation) in a red oak alley cropping system in the USA.
A study in a willow-grassland alley cropping system in central Germany found a reduction of 10–25% of the photosynthetically active photon flux density close to the tree rows in the first two years after establishment, whereas no significant differences with the control site were observed for the centre of the 9 m wide crop alley.
Carrier et al. (2019) did not observe a significant reduction in light intensity in the centre of 25–90 m wide crop alleys, whereas Yang et al. (2021) measured a reduction of 12% in crop alleys with a width of 6 m.
Tree height likely plays a role as well , as trees in the agroforestry system studied by Yang et al. (2021) were up to 5.1 m high, while trees in the systems studied by Carrier et al. (2019) and Ehret et al. (2018) were 3.5–12.7 m and 0.8–4.0 m high, respectively.
For temperate alley cropping systems in the northern hemisphere, significant shading of crops on the northern side of the tree rows would result in uneven development of the crop.
North-south tree lines are therefore recommended for temperate agroforestry systems at high and medium latitudes.
A reduction in light intensity might be minimized through the use of trees with a beneficial crown architecture or by manipulation of the crown through pruning or pollarding.
For example, Dufour et al. (2020) measured a smaller reduction in incident photosynthetically active radiation relative to full sun conditions at 6.5 and 11 m north of east-west planted tree rows after pollarding (0–10% reduction) compared to trees that were not pollarded (8–18%).

Wind speed

The physical structure of tree rows in cropland does not only intercept light, but also affects wind speed.
The effect on wind speed is determined by the length, density, height and orientation of the tree row or hedgerow relative to the predominant wind direction.
The tree rows reduce the air velocity at the windward and leeward side in proportion to their height.
This effect generally reaches further than their effect on temperature and incoming solar radiation.
At the leeward side, wind reduction at a distance of up to 12 times the height of the windbreak has been reported.
Even relatively young tree rows (3 m height) were able to reduce wind speed across 48 m wide crop strips, suggesting that beneficial effects can be observed shortly after establishment of an agroforestry system.
At the windward side, reductions in wind speed have been observed at distances of 0.5 to 3 times the height.
The porosity of the windbreak or tree row influences the degree of protection, whereby a porosity of 40–60% was found to be most effective.
A lower porosity could result in low air pressure at the leeward side, thus enhancing the occurrence of turbulence.
Tree lines with higher porosity, such as widely spaced or pruned trees could also enhance the development of turbulence and lead to increased damage to the crops.
Integration of windbreak elements, such as shrubs, within such tree lines is therefore recommended.
Turbulences are also related to the length of the windbreak, whereby a length of at least 10 times the height of the windbreak is recommended.
Direct benefits of reduced wind speed include reduced thigmomorphogenesis, i.e. the response of plants to mechanical stress, reduced damage to the surface of the crops and avoidance of wind-related soil erosion.
A negative or non-existent effect of tree rows on wind speed is usually observed in cases where the windbreaks are not properly implemented.
Windbreaks can influence other microclimatic variables, such as vapour pressure deficit and crop evapotranspiration.
Relative humidity can increase due to reduced air mass exchange, which influences the water use of plants.
For example, the water use efficiency of wheat protected by a windbreak was found to be 64% higher than in an unprotected crop field.
Windbreaks can also cause changes in localized precipitation patterns within the site.
Where the crops are protected from the wind, dew development occurs more frequently and precipitation is stronger.
These benefits of agroforestry are especially relevant for regions with a coarse soil texture, which dry out faster and thus will experience a stronger effect of the expected climate change.

Air temperature

Air temperature plays an important role in crop growth and development, as these processes are influenced by the deviation from the optimal growth temperature of the crop, as well as the diurnal temperature variability.
The latter is especially high in continental climates and can be further enhanced by climate change.
The nightly radiation and the daily incoming solar radiation are reflected and absorbed by the trees, such that the daily temperature fluctuations within the agroforestry system are generally reduced.
More energy is removed by the trees through evapotranspiration (i.e. latent heat flux), with less energy available as sensible heat to heat up the vegetation, soil and air, resulting in lower temperatures in and close to the tree rows.
Not all agroforestry systems are able to provide this temperature buffering effect.
Swieter et al. (2021) measured air temperature in a short rotation coppice agroforestry system in Germany with tree rows in north-south orientation, almost perpendicular to the prevailing wind direction (west/south-west).
They observed generally lower temperatures in the crop alleys in the morning compared to the reference crop field, and higher temperatures during the afternoon, resulting in a higher diurnal temperature variability.
These higher temperatures could be an effect of reduced wind speed, as it plays a central role in determining the resistance to heat transfer and evaporative cooling.
The air temperature close to the tree rows was lower than in the centre of the crop alley (Swieter et al., 2021).
Kanzler et al. (2019) observed the temperature buffer effect within poplar tree strips, but not in the crop alleys in a similar system elsewhere in Germany.
Ramananjatovo et al. (2021) also measured reduced daytime temperatures close to fruit trees grown among vegetables in a French garden-orchard system.
Higher minimum air temperatures could result in lengthening of the vegetation period during colder years.
Early germination of the crop could reduce unproductive water loss through soil evaporation and reduce shading effects, as crop development could start before the trees are in full leaf.
The tree rows could also create a higher spatial variability in air temperature, which could result in uneven ripening of the crop.
In a Danish short rotation coppice agroforestry system, Foereid et al. (2002) observed, for example, an earlier anthesis date and crop development close to tree rows where the temperatures were slightly higher.
On the other hand, Swieter et al. (2021) reported a delay in phenological development of winter wheat in the transition zone from crop field to tree strip in a German short rotation coppice agroforestry system.
The potential of agroforestry systems to buffer extreme temperatures therefore seems to be strongly related to the design of the agroforestry system and the choice of annual crops (including photoperiod and water status).
Although air temperature effects in temperate alley cropping systems are relatively well investigated, systematic research on the effect of the layout of the system as well as interactions with shading and wind speed are required to design alley cropping systems with a desirable microclimate effect.

Relative humidity

In addition to temperature and light, the growth and development of crops are influenced by the relative humidity or the vapour pressure deficit through its influence on evapotranspiration, water use and crop temperature.
Evapotranspiration and advection are the main sources of water vapour within the system, and they are strongly interlinked with the microclimate (wind speed, air temperature) as well as vegetation type and cover.
In parts of the alley cropping system, where the air temperature is higher compared to a sole cropping system, the vapour pressure deficit increases as warmer air can hold more moisture, thus increasing the transpiration of the crops.
This is supported by data from Kanzler et al. (2019), who observed lower air temperature and vapour pressure deficit in poplar tree rows and within 3 m of the tree rows compared to an open field, while higher temperatures and vapour pressure deficit were observed in the remaining part of the crop alley.
Compared to the open field, the vapour pressure deficit in the agroforestry system was more suitable for plant growth (0.9–1.0 kPa) for up to 29% of the time, whereas vapour pressure deficits resulting in water stress (> 2.0 kPa) occurred 36% less frequent.
This shows the potential of agroforestry to reduce stress during heat waves under water limiting conditions.
Other studies reported that the minimum relative humidity was 19% higher in a German alley cropping system than in the reference crop field without trees and 5% higher at 1.5 m from apple trees planted among vegetables than at 5 m distance.
Also Swieter et al. (2021) found higher relative humidity in the shaded part of a short rotation coppice system in Germany.
The effect was smaller on cloudy days than on sunny days.
However, one has to take into account that increased relative humidity can also induce uneven ripening of crops and create a more suitable environment for certain diseases.
The effect of changes in relative humidity on water use by crops and trees and potential competition for water resources is discussed in Section 3.2.3.

Water Balance

Precipitation, Throughfall, and Stemflow

The altered vegetation structure in agroforestry systems leads to differences in how precipitation is divided between vegetation and soil.
Precipitation interception, the capture of rainfall on plant surfaces, relates to the surface area of aboveground plant parts and the leaf area index (LAI).
Temporal changes in LAI result in seasonal interception variations, with examples showing interception values of 28% during vegetation and 12% when leafless.
The fraction of intercepted precipitation depends on canopy structure and decreases exponentially with increasing rainfall intensity.
Throughfall, water dripping through canopy gaps, and excess water reach crops and soil beneath trees, emphasizing the critical role of canopy structure.
Dense tree crowns with small leaves retain more water than sparse crowns with larger leaves, although this effect might be limited in the early years after establishment when crown cover is minimal.
Trees also affect spatial precipitation distribution by concentrating rainfall near stems via stemflow.
When canopy structures are organized stemflow can induce preferential flow through soils, transporting nutrients to biogeochemically active areas.
Stemflow yield depends on complex variables like meteorological conditions, seasonality, species-specific traits, stem density, and canopy structure, and rougher-barked trees generally produce less stemflow.
The choice of tree species affects the quantity and spatial distribution of rainfall reaching crops and soil directly, influencing water availability for annual crops near tree rows, but studies on throughfall and stemflow in temperate alley cropping systems are currently lacking.

Infiltration and Surface Runoff

After precipitation reaches the surface, it can evaporate, infiltrate the soil, or become surface runoff.
Tree rows with continuous vegetation can divert and slow water, reducing the erosive force of surface runoff and increasing infiltration.
During and after extreme precipitation, the risk of surface runoff increases with decreasing soil cover, increasing slope, and soil movement during tillage.
Agroforestry systems can substantially reduce soil erosion via raindrop interception by tree litter and enhanced water infiltration due to extended and deeper rooting systems.
The water erosion cover-management factor (C-factor) is between 0.03–0.13 (no units) with agroforestry, compared to arable land values of 0.233.
Reduced surface runoff could incentivize farmers to adopt agroforestry.
Spatial variability in soil type and macropore occurrence determines the extent to which agroforestry reduces surface runoff.
A German agroforestry system combining poplar and high-value timber trees reduced surface runoff by 90%, while a Swiss study indicated that precipitation lost to runoff reduced from 2.3 to 1.8% when comparing arable land/grassland to agroforestry landscapes.
Soil erosion occurs in crop rows more than in tree strips in alley cropping systems with different oak species, and tree rows increase water uptake and reduce runoff despite similar infiltration rates in tree rows and crop alleys.
Saturated hydraulic conductivity is 3 to 14 times higher under tree rows than in crop alleys, most likely related to tree age and root system depth.
Data is lacking on the time required for soil hydraulic properties and infiltration rates to change after tree row establishment.
Nutrient leaching could also be affected by higher infiltration rates.
Although deep-rooting trees can retrieve leached nutrients from deeper soil, current knowledge is limited on the role of tree rows in reducing leaching from agricultural land.

Evapotranspiration and Water Use

The largest water losses in agroforestry systems result from system evapotranspiration with plant transpiration being the greatest share, followed by interception losses and soil surface evaporation.
Plant transpiration is driven by radiation, wind speed, air temperature, and vapor pressure deficit, all of which are affected by integrating trees into agricultural land, and these factors can be modified to reduce crop evapotranspiration.
Coussement et al. (2018) estimated lower crop evapotranspiration rates near a mature poplar tree row, relative to crops grown without trees.
Potential evapotranspiration in arable crop areas of a short rotation coppice agroforestry system in Brandenburg, Germany, was up to 25% lower than in a neighboring arable field without trees.
However, trees generally have higher transpiration rates than crops, causing higher water use.
Petzold et al. (2009) compared water consumption between a poplar short rotation coppice agroforestry system and winter wheat, finding poplar had higher water use due to a two-month longer vegetation period.
The generally higher transpiration rates of trees lead to concerns regarding competition for water resources, especially during droughts.
The potential for competition and complementarity depends on root system and water uptake depth of trees and annual crops.
Jose et al. (2000) found that tree roots invaded the crop alley when no barrier was in place, leading to higher water uptake by trees, and competition was observed up to 15 meters from a row of 40-year-old poplar trees, the zone being function of the tree species and crops.
Younger trees likely compete more with agricultural crops for water than older trees because their roots are mostly in shallow soil.
Mature agroforestry systems could have less impact due to established root systems.
Careful consideration of tree and crop rooting characteristics, as well as the distance between the trees and the crops, is crucial for developing successful, resilient agroforestry systems.
Certain tree species can shift water uptake depth to deeper soil layers later in the growing season.
Trees can increase the overall production of the system via hydraulic redistribution, partially offsetting the negative aspects of competition.
Management practices like root pruning can alleviate water competition between trees and crops.

Water Storage

Spatial variations in interception, throughfall, infiltration, water use, and microclimatic factors within agroforestry systems affect soil moisture and water availability for crop and tree growth.
Beule et al. (2020) observed higher soil moisture content in and near tree rows in a German poplar-based alley cropping system, and beyond 7 meters from poplar tree rows, soil moisture was similar to a sole-cropped field.
Similar observations were made in a willow-grass alley cropping system in central Germany.
In oak tree rows in the USA, higher soil moisture was observed in the upper soil layer, attributed to higher porosity from root development.
Peng et al. (2009) found no differences between soil moisture in a sole-cropped maize field and an agroforestry system with walnut and plum trees.
Increased water absorption capacity and infiltration rates could enhance soil moisture in tree rows, reporting faster soil moisture increases in tree rows during recharge periods, with moisture variation across agroforestry systems being more prominent in the upper soil layers.
Rainfall exclusion experiments in walnut intercropping systems showed that soil moisture content was more reduced at 20 cm depth than at 50 cm depth when comparing the agroforestry system with a sole cropping system.
In contrast, high water use by trees has often been reported to reduce soil moisture content in and near tree rows, and several studies report a strong drying effect in summer in various temperate alley cropping systems.
Spatial soil moisture content variations can decrease in wet periods as competition is reduced.
Barriers or trenches to reduce tree root invasion in crop alleys significantly decreased soil moisture within tree rows but increased it in crop alleys, emphasizing the effect of competition.
Fast-growing/closely spaced trees had a stronger impact on soil moisture dynamics, performing better under drought stress as they developed deeper rooting systems.
Tree and crop species could be chosen to align seasonal growth to reduce competition for water and nutrients.

Landscape-Scale Effects

The impact of integrating trees and shrubs in agricultural land extends beyond field boundaries.
Increased tree cover is linked to enhanced regional precipitation, and it is simulated moisture recycling and cooling effects with models.
Most agroforestry models do not well represent biosphere-atmosphere interactions.
Silvoarable agroforestry could reduce soil erosion up to 70% and nitrate leaching by 20–30%. It could also sequester up to 140 tonnes C per ha over 60 years, and landscape diversity could increase up to 4 times.
The landscape configuration determines the magnitude of these effect.
Agroforestry can significantly reduce wind erosion in simplified cropping systems.
The net evapotranspiration of an agroforestry system depends on changes in soil evaporation and the sum of crop/tree transpiration, which generally increases due to the trees.
Small-scale differences within agroforestry system compensate each other.
Agroforestry sites had increased surface roughness and turbulence ,which resulted in higher evapotranspiration.
During wet periods oil mallee agroforestry and legume pasture had similar evapotranspiration rates however agroforestry has increased rates during dry spells.
In densely spaced Sitka systems, latent heat fluxes and evapotranspiration fluxes are higher.
Energy balance at larger scales decreases the sensible heat flux and the albedo, due to lower albedo in trees.
Tree rows, as well as continuous vegetation covering them, could lead to reduced ground water recharge when they intercept rainfall and use water. The effect of groundwater recharge relies heavily on crop type and how much precipitation/water it uses.
Likelihood of summer droughts necessitate understanding (large-scale) impacts of agroforestry on groundwater recharge to determine what benefits agroforestry has for agriculture during droughts.
Decision-makers can create beneficial impacts beyond the farm by developing regulations and allocating funds to promote the adoption of more agroforestry.

Implications for Resilience to Climate Change

Studies on temperate alley cropping show that agroforestry could be a good way to increase the climate resilience of temperate cropping systems.
Alley cropping systems can buffer any mechanical damage to crops from rainfall where extreme rainfall is recurrent, and they diminish soil erosion by reducing how rainfall goes across the ground.
This especially applies to tree rows in more mature systems, where the roots have increased how much water can be absorbed/stored.
Aligning tree rows with field contour lines helps decrease runoff, though it may conflict with having trees face north-south to reduce shading problems.
Agricultural machines are limited by how curvy they can go amongst the tree lines.
It is unavoidable that compromising must occur to optimize various functions (e.g. shading, lessening soil erosion/windiness) when constructing agroforestry systems.
Where trees have been planted in copious amounts, agroforestry may worsen droughts.
In a study by Nasielski et al. (2015), the soils of agroforestry systems had similar or increased reduction in moisture compared to a similar area that implemented sole cropping, but the yields in the agriculture were still decent.
The effect of early spring droughts are lessened by agroforestry practices. These studies tell us that agroforestry systems don't have a negative effect on the agro productivity or are supplemented by another way that the microclimate and water is balanced.
The positives from shading occur when stresses from heat are expected to be seen often, in extended droughts/heatwaves.
Shaded sections of a farm can improve the moisture in the air and reduce dryness.
High variations in how crops develop creates harvesting issues, but can also act as a risk reduction way to hedge your bets as these crops can maintain production under climactic stress.
Since alley cropping has a good amount of diversification, diversifying the goods/services can mirror how it reduces risk or makes farmers more equipped to handle climate change.

Synthesis and Outlook

This review summarized data from field studies about how the introduction of agroforestry systems affect microclimates and the movement/storage of water.
When comparing alley cropping practices to sole cropping, there can be a clear direction of change for certain components (less light/wind), other parameters have conflicting evidence.
This has been occurring often. Parameters, such as soil moisture, air temperature, vapor pressure deficit, showed uncertain signs or varying data between studies.
This could come from how different design and landscape contexts are.
Also, microclimatic variables have many interactions as well. Using factorial experimental designs is also not helpful in the common ways that agroforestry compares to sole cropping systems.
Using system modeling, as opposed to field tests, may make it easier to tell apart microclimate effects and signs in alley cropping.
Ideally, studies would require long term monitoring of microclimate and water balance, starting from the beginning of the alley cropping systems.

Research Questions

To what extent does the site context (e.g., soil type, climate, topography) influence the effect of agroforestry on field-scale microclimates and the water balance?
What is the effect of agroforestry on the microclimate and water balance at the landscape scale? Is there a certain threshold (e.g., % of agricultural land under agroforestry) beyond which an effect is measurable?
How can the design of alley cropping systems be optimized such that beneficial microclimate effects are achieved and negative effects on productivity are minimized?

Conclusion

Given the expected consequences of climate change for agricultural productivity in Central Europe, there is an urgent need to identify suitable agricultural practices to enhance the resilience of agriculture to droughts and extreme events.
Designing resilient alley cropping systems would be a first step towards improving the sustainability of farming systems in Central Europe.
Farmers need to make complex decisions that account for economic factors, water use and microclimates under increasingly uncertain climate conditions.