Agroforestry is one of the key soil health and land management practices of the Regenerative Organic Certification. The practice, as defined by the US Department of Agriculture (USDA), involves "the intentional integration of trees and shrubs into crop and animal farming." Trees provide a number of below- and above-ground services which may be of benefit to the viticulturist (documented by Favor). I have previously reported on the impact of trees on vineyard water parameters and soil-based nutrients. I continue herein with an examination of the effect of trees on vine root systems. Portions of this post draws heavily on the research reported out in Favor's work.
Soil Structure
The vineyard site is a key variable in the production of high-quality grapes, which is, itself, a key requirement for the production of high-quality wine. According to Wolf and Boyer (Vineyard Site Selection, Virginia Cooperative Extension), the best vineyard soils "permit deep and spreading root growth" and provide a moderate supply of water year-round. Katherine Favor posits that "much of the ability of vines to absorb both water and nutrients in the face of competition depends on the health and spatial distribution of the vine's root system" with research suggesting that "the depth and expansion of grapevine roots is highly dependent on soil structure and permeability ... and that grapevine root plasticity is also influenced by planting density and competition."
What are the soil characteristics that will permit "deep and spreading root growth" and year-round access to water? The most important requirements are internal water drainage and water-holding capacity. Geologic permeability (the capability of a porous rock or sediment to permit the flow of fluids through its pore spaces -- Dictionary.com) is seen by Wolf and Boyer as perhaps the most important consideration in a candidate vineyard's soil. Mark Chien (Soil and Site Selection Considerations for Wine Grape Vineyards, Pennsylvania State University) sees well-drained soils as a common denominator among all great vineyard sites. These soils "strike a balance between adequate depth and drainage and water-holding capacity" and vines deployed therein will have adequate water access during the summer and can rapidly drain water from the soils in the event of rainfall during the grape-ripening period. According to Favor, soil porosity is an even more important determinant of grape quality than is nutrient availability.
Another key soil feature is effective rooting depth. According to UCDavis, about 60% of the root structure of a vine plant can be found in the first two feet of the surface but individual roots can grow as deep as 20 feet, depending on soil permeability, the level of the water table, and the rootstock variety.
According to Favor, the above characteristics, among others, are influenced by soil structure. Soil structure -- the spatial arrangement of individual soil particles, their aggregates, and the pore space between them -- "affects soil strength, water holding capacity, nutrient retention, aeration, friability, erodibility, plant root movement and biological activity." A high-quality soil structure allows for deeper and stronger vine root systems and higher grape production quality. Soil quality characteristics are summarized in the chart below.
Agroforestry's Impact on Soil Structure
Agroforestry has been shown to improve soil structure through a variety of mechanisms (Favor):
- A mulching effect from litterfall and pruning materials can have beneficial effects on topsoil structure
- Soil cover improves soil structure by reducing raindrop and irrigation effect → conserved surface macroporosity → greater water infiltration rates → improved soil penetration
- High amounts of root biomass produced by the trees
- Tree roots improve soil macroporosity by breaking up compacted soils and leaving behind old root channels that grapevine roots are able to occupy for greater rooting depth capability
- Finer roots contribute to improved soil structure
- Tree buffer treatments produced higher porosity, increased mesoporosity, and improved soil structure
- Increased organic matter (OM) content
- Soil structure is largely influenced by the amount of OM in the soil
- Higher levels of OM → higher aggregate stability and overall improved structure
- Higher levels of OM contribute to increasing water infiltration and fertility
- Organic matter is able to use "sticky substances" to glue soil particles together → stable soil pores.
In summary, the agroforestry-related increase in OM → increased soil structure → increased rooting capability → higher yields and higher quality fruit production (Favor).
Competition Between Tree and Grapevine Roots
According to Favor, "much of the ability of vines to absorb both water and nutrients in the face of competition depends on the health and spatial distribution of the vines' root system."
Research has shown an overlap in the activity spheres of plants and vines: 63% of grapevine roots are in the upper 60 cm of soil compared to 77% of conifer roots while 80% of grapevine roots are in the upper 1 m of soil compared to 91% for trees. Grapevines might have a deeper concentration of roots at deeper soil profiles as their remaining roots extend to depths of as much as 12 m. Grapevine roots can spread out up to 10 m laterally from the trunk.
The most frequent agroforesty applications to date are olive trees with grapevines. Olive trees have similar lateral-root experiences as does the grapevine but its vertical roots grow even deeper. Similar to the grapevine, olive trees absorb the majority of nutrients in the top 1 m of soil and water in the top 1.2 - 1.7 m of soil. As Favor concludes, there may be substantial below-ground overlap between these systems.
Competition notwithstanding, Favor concludes that:
The positive effects that trees impart on soil structure, soil quality and root plasticity allow for deeper and stronger root systems that can better absorb nutrients and water despite competition from trees ... Fracture lines left behind by tree roots allow opportunities for grapevines to grow even deeper than they otherwise would have ... Additionally, competition from tree roots can trigger grapevine root plasticity, which results in increased root length density and increased nutrient and water absorption capacity per cm of soil. Tree roots and grapevine roots are indeed able to adapt to competition and thrive despite occupying overlapping niches.
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This concludes our examination of the below-ground services provided by trees in an agroforestry system. As per Favor, "there is growing evidence that incorporating trees into vineyards could play a valuable role in the future of viticulture in the coming years."
I will next turn to examination of the above-ground services provided by trees in an agroforestry system.
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