This week brought some much welcome relief in the interior of British Columbia (where I call home) from a long, dry, hot spell of weather. With temperatures into the upper 30s C, wildfires are raging, most soils have dried to a powdery consistency and ground vegetation has turned various hues of yellow and brown, or has already completely withered away for the year. On a walk through a sub-boreal forest however, I was reminded of the powerful benefits of an overstory to moderate extremes and balance out production relative to the open. There in the midst of one of the heat wave were pockets of green grasses and forbs, and plump berries ripening on saskatoon, pin cherry and high-bush cranberry growing in the partial shelter of mature spruce, trembling aspen, and paper birch.
Conventional “wisdom” in the applied plant sciences (agriculture and forestry) dictates that plant-to-plant interactions are driven by competition for limited resources. Following this philosophy, land managers tend to simplify systems in order to minimize competitive effects. This type of thinking leads some to conclude that the best path to maximize forage, horticultural or other production is to eliminate all tree and shrub cover not of direct economic interest. But ecosystems and their productivity are not solely the function of competition, but rather a complex mixture of negative effects (including competition) and positive, or facilitative processes whereby the presence of one species improves and promotes the growth of another. And it is usually in times of environmental extremes that the facilitative effects make their presence known, providing a strong ecological rationale for preserving some overstory in an agricultural setting.
Pole to mature-sized trees, by virtue of their size and superior canopy position, are the dominant above-ground structural elements in mixed systems and the therefore trees strongly influence resource availability and the microclimate in their understory. Tree canopies reduce sunlight penetration, precipitation and airflow to their understory, and singularly or in combination, these modifications are responsible for several environmental differences in the understory in comparison to conditions in the open.
Some of these modifications can clearly be viewed in the negative from a plant production standpoint. Reducing light (both the amount and the spectral quality), reduces photosynthesis and hence plant growth. But it is still important to remember that for a given species, plant growth has both upper and lower thresholds in response to light; that is, photosynthesis responds to a range of light levels, above and below which there is little change. At light saturation, additional units of light do not increase photosynthetic activity, whereas at the light compensation point, light levels are insufficient to generate photosynthesis in excess of the base respiration needs of the plant. The implication of this is that we can retain some tree cover in an agroforestry setting and have no, or minimal negative impacts by keeping the light levels at or slightly above the light saturation level of the understory crop.
Trees and shrubs also intercept precipitation and being large plants, can competitively deplete soil moisture and nutrients. This too, can have obvious negative implications for understory plant growth. There are however, also many modifications to the understory microclimate and soils that can facilitate plant growth. Here are some of the major positive influences that tree and shrub cover can have:
1. Reduced air temperature. By blocking sunlight, they reduce the air temperature in the understory. In extreme temperature events (where surface temperatures rise above about 40*C) leaf protein and tissue degradation can occur with eventual ‘burning’ of the foliage with prolonged heat exposure. Damaging surface temperatures can even occur in a boreal environment. Trees and shrubs cool the air and protect plants from searing heat.
2. Increased humidity. Under warm, dry atmospheric conditions, the combination of high temperatures and low atmospheric humidity result in a strong gradient for the movement of water from a plant to the atmosphere and can result in desiccation or suspension of photosynthesis. Elevated humidity in the understory slows water loss from cool season plants.
3. Decreased soil temperature resulting in improved nitrogen availability. High soil temperatures affect the bacterial populations responsible for converting nitrogen in the soil into a form usable by plants. By shading the soil, trees and shrubs promote a higher microbial biomass and greater mineralized soil nitrogen.
4. Reduced evaporation from the soil. A function of both slower wind speeds and less solar energy reaching the understory. This water conservation can often compensate for the soil water use by trees and shrubs.
5. Increased soil water holding capacity. Trees and shrubs add large amounts of organic matter to the soil resulting from litterfall and root turnover. This adds to the water holding capacity of the soil to the benefit of both under and overstory crops.
6. Decreased soil water losses. This is accomplished through root interception of run-off or deep drainage, allowing some for shallow-rooted plants to access water that would otherwise be carried by gravity too deep for them to utilize.
7. Hydraulic lift of soil water. Deeply rooted trees and shrubs draw water via their transpirational pull from deep soil profiles and release it in shallow profiles, again to the mutual benefit of understory and overstory crops.
8. Delayed maturing of understory plants. This, admittedly, could be viewed as either a positive or negative impact. But certainly for livestock producers have a portion of the forage crop maturing later in the season, or in the depths of a drought, can provide forage that is both more palatable and of higher quality, thereby extending the grazing season and / or offsetting supplemental feed purchases.
Clearly there are some advantages to retaining and integrating tree and shrub cover in our agricultural landscapes to benefit from their facilitative effects. Land managers would do well to heed the words of Harold Arlen and Johnny Mercer (1945):
“You’ve got to ac-cen-tuate the positive
E-lim-inate the negative
Latch on to the affirmative, and
Don’t mess with Mr. Inbetween”