Facilitation occurs when one species increases another species fitness. It is different than a mutualism because one species benefits, while one remains unaffected, as opposed to both parties benefiting from each other’s presence. One of the most influential, ecosystem altering mechanisms of facilitation is a process called hydraulic lift. This occurs when plants with deeply rooted roots access water far past where other plant species can reach. If the soil near the surface is dryer (has more negative water potential) than the aboveground plant that accessed the water, water will leave the roots and enter the dry soil horizon. This hydraulic lift benefits plants that don’t have deep taproots, as the otherwise inaccessible water is lifted to the upper soil horizons. As it turns out, the mycelial networks of fungi have an analogous function, transporting water beneath the forest floor to places that would remain dry. This facilitates other species with imperative functions in forest ecosystems.
Hydraulic lift is increasingly important in more arid climates. A long-term study carried out in the Sonoran Desert highlighted the difference between a plant living under dead shrubs, vs. living shrubs of the same species. Palo verde trees occur in the Sonoran Desert, and have extremely deep roots. This desert adapted species accesses the water table, unlike desert cacti that evolutionarily opted for water storage, instead of water scavenging. Dead palo verde trees still create shaded areas that limit soil evaporation, but this has a diminutive effect on plant communities. Young saplings that germinated under dead palo verde were not facilitated by the conditions presented by the deceased tree. Oppositely, living palo verde greatly enhanced sapling survival through the processes of hydraulic lift.
Sonoran Desert
In March, earlier this year, Anja Worrich and her team published work that uncovered facultative processes similar to the processes occurring in the Sonoran Desert. Using artificial microcosms, they discovered that networks of fungal hyphae can translocate water and nutrients to dry and nutrient poor areas. The movement of water and nutrients via fungal networks were found to be a facultative process which helped stimulate a vital group of organisms living from the forest floor; soil bacteria.
Soil bacteria are fundamental in forest ecosystems through their ability of chemical breakdown, nitrogen fixation, decomposition, and nutrient mobilization. Unlike fungi, most single celled bacteria can’t form networks, which limits them to a small subset of soil resources the bacteria are in direct contact with. This paper reveals that these bacteria are stimulated by nearby fungi, as both water and nutrients leach out of these fungal highways into patches of soil with potentially poor levels of nutrients and water. This experimental study showed that the transfer of these resources caused the spore forming bacteria Bacillus subtilis to break dormancy and germinate. So, not only do these networks facilitate functioning bacteria, but enhance soil processes by recruiting bacterial propagules that would otherwise remain dormant.
Synonymous to the palo verde tree making water accessible to shallow rooted plants, fungal hyphae stimulate the activity of a group of organisms vital to forest health. The facultative processes associated with soil fungi encourage both ecological resistance and resilience, by providing soil bacteria with nutrients and water. Together, both organisms influence the nutrient pool which radiates throughout forest ecosystems from the bottom-up. Facultative interactions are a dynamic interplay between species that for a long time were largely overlooked. Now, we are realizing how essential facultative processes are in maintaining the condition of the forest floor.