Roots as communication channels

Forests: Beyond the wood – VI

By Dr. Ranil Senanayake-September 9, 2018, 9:47 pm 

Living soil and tree roots have a dynamic relationship. Roots have three main roles. Anchoring trees in the ground to supply the tree. Acting as the collector of minerals, oxygen and moisture. Acting as an information exchange network. Roots of trees are always, exuding or releasing organic matter into the soil, approximately 20-30% of the total root weight is given to the soil each year. This is to feed and maintain a healthy soil ecosystem around their roots. Most trees have specific mycorrhizal fungi living as mutualists that grow in and around the fine, hairlike root tips of trees and help extend the mineral and moisture collecting capacity of the root by hundreds of times the length of that root hair

These mycorrhizal networks join the hairlike root tips of trees to form the basic links of the network, which appears to operate as a symbiotic relationship between trees and fungi. It has been demonstrated that trees are capable of exchanging chemicals and information along these networks. Studies of nutrient flows suggest that trees of the same species are communal, and will often form alliances with trees of other species. They also suggest that, forest trees have ‘evolved to live in cooperative, interdependent relationships, maintained by communication and a collective intelligence similar to an insect colony.’

The phenomenon of plants using their roots to communicate have been demonstrated often. In 2010, South China Agricultural University’s Ren Sen Zeng found that during attacks by nefarious fungi, plants would release chemical signals into the mycelia to warn other plants and in 2013 David Johnson of the University of Aberdeen and his colleagues showed that broad bean seedlings that were not themselves under attack by aphids, but were connected to those that were via fungal mycelia, activated their anti-aphid chemical defences. Those without mycelia did not.

Trees require three things for good rooting: water, oxygen, and soil compaction levels low enough (or with void spaces sufficiently large enough) to allow root penetration. Trees root in different in patterns, some have very deep roots often over 10meters , some are shallow laterally rooted others fibrous, thus a profile of the root zone of a forest demonstrates a complexity as great as the branching patterns above. This complexity not only acts in interspecies communication, but also helps to anchor the whole forest mass against episodic storms. It has been shown that for young saplings in a deeply shaded part of the forest, this network is literally a lifeline. Lacking the sunlight to photosynthesize, they survive because big trees, including their parents, pump sugar into their roots through the network. The behaviour ensures the survival of the forest, when a large tree falls, there are many juveniles to quickly fill the space.

Tree roots also affect the preferential flow of water in soil by creating root channels that are formed by dead or decaying roots, channels formed by decayed roots that are newly occupied by living roots, and channels formed around live roots. Rainwater reaching the ground moves along these channel to recharge the shallow aquifer. The different root systems such as the tuft root systems typical of grasses and bamboos, the taproot system typical of broadleaved trees like Mango etc. effect the flow of water in the soil. Further the direction of growth or architecture of the root have differential the effects on preferential flow. For instance, a downslope root orientation is more efficient for transporting excess water but hard root extremities can represent dead-end paths for water flow. Often, root branching may divide or concentrate flow or (Clusters of roots act as sponge-like structures and concentrate high water pressures. Thus the difference between even aged monoculture plantations that do not firm a diverse rooting system and a forest becomes obvious.

At the other end of the forest is the canopy. The canopy of mature tropical forests contains a distinct ecosystem comprised of plants and animals that do not live on the ground, but spend their entire lives high above it. This epiphytic canopy ecosystem develops and matures from the early seral stages found on young trees to mature diverse ecosystems which have been recorded to attain over sixteen tones of dry matter per hectare in some areas. Restriction to the tops of tall trees places unique constraints on the distribution and life strategies of the plant inhabitants of this ecosystem. The way that these constraints are overcome is analogous to the life strategies adopted by reefs and reef dwellers. Important decisions need to be made when choosing species and location for reforestation.

Coral reefs are specialized ecosystems that can only colonize suitable locations or substrates in the ocean. The major coral dispersal mechanism is the ocean current. Organisms that comprise the reef produce vast quantities of spores or propagules that are ejected into the water. These propagules travel in currents until they arrive at a suitable substrate where they settle and begin to grow. The early colonizers in turn create favourable conditions for the spores of more specialized organisms which settle in the maturing reef ecosystem. Sometimes fish and other large organisms transport the propagules of reef organisms as they visit different reef areas. All transport occurs via the water column.

Epiphytic communities in the canopies of forests use similar strategies to coral reefs in order to spread their propagules and colonize new areas. Most of the plants that make up the epiphytic canopy ecosystem, that is Bromeliads, Begonias, Peperomias, Gesneriads, lichens, mosses, ferns and orchids, have seeds or spores that are transported by the wind, some by animals. The plants release their seeds and spores to air currents, just as coral organisms release their spores into the water currents. These propagules travel in the wind until a suitable substrate is found. Further, just as certain species of animals whilst travelling from one reef area to another transport reef propagules, so do birds and mammals, moving from tree to tree and forest to forest, transfer seeds and spores.

Many species of epiphytes require trees with a specific bark character or chemistry to settle and begin growth. Thus planting unsuitable tree species is not sufficient to assist with the re-establishment of these ecosystems. The tree species have to be selected with care. There is likewise a succession of species, and species diversity, as the forest matures, with mature trees hosting the greatest number of ephiphyte species.

Each host tree species supports a particular set of epiphytes and each epiphyte reproduces at a certain time of the year. The propagules of wind-transported epiphytes travel on the prevailing wind at that time of year. The prevailing wind direction is therefore important when choosing the location, in relation to an existing forest, for planting any host tree species. If the conservation of a particular ephiphyte is the goal, then the time of year that it sends out propagules and the direction of the prevailing wind need to be known. Host tree species should be planted in a location downwind of the existing colonies of epiphytes.

To be continued…