Tag Archives: rainforest

In an unpredictable environment, trees network for stability

In the highly variable ridge, slope and valley mosaic that forms the Luquillo Mountains of northeastern Puerto Rico, Dacroydes excelsa, commonly known as Tabnuco, dominates the landscape. Though tropical forests are generally quite diverse and seen as ideal environments for plant growth, life in this rainforest can actually be quite challenging. Powerful hurricanes pass through the Caribbean annually and hit the Puerto Rican mainland every few years. Large swatches of the Luquillo forest were flattened several years back when hurricane Hugo struck in 1989.  Aside from directly damaging or wiping out forest stands, hurricanes cause landslides that severly erode the already shallow, nutrient depauperate soils.

On steep, harsh slopes that experience such frequent disturbance, what allows one tree species to gain a competitive advantage over the hundreds of others struggling to survive? Rather than compete fiercely for limited resources only to be at the mercy of the next devastating hurricane, Tabunuco trees have adopted an alternative strategy- cooperation and resource sharing through root grafting.

Root grafting, the joining of neighboring tree roots to produce a network, is a phenomenon that scientists have been aware of for decades, though the extent of its occurrence and the benefits that it provides trees are largely unknown. In Tabunuco forests, however, root grafting is widespread and many of its benefits obvious.

Tabunuco trees grow in dense stands and will graft roots with neighboring trees as they mature, forming unions that comprise anywhere from two to over a dozen trees. A clear advantage of this strategy in an environment that experiences powerful storms is structural stability. Trees that have entered unions increase their base of support and are less likely to be uprooted during a wind event or landslide. In increasing their wind-firmness, individual trees boost their survival chances during a storm. Fewer uprooting events also reduces the probability of a major landslide and helps ensure the retention of the surface organic matter that contains most of the forest’s available nutrients.

Root networks can also improve soil conditions during the off-season. Densely packed surface roots form “organic benches” which trap leaves and other decaying plant matter rather than allowing these important nutrient sources be washed downslope. Roots aerate the soil, facilitating decomposition and nutrient flow. They also “prime” the surrounding soil for productivity by releasing sugary compounds that stimulate beneficial microbial activity (the interaction between plants and microbes in the root zone known as the “rhizosphere” is another fascinating topic entirely, which I will do attempt to do justice to in the future).

Scientists are now discovering previously undetectable advantages of Tabunuco grafting that underscore the high degree of sophistication and evolutionary purpose in the development of these networks. It is now known that root networks can actually serve as conduits for the transfer of carbon and essential nutrients between trees. This can provide an immense competitive advantage over non-networked trees. Tabunuco trees that receive the most sunlight and produce the most carbon through photosynthesis can transfer carbon to neighboring Tabunucos to ensure the long-term health and survival of the community. Individuals of less common species, such as the Caribbean palm and Colorado tree are excluded from Tabunuco networks and must compete for growth given only the resources available in the vicinity of their roots.

Though in Tabunucos root grafting precludes the need for inter-tree competition, it is theoretically possible that trees could use grafting for more selfish purposes. Ecologists have speculated whether trees can gain a competitive advantage over their neighbors by leeching a neighbor’s nutrients, much as the fungal organisms that associate symbiotically with plant roots can become greedy and actually sap nutrients from their host under stressful conditions. Root networks may even serve as a conduit for disease or herbicide transfer, allowing trees that produce or tolerate a harmful compound to efficiently clear out their competitors.

Basnet, K., F.N. Scatena, G.E. Likens, and A.E. Lugo. 1992. Ecological consequences of root grafting in tabonuco (Dacryodes excelsa) trees in the Luquillo Experimental Forest, Puerto Rico. Biotropica 25:28-35.

sunset in the tropics

I was so struck by this description of watching the sun wane through the forest in eastern Amazonia that I just had to share. E.O Wilson is an ecologist blessed with an amazing gift for rendering what most of us consider to be insignificant details of the world into beautiful, potent truths.

“At Berhnardsdorp the sun passed behind a small cloud and the woodland darkened. For a moment all that marvelous environment was leveled and subdued. The sun came out again and shattered the vegetative surfaces into light-based niches. They included intensely lighted leaf tops and the tops of miniature canyons cutting vertically through tree bark to create shadowed depths two or three centimeters below. The light filtered down from above as it does in the sea, giving out permanently in the lowermost recesses of buttressed tree trunks and penetralia of the soil and rotting leaves. As the light’s intensity rose and fell with the transit of the sun, silverfish, beetles, spiders, bark lice, and other creatures were summoned from their sanctuaries and retreated back in alternation. They responded according to rerceptor thresholds built into their eyes and brains, filtering devices that differ from one kind of animal to another. By such inborn controls the species imposed a kind of prudent self-discipline.”

— In Biophilia, by E.O. Wilson– an ecologist, entomologist, professor and acclaimed science writer

urban jungles

a study published recently in Global Change Biology found that rainforests have been displaced as ecosystems that store the most carbon- by cities! cities store more carbon in their trees, buildings and dirt, than the densest and most productive tropical rainforests.

according to researcher Galina Churikina, who led the study, US cities store about 20 billion tons of organic carbon. most of this carbon is held in soils, though a sizable fraction is also contained within buildings constructed with wood. ironically, the key to city’s’ remarkable capacity to store carbon seems to be their artificial nature. buildings and asphalt “bury” soils, locking away carbon that was once part of a dynamic forest, grassland, or other natural ecosystem.

Shanghai, one of the world's largest cities, is an enormous carbon sink!

this is not to discount the importance of urban trees in both storing carbon and providing numerous ecosystem services. trees and other urban plants ameliorate temperatures, providing a cooling effect in summers that reduces the need for air conditioning. trees also directly take up CO2 emitted from cars, reducing the amount of pollution that enters the atmosphere from cities in the first place.

Urban trees such as those in Central Park, NYC, keep buildings cool, capture CO2, reduce stormwater runoff, and improve quality of life.