What does the mysterious cloud forest on Mount Sutro have in common with the strange and wondrous forests of the movie Avatar?

In that film, Pandora’s world encompasses huge old trees, vegetation that glows in the dark, and interconnected plants. Sutro Forest doesn’t have bioluminescent vegetation. It does have hundred-year-old trees, though, up to 200 feet tall. And it has — interconnections.

Interconnections? That’s science fiction, right?

Not quite. Many kinds of trees, when planted close to others of the same species, will intergraft their roots. The roots of individual trees join up to form an underground network.  Redwoods do it, so do lodgepole pines. So does eucalyptus. This helps all the trees in the group to survive. It’s one reason why even in a mixed forest, you tend to find trees in clusters by species, rather than evenly spread through the area.

In 120 years, in a densely-planted 80%-eucalyptus forest like this one, the root system is likely to be a massively intergrafted net. What this means for the forest is that, rather than being 50-60 thousand individual trees, it’s an entity that functions as an interconnected forest.

1. Trees support each other. Aside from merely providing a windbreak, the interwoven, interconnected mat of roots help provide stability to the trees. In the high wind conditions of Mount Sutro, wind throw is a major factor in tree death. Yet the number of downed trees is actually quite small considering the thousands of trees on the mountain.

[It has been suggested that another beneficial effect of intergrafting of the root systems of a pair or cluster of trees is the resulting stabilization of individual trees against wind throw (Loehle and Jones 1990). Basnet et al (1993) found that intergrafted trees of tabonuco (Dacryodes excelsa) underwent significantly less hurricane damage than isolated trees. — Horticultural Review 35, Article by K. Mudge, J Janick, S Scofield and E.E. Goldschmidt.]

2.  The intergrafted root network helps stabilize slopes. Even more than just the spreading roots of a few trees, such a network functions like a living geo-textile worked into the fabric of the mountain.

3. Trees share food. Research shows that carbohydrates – which are food for the trees – are transferred between trees. This may explain the density of the forest; the trees are co-operating, not competing.

[…large root grafts transferred proportionately more carbohydrates to the shaded trees than small root grafts. Carbohydrates transferred through root grafts could allow grafted trees to persist under conditions where non-grafted trees would be removed by competition. —  Carbohydrate transfer through root grafts to support shaded trees: Fraser, Lieffers and Landhausser.]

4.  Trees may share chemical signals. Research has shown that trees emit chemical signals in response to insect attacks or other changes in growing conditions.

Concludes one Australian arborist writing about root-grafting in eucalyptus, “Selective tree removal is not necessarily removing a competitor but a partner.”

But while the root grafting benefits the trees by sharing resources and providing support, it’s also a risk. Some types of infection can travel through intergrafted root systems. And when trees in the system are cut, and herbicides applied to prevent resprouting, the herbicides can travel back through the network and weaken other trees. Both Roundup (glyphosate) and Garlon (triclopyr) — the herbicides they plan to use in the forest — act this way.

[Root grafting occurs primarily within the same species, but may occur between plants within the same genus. This phenomenon can be of great importance. A herbicide can move (translocate) from a treated tree to an untreated desirable tree, killing or injuring it. Damage to desirable trees as a result of root grafting will occur from use of the following herbicides: amitrole, 2,4-D, dicamba, glyphosate, imazapyr, metsulfuron, picloram, and triclopyr. — Chemical Control for Woody Plants, Stumps and Trees By Stott W. Howard and Robert Parker (Washington State University)]

BREAKING THE CONNECTIONS

So what happens when you thin the trees from around 740 trees per acre to about 40 trees per acre, and add pesticides to prevent resprouting?

Parts of the root network start to die. It takes some years. But both because the remaining trees will not be enough to sustain the entire root network, and because of the toxins added to it, the network will thin out and decay. It will be not hold the soil as well; it will not support the remaining trees as well. Already more vulnerable to wind throw once the trees surrounding them are gone and wind velocities rise, they will lose the support of the intertwined root network as well.

The kind of cuts planned for Mount Sutro threaten to destabilize both the forest and the mountain.

[Edited to Add (Sept 2013): In addition to the intergrafted roots, there is also the mycorhizzal connection, where fungi connect trees to each other. Great article and video here from University of British Columbia: http://www.publicaffairs.ubc.ca/2011/07/07/at-the-root-of-the-problem/%5D