Managing a temperate cloud forest is quite different from managing a regular dry forest.

The most important thing is: Don’t open it up to dry out.

When the forest is opened up, the area immediately becomes dryer. Today, on a foggy day in June, we hiked up the Southern side of the forest to the Native Plant Garden on top. The trail on the South side was dry – but only a foot to the side, the soil was damp.

This is even more obvious when you compare the forest with the open Native Garden on the summit, which is bare of trees.
It has been planted with native plants theoretically drought-tolerant. And yet, the garden is very dry -even on a foggy day. It survives only because it has a sprinkler system. (This is referred to as the “intact Rotary meadow” on the Nature in the City website. Rotary it most certainly is, since they paid $100K for it; meadow it may be; intact, not so much.) [Edited to add: In Oct 09 we learned that the garden isn’t actually being watered; so presumably the plants are managing. Perhaps that’s why they look rather dry. The green bushes at the back, “black acacia” (we think) are part of the forest.]

Only steps away, the trails under the trees are wet. Without sprinklers.

Here’s how it works:
1. The tall trees catch precipitate moisture from the fog.
2. The moisture is absorbed by the duff, and protected from evaporation by the undergrowth.
3. The canopy of trees over all shades the undergrowth and further restricts evaporation.

Sutro Forest gets 8-12 inches annually of fog-precipitation moisture every year – nearly 50% of the regular winter rainfall. This fog-moisture is spread out through the year, particularly in the summer months. According to the 2001 report, there are around ten dry days each years. Ten.

[ETA: We kept a Fog Log for 2009. We found only seven dry days.]

What’s more, the duff, covered in rampant undergrowth, acts as a sponge and retains this moisture, releasing it gradually. That’s why the forest is green year round and never dries out. The moisture is trapped by the tall eucalyptus, and precipitates into the forest like rain. This water goes into the duff where it’s used and protected from drying by the undergrowth. And then the forest canopy keeps it shaded and prevents it from drying out.

As the volunteers opening trails on Mt Sutro have found, it’s the duff that holds in the water. The ground below it is dry. When all that duff and undergrowth is removed, the water will run straight off.