In this post last week I asked for your thoughts what might be contributing this green island effect underneath the trees in this photo I took while driving through California a few years ago.

I received a lot of responses. Thank you to all who shared your thoughts!

The most common response referenced the capacity of oak trees as dynamic accumulators and the contribution of leaf litter to the soil with its associated nutrient content and biological stimulation.

Other proposed possible answers included:

  • Shade from sunlight and heat
  • Moisture drip from the canopy, fog/dew accumulation
  • Slowing rainfall and increasing infiltration
  • Livestock concentrating under the shade
  • Mycorrhizal fungi
  • Protection from freezing
  • Nitrogen contribution of higher organic matter/leaf residue
  • Fertility contribution from nuts/mast crop
  • Root system pattern
  • Tree root exudates
  • Deep tree roots pulling up water
  • Different grass species, growing at different times
  • Most probably some combination of the above

My guess is that some of these factors are unlikely to be significant contributors in this local context, though they may be in other environments. Shade seems unlikely because the direction from the tree canopy is not uniform. There is no livestock grazing in this landscape (sadly), and freezing protection seems improbable given that it is central California.

The good news is, we don’t need to speculate what all might be going on. Thank you to Jeff Herrick and Leslie Roche for sharing references that point us in the right direction.

The abstract from the paper Blue oak enhance soil quality in California oak woodlands1 is succinct and to the point:

Blue oaks create islands of enhanced soil quality and fertility beneath their canopy. The quality of soil beneath the oak canopy is considerably better than that of the grasslands adjacent to the trees. We found evidence of improved soil quality under blue oaks for physical, chemical and biological soil properties. The type of vegetation (oak versus annual grasses) has a much stronger influence on soil organic matter and nutrient pools than does soil parent material. Removal of oak trees results in a rapid deterioration of soil quality with the majority of the loss occurring within 10 to 20 years after tree removal.

The article is interesting reading in its description of how oak trees contribute to soil fertility.

This observation and discussion raises lots of interesting questions, but the most obvious one is, “What are we missing by not including oak and other tree species in our agricultural landscapes?”


  1. Dahlgren, R., Horwath, W., Tate, K. W., Camping, T. & Others. Blue oak enhance soil quality in California oak woodlands. Calif. Agric. 57, 42–47 (2003).
  2. Eastburn, D. J., O’Geen, A. T., Tate, K. W. & Roche, L. M. Multiple ecosystem services in a working landscape. PLoS One 12, e0166595 (2017).