Mixed Evergreen Forest
Plant Communities of Mount Diablo State Park
Excerpted from MDIA's book Plants of the East Bay Parks, by Glenn Keator, Ph. D.
Madrone, Arbutus menziesii
Where oak canopies overlap, conditions favor a variety of other usually evergreen trees: California bay laurel, madrone, Douglas fir, tanbark oak (not a true oak but a lithocarpus), and California nutmeg. The Douglas fir and California nutmeg are not met with in our own region, while the other trees are.
These mixed forests represent habitats intermediate in winter rainfall and summer drought between redwood forests, where summer fogs and heavy winter rains rule, and oak woodlands , where we've already seen the the severity of summer drought. Often there will be no absolute line and mixed-evergreen forest nudges the borders. Generally mixed-evergreen forests occur on north-facing slopes where south slopes are home to oak woodland. However, mixed evergreen forests may carpet a canyon bottom alongside the narrow riparian corridor but give way to oak woodland or chaparral on adjacent slopes.
Often, too, the mixture of trees in these forests varies from locale to locale. Close to the coast, expect to see Douglas fir (Pseudotsunga menziesii) and California nutmeg (Torreya californica) in the forest; inland, expect to encounter canyon live oak. California black oak, California bay laurel, and madrone. The complex interactions of different trees from site to site are still not fully understood, for they also change with the age of the forest and its fire history.
With the exception of a few deciduous trees -- California black and Garry oaks, California buckeye, and occasionally bigleaf maple -- mixed evergreen forests have the leathery, tough evergreen leaves so characteristic of chaparral shrubs. Unlike those, however, mixed-evergreen forest tree leaves tend to be broader and -- at least on lower branches -- horizontally oriented, for purposes of more efficient light absorption for photosynthesis. Only near the tree tops and only in some species (such as madrones) are leaves obliquely inclined, with pale undersides held skyward to reflect away intense summer sun.
Although in mixed-evergreen forests as elsewhere wind pollination is used for the conifers and oaks, both madrone and bay laurel differ sharply, having insect-pollination strategies. Madrone produces abundant, nectar-rich white bells in mid-spring (bee favorites); bay laurel makes long lasting sets of small, pale yellow, saucer shaped flowers from mid-winter to early spring. Bay laurel is thus especially important in sustaining insects active at at time of year when most life is dormant. It joins ranks with the manzanitas in fulfilling this important role.
As to seed dispersal, strategies resemble those of oak woodlands; again with many nutrient rich stored foods in extra large seeds. Only the madrone makes bright red-orange berries, attractive to large numbers of birds. Where Douglas fir occurs, its seeds are winged and wind distributed. This makes good sense, for Douglas fir is taller than the other trees, and winds easily reach its tall branches laden with seed cones.
Not only do the roots of these trees extend outward for great distances to pick up as much of the winter rains as possible, but the competing understory plants -- shrubs, bunch grasses, bulbs, and perennial herbs -- seek water for later use. This intense competition for water means that the drier areas with least winter rainfall, where mixed-evergreen forest is marginal at best, have poorly developed understory vegetation. At the opposite pole, along the edge of redwood forests, the understory may be rich and varied. Most smaller plants are perennial; the annual life cycle is not favored by the relatively low light intensities. Many of these smaller plants extend into adjacent communities. The moisture-loving kinds extend into redwood forests and the droughty kinds -- especially the few bunchgrasses, such as melicas and California fescue --into oak woodlands.