What if you drove some thick bamboo poles into the clay when it's wet & mushy, stacked some cut Siberian elm branches behind it and then filled it in with dirt & compost to create a small terrace there. Obviously, topsoil is always better for any plant to grow in. And a terrace will also capture some water, that would otherwise just always runoff.

I would then recommend a Texas Persimmon (Diospyros texana). This large shrub/small tree not only produces copious crops of deliciously sweet berries, but is also a highly heat/drought-resistant xeriscape native that can still withstand low temperatures down to 5ð-10ð F. Moreover, it is an attractive ornamental complete with creamy bell flowers, multicolored trunks and rugged hardwood that can be used for woodworking.

I think it would fit all of your requirements. Note that is is dioecious though, so you need at least a male & female of each to bear fruit... Therefore, you would be best planting another 1 or 2 somewhere else on your property, as well.

Here is a link that might be useful: favorite underused trees

20-25' could be a medium to large shrub rather than a tree, so do not limit yourself to just trees, widen you search to larger shrubs as well. Most trees would come in larger than 25'. 15' from your home is also very close. Are wildfires an issue in your area? That is something to consider--you want any foliage as far back from the home as you can get for fire safety, but at least 10'.

Also check the SelecTree website that Cal Poly has. Use the "select tree by attribute". You can enter your requirements and come up with suggestions. For example Callistemon viminalis might be plenty big enough

Here is a link that might be useful: Tree Selection Guide from CalPoly

Thanks for the suggestions!

blakrab: Diciduous is ok. I'm surrounded by evergreens, so it's nice to have some diversity. not sure if we have any other open space to plant more though (property is overrun by coast live oak which sprout like weeds here. Every year I pull out about 20-30 saplings, and seems like I'm making no dent in the growth density here... every few years I have to have one cut down since they start crowding eachother too much). I'll take a close look at the Texas persimmon. Is that anything like the persimmons you find sold in stores? (orange fruit)? There are few of those in the area. if so, may be there'll be enough in the area without having to plant another in our lot...

hoobv: it's a pretty densely vegetated hillside property, so fire is always a concern. But never really thought bout tree-fire exposure (we're usually more concerned with shrub/grass fire). I said 15ft, but just looked at the survey map, and it seems like it's actually more like 25 ft from the house. making a terrace is an interesting idea.

the tree can be taller, as long as it's the type that is conducive to shaping/height reduction management. but taller tree usually means faster growth (may be) and hard to manage?

and thank you for the tree selection guide. I'll ahve to take a look at that.

An arborist who just came by recommended chinese pistachio. He said around here, it doesn't get much bigger than about 25ft or so, and has great fall foliage, drought tolerant.. doesnt require too much water in the summer so won't hurt the juniper either (I called it juniper, but arborist said cypress, so... hmm). The tree sounded good until I found out that it has a potential of growing to 60ft.. i'll have to do more research on that tree.

So, I was speaking to the arborist, and he said that the elm tree is "California Elm". Not the weakest elm, but not the greatest either and he recommended replacing it with something else.

I'll start another thread on this topic, but thought i'd mentioned what I found out here too..

"Is that anything like the persimmons you find sold in stores? (orange fruit)?"

Yes and no. It is technically a persimmon, but it is more of a wild, uncultivated type. Hence, the fruits are much smaller and black, not orange. They average in size from a nickel to a quarter, but can be smaller or slightly larger than that range too. Yet, they will grow prolifically and are very sweet!

An orange Fuyu persimmon tree would also be great...except they can grow muucchh taller than your spot would allow and I believe also need more water/care.

Whereas Texas persimmons grow on their own out in the wild even under very intense dry heat without any added watering - so are still naturally ruggedized for a spot like yours. They also tend to grow from maybe ~10'-20' tall - again, perfect for your spot.

Their overall shape and trunks sort of resemble crape myrtles'.
http://www.penick.net/digging/?p=21341

So to recap, these native trees are so beneficial and worth propagating because of their amazing fruiting ability (and other desirable traits) combined with extreme heat/drought (& some relative cold) tolerance.. Now, that is a rare combination indeed!

Here is a link that might be useful: Not only do they survive, but they still even bear fruit in deep drought!

Desert Willow might be another choice. Drought/heat resistant - flowers all summer. Height to ~ 25'.
Redbud - heat/drought resistant - hgt to 25', flowers in early spring.
Prairie Flameleaf Sumac - 15-25' height. Heat/drought resist. Can develop sucker growth from roots. Dependable red fall color.
Or plant another Cypress (Arizona Cypress?) to match what is already out there.