In article , [R]
wrote...

This tree is growing in a cultivated area of my yard planted with
dichondra in Central California and I would like to know what it
is. The photos were taken this February. The main plant (first
three images) is about 16" tall. The last five images are of a
newly sprouted one about 5" tall that was growing under the main
plant. The best I can determine, given my limited botanical
knowledge, is that it's some type of arrowwood?

http://webpages.charter.net/rpeterse/

It's a stinging nettle, a species of _Urtica_.
Probably Urtica urens. Those long pale hairs on the stems and
leaves can sting you. [This is an example of a venomous [not
poisonous] plant!] Weren't you stung by it when collecting those
stems and leaves?

It's a herbaceous plant, not a tree. If it's U. urens,
it's supposed to be an annual herb.

http://www.csupomona.edu/~jcclark/fl...ica_urens.html
http://dlp.cs.berkeley.edu/cgi/img_q...n=Urtica+urens
http://dlp.cs.berkeley.edu/cgi/img_q...3022+3910+0078
http://www.uib.es/depart/dba/botanic.../Urtica/urens/
http://www.kulak.ac.be/facult/wet/bi...nten/Urtica%20
urens%20-%20Kleine%20brandnetel/kleine%20brandnetel.htm#1

Also compare it with _Urtica dioica_, another nettle species listed
for California [this one's a larger, perennial species]:
http://dlp.cs.berkeley.edu/cgi/img_q...3022+3910+0074
http://dlp.cs.berkeley.edu/cgi/img_q...=Urtica+dioica

cheers