Fall. Luis Vallejo calls this tree a Spanish oak (Quercus faginea) though Wikipedia and some others refer to it as Portuguese oak (other names that popped up are Valencian oak and Lusitanian oak).  It belongs to Luis and resides at the the Municipal Bonsai Museum of Alcobendas in Spain.

Welcome to Luis Vallejo’s Bonsai Bark (just kidding). This post features four photos of a single tree over three seasons (winter twice; not sure what happened to spring). There’s a lot more we could say, especially about the Museo de Bonsai Alcobendas and Luis Vallejo’s (and others) involvement, but it’s a whole lot easier to simply invite you to click here and get it straight from the source 

Summer

Winter with companion

Winter with snow