the native canopy of conifers and big-leaf maples , is really as much forest now as an arboretum in some places , and close-up appreciation of the variety of the species sometimes is limited to the trees ’ bark . Even some more recent accessions , like silverleaf oak ( Q . hypoleucoides ) from the southwestern U . S . and Mexico ( dating to 1968 and added in the main Oak Collection area ) has grown tall and narrow , unlike in its native habitat , where it is small and shrubby with a rounded crown .
To extend the variety of the collection , Ray can ’ t help but contemplate some editing . This may also occur naturally . So far , the translocation of these exotic species to our climate has not yielded any widespread early senescence or losses . However , fast growth in Seattle conditions may ultimately be threatening to the humid-summer or cold-winter species . Climate change — bringing longer , hotter , drier summers to our region — may also take a toll .
How have plants from more intense , Mediterranean-like climates performed ? One of the challenges in the Arboretum is wet soil , and one of the victims , in Ray ’ s view , was a very old California black oak ( Q . kelloggii )— a rare montane California component of the Olmsted-era plantings that appears to have succumbed from poor drainage .
This specimen was planted near the new Birch Parking Lot , in an area that has long been wet and has a lot of clay in the soil about two feet down . New specimens of this species were planted in more suitable conditions in the Cascadia Forest of Pacific Connections and appear to be doing well .
Ray suspects some other species that inhabit hotter , dry climates , like the interior live oak of California ’ s foothills ( Q . wislizeni ), have — over past decades — lacked the summer heat they need to sustain good health . However , summers are getting hotter , and Ray is interested in building the collection of Mediterranean-climate oaks to test how they will fare here .
Two cork oaks ( Q . suber ), native to southwest Europe and northwest Africa , have grown to maturity in the Mediterranean section along Arboretum Drive since being planted in 1958 . The soils in this area are faster draining , and the specimens seem to be doing well . ( The surrounding
canopy was thinned back a few years ago to provide more afternoon sun .) Their thick , cracked bark — used for wine bottle stoppers and more — is always a sight to behold .
What paths forward ? For all the emphasis I ’ ve put on the earlyplanted ENA oaks , there is enough diversity of Quercus in the Arboretum to merit its recognition as a Nationally-Accredited Plant Collection through the Plant Collections Network of the American Public Gardens Association ( APGA ; see publicgardens . org / programs / about-plantcollections-network ). Ray is excited about the possibilities of collaboration with other network members in ex situ conservation of threatened species through the sharing of germplasm . This effort is also linked to the Global Conservation Consortium for Oak sponsored by Botanic Gardens Conservation International ( bgci . org ).
Quercus is a diverse and important genus across the northern hemisphere ( and dipping into the southern hemisphere in Indonesia ), with
18 v Washington Park Arboretum Bulletin