What gives Svalbard such a unique and diverse flora, is its key position in regards to warm ocean currents. The West Spitsbergen Current (WSC) brings warm water up from the Atlantic Ocean which spills into the inner fjords.
Another reason for such a diverse flora is the extensive bird life on Svalbard. Due to their large flocking areas on cliffs, the area beneath them receive large amounts of bird excrement, which acts as fertilizer for plant life.
The flora, as well as the fauna, is very resilient for surviving in such harsh climates, and has developed the necessary defence mechanisms to adapt to the environment.