Sentence:  Throughout much of northern Alaska, tundra is becoming <WSD>brushier</WSD> and giving way to forest. As tundra diminishes, animals that depend on it for food or nesting habitat—caribou and many birds—are likely to lose out. 
Meaning of the word marked with <WSD> tags:  Of the countryside: having thick vegetation, taller than grass but shorter than trees; having abundant brush; shrubby.