Passage Summary: Big white pine trees are so tall and thick that they block almost all sunlight from reaching the forest floor. Because they need light to grow, baby white pines can't survive in the shade of the older trees.

Conclusion: A forest consisting entirely of mature white pines will eventually be replaced by other tree species as the current ones die off.

Reasoning: White pines grow so tall and create such dense shade and dry ground cover that their own seeds cannot successfully grow into new trees beneath them.

Analysis: Since this is a 'Complete the Argument' task, we must treat the premises as absolute facts and find the most logical extension. The stimulus establishes a biological 'dead end': mature white pines create an environment where their own offspring cannot survive. Therefore, if you see a stand of only mature white pines, you are looking at a generation that cannot replace itself. Look for an answer that suggests these trees will be succeeded by different species that are more shade-tolerant or that the current trees all started growing at the same time before the canopy closed.