Explanation

The sentence explains why plant densities are lower at the northern edge of a species' range compared to the center. The conditions at the periphery must be worse for survival and growth.

Blank (i):
The northern limit involves harsher, more variable environmental conditions-such as extreme temperature swings, erratic weather, or shorter growing seasons.

Unpredictable fits perfectly: peripheral populations often face greater climatic variability and stochastic events, making establishment and persistence more difficult.


Blank (ii):
The structure "not only more $X$ but also markedly less Y " requires Y to be a positive quality that is diminished at the edge.

Productive (meaning biologically productive-more sunlight, nutrients, or longer growing seasons) is exactly the kind of benefit that drops off at range limits. Lower productivity directly explains lower population densities.


Why the others are wrong:
B. uniform - more uniform conditions would generally make survival easier, not harder, and don't explain lower densities.
C. diverse - while diversity might increase or decrease, it isn't as clear a negative factor as unpredictability; moreover, "less productive" is a stronger, more logical pairing.
D. inhospitable - "less inhospitable" would mean more hospitable, which contradicts the lower densities described.
F. marginal - "less marginal" would mean more central or favorable, again contradicting the lower densities. The word "marginal" describes the periphery itself, not a condition that varies between center and edge in this way.