How would you expect pigments to change over time in an evergreen tree like a loblolly pine that is green throughout the year

There is no change in the chlorophyll A and B over time, concentration would not decline in late autumn.

Explanation:

Evergreen are trees whose foliage is green throughout the year, so they do not shows senescence during autumn. This green color is due to the presence of chlorophyll, which is a photosynthetic pigment found in chloroplasts of plant cells. The pigment is located inside the membranes of the tilacoids of the organelle. It also fulfills the function of carrying out photosynthesis, using light energy and water to fix carbon and be able to manufacture glucose which is the source of energy.  

There are four types of chlorophyll:

These trees, having green leaves all year round due to the presence of chlorophyll A and B that is produced all year round, are capable of carrying out photosynthesis throughout the year, even in low sunlight, as occurs during autumn and winter. Because of this, it the tree looks the same at any time of the year.

So, there is no change in the chlorophyll A and B over time, concentration would not decline in late autumn.