Is pine straw caustic? ...

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Answers:    Nutrients (such as nitrogen, phosphorus, and potassium) in the pine needles are recycled as they rust and the trees take up these nutrients and use them to grow. Decayed pine needles become module of the soil's organic business which holds nutrients making them more available for trees and other plants. Nutrients are also stored in the litter; for example, by age 30, a pine forest have as much phosphorus in the forest floor as within the trees
Pine litter provides food and habitat for many animals. These animals (for example, earthworms and beetles) comfort decompose the litter and are a source of food for several species of wildlife such as birds, frogs, and turtles.
Pine litter helps insulate the soil from extremes within temperature and moisture. It also protects the soil from rainfall, reducing erosion.
The fresh and partially-decayed pine litter has great water-holding size, thereby providing water for tree growing. The litter veil can also reduce evaporation of marine from the surface making more water available for trees.
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By pine straw i guess you indicate pine needles. Yes they are acidic. You will have need of to lime the area to sustain neutralize the pH. I would advise against using them for any plants except azaelas.