Chemically, coumarin is a benzopyrone (1-benzopyran-2-one) which, apart from tonka beans, also occurs naturally in vanilla grass (Anthoxanthum odoratum), sweet woodruff (Galium odoratum), sweet clover (Meliotus L.), sweet grass (Hierochloe odorata) and cassia cinnamon (Cinnamomum aromaticum) among other species. In short, it's rather sweet, as you might have surmissed by now, and evokes cut grasses. You'd be correct to assume both facts, but that's not all: Although coumarin in perfumery does add a certain sweet note of mown hay or freshly cut grass with vanilla overtones, it's really bitterish in flavour in high concentrations (its -now banned- inclusion in food would attest that). Therefore theorizing its plant origin one would assume it's produced by plants in order to defend themselves from predation. After all it's also present in cherries, strawberries, and apricots, prime targets for birds. You might have even seen it featured in your rodent pesticide: don't be alarmed (coumarin is included in miniscule quantities in foodstuff anyway), but now you know why!