Why do we not sense a foul smell from dried leaves/dead trees as in the case of dead animals?

The smell from dead animals is due to the release of some gases during the process of decomposition. The type and amount of gases that are released depend on the composition of the organic matter being decomposed and the rate of decomposition respectively. The decomposition of animal body begins almost as soon as the animal dies. The animals own enzymes and intestinal microorganisms start the process. Animal carcass contains more protein and water than dead plant material. Proteins provide amino acids and nitrogen to the microorganisms to build their enzymes. Hence, the rate of decomposition is much quicker especially in warmer temperatures and when the carcass is not submerged in water or buried in the soil.

Image source: mayang.com

Decomposition of an animal body leads to the release of various gases including hydrogen sulphide, methane, cadaverine and putrescine as by-products of protein degradation. These gases are primarily responsible for the foul smell, but also attract insects and sometimes, smaller animals.Dead plant matter on the other hand is usually dry and contains very little protein. So, the rate of decomposition is slow. In addition, during plant decomposition very little or no cadaverine, putrescine or hydrogen sulphide is released. Methane and carbon dioxide are the gases that are primarily released from plant waste decomposition. In addition, dead plant material consists primarily of cellulose and lignin, which are decomposed by only a few species of microbes. This considerably delays complete decomposition. Interestingly, there are some plant species (for example, Titan arum, Amorphophallus titanum ) that produce an awful, dead animal-like smell in their flowers to attract certain insects to help in pollination. The above species produces a very large red flower and the smell can usually be felt from about a kilometre away. Luckily, it flowers rarely and releases the smell only once every few hours.

Source: thehindu.com