Hello, by non living factors you are referring to Abiotic factors. In a forest ecosystem what can cause stress to organisms involve water. Organisms require this to survive. Another one that I can think of is pH. The enzymes in the organisms have a specific pH at which they work at (optimum pH). When this pH is exceeded, the enzymes denature and the organisms cannot carry out basic processes like respiration as for example the enzyme Dehydrogenase is needed. ATP hydrolase hydrolyses ATP to produce ADP and pi with energy, so if ATP hydrolase (an enzyme) denatures then how will the organisms have energy?? In a desert ecosystem I would say water for dehydration and light intensity, as a high light intensity can lead to higher levels of evaporation and thus dehydration. I hope this helps :D