Lichens and mosses often live on tree branches in the forest canopy. They store large amounts of rain, fog, or dew, because they cannot reach water in the soil. However, we do not know what their most important source of water actually is. For example, is water uptake from wet bark important? This is hard to answer, because these lichens and mosses live high above us, so it is difficult to make direct measurements. Computer mathematical simulations can be useful to answer this question. We used a computer model of lichens and mosses to calculate how much water these organisms may take up from bark. We found that water from bark supports about 20% of the growth of lichens and mosses per year, so it is important for their survival. We also found that lichens and mosses are well adapted to taking up water from bark.