The ecotones that occur along altitudinal and hydrological gradients are some of the most distinctive and easiest to observe. When the water table moves further from the soil surface, sharp ecological transitions take place. As you move from a body of water to the land you may see reeds, grasses, and drought-hardy trees. On the shore now, you are in the riparian zone, a biologically diverse area where hydrophilic plants are adapted to wet soils and periodic inundation. The riparian zone typically occurs in the first 50-100 feet of land adjacent to the body of water. This gives way to fertile soils that can be dominated by either plains or forests.
Another example of an ecosystem with visibly distinct ecotones, are mountains. At the bottom of the mountain, relatively fertile soils give way to fast growing trees such as Tulip poplar (L. tulipifera), White ash (F. americana), and Sweet gum (L. styraciflua). As you hike up the mountain, soils become increasingly rocky and well drained, and fast growing trees are no longer favored. Stress tolerant trees dominate the area, such as Oaks (Quercus spp.) and Pines (Pinus spp.). Near the top of the mountain now, we are in the alpine ecosystem, an area of stunted trees and shrubs.
The species listed are generalized examples from eastern North America, but analogous relationships occur across the planet. Mountains and hydrological zones are great examples of stark ecotones that are easily observed. If you are not keen on identifying trees, stare at a mountainside in the fall, where you may see bands of monotone colors (or patterns) representing tree species or groups.
Notice the transition at the top third of the mountain where conifers begin to overtake the hardwoods below.