As a photographer, and particularly a landscape photographer, you need to be able to observe and adapt to the conditions. No matter how much you may plan and research a location or landscape things can and will change, and you need to be able to observe and adapt to these changes. These changes may happen during a shoot, or the changes may be in relation to your expectations from either a previous visit or from your research prior. Throughout a shoot there are many variables that can change and that you need to adapt to. An obvious thing that can change for a landscape is the weather conditions. Anyone who has spent any length of time outdoors will know that the weather can be a fickle thing. Obviously, you must be prepared for any conditions in terms of equipment and clothing, but you also need to be able to adapt your photography to the conditions as they change. Adapting what you are trying to capture is important as the weather, including rain, cloud cover, and even wind, will have a big impact on your final image so you need to be aware of its impact and adjust accordingly.
The lighting conditions are closely related to the weather and can change also, and sometimes quickly. The direction of light, the quality of light, and whether there is direct sunlight or cloud cover can change, and you must adapt to these changes as they happen. The lighting conditions are one of the most important factors to observe and adapt to in landscape photography as a landscape can take on a very different feel with different lighting conditions.
Perhaps a less obvious variable that you need to adapt to are changes to the environment itself. In a seascape for example, things likes tides, sand movement, and erosion can cause changes in the environment itself, similarly a waterfall, river or lake type landscape could be affected by the amount of water in the environment. Another example of a variable within the environment itself that you need to consider is the foliage on plants and trees. Changes in foliage won't usually happen within a single shoot however two different shoots at different times of the year can result in two totally different landscapes, so adapting to these different conditions is necessary for a successful image.
An example of adapting to the conditions was on my most recent visit to Robe. I was up early for sunrise expecting the weather to be partly cloudy - perfect conditions for a "traditional sunrise" type image, you know, the brilliant orange/red sky and all. Instead it was quite overcast and much more gloomy, so I had to adapt to these conditions and try to capture that gloomy feeling with my images. The "ghostly" image below arguably wouldn't work any where near as well if the conditions were bright and sunny, this image was a conscious choice to adapt to the "gloomy" feeling and try and work with it and capture it.
What can you do to help in observing a landscape and the conditions and variables that you need to work with during a shoot? Firstly, to be able to thoroughly observe and adapt I really like to spend some time at a location (without pressing the shutter button) and get a feel for the environment and the surroundings. Only once I have spent a little time observing and experiencing an environment can I get a feeling for the landscape and the mood or impression that it invokes. It is then that I can capture a representation of the landscape, and also be able to adapt to the current conditions of the landscape.
One thing that I find that also helps is to not view too many images of a particular landscape prior to a shoot. I find that if I view images of a particular landscape then I have preconceived ideas of that landscape. This can cloud my vision (pardon the pun) and affect what I observe of the landscape. Having a clear head and truly observing my surroundings lets me adapt to the current conditions and capture "my" vision much more honestly.