For a landscaper, there is little incentive to change, the base ISO 100 files are cleaner on the D7000. The Nikon modified Sony sensor in the D7000 is still the best crop sensor for DR and detail recovery. The big reasons to change would be AF, if you are doing birds in flight, sports etc since it is better in lock on, and tracking than anything Nikon has ever produced for DX.
Noticeable difference in low light, no. But the D7100 has large enough files to allow downsizing which can reduce noise. From your subjects mentioned, I see no compelling reason to change cameras but I would sure investigate faster lenses. A used VRI 70-200 is a bargain and fast to focus, and superior optically. If you are regularly pushing the low light limits of the D7000, you will also with the D7100 or any crop camera. A used D3s could be the answer or a new D600 for a lot less money will make more of a difference. What do you consider typical low light conditions? 3200 at 1/8 sec and f/1.4? or 800 1/500 at f/5.6? Some people would class both as low light. I regularly shoot in ballet or drama theaters where 12k 1/60 f/2.8 is pushing it and in dance clubs were external flash is possible and shoot in much lower light/bad light where 1/250(or 1/8 rear curtain) 320 iso and f/4 are typical manual settings. The secret to actual low light shooting is technique and creative problem solving more than camera performance.
The biggest single problem with low light photography is not low light, it is the desire to take low light shots and expect them to look like normal bright light shots, by using the camera as a light amplifier or night scope. It a truly low light condition exists the color and DR of our own eyes is very poor, so a realistic image is one that is rather dark, with little color information and very little detail. That is normal, not the highly amplified images that users are expecting, daylight images in night time conditions. Look in any art gallery that features photos and you will notice great art taken at all period of photography technology that uses low light to be represented by low light prints. Any of these cameras are capable of true low light photography where color and detail is subdued. Stan St Petersburg Russia