Identifying tree stand boundaries
Your question is very broad but try some kind of segmentation, for example GRASS i.segment. Your raster is probably to high resolution. I tried with 1 m and 5 m:
- i.segment
- Vectorize
- Convert to lines
- Simplify
The combination of input parameters are infinite.
To complement the comment above, there is a number of ways that you may want to tackle this issue - most can be run from within QGIS. As mentioned above, the GRASS GIS tool i.segment is a very powerful tool.
In addition, another option is the segmentation tools (e.g. otbcli_Segmentation) available in the Orfeo Toolbox, which are described in detail here. These include the Mean Shift and Watershed segmentation algorithms and are very efficient, particularly for processing large images.
Although not a specific QGIS tool (and if you are using [LiDAR] derived products), you might consider using the R package, lidR and specifically the watershed segmentation routine, which is very clearly described here. Figure below is reproduced from the lidR github page :
