I am trying to model stand structure of a forest plantation, so I intend to also assess the structure (either tree height or crown) of the stand using Lansat imagery (Remote Sensing)
Its unlikely that you will get much information relevant to these two measures unless you combine Landsat imagery with other data such as field measurements, lidar, or imagery from aircraft or UAVs--and even then, there will be substantial confidence intervals (that may or may not be suitable for a given application). Landsat is just too coarse resolution to distinguish crown structure and does not have any inherent information related to tree height.
Here's a recent articles describing using Landsat imagery combined with LiDAR data to try to gain insight about tree height:
Article Mapping tree height distributions in Sub-Saharan Africa usin...