I am drafting a research proposal on this topic:
Assessment of habitat condition and conservation status for lowland British woodland using Earth observation techniques.

like:
field data acquisition techniques (in particular the use of differential GPS and terrestrial laser scanners, as well as more traditional woodland survey),
• image analysis and LiDAR data processing using software such as ENVI, ERDAS Imagine,
• GIS and geospatial data analysis,
• Statistics and numerical modelling.

so Basically I am student of Plant Sciences, But earth observational techniques seems to be related with geography and geoinformatics. Is it possible for me learn them to successfully conduct my research on the said topic

I didn't have much experience of these techniques before my PhD (I was a biologist) but my thesis, which is just about done, is based on them. It was tough, my first year was a steep learning curve, and it depends on your prospective institution (there weren't many people I could go to in my department, but I had some outside contacts and help). So I had to pretty much teach myself, and am now a point of contact myself for a lot of other people in the department!

My only advice is don't be too ambituous - I had to drop the numerical modelling element of my project because learning both was just too much. By all means put it in the proposal, but you will have to prioritise. The terrestrial laser scanner stuff I think is something especially good to focus on, and pretty underused in Academia so far.

Most importantly, remember that a PhD is as much about learning research skills for future jobs, not make life-defining discoveries.

Drop me a PM if I can be of any more help.

Oh, and google 'Erdas tutorials' and the same for ArcView etc.