Hello everyone,
I'm a complete beginner in growing bonsai and I've grown what I believe to be a Japanese black pine (mixed seeds so this is what I think I've got). A couple of the ends have started to go brown as in my picture, should I be worried about this? Is it caused by too much sun? It's been very hot the last couple of weeks and it lives on my windowsill. Any advice would be greatly appreciated.

It looks very wet to me. Also, placing it outside might help.

I'd put your little pine outside in partial sunlight. Ideally a spot where it only gets 3-4 hours of direct sun a day then the rest is semi shaded. If you can't put it outside, open the window so it gets airflow and place it on a window-ledge / position it so it only gets sunlight for a few hours a day.

Little plants like this just don't require that much light to grow and their root system isn't huge. I'd think because the plant has been in direct sunlight the soil is drying out really fast.

When the plant grows up it'll really want to be outside. Pines don't thrive indoors, so you'll have to identify that point for yourself.

Everything you've said really does make sense especially regarding it drying out fairly quickly. I'll certainly look for a suitable spot outside as my windowsill is pretty much full sunlight all day. Thanks for your advice!

I had only just watered it, I believe you're right and I will be putting it outside, thanks!

If you are certain this is a pine, it should get lots of sun. Not just a few hours, unless you are in a desert-like climate and you really cannot keep it from drying out comepletely.

What will help agains the drying out is to plant the little organic fibre pot you have there in a pot made out of plastic. That will loose water soo much less quickly. These organic pots are not ideal for growing slow growing tree seedlings.