Water and crud is obviously collecting in that area causing the rot. As it has a TPO you'd have to apply to the tree officer to completely remove that stubby limb and hope you can open up the area enough that it stops collecting water and detritus. Then you'd need to hope it heals at both the new and old wound areas. As others have pointed out birch are short lived and judging by the pictures yours looks quite mature and definitely compromised at the base.
Water and crud is obviously collecting in that area causing the rot. As it has a TPO you'd have to apply to the tree officer to completely remove that stubby limb and hope you can open up the area enough that it stops collecting water and detritus. Then you'd need to hope it heals at both the new and old wound areas. As others have pointed out birch are short lived and judging by the pictures yours looks quite mature and definitely compromised at the base.
From the pics, I would be pretty certain that the rot has already got into the main tree. I wouldn't like to guess how quickly it will spread, but silver birch do not have the most rot-resistant wood.
location: Surrey Hills, England, ex-woodland acidic sand. "Have nothing in your garden that you don't know to be useful, or believe to be beautiful."
What's the definition of short-lived? The two birches just outside our boundary are nearly 50 years old!
I guess when viewed in the context of a relatively short human life the determination of a trees life span is subjective. Those trees might seem like they've been there for yonks but 50 years would be young for a broadleaf hardwood like Oak yet old for a pioneer species like Birch.
But don't take my word for it, according to treesforlife.org birch typically only live for 60-90 years. Without wanting to draw pedantic responses its not usually old age that kills a tree but environmental factors like rot (disease) or bad weather.
It looks like it's already progressed to far for it to be chopped back. If it didn't have a tpo order on it then you could strip back the decay and cover the hole like they do in bonsai but with a tpo you are probably limited to just watching how far it rots back to and dealing with things when the time comes.
I recall the (former I think) Kew tree expert say that scars over a few inches are too big for a tree to heal and it is the start of their decline towards death when they lose such a limb.
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https://www.neavegroup.com/blog/landscaping/7-things-might-know-birch-trees/
Billericay - Essex
Knowledge is knowing that a tomato is a fruit.
Wisdom is not putting it in a fruit salad.
"Have nothing in your garden that you don't know to be useful, or believe to be beautiful."
But don't take my word for it, according to treesforlife.org birch typically only live for 60-90 years. Without wanting to draw pedantic responses its not usually old age that kills a tree but environmental factors like rot (disease) or bad weather.