Can someone help me diagram this sentence?

Solution 1:

I don't know about your terminology, so perhaps my analysis won't be of use to you; but this is how I'd parse your example according to traditional grammar.

[It is] no matter [what] the season [is]: these features present...

The "no matter" phrase is an elliptical clause, to be completed hypothetically as above. It is simply an independent main clause—no part of the co-ordinate main clause "these features present". I agree that parsing this phrase is problematic and that other labels are possible. As an alternative you could parse it as an anacoluthon or parenthesis: no matter the season—these features represent....

...a challenge, found nowhere else in Britain.

"Found" is a participle that modifies "challenge". Since participles are best considered both verb and adjective, its adjectival function is to modify "challenge", its verbal function to govern "nowhere else in Britain", a satellite of location (where is it found? nowhere else ...).

Solution 2:

Just to add some specificity to the other answers, here is an attempt at a parse tree:

The image was created by using

• phpSyntaxTree

with the input:

• [S [AdvP [ADV no matter the season]] [NP [DET these] [ADJ combined] [N features] ] [VP [V present] [NP [ART a] [AdjP [ADV uniquely] [AdjP [Adj varied] and [Adj demanding]]] challenge [AdjP found [AdvP nowhere else ]]]]]

which I created out of thin air. Please critique/come up with an alternative. I have not yet found a reasonably accurate online English parser.

Note that I'm not even trying to parse 'No matter the X'.