Before we begin, be ware, that the kind of a word and its grammatical usage are two different things:
- Adjective: This is a kind of a word, similar to noun or verb. It's a category to which a word belongs, independently from its usage in a sentence. When you look up a word in a good dictionary, the dictionary will tell you to which kind of word the word belongs, and it can do this, because you don't need the surrounding sentence to know this category.
- Attribute: This is a grammatical function, i.e. a kind of usage in a sentence, similar to object or predicate. You need the sentence in which a word is contained to be able to tell its grammatical function. A dictionary can't help you with that, because it doesn't know the sentence.
Adjectives are very often used as attributes, but they can also be used as adverbials or predicative. And also attributes are very often adjectives, but as we soon will see, also other things (single words that are not adjectives and groups of words) can also be attributes.
In this sense, your question is not about adjectives, but about attributes.
1. Multiple attributes of the same noun
You are right: the adjective »flachen« is an attribute of the noun »Landstrichen«, but it's not the only one. Also »meist gerodeten« is an attribute too. So, we have multiple attributes. Sometimes we make a comma between these attributes, and sometimes we don't:
If the attributes are of equal rank
Attributes are of equal rank, if you can swap them and you still have a meaningful sentence. Also adding the conjunction »und« doesn't change the meaning:
Gurken sind grüne, längliche Früchte.
Gurken sind längliche, grüne Früchte.
Gurken sind grüne und längliche Früchte.
Gurken sind längliche und grüne Früchte.
Cucumbers are green, elongated fruits.
These sentences mean all the same. It doesn't matter, if you say first »grün« and then »länglich« or the other way round, and adding or omitting »und« also doesn't change the meaning. If you can do so, then the attributes (here: »grün« and »länglich«) are of equal rank, and if you don't use the conjunction »und«, you have to add a comma.
In such a case, the attributes are just a normal enumeration, that you also can have in other situations. The general rule for enumerations is: Write commas between all elements of the enumeration. Only immediately before the very last element you can replace the comma by a conjunction (usually »und« or »or«, depending on the context), and in many situations this conjunction is even mandatory, but in an enumeration of attributes of a noun its optional. Such enumerations of attributes can have any length:
Diese grünen, länglichen, völlig überteuerten, von Martin gezüchteten und überaus hässlichen Gurken werde ich nicht kaufen.
I will not buy these green, elongated, completely overpriced, Martin-grown and exceedingly ugly cucumbers.
In this sentence we have these attributes, which are all of the same rank and which are all attributes of the noun »Gurken«:
- völlig überteuerten
- von Martin gezüchteten
- überaus hässlichen
You need commas between all of them, and only between the last element and its predecessor you can write »und« instead of the comma. You can also learn from this example, that attributes can also be other things but only adjectives (in »von Martin gezüchteten« is no adjective, but a participle of a verb), and that attributes can consist of more than just one word, which will be topic of the second part of my answer.
If the ranks of the attributes are different
Look at this sentence:
Der neue französische Käse hat einen herrlichen Duft.
The new French cheese has a wonderful aroma.
Both adjectives, »neue« and »französische«, are attributes of the noun »Käse«, which you can see, if you try to use them alone:
Der neue Käse hat einen herrlichen Duft.
Der französische Käse hat einen herrlichen Duft.
These are correct and meaningful sentences. But these sentences are not possible:
Der französische neue Käse hat einen herrlichen Duft.
Der neue und französische Käse hat einen herrlichen Duft.
Why is this? Well, the answer is, that »neue« is not an attribute of »Käse« but of »französische Käse«. The term »französische Käse« is a conceptional unit, and »neue« is an attribute of this unit. I try to visualize this with brackets:
- ((grüne, längliche) Gurken)
- der (neue (französische Käse))
In the first example, »grüne« and »längliche« are elements of an enumeration, and all elements of this enumeration are equally ranked attributes of the noun.
In the second sentence, »französische Käse« is a conceptual unit, and »neue« is an attribute of this unit. And because of this, there is no comma and no conjunction allowed, and for the same reason, it's not allowed to swap the two attributes. Because if you write »der französische neue Käse« you would destroy the conceptual unit.
Whether a noun together with an attribute forms a conceptual unit or not is often not quite clear. It is a vague concept that cannot be defined by fixed rules. Thus, in some combinations of attribute, attribute and noun, some people think that there should be a comma between the attributes, while others think that it is forbidden to put a comma there.
2. Attributes of attributes
Some examples for the pattern attribute attribute noun:
das erstaunlich feine Gewebe
das sehr helle Licht
der völlig lächerliche Hut
die von Martin gezüchteten Gurken
über meist gerodeten Landstrichen
Note, that not all of these attributes are adjectives (»gezüchteten« and »gerodeten« are a participles, which are very similar to adjectives, »sehr« is a degree particle and »von Martin« is a prepositional group), but most of them are, and this is true for the most sentences you will read or hear.
The point is, that here we have attributes, that do not describe nouns, they even don't describe conceptional units that contain a noun, but they describe an attribute of a noun.
It makes no sense to talk say »der völlige Hut« This is complete nonsense (Das ist völliger Unsinn), although »völlig« is a quite normal adjective. It just fits to just a small number of nouns, but it is very often used as attribute of other adjectives. The hat is not completely. But the hat is ridiculous, and the adjective völlig (completely) describes a property of »ridiculous«: the completely ridiculous hat.
All examples given here match this pattern, also that from your sentence:
über meist gerodeten Landstrichen
The word »meist« (Engl: most) is a normal adjective, that can be used as attribute of many nouns (»Die meisten Menschen leben in Asien.« »Das meiste Geld ist in die Erneuerung des Fuhrparks geflossen.«) and Landstriche is one of them: »Die meisten Landstriche der Südsteiermark sehen aus wie jene aus der Toskana.«
But here it doesn't describe the noun Landstrichen. It describes a property of gerodet (cleared): Not the countrysides are most, but the countryside are mostly cleared.
When we put this together, we get:
über flachen, meist gerodeten Landstrichen
over flat, mostly cleared countrysides
- flachen (a form of flach, Engl: flat) is an attribute of Landstrichen (countrysides)
- meist gerodeten (Engl: mostly cleared) is an attribute of Landstrichen (countrysides)
These two attributes are of equal rank. You can swap them and you can write a conjunction like (»und«) between them.
- meist (Engl: most, mostly) is an attribute of gerodeten (cleared). It is not directly connected to Landstrichen (countrysides)
- gerodeten (a form of the participle gerodet, Engl: cleared, which itself is a form of the verb roden; Engl.: to clear, to strub) is an attribute of Landstrichen (countrysides) and is itself further described by the attribute »meist«.