Above and below can be used as both an adverb and an adjective to indicate an earlier or a later part of a piece of writing respectively. However, adjective below is rare compared to adjective above (and mostly not a preferred usage).
OED definitions of the adjective forms:
above:
Mentioned, described, listed, etc., further up on the present page or previously in the text. Cf. above-mentioned adj.
Last two citations from OED:
1959 A. Arnold D. H. Lawrence & Amer. ii. 24 Ten days after the above letter was written, Lawrence learnt of the suppression of The Rainbow by the English authorities.
1990 T. Cunliffe Easy on Helm xiv. 120 Should you be faced with the above horror show.., you'll have to use the single controlling line technique.
below:
Mentioned, described, listed, etc., further down on the present page, or subsequently in the text. Cf. below-mentioned adj.
rare in comparison with above adj.
Last two citations from OED:
1996 Frank 8 May 3/3 David's many Canadian admirers are invited to visit he and Danielle..at the below address.
2016 M. Sofiev & M. Prank in P. J. Beggs Impacts of Climate Change on Allergens & Allergic Dis. iv. 53 In the below analysis, we shall..make some basic statistics based on meteorological past-time data.
There is also the noun usage of above and below (with "the") with a similar definition.
OED definitions:
above:
Chiefly formal. With the: the preceding passage, paragraph, etc.; the person, thing, or set of things mentioned or discussed in the preceding passage, paragraph, etc.
below:
Chiefly formal. With the: the following passage, paragraph, etc.; the person, thing, or set of things mentioned or discussed in the following passage, paragraph, etc.
rare in comparison with above n. 3.
Why is the adjective "below" rare compared to adjective "above"? (the noun usage also)
Would it be possible to find a historical reason? Could it be related to how frequently they appeared in publications or influential works? Or, could it be related to the distribution of the adverb senses?