Estimate litter production
The 1967 International Biological Programme has a handbook of suggestions for various methods to calculate primary productivity of forests. A subset of these measures would be effective for measuring litter production.

Regarding leaves and other falling litter, 


  Bags suspended from hoops (Ovington &amp; Murray 1964) about 1 m above the
  soil. The hoops would be at least 0.5 m in diameter. The bag should be
  freely permeable to water (nylon mesh, cheese cloth, sail cloth, etc.)
  to reduce moisture and decomposition inside the bag. Care must be
  taken with this type of litter trap that the hoop does not incline out
  of the horizontal and the bag must be pegged or weighted to prevent it
  blowing inside out.


Further, the handbook suggests no less than 20 leaf traps in one sample area, and checking contents every week especially in moist areas. Also, to account for branch fall, the weekly collection should check for and weigh fallen branches in a 20m X 20m area. 

For fine roots:


  An additional estimate of fine roots can be obtained either by
  excavation of soil monoliths, or by taking soil cores (e.g. Bray,
  Lawrence &amp; Pearson 1959) down to a depth of at least 50 cm and washing
  out the fine roots over a sieve. So far as is possible only live roots
  should be included.


For ground cover:


  Where distinct plants of a single or only a few species are present,
  the best procedure will be to collect a number of individual plants of
  each species (preferably including subterranean organs) at monthly
  intervals through the year. The collected plants are separated into
  leaves, flowers, stems and roots, which are dried and weighed. These
  individual plant data can then be combined with density (number of
  individual plants/area) data for conversion to an area basis. The sum
  of (Species A max. biomass - min. biomass) + (Species B max. biomass min. 
  biomass) ... gives an estimate of net production.


I believe that should account for the significant components of litter production.