I´m looking for different ways to find relationships between litter production and the ecosystem services in a process of restoration ecology and know how can I size and quantify it.
Litter production is definitely important with respect to restoration ecology, as well as ecosystem functions and services. I think this was well reviewed by Facelii and Pickett in 1991 (Bot. Rev. 57: 1-30, http://www.jstor.org/stable/4354158). The full paper is available on Researchgate.
The second part of your question (how to measure it) is more difficult of course because this will depend to some extent on the type of ecosystem you are trying to restore and the experimental approach you decide to take.
Methods for measuring the success of ecosystem restoration projects was reviewed by Ruiz-Jaen and Aide in 2005 (Restoration Ecology 13: 569-577). Their data table lists "litter" as a variable measured in some studies, and so you may be able to track down a method used in a previous study of an ecosystem comparable to the one you are trying to restore. This paper is freely available on the University of Texas website and elsewhere, e.g.
I have tried to attach the paper here, but I am not sure that the paper has been successfully uploaded (!) Nevertheless, I am confident you can find it.
From the point of view of coastal ecology, litter production is important for ecosystem services. Litter, produced in mangroves and saltmarsh, supports nearshore fisheries. An early review examined the outwelling of mangrove litter. Please refer to the reference as below.
In addition, litter production contributes to sediment carbon accumulation in coastal ecosystems. Please also refer to my review paper re sediment carbon burial of saltmarsh.
Yes definitely litter production is very important in restoration ecology. In restoration of degraded lands, litter production and decomposition of the speceis planted in the initial years have a profound influence in the humid tropics. Litter production plays crucial role in soil conservation, moisture conservation, release of soil nutrients etc. Thus, litter production is one of the important aspect in microsite enrichment / modification by trees on degraded lands.