First of all, you can define a set of sustainability indicators (environmental, social, economic,...) and the weights associated to them. After this, apply this set in your alternatives regarding NTFPs and select the most sustainable one. 

Hola Patricia Negreros-Castillo, yo evalué la diversidad de huertos familiares en la costa de Tabasco, ahora estoy interesado en evaluar su sostenibilidad. El documento que me sugirió podría servir mucho para desarrollar algo así en doctorado en Agroecosistemas Tropicales, del Colegio de Postgraduados, Campus Veracruz. Saludos

Muchas gracias

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If the Produce available from year to year is not depleting, it is a good indicator of sustainability.

Article Non-timber forest product utilization under community forest...

This would be helpful.