Background
An increasing evidence base is improving our understanding of how forests and trees provide important ecosystem services to agriculture. However, the specific functions and contributions forests and trees make to agricultural systems is far from being fully understood. This review assesses the strength of the evidence that reports how forests and trees contribute to agricultural (food) production in order to prioritize further research for better decision-making. We consider there may be significant gaps in the literature with regard to, 1) Which ecosystem services are provided by forests and trees within a landscape, 2) Over what spatial scales are these services transferred and, 3) To what extent are these services ultimately translated to increased food production? The contributions of trees to agriculture have often been poorly understood and poorly integrated into agriculture and conservation policy and practice.
Methods
The primary question of this systematic review is: To what extent does the presence of forests and trees contribute to food production in humid and dry forest landscapes? The search strategy will employ terms from studies on forests, agroforestry, ecosystem services and agriculture. A scoping exercise in CAB Abstracts, Scopus and ISI Web of Knowledge was used to understand the breadth of ecosystems literature, and further to conduct a preliminary scoping study. An equivalent search in Google Scholar will be used to cross-reference studies retrieved to ensure that relevant studies are not missed. Specialist searches at universities, relevant agricultural and forestry organizations websites, and a call for unpublished studies will identify important grey literature. Retrieved articles will be screened by title, abstract and full text and inclusion/exclusion exercise will generate the final list of studies. Data from these studies will be extracted using a coding tool. Due to anticipated heterogeneity in the retrieved data, we will group findings into appropriate categories as an initial presentation of the data. Sub group meta-analysis by types of ecosystem services and other appropriate predictors will be conducted to show the positive or negative effects of forests and trees on food production.
