Systematic review of field research reveals critical shortfalls for restoration of tropical grassy biomes

Scientists and policymakers are becoming aware of the pressing need to restore tropical grassy biomes (TGB), which are home to unique biodiversity and provide essential ecosystem services to hundreds of millions of people. However, TGB face increasing threats, including the forest- and tree-centric approaches that promote their degradation though we still lack a systematic assessment of where and how TGB restoration research has been done to guide policy and practice.

We synthesised knowledge on field restoration experiments by conducting a systematic literature review to map TGB restoration field studies, examine the association of restoration techniques and degradations sources, and investigate the diversity of indicators used to monitor restoration outcomes. 

TGB restoration was concentrated in Brazilian and Australian savannas, with large blindspots in Asia, Africa, and northern and western South America. Studies were largely context-dependent, with an inconsistent usage of restoration techniques to different sources of degradation. Less than half of the indicators evaluated were monitored consistently through time, often using a low-dimensional approach related to ecosystem functioning. Few studies manipulated fire, herbivores, and soils, the key drivers for the re-establishment of TGB dynamics. Unfortunately, many studies lacked negative (degraded ecosystems), positive (reference ecosystems) controls, or both, impairing attempts to robustly determine restoration outcomes. 

Our overview of field research on TGB restoration highlights that research needs improvement to refine our ability to assess, plan, implement, and monitor restoration. Severe issues with experimental designs and data reporting are identified as barriers to finding generality and upscale TGB restoration to meet the goals of the UN Decade on Ecosystem Restoration. 

Synthesis and implications: Our synthesis calls for enhanced field experiments, transparent data reporting, and quantitative syntheses to guide large-scale TGB restoration. The overall lack of knowledge on improving resilience and measuring outcomes hampers meaningful comparisons between studies and hinders synthetic views essential for determining appropriate restoration techniques for different degradation sources and suitable monitoring indicators. To overcome the scarcity of reliable and transparent data supporting TGB restoration, we propose a simple checklist for minimum research reporting information and a more complete multilingual standardized guideline.

Methods
We conducted a systematic literature review about field research restoration in Tropical Grassy Biomes (TGB) and retrieved information from the selected papers. We presented such information in two main datasets: a general dataset (data_review: geographic coordinates, biome type, sources of degradation for each restoration site, degradation and restoration time length, manipulation of TGB main drivers, and restoration techniques) and another dataset for the indicators used to measure restoration success and their monitoring frequency intervals and length. We also present datasets used to standardize degradation source and restoration techniques, which were used to make the alluvial graphs.