Discussion

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            Normally TDF restoration studies occurred within the US or Mexico, with most of them happening in Hawaii and lasted less than three years.  Plant material for restoration was usually locally sourced and seedlings were preferred over seeds with mixed assemblages, instead of a single species of plant material.  Studies usually took place in protected areas with domesticated or feral grazing livestock present, as most sites were abandoned pastures.   Most papers focused on the rate of success of through individual plant performances to determine which methods were most effective. 

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            The most common performance metric was survival of outplanted seedlings.  Conventional forestry restoration usually favors seeds for plant reintroduction, but across most studies, it was shown that seedlings have a higher performance in restoration when compared to seeds.  It was also found that outplanted plots of land had nearly twice the native plant cover than seeded plots, creating higher species richness in an area.  It was also observed outplanted plots had relatively low survival rates due to low establishment of the plant in the environment as compared to seeded plots.  Also noted that these trends may be due to high variation among treatments performed.  Survival rates and quantitative data also vary in the amount of care and intensity in which the studies were conducted (Dimson & Gillespie, 2020).  

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            It was observed that the survival rates metric overshadowed other important metrics such as reproduction, which is important for the long-term survival of TDF projects.  Few studies noted positive performance results in terms of reproduction.  One source observed higher reproductive rates for tall canopy plant species.  Researchers speculate that the low recruitment rates for reproduction may be the low establishment of pollinators in the newly planted TDF, resulting in poor survival rates in such sites.  This may also be the result of short monitoring periods which don’t allow newly established TDF to become properly settled in to produce more reliable data.  The cost of the restoration projects and studies were also largely unreported, only mentioned in 20% of papers, as there were social and economic implications from each study.  Land management, monitoring, and the undertaking of the projects took up approximately 98% of the costs and two additional years of land management represented 2% of the costs.  The restoration projects being costly, volunteers were utilized in many of the projects (Dimson & Gillespie, 2020). 

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            In order to create better research and practice studies, there needs to be more transparency in methods.  For example, while focusing on one performance metric such as survivability, other metrics such as reproduction and costs are largely ignored in many studies.  When producing new studies here onward, there have been many new tools at disposal for more accurate and rounded data such a remote sensing which would allow for greater monitoring ability and Thermal Radiometer Experiment Sensor (ECOSTRESS) which provides high resolution images from space depicting surface temperature in correlation to land stress and use.  In addition, there have been many new data bases pertaining to land use and vegetation such as Vegetation Index (VDVI) which would give researchers more information to work with (Dimson & Gillespie, 2020). 

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Conclusion

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            The study concluded that most TDF restoration papers were empirical studies with few published long-term projects.  Most studies utilized locally sourced seedlings in heterogeneous assemblages because of the higher success rates they yielded verses homogeneous seedling assemblages.  It was also noted that land management of invasive species and herbivores was common among studies in order to maintain a controlled environment.  In order to evaluate each study, metrics such as germination, growth, and survival were utilized to measure success of each treatment.  The reviewed studies also lacked uniform strategies for how they carry out their studies as there was a high level of variation in layout and design of studies.  They also failed to mention important metrics such as social and economic impacts of their studies in the area in the long run, the costs of conducting the project, and labor.  Long term outcomes were also excluded from most studies (Wilson & Peter, 2016).  In order for their to be a better understanding of TDF restoration, it is shown that there has to be more uniform and in depth project design as well as data collection; as well as including or providing better detail on under evaluated metrics such as social and economic impacts, and more long term studies (Dimson & Gillespie, 2020).