Surface temperature as an indicator of plant species diversity and restoration in oak woodland

As ecological restoration projects increase in size and complexity, scalable indicators of biodiversity change are needed for monitoring and evaluation. Through three experiments, we tested (1) if relative surface temperature decreased over time across 31 fields initially restored from agriculture to oak woodland between 2006 and 2013, (2) if diurnal temperature variation decreased with time since restoration for the same fields, and (3) if plant species diversity affects relative surface temperature change when controlling for biomass and shade. Thermal imagery was acquired from Landsat satellites 5, 7 and 8 for each growing season in 2002-2018 (excluding 2012) and from the ECOSTRESS thermal instrument on the International Space Station (ISS) for 2018. Three of the 31 fields were surveyed annually 2007-2018 for plant species diversity, ground and canopy cover, and number of woody stems. All surface temperatures were measured in percentage difference relative to adjacent mature forest areas that exhibited stable temperature responses over time. We found (1) a mean decrease in temperature of 1.5 percentage point per year since restoration; (2) a decrease of 4 percentage points of diurnal temperature difference per year since restoration for the same fields; and (3) when controlling for ground and canopy cover, stem count, and Normalized Difference Vegetation Index, an increase of one 'effective number' of plant species diversity decreased relative temperature by 5 percentage points. These results correspond to a decrease of daytime temperature of 4.5 degrees C over 12 years, a decrease of diurnal temperature variation of 5 degrees C in 8 years and a 0.3 degrees C decrease per extra plant species. Our results offer compelling evidence that relative temperature has potential to be used as an indicator to measure ecosystem change resulting from restoration.