Analyzing the relationship between historic canopy dynamics and current plant species diversity in the herb layer of temperate forests using long-term Landsat time series

Current plant species diversity of the forest herb layer is influenced by site conditions, seed banks, stand age and historic canopy dynamics, induced for example by natural and anthropogenic disturbances. Long-term Landsat time series allow for analyzing forest canopy dynamics over several decades at a spatial resolution of 30 m. These dynamics have not been related to plant diversity in the herb layer of forests yet. Here, we related canopy layer dynamics of 132 one hectare temperate forest plots derived from Landsat time series (1985-2015) to herb layer plant species diversity observed in 2015. To quantitatively characterize the dynamics in the canopy over this period, we applied Symbolic Aggregate ApproXimation representation (SAX) to yearly NDVI time series of the plots and calculated the frequency of the representations of similar time series patterns, hereafter called SAX-metrics. We used the SAX-metrics as predictors to estimate with beta regression models the Simpson's diversity index of the herb layer of our forest plots subdivided into six forest types. Models including SAX-metrics, describing abrupt decreases in NDVI, and high, medium, or low NDVI values had pseudo R-2 between 0.13 and 0.63. We conclude that disturbances, canopy closure, as well as stand age, as represented in NDVI time series, influence the diversity in the herb layer in five of the six examined forest types. Our study shows that information on historic canopy dynamics, detected from Landsat time series, can contribute to a better understanding of current herb layer diversity in some forest types. This study gives first indications on the potential of temporal metrics of Landsat time series in biodiversity research.