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Investigating the Rainforest Canopy Phenology Variation and Productivity Responses to Drought by Using Digital Camera Images
Among the natural disturbances, drought may cause significant change in forest ecosystems by shifting phenology and productivity. Digital cameras have been used in phenological observations for their high accuracy and the colour index values (digital-number of red, green and blue) derived from long-term continuous digital camera imagery are useful as proxies for investigating a forest canopy’s response to drought. Here, we examine the interaction between colour indices (the strength of red (Sred), the strength of green (Sgreen), green excess index (GEI)), productivity (gross primary productivity (GPP)) and drought (standardised precipitation evapotranspiration index (SPEI)) and climatic factors. We use forest canopy images derived from a digital camera and flux tower-based productivity from 2010 to 2016 to show the rainforest’s responses to drought in phenology and productivity. The SPEI indicated the occurrence of drought condition in 2014. The lowest values of the SPEI (-0.403 mmday-1), and total precipitation (1062 mmyr-1), and the highest values of average air temperature (21.3°C), potential evapotranspiration (3.31 mmday-1) and rain use efficiency (2.26 gCL-1) were found in 2014. Leaf color variation period (CVP) become longer with an advance in foliage green-up after drought. The peak GEI values were found at the end of the CVPs. The GPP and the Sgreen had positive and the Sred had negative relationship with drought index. The GPP dropped during the drought and bounced back after the drought due to a longer leaf CVP. The Sgreen and GEI were significantly (p<0.05) related to GPP during the drought. During the CVP of the drought, the Sgreen and GEI were significantly correlated (p<0.05) with total P, PET and average Ta, meanwhile, there was only a significant relationship (p<0.05) between the GPP with PET and avg Ta. Among the variables, the GPP was more significant (p < 0.001) with avg Ta. There was also a relationship between the colour indices and GPP with climatic factors on a yearly time series. Our results indicate an understanding of the phenology and productivity response of rainforests to drought, which might be useful for ecologists when predicting the effects of future climatic change on rainforest phenology and productivity.