Global changes such as climate change and land use change are significant factors that can impact species survival.Proper assessment on impacts of these factors on biodiversity calls for long term demographic studies of species of concern because population vital rates,i.e.survival,reproductive and growth rates,reflect the overall responses of the species to its living environments.Recognizing that population vital rates are plant-size dependent in orchid species,as in many other plants,I advocate field techniques that allow for random sampling of a population,consistent individual plant marking throughout study periods,and proper size classification.Sophisticated analytical techniques such as population matrix model projection or Integral Population Model (IPM) will yield credible results only if they are based on long term,good quality field data.I will illustrate some of these techniques and their applications on assessing impacts of extreme weather events and restoration efforts on wild orchids in China and the United States.