Temporal changes in plant biodiversity serve as critical indicators of vegetation response to global change drivers. However, these observed shifts only capture a fraction of the broader temporal trends. To comprehensively assess the conservation status and anticipate future responses to environmental changes, it's essential to contextualize observed biodiversity changes with respect to the potential species pool of given sites. This study addresses this gap by employing a resurvey approach across 320 grassland plots, categorized along a soil moisture gradient, over a minimum period of 34 years. Species pool was assessed by novel hypergeometric methods based on species co-occurrence. Through taxonomical and trait-based analyses, the study reveals a decline in species diversity and completeness with respect to the potential species pool. Moreover, evidence points to a significant rise in functional uniqueness among species, indicating a potential decline in community redundancy. These findings indicate a transition towards a simplified ecosystem, potentially reducing resilience to environmental changes. The study underscores the importance of integrating the potential species pool into assessments of temporal biodiversity shifts to gain a more nuanced understanding of ecosystem dynamics and inform effective conservation strategies.