Fire and hydrologically mediated diversity change in subalpine forests through the Holocene

Questions We consider whether: (a) fire frequency and hydrologic trends drive changes in subalpine forest community diversity and; (b) if there are cumulative effects on diversity when frequent fire and hydrologic drying occur synchronically? Location We use three high elevation sites from Wyoming, USA to investigate the response and resiliency of subalpine forests in the Rocky Mountains to centennial‐to‐millennial drought and fire disturbances in regards to shifts in pollen richness, evenness and diversity. Methods ‘We examine ecological changes measured by pollen richness, evenness and diversity between subalpine communities under drying and wetting conditions to determine if there is an amplified effect when differing fire frequencies are considered. Using the inverse Simpson index of pollen diversity on pollen percentages as an estimate of biodiversity. Results Periods of frequent fire display significantly higher diversity ( p  < 0.05) than periods of infrequent or intermediate fire return intervals regardless of hydrologic trends at Little Windy Hill Pond. Further, sites in the northern and southern Rocky Mountains appear to operate under differing fire regimes with sites in the northern Rocky Mountains not recording periods of frequent fire (>225 year mean fire return interval) through the Holocene. Conclusions Changes in past pollen abundance and diversity have the ability to capture past landscape structure and ecosystem turnover prior to European settlement of the Americas. If fire regimes in subalpine forests of the Rocky Mountains continue trending towards higher frequencies there is the potential for these areas to experience an increase in vegetation diversity.