Biotic and abiotic drivers of life-history and demographic variation in boreal orchids

Sammanfattning: Environmental interactions, including both abiotic and biotic factors, are key drivers of life-history and population dynamics. Despite this, most population studies do not explicitly link variation in environmental factors and vital rates. Here, I combined long-term demographic data and short-term experiments in four species of long-lived boreal orchids (Dactylorhiza incarnata, Dactylorhiza lapponica, Dactylorhiza maculata, and Gymnadenia conopsea) at two sites (Nordmarka, a coastal site, and Sølendet, an inland site) in Norway to understand the effects of biotic and abiotic drivers of variation in vital rates, population dynamics and life history. More specifically, I examined if (I) demographic responses to climate are sensitive to small changes in species life history or location, (II) floral traits are under selection due to limitations of resources or pollinators, (III) pollen limited orchid species compensate for short-term reductions in pollination success, and (IV) there is a demographic advantage to multi-year gaps in reproduction.I found that relationships between demography and climate were highly variable, both among species and sites. This highlights the difficulty in transferring data from similar populations for viability analyses and for models predicting range shifts. In chapter II, I documented strong pollen limitation but no significant resource limitation of female fitness in D. lapponica, and found that pollinator-mediated selection was far stronger than resource-mediated selection on floral traits. Chapter III revealed that individuals of D. lapponica and D. incarnata did not compensate for a temporary reduction in pollination success, though exact population responses varied both by species and reproductive cohort. This shows that long-lived plants may be unable to compensate for poor pollination by increasing future reproduction, and suggests that declines in pollination may have negative demographic consequences. In chapter IV I found no evidence for individuals of D. lapponica being able to use resources saved in reproductive gaps to increase future reproductive output, suggesting that gaps are simply plastic responses to resource constraints. Overall, these results show that population-level responses to abiotic and biotic factors vary among species, locations, and with timing. This implies that considering the effects of multiple environmental drivers across multiple populations is key to further our understanding of population dynamics.