Which abiotic and biotic factors influence reproductive output in alpine populations of Ranunculus acris L.?

Reproductive output in alpine plants depend on abiotic and biotic factors. Alpine topography creates microhabitats for plants, with abiotic factors, like temperature, snow cover and soil moisture playing an important role in creating this heterogeneity on a small scale. Further, snow cover and temperature determine the timing of snowmelt and thus affect the length of the growing season. These abiotic factors also affect biotic factors, like pollinator activity, production of above ground biomass and flower abundance. This study investigates how abiotic and biotic factors influence reproductive output in alpine populations of Ranunculus acris L. I established ten snowmelt gradients in an alpine area at Finse, southern Norway. Each gradient contained three stages (early, mid and late) representing three different timing of snowmelt. Timing of snowmelt and temperature, two abiotic factors, were measured along the gradient. Three biotic factors were also measured, R. acris plant biomass, abundance of R. acris flowers in the surroundings and pollen limitation. The total aboveground plant biomass per individual was weighed and the abundance of R. acris individuals per stage was counted throughout the growing season. A supplemental pollination experiment was conducted on R. acris to test for pollen limitation. I used seed mass (g) and seed:ovule ratio (the number of developing seeds divided by the initial number of ovules in one flower) as measures of reproductive output. Late stage had the lowest average seed mass per plant in the first year, suggesting that seed mass decreases throughout the growing season. Early emerging plants have more time for fertilization and seed maturation, possibly causing the difference in seed mass produced in the different stages. Higher temperatures were correlated with higher seed mass in the second year, highlighting that temperature is important for seed production. Plants with a higher biomass produced heavier seeds in both years of this study. Plant biomass production depends on different abiotic factors, like temperature, nutrients and light. Even though temperature only had a direct effect on seed mass in the second year of the study, indirect effects from abiotic factors through the plant’s biomass is just as important. A higher R. acris abundance in the surrounding vegetation had a decreasing effect on seed mass in the second year. An intraspecific competition for light is suggested to be the reason for the low seed mass when abundance of R. acris plants in the surroundings increases. Average seed mass per plant seems not to be affected by pollen limitation, and I found no relationship between seed:ovule ratio and any of the abiotic or biotic factors measured. This study highlights that reproduction in alpine plants are determined by a combination of both abiotic and biotic factors. Especially, biomass, temperature and the abundance of other R. acris individuals was important for reproductive output in R. acris. In a warmer world biotic factors, like timing and length of flowering, is expected to change. This can affect reproductive output and have further implications for community composition and interactions between plants and pollinators