Experimental evidence that seed availability and microsite conditions influence post‐fire seedling recruitment

Abstract

In many historically fire‐dependent ecosystems, the abundance of newly emerged seedlings increases after fire. Greater post‐fire seedling recruitment has been widely attributed to environmental conditions that promote seedling emergence (i.e. seeds germinate and exhibit above‐ground growth) and post‐emergence survival. However, fire increases flowering and seed production in many species making it difficult to assess the extent to which post‐fire seedling abundance reflects microsite conditions versus seed availability. Experimental manipulation of seed inputs can elucidate how seed availability versus microsite conditions contribute to post‐fire seedling abundance. We experimentally manipulated years since fire across 36 patches of remnant tallgrass prairie within an agriculturally fragmented 6400 ha landscape in western Minnesota (USA). Over 2 years, we sowed 10,962 Echinacea angustifolia (Asteraceae) seeds across 84 randomly placed transects. We then located and tracked the 974 newly emerged seedlings to evaluate how time since fire influenced seedling emergence and survival after controlling for variation in seed inputs. We also evaluated the association between six environmental covariates and seedling emergence and survival. Fire influenced both seedling emergence and survival between spring emergence and late summer. Seedlings emerged from approximately 1% of all seeds sown prior to experimental burns while seeds sown 1 year after experimental burns emerged at more than 10 times this rate. Emergence rates then declined as years since fire increased. Sowing seeds at high densities reduced rates of seedling emergence but nonetheless increased overall seedling abundance. Increases in litter depth were associated with reduced emergence. Meanwhile, the probability that seedlings survived to late summer was greatest 0 or 1 years after fire. The probability of seedling survival decreased with litter depth and increased with the local density of conspecific seedlings. Synthesis . Our experiment demonstrates that fire enhances seedling recruitment by generating microsite conditions that favour seedling emergence and survival. Nevertheless, seedling abundance also increased with seed inputs, suggesting that both seed availability and microsite conditions influence post‐fire seedling recruitment. Experiments like ours that explicitly discriminate between seed‐ and microsite‐limitation of recruitment are crucial for revealing processes that influence plant demographic responses to fire in many historically fire‐dependent ecosystems.