The importance of biotic filtering on boreal conifer recruitment at alpine treeline

Abstract

Treeline, the ecotone where forest transitions to alpine or tundra ecosystems, is considered the thermal limit to tree growth and survival. Despite temperature increases across mountainous areas and high latitudes globally, there has been no ubiquitous change in treeline position. The process of range expansion must initially depend on increased recruitment at, or beyond current range limits and recruitment limitations have been hypothesized as a mechanism for the variable response of treeline position to climate warming. We conducted a unique series of observational and experimental studies to quantify early-life stage constraints, from seed production to seedling establishment, on black spruce Picea mariana and tamarack Larix laricina recruitment at a model alpine treeline in Newfoundland, Canada. We found recruitment at treeline to be simultaneously seed and establishment limited. The treeline population produced fewer seeds than the forest population and black spruce seeds produced at treeline were less viable. Tamarack was more seed limited than black spruce where seed viability was low regardless of altitudinal position. Post-dispersal seed predation greatly constrained recruitment across the altitudinal gradient; however, black spruce seeds experienced the lowest levels of invertebrate seed predation on the lichen mat at treeline. If seeds were not consumed, individuals at treeline were establishment limited where germination and seedling establishment was both less abundant and delayed on lichen substrate. Our study highlights the need for multiple factors to align temporally for significant recruitment at treeline to occur.

A global framework for linking alpine‐treeline ecotone patterns to underlying processes

“…Although not technically a form of mortality (in contrast to e.g. seed predation and germination failure, Crofts and Brown 2020), a lack of seed rain or clonal dispersal is, at the very general first level, an alternative form of ‘mortality', because both lead to an absence of trees. Many seed‐dispersal types lead to spatial patterns that cannot be unequivocally distinguished from patterns caused by early‐stage mortality.…”

“…assumed clonal tree islands turned out to be non‐clonal after genetic analysis, and even local seed dispersal seemed to play a small role, so that the patterns may have been due to microclimate‐based positive feedback, Johnson et al 2017). However, to distinguish the importance of dispersal limitation from that of other processes leading to establishment failure, spatial patterns are not sufficient and more detailed experiments are needed (Crofts and Brown 2020).…”

Early afforestation on islands of proglacial Lake Ojibway as evidence of post-glacial migration outposts

“…For primary succession to be successful on a bare island on a proglacial lake, several biotic and abiotic barriers must be overcome (Crofts and Brown, 2020). Climatic and meteorological factors are at play, substrate characteristics are also important, but distance to seed sources is the main hurdle.…”

Lagged climate-driven range shifts at species’ leading, but not trailing, range edges revealed by multispecies seed addition experiment

“…Ongoing range shifts can proceed at slow, undetected rates, particularly for long-lived species, as methods for detecting range shifts are often based on adult occurrence (e.g., species distribution models, Elith et al, 2010; historical plot re-surveys, Wilson, 2017). Beyond leading edges, expansion into newly macroclimatically suitable areas can be limited by dispersal (Zimmer et al, 2018), microsite availability (Goodwin & Brown, 2023), resident competition (Solarik et al, 2020), seed predators (Crofts & Brown, 2020) or a lack of mutualist partners (Benning & Moeller, 2021). These bottlenecks can create ‘colonization credits’ (Jackson & Sax, 2010) where colonization into newly suitable habitat beyond leading edges is likely but has yet to be realized.…”