Assessing pattern‐process expectations for the krummholz island treeline form: Seedling survival and tree stature across a boreal alpine treeline

Alpine treeline ecotones, where forests transition to alpine ecosystems, display a variety of spatial patterns. These patterns, or treeline forms, are considered indicators of the processes that control treeline dynamics and may provide insight into future abundance, distribution and stature of tree species in montane ecosystems. However, consistent links between treeline forms and proposed variation in the relative strength of controlling processes are not well established.

We examined whether the processes expected for krummholz island treelines (one specific treeline form) under the treeline pattern‐process framework operate at a treeline in Newfoundland, Canada. For this form, the framework hypothesis states that recruitment is limited, with changes in tree cover dependent on facilitation by established trees or krummholz. Additionally, it is hypothesized that growth and dieback limit adult tree statures, thereby causing established trees to form krummholz.

To assess the effects of established individuals on recruitment, we transplanted black spruce ( Picea mariana ) and tamarack ( Larix laricina ) seedlings along transects leading from established krummholz islands (presumed facilitators) into alpine heath (presumably lacking facilitation) and monitored their survival. To explore biomass maintenance limitations among established, adult individuals, we quantified the presence and distribution of emergent stems extending above the krummholz mat.

Seedling survival was high and despite changes in microsite conditions with distance from established individuals, we found no trends in survival. Assuming recruitment at other life stages is not limiting, tree cover may increase not only through the expansion of krummholz islands but also through the infilling of open vegetation patches.

We observed that stems within krummholz islands were similar in height regardless of their position within the island, suggesting that their ability to maintain biomass is more likely driven by climatic conditions than sheltering from neighbouring individuals. This could indicate that this treeline may be undergoing krummholz release, a shift from stunted towards upright forms.

Synthesis : Our study highlights a potential limitation of the treeline pattern‐process framework: spatial patterns represent snapshots of dynamic systems. Monitoring treeline populations will advance understanding of how treeline form develops and changes in response to climate change.