Forest features shape the occurrence and abundance of a threatened boreal orchid <i>Calypso bulbosa</i> via microclimatic conditions and vegetation composition

<jats:p>Biodiversity is declining in the boreal forest biome. To halt this loss, deeper understanding of the mechanisms driving the occurrence and abundance patterns of boreal forest species is needed. It is increasingly acknowledged that forest features, such as canopy cover, influence abiotic and biotic conditions within forests. However, the importance of these mechanisms for boreal forest species remains poorly understood. Here, we assess three hypothesized mechanisms by which forest features influence the occurrence and abundance of &lt;i&gt;Calypso bulbosa&lt;/i&gt;, a declining boreal orchid. Using Structural Equation Models and a unique dataset collected across 75 presence and 53 absence plots in northern Finland, we test whether canopy cover influences the species by altering microclimatic conditions (hypothesis 1), light and vegetation composition (H2), or whether deadwood, old trees and tree species diversity impact &lt;i&gt;C. bulbosa&lt;/i&gt; (H3) by, for example, altering the availability of symbiotic partners. Our results provide support for all three hypothesized mechanisms. Occurrence peaked at intermediate soil moisture, whereas soil moisture was shaped by tree species diversity: higher diversity increased soil moisture, likely due to a buffering effect of denser, mixed canopies (H1). A higher cover of animal-pollinated plants increased occurrence, likely by facilitating the pollination of &lt;i&gt;C. bulbosa&lt;/i&gt;. The cover of animal-pollinated plants, in turn, peaked at intermediate canopy cover and increased with deadwood, potentially reflecting the effects of light availability (H2). Occurrence was also directly affected by canopy cover, with intermediate cover providing the most suitable (likely light-driven) conditions (H2), while the abundance of the species increased with the number of old trees, potentially due to greater availability of symbiotic partners (H3). These findings offer valuable insights for ecological research and conservation practice. They also emphasize the need for a deeper understanding of how forest management, together with climate change, will affect declining boreal forest species.</jats:p>