Species identity and spatial scale drive context‑dependent tree diversity effects in a Finnish forest experiment.

Tree species diversity is known to affect tree growth and leaf traits, which in turn can influence various ecosystem processes. However, the reported direction of these tree diversity effects is inconsistent, indicating that their outcomes depend strongly on ecological context. Using the long‐term Satakunta forest diversity experiment in Finland, we investigated how the effects of tree species diversity on growth and leaf traits vary with species identity, stand density, and spatial scale. By comparing the responses of light‐demanding Scots pine and shade‐tolerant Norway spruce , we show that the factors influencing diameter at breast height (DBH) and leaf traits differ between species with contrasting life‐history strategies. Pine needle terpene concentrations were lowest in neighbourhoods dominated by conspecifics, while spruce needle terpenes were unaffected by tree species composition. Increasing canopy cover reduced spruce dry needle mass but had no effect on the dry needle mass of pines. Likewise, the factors that influenced tree growth differed between species; spruce DBH was lowest in thinned stands but was unaffected by tree species composition, whereas pine DBH did not vary significantly with plot density but was reduced in neighbourhoods containing silver birch . Our findings also indicate that diversity effects on leaf traits primarily operate at the local scale, as statistical models assessing the effect of immediate neighbours on pine terpenes yielded significant results, whereas models assessing diversity effects on a plot‐level did not. In contrast, both pine and spruce DBH responded to plot and immediate‐neighbour level factors, implying that tree growth is influenced by broader stand‐level conditions, while leaf traits responses to diversity are more localised.