Tree growth phenology shifts in response to trait‐based neighbourhood effects in a large subtropical forest biodiversity experiment

Seasonal dynamics of tree growth, specifically intra‐annual variation in growth timing and rates, are critical for forest carbon sequestration. However, the mechanisms by which neighbourhood interaction affects these responses remain unclear in species‐rich subtropical forests. We monitored radial growth of 2478 trees from 35 species over 3 years in the BEF‐China experiment to examine how neighbourhood functional composition, that is functional dominance and diversity influence growth phenology and annual growth. We used gradients of traits related to resource acquisition (e.g. specific leaf area, leaf phosphorus, xylem conductivity) and water‐use control (e.g. wood density, stomatal density/index) to quantify functional dominance (community‐weighted means; CWM) and functional diversity (Rao' Q ) in 8‐tree neighbourhoods. Our results showed that neighbourhood effects depended on focal‐tree trait syndromes. Trees with resource‐acquisitive traits started and ended fast growth earlier but reached a lower maximum daily growth rate. In contrast, conservative water‐use trees delayed the end of fast growth and achieved a higher maximum daily growth rate, consistent with temporal niche differentiation. Functional diversity interacted with focal traits: in acquisitive trees, higher functional diversity along both trait axes tended to delay the start and end of fast growth, whereas in conservative water‐use trees, higher functional diversity advanced start and increased maximum daily growth rate, particularly in neighbourhoods dominated by such water‐conserving traits (high CWM on the water‐use axis). Structural equation models showed that earlier start, later end and higher maximum daily growth rate together increased annual growth. Among these, the effect of maximum daily growth rate was the strongest. Fixed effects of focal and neighbourhood traits accounted for relatively limited 8–19% of variation in phenology metrics (marginal R 2 : start = 0.082, end = 0.191, maximum rate = 0.131). These findings suggest that neighbourhood functional composition is one of multiple factors driving phenological timing and productivity. Synthesis . Shaped by focal‐tree trait syndromes, neighbourhood functional diversity was associated with growth phenology and annual productivity in subtropical forests, which points to temporal niche complementarity as a contributing mechanism in biodiversity–productivity relationships.