Tree community composition modulates early-stage decomposition of standard litter through chemical and physical engineering

Litter decomposition is an essential ecosystem process influenced by multiple factors, including substrate quality, climate, edaphic environment, and decomposer communities. However, the role of canopy species identity and diversity on leaf litter decomposition in forests remains understudied. By controlling for macroclimate, soil properties, and litter substrate in a mature common garden, we investigated whether a three-month tea bag incubation of standardized green and rooibos tea substrate is driven by canopy tree species characteristics and diversity. Our study hypothesized two primary pathways: a chemical engineering effect, where trees alter soil properties and decomposer communities through litter input, and a physical engineering effect, where tree canopy structure modulates the local microclimate. The results showed that even under uniform macroclimatic and initial soil conditions, mass loss rates varied widely for green tea (27.4%–73.2%) and rooibos tea (6.1%–34.7%), comparable as found in other research between distinct biomes. While substrate quality was the dominant factor, both engineering pathways and, to a minor extent, tree diversity modulated mass losses. For green tea, tree chemical and physical characteristics seemed equally important, while the physical environment showed an increased importance for rooibos. Incubation depth played a key role, where forest floor decomposition rates are more susceptible to temporal climate variations, and soil-layer decomposition rates are less susceptible to climate variations and more determined by tree species identity. Our findings suggest that tea bag experiments focusing solely on topsoil burial may underestimate processes in the forest floor and the mineral-organic boundary layer. This study underscores the critical role of litter substrate quality in decomposition while demonstrating that tree community composition and the associated herbaceous layer, through both chemical and physical engineering pathways, strongly modulate decomposition rates.