Temporal dynamics and spatial variability in the enhancement of canopy leaf area under elevated atmospheric CO₂

Increased canopy leaf area (L) may lead to higher forest productivity and alter processes such as species dynamics and ecosystem mass and energy fluxes. Few CO ₂ enrichment studies have been conducted in closed canopy forests and none have shown a sustained enhancement of L. We reconstructed 8 years (1996-2003) of L at Duke's Free Air CO ₂ Enrichment experiment to determine the effects of elevated atmospheric CO ₂ concentration ([CO ₂]) on L before and after canopy closure in a pine forest with a hardwood component, focusing on interactions with temporal variation in water availability and spatial variation in nitrogen (N) supply. The dynamics of L were reconstructed using data on leaf litterfall mass and specific leaf area for hardwoods, and needle litterfall mass and specific leaf area combined with needle elongation rates, and fascicle and shoot counts for pines. The dynamics of pine L production and senescence were unaffected by elevated [CO ₂], although L senescence for hardwoods was slowed. Elevated [CO ₂] enhanced pine L and the total canopy L (combined pine and hardwood species; P <0.050); on average, enhancement following canopy closure was ∼16% and 14% respectively. However, variation in pine L and its response to elevated [CO ₂] was not random. Each year pine L under ambient and elevated [CO ₂] was spatially correlated to the variability in site nitrogen availability (e.g. r ²=0.94 and 0.87 in 2001, when L was highest before declining due to droughts and storms), with the [CO ₂]-induced enhancement increasing with N (P = 0.061). Incorporating data on N beyond the range of native fertility, achieved through N fertilization, indicated that pine L had reached the site maximum under elevated [CO ₂] where native N was highest. Thus closed canopy pine forests may be able to increase leaf area under elevated [CO ₂] in moderate fertility sites, but are unable to respond to [CO ₂] in both infertile sites (insufficient resources) and sites having high levels of fertility (maximum utilization of resources). The total canopy L, representing the combined L of pine and hardwood species, was constant across the N gradient under both ambient and elevated [CO ₂], generating a constant enhancement of canopy L. Thus, in mixed species stands, L of canopy hardwoods which developed on lower fertility sites (∼3 g N inputs m ^-2 yr ^-1) may be sufficiently enhanced under elevated [CO ₂] to compensate for the lack of response in pine L, and generate an appreciable response of total canopy L (∼14%).