Overstory Community Composition and Elevated Atmospheric CO2 and O3 Modify Understory Biomass Production and Nitrogen Acquisition

Abstract

Elevated atmospheric CO 2 and O 3 have the potential to affect the primary productivity of the forest overstory, but little attention has been given to potential responses of understory vegetation. Our objective was to document the effects of elevated atmospheric CO 2 and O 3 on understory species composition and biomass and to quantify nitrogen (N) acquisition by the understory vegetation. The research took place at the aspen free-air CO 2 and O 3 enrichment (FACE) experiment, which has four treatments (control, elevated CO 2 , elevated O 3 , and elevated CO 2 +O 3 ) and three tree communities: aspen, aspen/birch, and aspen/ maple. In June 2003, each FACE ring was uniformly labeled with 15N applied as NH 4 Cl. Understory biomass was harvested in June of 2004 for productivity, N, and 15N measurements, and photosynthetically active radiation (PAR) was measured below the canopy. The understory was divided into five species groups, which dominate in this young aggrading forest: Taraxacum officinale (dandelion), Solidago sp. (goldenrod), Trifolium repens and T. pretense (clover), various species from the Poaceae family (grass), and composited minor components (CMC). Understory species composition, total and individual species biomass, N content, and 15N recovery showed overstory community effects, but the direct effects of treatments was masked by the high variability of these data. Total understory biomass increased with increasing light, and thus was greatest under the open canopy of the aspen/maple community, as well as the more open canopy of the elevated O 3 treatments. Species were different from one another in terms of 15N recovery, with virtually no 15N recovered in clover and the greatest amount recovered in dandelion. Thus, understory species composition and biomass appear to be driven by the structure of the overstory community, which is determined by the tree species present and their response to the treatments. However, N acquisition by the understory does not appear to be affected by either the overstory community or the treatments at this point.

ElevatedCO2concentrations reduce C4cover and decrease diversity of understorey plant community in aEucalyptuswoodland

“…Light is also generally considered to be one of the most important determinants of succession and composition of understorey plant communities (Canham, Finzi, Pacala, & Burbank, ; Facelli & Pickett, ; Reich, Frelich, Voldseth, Bakken, & Adair, ; Valladares & Niinemets, ). At Aspen FACE, for instance, eCO 2 was reported to have indirectly affected understorey species competition via increased tree canopy area and an associated reduction in light availability (Awmack et al., ; Bandeff et al., ). However, this was not the case in our study as there was no evidence that soil moisture (Figure ; Pathare et al., ), canopy leaf area or understorey light availability differed between CO 2 treatment (Duursma et al., ; Figure ).…”

“…For example, eCO 2 accelerated succession from an herbaceous‐ to a woody‐plant dominated community in a sweetgum plantation (Oak Ridge FACE) after 10 years of CO 2 fumigation (Belote, Weltzin, & Norby, ; Sanders, Belote, & Weltzin, ; Souza, Belote, Kardol, Weltzin, & Norby, ). In contrast, there was no evidence of a CO 2 effect on the relative abundance of contrasting PFGs in the understorey of a mixed aspen, birch and maple plantation following 2–6 years of CO 2 treatment at Aspen FACE (Awmack, Mondor, & Lindroth, ; Bandeff, Pregitzer, Loya, Holmes, & Zak, ) nor in a pine plantation at Duke FACE following 15 years of CO 2 fumigation (Kim, Oren, & Qian, ). Moreover, whilst the majority of large‐scale and long‐term eCO 2 studies have been conducted in Europe or USA in young plantations where nitrogen (N) is the primary limiting nutrient for plant growth (e.g.…”

Elevated carbon dioxide and ozone alter productivity and ecosystem carbon content in northern temperate forests

“…This effect is consistent with an earlier increase in the production of fine roots, which are ephemeral, under elevated O 3 in this community (Pregitzer et al ., ). Aside from this community × O 3 interaction, it is notable that none of the other previously observed treatment effects on fine root, groundcover, and leaf production (King et al ., ; Bandeff et al ., ; Pregitzer et al ., ; Talhelm et al ., , Table S3) influenced the relationship between cumulative NPP and the quantity of C in pools other than the mineral soil because these tissues represented approximately 40% of cumulative NPP.…”

Response to CO2 enrichment of understory vegetation in the shade of forests

“…Similarly, consistent with higher photosynthesis, elevated CO 2 in a L. styraciflua plantation increased aboveground biomass of understory vegetation by 25% (60% for woody saplings only) after 10 years of CO 2 enrichment, with the contribution of woody species increasing over time (Souza et al ., ). However, such CO 2 ‐induced response was not detected in stands of aspen ( Populus tremuloides ) and mixed aspen‐birch ( Betula papyrifera ) overstory (Aspen FACE; Bandeff et al ., ). At that site, both above‐ and belowground biomass of red clover ( Trifolium pratense ) decreased due to lower light levels under elevated CO 2 (Awmack et al ., ).…”