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

Abstract

Compared to tree responses to elevated (e)CO2, little attention has been paid to understorey plant community responses in forest ecosystem studies, despite their critical role in nutrient cycling and the regeneration of overstorey species. Here, we present data on understorey responses from a 3‐year Free‐Air CO2 Enrichment experiment in a native, phosphorus‐limited Eucalyptus woodland in Australia (EucFACE). We conducted repeat surveys of the understorey plant community from 2012 to 2016, recording cover at the species level. Three years of eCO2 significantly decreased the diversity (Shannon‐Weaver; −30%) and species richness (−15%; c. −1 species per 4 m2 plot) of graminoid species, and the cover of C4 graminoids in both dominant (−38%) and subordinate (−48%) groups, relative to ambient conditions, leading to a significantly lower graminoid C4:C3 ratio (−59%) in the understorey plant community. The ratio of C4:C3 graminoids was negatively associated with soil nitrogen (N) availability suggesting that previously reported eCO2‐associated increases in N availability may contribute to (or be a consequence of) shifts in the composition of the graminoid community at the study site. There was, however, no effect of eCO2 on the diversity of forb species, which represented the most species‐rich functional group but only c. 1% of the understorey biomass. Synthesis. Our results suggest that eCO2 influences competition between C4 and C3 graminoid species both directly and indirectly via increasing N availability. The shift towards lower C4:C3 ratios and enhanced dominance by C3 species with their generally higher tissue N concentrations could further change soil nutrient availability and potentially accelerate community succession. Thus, eCO2 has altered the diversity and composition of the understorey plant community in this woodland, with the potential for cascading consequences for trophic interactions and ecosystem function.

Biomass increases attributed to both faster tree growth and altered allometric relationships under long‐term carbon dioxide enrichment at a temperate forest

“…Our study helps address the question of whether the effect of eCO 2 on allometry is similar to an acceleration of stand development (Hasegawa et al, 2018;Körner, 2006;Pretzsch et al, 2014). Our results mostly support this idea.…”

Water availability drives fine root dynamics in a Eucalyptus woodland under elevated atmospheric CO2 concentration

“…The fine root population comprised a diverse mixture of shrub, grass, forb and tree species, which is an important consideration when interpreting findings and comparing below‐ground CO 2 responses with other studies. Whether the observed trends under eCO 2 are responses of individual species or result from changes in the species contributing to the fine root population (Hasegawa et al., 2018) is unknown. Previous studies at EucFACE showed similar relationships between above‐ground production and soil moisture in tree and understory productivity (i.e.…”

“…Microlaena stipoides , Cynodon dactylon and Paspalidium distans ) and forbs (e.g. Commelina cyanea and Lobelia purpurascens ) as common species (Hasegawa et al., 2018). Grass and forb live green cover ranged from 10% to 60% of ground area during the course of the experiment (Collins et al., 2018).…”

“…First, we examined the effects of soil water content and soil temperature on fine root dynamics (i.e. FRB, FRP ig and FRT) and chemical composition, through multiple regression analysis in a linear‐mixed effects model framework (Hasegawa et al., 2018). To do so, for each sampling date, we summed fine root values from both soil depths and incorporated ring‐averaged values of fine roots as response variable, with soil water content (VWC) and soil temperature (Soil Temp) as fixed predictors in the analysis.…”

The fate of carbon in a mature forest under carbon dioxide enrichment

“…Permanent groundwater depth is ∼11 m below the soil surface 38 . Understorey vegetation is a diverse mixture of 86 species including forbs, graminoids and shrubs 39 . The dominant understorey species is Microlaena stipoides , a C3 perennial grass that accounted for ∼70 % of herbaceous biomass, and responded rapidly to rainfall variability 40 .…”