Baremos ó escalas para la medición de dependencia.

Previous studies have reported increased C3 forb biomass in response to eCO2 (Polley et

al., 2003, Reich et al., 2001, Teyssonneyre et al., 2002). However, lack of response (Dijkstra et al., 2010, Polley et al., 2012a) or even reduced biomass and relative abundance under eCO2 in the forbs has also been reported (Niklaus & Körner, 2004, Zavaleta et al.,

2003). In the current study, the C3 grasses and C3 forbs varied significantly in overall

biomass as well as biomass allocation responses to eCO2. Specifically, leaf area ratio and

leaf biomass decreased significantly under eCO2 in the two C3 forbs (Fig.4.5). In contrast,

there was a significant decrease in leaf mass per area and increase in leaf area ratio under eCO2 in the two C3 grasses, which was accompanied by no change in the leaf biomass as

well as total biomass (Fig.4.5 and Fig. 4.6). These results suggest that under eCO2,

adjustments in leaf area occur in the C3 grasses which may help the plants in optimizing

resource capture and use in response to changes in resource availability (Poorter et al., 2012, Tilman & Wedin, 1991). In particular, decrease in LMA and increase in leaf area ratio without a corresponding increase in total leaf biomass indicates a decrease in leaf density or the production of thin leaves in the C3 grasses under eCO2. This decrease in

LMA could also be responsible for decrease in leaf N as well as P content under eCO2

especially in the two C3 grasses (Fig. 4.3 and Fig. 4.4). Since, the Rubisco enzyme

represents a larger fraction of leaf N in thin leaves (Hassiotou et al., 2010, Poorter & Evans, 1998), the decrease in LMA under eCO2 in the current study suggests a strategy

of the C3 grasses to allocate leaf N efficiently to photosynthesis under eCO2. In accordance

with this I observed higher fN-Rubisco under eCO2 in the grasses, despite decreases in Narea

(Fig.4.3). Taken together, results from the current study suggest that differences in fN- Rubisco responses to eCO2 coupled with changes in above-ground biomass allocation

patterns via leaf area adjustments likely affected the CO2 responsiveness in these species,

in particular ability to maintain fN-Rubisco and avoid down-regulation by the grasses but not

by the forbs. These differences in N allocation patterns and leaf area adjustments among different species have important implications for nitrogen-use efficiency and species responses to eCO2 in nutrient-limited sites (Ellsworth et al., 2004).

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Despite stimulation of photosynthetic rates and maintenance of photosynthetic capacity, there was no significant increase in total biomass in the C3 grasses under eCO2 (Fig. 4.6c).

Such discrepancy between photosynthesis and biomass responses to eCO2 has been

observed previously for trees as well as herbaceous species (Ellsworth et al., 2017, Norby et al., 2010, Reich & Hobbie, 2013, Sigurdsson et al., 2013) and could be attributed to increase in carbohydrate availability exceeding the plants’ capability to utilise it due to nutrient and inherent growth limitations (Kirschbaum, 2011). In contrast to the C3 grasses,

total biomass decreased significantly under eCO2 in the C3 forbs (Fig. 4.6c) and was

correlated with the lack of stimulation in photosynthetic rates (Fig. 4.1) and significant down-regulation of photosynthetic capacity (Fig. 4.2). Overall, results from the current study suggest that eCO2 had a negative effect on biomass of the two C3 forbs, but not

grasses. A negative eCO2 effect on biomass has rarely been reported, and if so, has been

observed under low nutrient availability (Inauen et al., 2012, Zavaleta et al., 2003). Under low soil nutrient availability, plants exposed to eCO2 may allocate more biomass to roots

in order to increase the root foraging capacity (Sigurdsson et al., 2001, Suter et al., 2002). For instance, Inauen et al., 2012 observed a significant decrease in above-ground biomass under eCO2 in glacier fore-field forb plants, which they indicated was a consequence of

higher biomass partitioning to roots. In the current study, there was a significant decrease in shoot biomass under eCO2 in both the C3 forbs. However, I did not observe a concurrent

increase in the root biomass under eCO2 (Fig. 4.6b; Piñeiro et al., unpublished data). Thus,

there was no evidence of biomass partitioning in favour of root growth under eCO2 in the

C3 forbs in current study.

4.5.4 Conclusions

In summary, the main goal of my experiment was to examine the differential photosynthesis and biomass responses to eCO2 in the dominant C3 grasses and C3 forbs

growing under similar nutrient availability and unlimited water inputs. The results suggest that magnitude of eCO2 effect on photosynthesis and hence biomass accumulation varied

among the species. Photosynthetic capacity and total biomass decreased in the C3 forbs

under eCO2, but were maintained in the C3 grasses. Lower leaf N content and inability to

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capacity and biomass in the two C3 forbs under eCO2, as was found by Crous et al. (2010)

previously. Such differences in photosynthesis and biomass responses to eCO2, between

the C3 grasses and C3 forbs, may lead to less diverse herbaceous communities, possibly

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4.6 Supplementary information

4.6.1 Supplementary figures

Fig. S 4.1 Glasshouse growth conditions for the daily time period from 8 am to 4 pm during the duration of experiment.

Panel (a) shows CO2 levels under the ambient (aCO2, blue dots) and elevated CO2 (eCO2,

red dots) treatments; (b) shows relative humidity averaged across all the glasshouse chambers and (c) shows PPFD averaged across all the glasshouse chambers. Black solid lines indicate the gam fits with shaded confidence interval of 95%.

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Fig. S 4.2 Daily glasshouse temperatures during the duration of experiment across all the four glasshouse chambers.

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Fig. S 4.3 Effects of CO2 treatment on volumetric soil water content (VSWC) in C3

grasses (Msti and Nnie) and C3 forbs (Lpur and Smad).

Grey bars indicate ambient CO2 and black bars indicate elevated CO2. The percentages

above a pair of columns denote changes with eCO2. Results of split plot ANOVA with

CO2 and species as main effects are shown in the panel. Within a species, differences in

parameter between CO2 treatments (paired t-test) are denoted by ‘*’ when P ≤ 0.05 and

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Image 4.1 Four herbaceous plant species growing in pots in the glasshouse during the current study.

(a) Microlaena stipoides Labill. (Msti) - a native C3 grass (b) Nasella neesiana (Trin. &

Rupr.) Barkworth (Nnie) - an invasive C3 grass (c) Lobelia purpurascens R.Br (Lpur). - a

native C3 forb (d) Senecio madagascariensis Poir (Smad). - an invasive C3 forb. Images

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: Synthesis