the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Aug 2024
A conservative resource use strategy in agricultural grasslands counteracts lower productivity and water use efficiency under drought conditions
Abstract. Grassland response to changes in water availability is closely tied to the traits of the plant community which determine plant water uptake and resource use. Plants can adopt either moderate and efficient (conservative) or rapid and demanding (acquisitive) resource use strategies. These strategies combined with the plant interactions with microbes, such as arbuscular mycorrhiza fungi (AMF), determine the grassland productivity and efficiency. This study aims to compare the drought response of two agricultural grasslands that differ in their resource use strategies. In an experimental garden 12 small-scale lysimeters were installed with two different agricultural grassland types (conservative and acquisitive) and two different irrigation levels (wet and drought). We measured water fluxes, above- and belowground phytomass productivity and AMF spore productivity in these two grassland types throughout a drought of 64 days. Despite differing resource use strategies, both grassland types exhibited similar reductions in evapotranspiration and aboveground phytomass under drought. However, the conservative grassland showed higher water use efficiency (WUE) when considering only aboveground phytomass, and a less pronounced compositional shift towards greater grass phytomass. Furthermore, in acquisitive grasslands the root:shoot ratio of grasses and AMF spores abundance in the soil were greatly reduced than in conservative grasslands. We also identified differences in legume productivity, rooting system, and AMF spores community composition as key factors influencing WUE. In a changing climate with greater frequency and severity of droughts in the European Alps, opting for grassland mixtures with more species with conservative characteristics should be considered, as they i) reach the productivity of acquisitive grassland even under wetter conditions and ii) show higher efficiency and longer vitality under drought conditions.
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RC1: 'Comment on egusphere-2024-2435', Anonymous Referee #1, 01 Sep 2024
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I have reviewed “A conservative resource use strategy in agricultural grasslands counteracts lower productivity and water use efficiency under drought conditions” by Tello-García et al. I think the research question/topic – how grassland communities cope with drought in terms of productivity, water use efficiency, and the mechanisms underlying this (traits, functional groups, and connections to AMF)- is very interesting and relevant. I appreciate the hard work of the authors, carrying out this field experiment and the analysis of AMF spores, root washing, and stomatal conductance. I’ve added quite a few smaller comments below addressing the Abstract – Results, but only briefly reviewed the Discussion as I think there are two major issues that need to be addressed first:
- Currently, the paper is framed around conservative versus acquisitive grasslands (and their drought response). However, for me it is uncertain if the grasslands plots actually differ in their strategies. Functional traits were used to determine the CWM of the mixtures, with the abundance of each species coming from the proportion of seeds in the mixture. In Table 1, I can see that both seed mixtures should be grass dominated, the seed mixture for dry locations (the ‘conservative’ mixture) by Festuca arundinancea, and the humid mixture (‘acquisitive’) by Festuca rubra/Lolium perenne/Poa pratensis. However, from Figure 3, I see that both grasslands have almost exclusively legume biomass (x10 higher than grass biomass). Therefore, I am not sure that how you calculate CWM holds – because there were obviously some establishment issues with the grasses (and perhaps with some of the legume species) which means that the proportion of the different species in the seed mixture is very different than the community that established (and where all of your measurements are based). Essentially, you have two legume dominated stands that respond differently to drought. I think this needs to be re-framed with more of a focus on the legumes – because it is their response that you are determining (rather than the community composition in the seed mixture).
- I’m also not sure about the argument that plants invest more carbon belowground and therefore there is higher AMF spore productivity, and this feeds back to improve drought tolerance. Determining belowground carbon investment in AMF requires a different approach I think – e.g. pulse labelling of 13CO2 to trace plant-derived carbon into AMF (fatty acids – NLFAs or DNA). To determine the role of AMF in drought response, it would be nice to see colonization – if there is higher colonization in one grassland or the other (although I think Trifolium species are highly mycorrhizal, so likely you would have very high colonization overall). As you note in the introduction (lines 88-91) AMF spore production may not be a good indicator for AMF activity, but rather a proxy for stress, and not necessarily derived from drought stress of the plant (it could be due to a direct effect of water limitation, right?) In the introduction, I miss an explanation on why AMF spore abundance or abundance of different sizes or pigmentations would affect plant WUE or drought response. What was the rationale behind this investigation and these analyses (e.g. results line 393-396). Also in the introduction, there are no references (as far as I understand) that use AMF spores/spore productivity as an indicator of plant C investment, or that AMF spores would affect plant drought response. For me, these are two missing links that undermine your line of argumentation.
Because of these two points, I would recommend reframing the paper, away from the conservative – acquisitive resource acquisition spectrum, and perhaps also away from linking AMF spore productivity to plant carbon investment and plant drought response.
I’ve made some more comments below that I hope help improve the manuscript:
Abstract
L18 and L32: efficiency in terms of what? Perhaps specify here if you mean water use efficiency, nutrient use efficiency, or something else
L18: perhaps ‘In this study, we compare …’
L28: ‘compared to conservative grasslands’
Intro
L61: perhaps spell this out a bit more – that acquisitive species do not adjust their water use strategy, and this becomes wasteful as the soil dries. Are there specific studies looking at WUE that you could site here?
L79: AMF would obtain all of their C from the plant host, right?
L81: is stomatal conductance a plant trait?
L82: perhaps include more recent references on AMF’s role in drought tolerance, e.g. XXXXX
L82: compared to no colonization? Or reduced colonization to higher colonization rates?
L83: ‘Decreases in AMF productivity include…’ Also, it is unclear what is meant by AMF productivity, could you elaborate here?
L86: ‘host-independent physio-chemical soil properties’ is a bit complex, perhaps instead something along the lines of ‘soil physical chemical properties’
L87: the plant community composition?
L90: nutrients or carbon? Perhaps back this up with a reference showing how drought affects plant-AMF C transfer
L93: Plant traits and plant-AMF interactions… (perhaps less complex).
L104-111: I think this text can be shortened and moved to after your hypotheses. Start this paragraph your hypotheses, and then add a few lines of how you tested them. You could also start with a short opening: e.g. “We investigated grassland drought responses in two model grassland systems differing in their functional traits and resource acquisition strategies in a field experiment in the Austrian Alps. We hypothesized that (i)….(ii)…. We determined plant drought resistance, investment in root biomass and water use efficiency. Belowground, we quantified AMF spore productivity and hyphal biomass.” I think this would make the paragraph sharper and the reader would be able to more quickly grasp your hypotheses and experiment. Introduce the conservative/acquisitive grasslands above, and then here you can directly refer to them. The details on the seed mixture can go in the methods
113-114: ‘…higher investment in root and AMF sporulation.’ Makes it sound like the plant is in control of AMF sporulation, which is not correct right? Perhaps it would be clearer to specify what are the plant responses and what are the soil responses. E.g. We hypothesized that (ii) conservative grasslands would respond faster to drought in terms of stomatal closure, higher investment in root biomass and higher WUE than fast-strategy grasslands. (ii) belowground, we expected that in conservative grasslands, AMF spore productivity would be higher…
And perhaps a question – how did you measure ‘faster’ investment in root biomass? From the methods I see that you washed roots at one timepoint, so I don’t think much can be said about the speed of investment. Instead I would say something along the lines of ‘greater investment in root biomass during the drought’.
L113-118: I think it is good to include these responses in your hypotheses, but I would incorporate them into your hypotheses rather than listing them as possible responses.
Methods
L122: Remove the quotation marks from Stubai Valley (capital V) and Tyrolean Alps, and it is: Neustift im Stubaital, Austria. Perhaps after Austria, start a new sentence describing the site.
L124: ‘It’ is ambiguous, perhaps ‘The site…’
L125: Here, and everywhere in the paper, switch to the active voice. E.g. instead of There, 12 small lysimeters were installed, change to ‘We installed 12 lysimeters’ and in L126-126: ‘We carried out our experiment in June-August 2018’
I think it is also useful in this section to specify the irrigation levels – in the figure I see ‘drought’ and ‘wet’. In the text (main and fig description) perhaps specify shortly what these were – my interpretation would be that ‘wet’ = waterlogged, so I wonder where the control is. I’d list the treatments and then in brackets write the target soil moisture/WHC target.
L134-139: I think you can tighten up this section ‘We used two seed mixtures commonly sown in European alpine grasslands: ‘seed mixture for humid locations’ (SR037) and ‘seed mixture for drier locations’ (SR032) (company).
L144: It would be good to show the PCA that you made to determine acquisitive vs conservative. Also, perhaps an explanation on why you used these traits. The original leaf economic spectrum (Reich, Wright) is SLA, LDMC, leaf N concentration. There is also the root economic spectrum with SRL, RTD, root N, and in some cases AMF (Bergmann, Roumet, Weemstra) and also a whole plant economic spectrum, but it would be good to specify why these traits (and also why not simply SLA, LDMC, and leaf N?) and also this is a bit of reference stacking as not all of these references back up the traits you’ve listed here. Perhaps just choose the reference that includes all and only these traits. Or, if you are adding/excluding traits, give a reason why.
L146: What is a CWM proxy? Or did you just calculate the CWM?
L148: Was there differences in germination? It could be that more seeds of a certain species would be added to compensate for poor germination. Although, I suppose if you do not have the proportions based on biomass, calculating CWM becomes difficult. Are there data (from the seed company) on germination success?
L149: displayed? Or had?
L150: To clarify: the spectrum was not determined as such? Just to note that the references you are using here do this in a different way. To integrate the traits, and quantify the tradeoff, usually PCA scores are used.
L153: what is seed share?
L154: Calculated or determined (i.e. did you measure this?) if you measured, specify how? Was this statistically different (i.e. did you measure SLA for each plot and then statistically compare your conservative vs acquisitive community)?
L159: Can you specify what you mean by root density (classes)? It would be helpful to have units here, as root density could be root tissue density (volume/weight) or rooting density (root length/volume soil). It would also be good to include this first in the text (rather than the Table heading)
L177: pH in water?
L178: report the other information here, or specifically refer to a supplementary table or figure
L200: specify that this is the height
L203: do you know the moisture level (volumetric water content) that field capacity was?
L218: Perhaps specify how many days after the drought began this was (e.g. Beginning 3 weeks after the start of the drought treatment…)
L221-222: I think you could say ‘… due to difficulty identifying plant species’
L226: ‘At the end of the experiment, grass and legume individuals were counted’ Perhaps also specify how long the drought treatment had been running (e.g. ‘At the end of the experiment (after 8 weeks of drought), …)
L230: How deep did you extract?
L234: You visually assessed necromass, but was this also determined (by weight) at the harvest?
L264: Why analysis of covariance (ANCOVA) instead of ANOVA? Perhaps specify that interaction between grassland type and irrigation level was tested (instead of ‘variables’). Which function and package did you use? In this section, mention these and reference the packages.
L267: what does ‘tested independently’ mean?
L279-284: This sounds like a nice analysis – can you reference also a paper that uses it in this way?
Results
Figure 2 – The stats you report here are for the ANCOVA, I assume because you are looking at the change between factors over time. Why did you use ANCOVA and not a generalized additive model (GAM, e.g. from package mgcv in R) which would be a more powerful model to capture the changes in SWC and ET over time? Also, you mention that the shaded area represents standard error – how many points per timepoint were measured?
L306: This can be simplified a bit. Perhaps something like: ‘Legumes had higher stomatal conductance than grasses (P < 0.001, Table S1), and responded differently to drought (). Grassland type affected how legumes responded to drought (): legumes in the acquisitive grassland xxx, while legumes in the conservative grassland xxx.
L308: “Legumes in the acquisitive grassland” (add ‘the’ throughout, or else it would need to be: legumes in acquisitive grasslands’)
L309: two weeks after the start of the drought? When was the point when the drought became ‘extreme’?
L310: More direct: Drought reduced grass stomatal conductance after 2 weeks of drought in both grassland types ().
L313: tended? (was this difference significant or not?)
L314: ‘Impacts’ is vague, could you reword this?
L317: Decreased, or ‘was significantly lower’
L319: Perhaps more direct: “Grasses and legumes significantly differed in their total aboveground phytomass (), which depended on grassland type (interaction) and irrigation level (interaction).”
L322: ‘while’ or ‘in contrast’ and specify what ‘it’ is
L322: no space between the % and the number
L322-323: Am I understanding this correctly that grasses were only 2-4% of your grasslands? Is this typical for this seed mixture?
L327: ‘reduced’ is in the wrong place here I think. Instead, something like: ‘In both grassland types, drought significantly reduced belowground biomass (). Unless there is a major difference in the magnitude of the reduction, I would not add the numbers as personally I think that numbers in text trip up the reader (unless they are comparing to other studies, or you want to specifically highlight them).
L337: 80% of the aboveground biomass was necromass? Or 80% of the grass individuals had died (i.e. did not regrow)? Specify this because there can be a lot of necromass and the individual can still grow back
L338: Increased with time (duration assumes that you have different droughts with different lengths)
L339: Instead of the percentages, perhaps state if this was different – was there sig more necromass in the acquisitive grassland?
Figure 3 – perhaps add in the legend that the scale of (a) between legumes and grasses differs – at first glance it looks like grasses have slightly less biomass, but this is actually an order of magnitude lower. Also specify which posthoc test you used.
L350: inoculum implies that the AMF were added, but this refers to the initial soil right? Perhaps then ‘compared to the initial AMF spore density’
L352: AMF spore size?
L353: ‘We found slightly (but non-significant, P < ..) higher AMF spore productivity in the conservative versus acquisitive grassland’… I would only report this if the significance was close, if not, this is no difference.
Figure 4 (b) – I like this figure, but wonder if these different colors can be seen by someone who is red-green color blind.
L379: I think this could be more direct: ‘We found that WUEap and WUEtp responded differently to drought (Fig 5).
L379: also more direct: ‘We found that grassland types had significantly different WUEap at the end of the drought’
L394: how were the variables condensed? This part is not clear to me, and I think this paragraph needs to be clarified. It makes sense that productivity is correlated to WUE, but is this not a spurious correlation (aboveground productivity is used to calculate WUE right?)
L400: move the ref to the figure and table to the end of the sentence
Discussion
L416: this was only one field site correct? Then why ‘in two climatically different areas of the Alps’?
L417: as far as I see, they were categorized based on individual traits rather than ‘strategies’ (there was no PCA/PCA scores as far as I can see?)
L418-420: This line reads a bit like droughts are not a big issue, when in fact they are (see Smith et al - https://www.pnas.org/doi/abs/10.1073/pnas.2309881120 drought effects are underestimated).
L415-420: reword this paragraph to show your key results and the main message.
Section 4.1. – rearrange this section so that you start with your most important results, and with the heading (water stress effects on aboveground productivity). Then move in to differences in stomatal conductivity as an explanation for how grasslands with different strategies deal with water stress (and what this eventually means to their productivity, and then why this is important). In the title you refer to ‘aboveground productivity’ and in the text ‘phytomass’, keep this wording the same throughout and the text will be more clear/readable.
L444-447: grasses are typically more ‘conservative’ than legumes, but why not compare within functional groups? Especially between grasses and legumes, they are functionally very different, so the difference between functional group (rather than strategy) would always be large, wherease there are more conservative legumes (Lotus corniculatus compared to an acquisitive like Trifolium repens or pratense) and the same situation with grasses (think of a species like Nardis stricta versus Lolium perenne or Dactylis glomerata). I don’t know how useful it is to compare between legumes and grasses in terms of ‘resource acquisition strategy’ because the functional group classification already covers this difference.
L451: was this difference apparent between the drought and the control as well? I.e. after drought conservative grasslands were almost exclusively legumes, but in both, there was still a significant decrease (Fig 3 a) and the legume biomass between acquisitive and conservative looks statistically the same right?
L456: big = significant?
L460: no doubt, drastic reductions in legume biomass in grasslands is a bad thing, but since your grasslands are almost completely legumes, I wonder if these arguments hold here? Grazing (or making hay from) a grassland with 98 or 80% legume biomass would not be safe for ruminants due to the high protein.. I’m wondering if the discussion could stick a bit closer to the water dynamics and reasons for these differences.
L480-483: I think you can remove this sentence. This paragraph could also be merged with another (possibly the one above)? It contradicts the title of the section, so perhaps first start with the result that is the reason for that title (the AMF spore abundance?) and then move on to mention the non-significant shifts in root:shoot.
L486-489: AMF, yes – but are spores a good indication of plant-AMF water/nutrient transfer? Or AMF impact on plant stress responses?
L493: slightly or significantly?
Citation: https://doi.org/10.5194/egusphere-2024-2435-RC1
Data sets
Data from: A conservative resource use strategy in agricultural grasslands counteracts lower productivity and water use efficiency under drought conditions Elena Tello-García et al. https://doi.org/10.23728/b2share.33d070b2e3d541ab8d147e80ebba9eb4
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