Root distributions predict shrub-steppe responses to precipitation intensity
Abstract. Precipitation events are becoming more intense around the world, changing the way water moves through soils and plants. Plants that have, or create, roots that absorb more water under these conditions are likely to become more abundant (e.g., shrub encroachment). Yet, it remains difficult to predict species responses to climate change because we typically do not know where active roots are located or how much water they absorb. Here, we used water tracer injections in a field experiment to describe forb, grass, and shrub root distributions under low and high precipitation intensity treatments. To estimate how much water different active rooting distributions can absorb over time, we used a soil water flow model, and we compared our estimates of water uptake to aboveground plant growth. In low precipitation intensity plots, deep shrub roots were estimated to absorb the most water (93 mm yr−1) and shrubs had the greatest aboveground cover (27 %). Grass root distributions were estimated to absorb an intermediate amount of water (80 mm yr−1) and grasses had intermediate aboveground cover (18 %). Forb root distributions were estimated to absorb the least water (79 mm yr−1) and had the least aboveground cover (12 % cover). In high precipitation intensity plots, shrub and forb roots moved in ways that increased their water uptake relative to grasses, predicting the increased aboveground growth of shrubs and forbs in these plots. In short, water uptake caused by different rooting distributions predicted plant aboveground cover. Our results suggest that detailed descriptions of active plant root distributions can predict plant growth responses to climate change in arid and semi-arid ecosystems.
Andrew Kulmatiski et al.
Status: open (until 12 Apr 2023)
RC1: 'Comment on bg-2023-13', Anonymous Referee #1, 16 Mar 2023
- CC1: 'Reply on RC1', Andrew Kulmatiski, 22 Mar 2023 reply
RC2: 'Comment on bg-2023-13', Anonymous Referee #2, 17 Mar 2023
CC2: 'Reply on RC2', Andrew Kulmatiski, 22 Mar 2023
RC3: 'Reply on CC2', Anonymous Referee #2, 23 Mar 2023
- CC3: 'Reply on RC3', Andrew Kulmatiski, 24 Mar 2023 reply
- RC3: 'Reply on CC2', Anonymous Referee #2, 23 Mar 2023 reply
- CC2: 'Reply on RC2', Andrew Kulmatiski, 22 Mar 2023 reply
Andrew Kulmatiski et al.
Andrew Kulmatiski et al.
Viewed (geographical distribution)
The manuscript “Root distributions predict shrub-steppe responses to precipitation intensity” describes results from a multi-year precipitation manipulation experiment and labeled-isotope tracer injection study, combined with soil water flow modeling, to explain how root distributions and water uptake patterns relate to plant abundances in sagebrush steppe. This experiment has been the subject of previous published work as well (e.g. Holdrege et al 2021, cited in the text), and there are places where a bit more could be done to articulate what novel addition is being made by this new contribution, but I also appreciate the way this leverages previous efforts to make the most of a multi-year experiment. I found it to be an interesting and well-conducted study which will make a useful contribution to the literature on global change, ecohydrology, and plant ecophysiology in shrubby ecosystems. I list major and minor concerns and suggestions for revision below.
My greatest overarching concern/question pertains to the plant functional group distinctions made in this study. It makes sense to separate shrubs, forbs, and grasses from each other; however, I was surprised to see annual and perennial grasses lumped this way. Sagebrush systems are home to a variety of grass growth forms, with the most notable contrast being between the native, deep-rooted perennial bunchgrasses, and the shallow-rooted exotic annual grasses (e.g. Bromus species) that are a major ecological threat throughout the biome. It sounds like both annual and perennial grasses were present at the study site, and species-level samples were collected after tracer injections, so would it be possible to separate these for the data analysis? I expect that would be more meaningful than a lumped “grass” category, and could be a very informative in a system where ecologists and land managers are at least as concerned about the future of annual grass invasion under global change as they are about woody encroachment. Some of the seasonal shifts in tracer uptake for grasses between the May and July periods could also perhaps be explained by phenological turnover between different grass species, rather than the same plants changing their rooting behavior – e.g. in Fig. 3a-b, could the high uptake from the shallowest depths be attributed to annual grasses active in May, while the shift toward the second-shallowest depth in July reflects annual grasses senescing while deeper-rooted perennials remain active?
I have a few wording quibbles that apply throughout the manuscript:
Minor comments by line number:
L58-59 – I believe that not all of these studies (e.g. Case et al 2020) used tracer techniques – check which ones are actual examples of this method, vs. examples of water uptake depth estimates along a continuum rather than a shallow vs. deep approach?
L74 – A previous paper is referenced that seems to have had very similar findings from the same experiment. Add a bit more explanation here of what this new paper adds – if it’s just additional years of data, what’s the question being answered by extending the time series? If it’s also that a new method or approach was applied here, explain that novel addition more clearly too.
L109-110 – Clarify whether these adjustments to event sizes meant all events were a fixed size, or if setting a minimum means larger events could also be delivered. A supplemental figure showing event size distributions for the different treatments might be helpful.
L119-122 – On what basis were the high vs. low cutoffs set? At a glance, the 4mm treatment seems much more similar to the 1-3 mm treatments than the 8 or 18 mm treatments, yet it’s lumped with those much higher values. Perhaps there’s extra information that can be shared from the first few years of the experiment to justify this split?
L126 – Clarify whether the increased soil water availability noted here was overall, or at certain depths but not others.
L167-169 – How were the two different seasons of tracer uptake data incorporated into the modeling?
L221-222 – “early season low intensity treatments,” etc., sound like a description of the treatment (low intensity rainfall manipulated early in the growing season) rather than the timing of sampling + event intensity of the treatment. Suggest re-wording to clarify this.
L298-300 – Is this circular, to say that root distributions weighted by existing plant cover correctly predicted the rank order abundance of aboveground plant cover?
Fig 3 – Make line thicknesses consistent among the four panels of this figure (the lower panels have thicker lines, which is distracting). I would also suggest putting panels for different treatments side by side (rather than seasons side by side, treatments in rows) to be consistent with the arrangement of later figures and easier to compare between treatments.
Fig 4 – The legend here seems to be incorrect – should the left panel say “Forb low,” etc.? This is also a place where I wonder about the definition of competitive advantage as simply meaning greater water uptake in a layer, where the other groups may also be using nearly as much water. Does the right panel imply there’s no niche for forbs in the high intensity treatment?
Fig 7 – The forb graphic inset is positioned such that it looks like a label for the vertical dotted line – I suggest moving it elsewhere within the figure panel to avoid confusion.