Herbivore-shrub interactions influence ecosystem respiration and BVOC composition in the subarctic
Abstract. Arctic ecosystems are warming nearly four times faster than the global average which is resulting in plant community shifts and subsequent changes in biogeochemical processes such as gaseous fluxes. Additionally, herbivores shape plant communities and thereby alter the magnitude and composition of ecosystem respiration and BVOC emissions. Here we determine the effect of large mammalian herbivores on ecosystem respiration and BVOC emissions in two southern and two northern sites in Sweden, encompassing mountain birch (LOMB) and shrub heath (LORI) communities in the south and low-herb meadow (RIGA) and shrub heath (RIRI) communities in the north. Herbivory significantly decreased ecosystem respiration at RIGA and altered the BVOC composition between sites. However, plant community composition had a larger effect on ecosystem respiration as RIGA had 35 % higher emissions than the next highest emitting site (LOMB). Additionally, LOMB had the highest emissions of terpenes with the northern sites having significantly lower emissions. Differences between sites were primarily due to differences in exclosure effects, soil temperature and prevalence of different shrub growth forms. Our results suggest that herbivory has a significant effect on trace gas fluxes in a productive meadow community and differences between communities may be driven by differences in shrub composition.
Cole G. Brachmann et al.
Status: final response (author comments only)
RC1: 'Comment on bg-2023-14', Anonymous Referee #1, 21 Feb 2023
- AC1: 'Reply on RC1', Cole Brachmann, 23 Mar 2023
RC2: 'Comment on bg-2023-14', Kathy Kelsey, 10 Mar 2023
- AC2: 'Reply on RC2', Cole Brachmann, 23 Mar 2023
Cole G. Brachmann et al.
Cole G. Brachmann et al.
Viewed (geographical distribution)
In their manuscript “Herbivore-shrub interactions influence ecosystem respiration and BVOC composition in the subarctic,” the authors report differences in ecosystem respiration (ER) and biogenic volatile organic compounds (BVOCs) flux and BVOC composition between plots that were either open or excluded to mammalian grazing in four sites across a Swedish mountain range. The authors attempted to use herbivory-driven changes in plant communities to explain differences in ER, BVOC flux, and BVOC composition and found that differences in ER and BVOC composition were putatively related to plant community composition. The links between plant community and BVOC composition are convincing because they are supported by redundancy analysis indicating that treatment and plant functional type both affected BVOC composition.
In contrast, the authors’ claim that ER was related to plant composition is not convincing, and additional analysis is necessary to strengthen this finding. The authors did not directly include plant community composition in their analysis of ER, despite the fact that they appear to have robust vegetation data for each plot. The reason for the exclusion of vegetation data from these models is unclear and should be clarified by the authors. Instead, the authors appear to simply rely on the “treatment” term in their linear models to serve as an indicator of differences in plant communities. Using this approach, the authors found differences in ER between plots open vs excluded to grazing in only one site, and attribute this difference to changes in plant composition. However, other sites had similar differences in plant composition but did not differ in ER. The authors attribute this discrepancy to differences in productivity between the sites. However, the authors do not provide any statistical evidence to support the claim that productivity influenced their findings (nor do they even provide any data showing that productivity differs between sites). The authors should provide additional evidence that changes in ER are related to plant communities to support their claims.
Specific and Technical Comments:
Line 43-46: This sentence is really the only one in the introduction that explains the importance of BVOCs to climate change. Arguably, the results of this study related to BVOCs are the most robust and interesting aspect of the study, but most readers will not be very familiar with their importance to climate change. Could the authors provide additional information about the relevance of BVOCs to climate change?
Line 75-77: Why will excluding herbivores shift plant dominance to deciduous shrubs? The introduction does not provide specific background about why this should occur and should be revised to include this information.
Line 77-79: This hypothesis does not reflect your statistical approach. As stated in the general comments section above, plant functional type is not included in your statistical model for ER, and soil temperature is the only soil characteristic considered. Furthermore, it doesn’t appear that climate properties were included in any models for either ER or BVOCs.
Line 95: What does it mean for a density estimate to be “tentative”?
Line 97-98: The sentence beginning “Langfjallet is an area…” is unnecessary because the previous sentence already reports reindeer density, and the proximity of RIGA and RIRI is reported earlier in the paragraph.
Lines 106-121, Table 1: It is unnecessary to report climate conditions both in the paragraph and as a table. I would recommend keeping the table and eliminating the climate description from the paragraph.
Lines 128-131: How many pairwise plots were present at each site? It seems as if there are three at most sites, and two at RIGA. Is this correct? Please explicitly state the number of plots at each site.
Line 221-222: This sentence seems to be missing a word, making it difficult to interpret. However, I don’t think this justification is necessary as using a linear mixed model for categorical data is not an unusual approach.
Line 226-229: This sentence appears to be missing some punctuation that makes it difficult to read.
Throughout the discussion, the authors attempt to relate the differences they observed in ER and BIVOCs to differences in plant composition between grazed and ungrazed plots. However, they never report how plant composition varies between grazed and ungrazed plots, nor provide statistical support that composition varies. It appears that results related to plant composition have been reported in two previous publications by Vowles et al, so perhaps the authors don’t want to duplicate those results here. However, it would greatly aid the reader in interpreting the findings of the current study if the authors could include a table or description of how vegetation communities varied between grazed and ungrazed sites, and confirm that differences are statistically significant.
Line 235: There should be a comma after “sites”
Line 239: I recommend changing “fences” to “fenced” here and throughout.
Lines 305– 315: Here, the authors attempt to link their findings of differences in ER to plant community composition, but, as stated in the general comments, these claims are not well supported statistically by the authors’ models. The authors should either explicitly include plant composition in their statistical approach, or consider additional explanations for the observed patterns in ER. For example, the authors found that temperature was a significant predictor of ER, and RIGA, the only site with differences in ER, is also the only site where soil temperature in ambient plots is consistently lower than in exclosure plots. Doesn’t this suggest that temperature is more likely than plant composition to explain these patterns? Why is the influence of temperature not discussed?
Lines 305 – 331: This is a long paragraph that is difficult to follow because it jumps from topic to topic in a fairly rapid fashion. I would recommend restructuring this paragraph to keep related topics together, and perhaps consider breaking it up into two separate paragraphs.