Cost-benefit Quantification of Leaf Carbon Economics to Disentangle Responses of Plant Assemblages to Deer Herbivory


 Although the plant carbon cost-benefit balance is known to be related to individual plant growth, reproduction, and population expansion, the association with plant community differences is not well understood. In this study, we examined how the leaf carbon cost-benefit metrics were associated with the assembly process of forest understory plant communities in areas highly affected by deer browsing. We calculated these metrics from plant physiologically parameters for 14 forest floor plant species growing in deer presence/absence site to detect the relationship between species dominance and leaf carbon cost-benefit metrics. As a result, the patterns of interspecific variation in benefit along the plant dominance rank differed in deer presence/absence sites, contributing to the marked differences in species composition and diversity observed at the two sites. In the absence of deer, where competition was the dominant determinant of plant community composition, carbon benefits among species were positively related to the plant dominance rank, indicating that species able to acquire more carbon were at an advantage. On the other hand, under deer herbivory, differences in carbon benefit between species were not strongly apparent and were not related to the plant dominance rank, indicating few differences in reproductive and expansion ability (plant fitness) between species. This process contributes to the high species diversity of plant communities observed in the presence of deer. Our results emphasize the possibility of connecting different fields of studies, physiological ecology, community ecology, and the plant carbon cost-benefit balance of single leaves to explain plant community composition differences.

. 60 Importantly, plant carbon benefits are often quantified on the basis of individual leaves. 61 Researchers have been trying to build the "cost-benefit model" of leaf photosynthetic carbon 62 production to estimate the optimal plant strategies for leaf longevity in growing environments  The acquisitive plant strategy is characterized by low investment in leaf structure and high 77 photosynthetic efficiency. This strategy is advantageous when the risk of losing leaves is 78 relatively high (a short leaf lifespan), especially under disturbance pressure, because it allows 79 for the offset of carbon investment in leaf structure in a short time considering the carbon 80 cost-benefit methods. This strategy is often considered the fast end of a plant economic 81 spectrum. In contrast, the slow end of the spectrum is characterized by a conservative strategy 82 with high investment in leaf construction and maintenance, requiring a long leaf lifespan.  Forest floor plant communities are known to account for much of the diversity in 90 forests due to their spatial and temporal environmental heterogeneity (Gilliam, 2007). In 91 recent years, there has been an increasing recognition that anthropogenic influences can 92 substantially alter these ecological communities due to land conversion, biological invasion, 93 and an overabundance of herbivores (Whigham, 2004     The exclosure plot was protected from deer herbivory by the fences, and the control plot was visually estimated in units of 1%. Here, we defined "forest floor plants" as herbaceous plants, 153 ferns, and vines, and excluded the seedlings of semi-tall tree species. In the case of species 154 coverage less than 1%, we recorded the appearance. We conducted this study during the plant 155 growth period (from the end of June 2020).

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Calculation of expected leaf carbon benefit and cost metrics 158 We used 14 forest floor species that appeared in both plots in the field study (supplementary 159 material: S1) to evaluate the difference in leaf carbon gain and cost metrics in the deer 160 presence and absence sites. We collected five leaf samples for each species for each treatment  Parameter 2. Leaf area change with growth 183 We measured the leaf area of collected samples from scanned images using ImageJ software 184 (Rasband 1997(Rasband -2014, and this size collected in August was assumed to be the fully grown where d is the value used to convert the unit of measure from µ mol CO2 s -1 to grams. The

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DCG estimates daily carbon gain (g) on day t using PAR at every 10 minutes interval (i).    Leaf carbon gain 312 Comparing each parameter that we used to calculate the metrics of leaf carbon benefit and 313 cost between inside and outside the fence for each species, the leaf life span was longer, and 314 the leaf carbon concentration was higher in deer absence plots than in deer presence plots 315 (Supplementary materials: S2 and S6).
316 Figure 1 shows that species with higher total carbon gain and carbon benefit were 317 more dominant in the exclosure plots. On the other hand, in control plots, total leaf carbon 318 gain was higher for less dominant species than inside the fences, and there was no significant 319 relationship between leaf carbon benefit and the dominance rank. Figure 2 shows that species  In the exclosure plots, species with a more significant total carbon gain and benefit had a  Availability of data and material 424 These are available from the corresponding author upon reasonable request.

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Code availability 426 These are available from the corresponding author upon reasonable request. Correlation between total carbon gain (gray line) and bene t (colored line) and plant species abundance rank. Orange and green points show the data measured in the control plot and in exclosure plot, respectively. Signi cant are evaluated by Kendall rank correlation coe cient (n.s: p> 0.05, *p <0.05, **p < 0.01, ***p < 0.001).

Figure 2
Correlation between carbon cost and plant species abundance rank. Orange and green points show the data measured in the control plot and in exclosure plot, respectively. Signi cant are evaluated by Kendall rank correlation coe cient (n.s: p> 0.05, *p <0.05, **p < 0.01, ***p < 0.001).

Supplementary Files
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