Grazing and light modify Silene latifolia responses to nutrients and future climate

by 12.8% (Table 2, S1 Fig) and fertilization alone decreased foliar C:N (i.e., increased relative foliar N content) by 21.9% (Table 2, Fig 2). Future climate had no main effect on Silene SLA and foliar C:N (Table 2). Discussion Testing how single plant species respond to joint effects of nutrients and lights in combination with changes in grazing patterns and climate is important to better understand changes in plant community composition and diversity. We found that Silene grew taller under fertilized and future climatic conditions without additional light than when light was added. This find- ing demonstrates that competition for light is an important mechanism affecting plant invest- ment into height under densely vegetated conditions, which has not been previously shown using direct manipulation of light in the field. Light was also an important modulator affecting SLA and C:N responses to climate and nutrients, highlighting that competition for light affects trait expression. In addition, future climate increased Silene biomass but this impact was con- ditional on the absence grazers, highlighting that individual species’ responses to climate change can strongly depend on grazing. In general, strong responses to grazing highlight the important role of grazing animals for performance and traits of highly palatable forbs in grass- land systems, especially under future climatic conditions. We found that Silene transplants grew tallest under future climate when fertilized but not lighted. These effects were clearest in unfenced plots, as transplants were almost completely

eaten when exposed to grazing. In these plots, general vegetation biomass was high and light levels low (S2A and S2B Fig, Eskelinen et al., submitted). This suggests that Silene invested in height in densely vegetated and shaded conditions (S3A and S3B Fig) as tall stature can increase the plant’s ability to compete for light [13,86]. Because our experiment directly adds light into nutrient-enriched, fenced and climate-manipulated grassland communities, where light levels are low, we suggest that tall stature is a direct response to increased competition for light (S4 Fig). We also found that the increase in transplant height in fertilized and unlighted plots was limited to future climatic conditions. Increased spring precipitation and warmer temperatures in the future climate treatment may have promoted transplant growth during the early growing season and life stage [87,88] due to more favorable growing conditions than under ambient climate. Our finding suggests that changes in seasonal precipitation and tem- perature patterns can be a decisive factor for species’ ability to grow tall and compete for light under altered nutrient and grazing conditions [56,89]. We also found that the addition of light increased SLA especially in fenced plots. This effect also depended on the climate treatment such that the positive response of SLA to light addition was prevalent in ambient climate only. Future climate with lower summer precipitation and higher temperature may have caused the transplants to invest less on expanding leaves during

the summer months, resulting in lower SLA due to water scarcity [52]. Nevertheless, the observed increase in SLA in response to light addition seems counterintuitive, as plants are expected to increase SLA in response to light limitation [86]. However, also other factors than direct light intensity could have driven SLA responses which is supported by the fact that we did not find a significant relationship between light intensity and SLA (S5A Fig). Instead, SLA was positively related to percentage total vegetation cover (S5B Fig), suggesting that dense veg- etation with high competition for space can be a decisive factor for foliar SLA manifestation. Competition for water, nutrients or space due to increased abundance of species with low SLA and leaf water content (LWC) (Eskelinen et al. submitted) may also contribute to explain the tendency of Silene total biomass to decrease in lighted plots. In addition, plants growing tall in dense vegetation may also grow thinner and thus produce less biomass [90]. Furthermore, also PLOS ONE Transplant performance and trait responses to abiotic and biotic changes PLOS ONE | https://doi.org/10.1371/journal.pone.0276789 November 8, 2022 9 / 16