Evolutionary effects of nitrogen are not easily predicted from ecological responses

Abstract

Premise: Anthropogenic nitrogen (N) addition alters the abiotic and biotic environment, potentially leading to changes in patterns of natural selection (i.e., trait-fitness relationships) and the opportunity for selection (i.e., variance in relative fitness). Because N addition favors species with light acquisition strategies (e.g., tall species), we predicted that N would strengthen selection favoring those same traits. We also predicted that N could alter the opportunity for selection via its effects on mean fitness and/or competitive asymmetries. Methods: We quantified the strength of selection and the opportunity for selection in replicated populations of the annual grass Setaria faberi (giant foxtail) growing in a long-term N addition experiment. We also correlated these population-level parameters with community-level metrics to identify the proximate causes of N-mediated evolutionary effects. Results: N addition increased aboveground productivity, light asymmetry, and reduced species diversity. Contrary to expectations, N addition did not strengthen selection for trait values associated with higher light acquisition such as greater height and specific leaf area (SLA); rather, it strengthened selection favoring lower SLA. Light asymmetry and species diversity were associated with selection for height and SLA, suggesting a role for these factors in driving N-mediated selection. The opportunity for selection was not influenced by N addition but was negatively associated with species diversity. Conclusions: Our results indicate that anthropogenic N enrichment can affect evolutionary processes, but that evolutionary changes in plant traits within populations are unlikely to parallel the shifts in plant traits observed at the community level.

Nitrogen enrichment alters the resistance of a noninvasive alien plant species to Alternanthera philoxeroides invasion

“…This trade-off of N utilization in A. philoxeroides in our study strongly supports the adaptive growth hypothesis (AGH), which is that maintaining a high C:N ratio in a low N environment will help invasive plants to improve their nutrient utilization efficiency and prioritize survival, while maintaining a low C: N ratio in a high N environment will enable invasive plants to obtain more resources and achieve rapid growth (Sun et al, 2020;Zhang et al, 2020b;Geng et al, 2023). However, a high N content in plants will increase their palatability and thus their susceptibility to herbivores by changing the interactions with high-trophic-level organisms; this might weaken the defensive ability of A. philoxeroides to resist insect feeding compared to O. articulata in their symbiotic communities (Lu et al, 2015;Liu et al, 2018;Waterton et al, 2022). NBI, the chlorophyll/flavonoid ratio of plants, is an important indicator for evaluating plant growth (Ilyas et al, 2022).…”

Microbial responses to stress cryptically alter natural selection on plants

“…The variance in relative fitness is equal to the ratio of variance in absolute fitness to squared mean absolute fitness (Wade & Shuster, 2005). Therefore, to test whether changes in I were due to changes in mean fitness vs changes in the variance in fitness, we calculated each of these for each mesocosm and conducted similar analyses to those described in the previous paragraph (Waterton et al ., 2022).…”

What determines the evolutionary trajectories of wild plant species? Approaches to the study of quantitative fitness‐related traits

“…It also permits the detection and measurement of the three distinct modes of selection on individual traits: linear, stabilizing (or optimizing), and disruptive selection. In this issue, many of the investigators used selection analysis to examine or to infer the process of selection on traits such as germination time (Muir et al, 2022), flowering phenology (Johnson et al, 2022, MacTavish and Anderson, 2022, Valdés et al, 2022), flower size and height (Chen and Pannell, 2022), plant size at flowering (MacTavish and Anderson, 2022), specific leaf area and plant height (Waterton et al, 2022), and physiological performance (Mazer et al, 2022; see also Johnson et al, 2022).…”

“…In another experimental approach designed to determine how and whether patterns of selection on functional traits are context‐specific, Waterton et al (2022) investigated the effects of nitrogen (N) enrichment on phenotypic selection in replicated experimental populations of the invasive annual grass Setaria faberi (giant foxtail; Poaceae). This study is particularly interesting given that anthropogenic N supplementation can alter both biotic and abiotic conditions through its effects on soil nutrients, the density and height of ground cover (and therefore the light received by plants), and community composition.…”