Phenological and physiological advantages of invasive annuals are strengthened by nitrogen enrichment

Anthropogenic nitrogen (N) deposition exerts profound influence on the
composition and function of native ecosystems, commonly resulting in
reduced plant richness and increases in nonnative plant invasion.
Identifying differences in plant N responses and functional traits could
help to explain the ongoing invasion and biological impoverishment of
ecosystems subject to N deposition. We compared the growth, emergence
phenology, and functional traits of thirty native and invasive annual
plant species under high and low N availability, with the goal of
evaluating if invasives exhibit functionally dissimilar responses to N
compared to the native species they displace. Invasives grew larger than
natives, especially under increased N availability. Overall, invasives and
natives did not differ in emergence phenology. However, these groups of
species differed in relationships between growth and days to emergence,
and N addition further strengthened these differences; invasives
consistently grew larger than natives that emerged around the same time,
while natives showed a strong negative effect of delayed emergence on
plant growth. Relative N responses for measures of plant growth and
allocation did not differ between native and invasive species, but
invasives exhibited greater plasticity in physiological traits. This
included significantly lower leaf N content, higher CN ratios, and greater
water-use efficiency (WUE) in plants grown under high N. Invasives also
showed functionally different patterns of growth and water use. Across
species, invasives showed a positive relationship between shoot growth and
WUE. Native species showed a tradeoff between root allocation and WUE,
while invasives displayed high values for WUE even despite reduced
root:shoot ratios under high N. Our results highlight the ability of
invasives to rapidly accrue biomass, disproportionately benefit from early
emergence and N addition, and use water efficiently despite rapid growth.
Natives, on the other hand, pay a higher cost for delayed emergence and
may experience tradeoffs between growth and water-use. The more pronounced
functional differences between natives and invasives observed under high N
likely play an important role in driving invasion and reduced species
diversity under N enrichment. These differences could be used to inform
successful management strategies for N-impacted and water-limited
ecosystems.