DISSERTATION RESEARCH: Do tradeoffs between water use efficiency and growth explain plant community response to nitrogen deposition?

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Nitrogen is a largely unrecognized component of air pollution and can negatively impact ecosystems. Excess nitrogen in the air is eventually deposited in the soil. While more nitrogen in the soil may seem beneficial for plant growth, many plant species are adapted to lower nutrient levels. Indeed, previous studies have shown that nitrogen deposition can be detrimental to ecosystem health. Southern California, which is notorious for its nitrogen-containing smog, has high levels of nitrogen deposition in the soil and many invasive plant species that also reduce native biodiversity. Nitrogen deposition may magnify the impact of invasive species because it often promotes growth of invasive annual plants over native shrubs and wildflowers. One possible explanation for the loss of native species is that added nitrogen makes them grow faster and use water less efficiently, causing them to be more susceptible to drought. Invasive plants on the other hand, may be able to grow fast with added nitrogen and still use water efficiently. The purpose of this research is to compare growth and water use by native and invasive plants under different levels of nitrogen addition. This work will lead to a better understanding of the environmental effects of nitrogen deposition, especially in dry habitats with problematic invasive species.Identifying how trait differences between native and invasive plant species influence community composition over environmental gradients is critical to a mechanistic understanding of how ecosystems will respond to global change. Nitrogen deposition is reported as major driver of plant diversity loss, invasion and vegetation-type conversion in some areas of Europe and North America. In arid systems, nitrogen and water will have interactive effects on water-use efficiency and growth, and these responses may mediate survival. Trade-offs among plant traits, such as water-use efficiency and relative growth rate, are known to play an important role in community assembly and species coexistence. Native and invasive plants may differ in this trade-off, and added nitrogen may influence these dynamics. The use of stable isotopes (13C) for the estimation of integrated water-use efficiency in conjunction with a plant functional trait-based approach and community-level data will allow for exploration of this important tradeoff as a mechanism of invasion under N deposition. The researchers will address this hypothesis by measuring the growth and water-use efficiency of native and invasive plants along an experimental gradient in nitrogen deposition in the Santa Monica Mountains of southern California.