EXPANSION OF NON-NATIVE INVASIVE PLANTS IN EASTERN UNITED STATES 
Expansion of non-native invasive plants have been a major challenge for various ecosystems worldwide. Despite the importance of understanding of invader expansion patterns to manage those species, the spatio-temporal progression of invaders is not well understood.  In the Fei lab at Purdue University, using herbarium records in the eastern USA, we constructed invader expansion networks over the past two centuries and examined how human mediation and plant traits affect the expansion process. 


ABOVE- & BELOW-GROUND RESOURCE-USE STRATEGIES OF INVASIVE SPECIES 
During the PhD program in the Frank and Fridley labs at Syracuse University, I aimed to examine how the coupled carbon (C) and nutrient-use strategies of invasive plants differ from natives and how the unique plant C and nutrient use patterns of invasives affect soil C and nutrient processes. 

Because C and nutrient processes are inextricably coupled in plants, the greater C uptake and use patterns of invasives likely are linked to a different resource use strategy than that of native species. I found that non-native shrubs had a greater productivity. Interestingly, non-natives had lower leaf nitrogen (N) resorption rates but higher leaf N concentration and specific root length (SRL) than natives. These results suggest that non-native shrubs have a greater capacity to take up soil nutrients but a less conservative nutrient use strategy compared to natives (Jo et al. 2015)


Linkage between above- and below-ground resource use strategies. Illustration by Insu Jo


DO INVADERS HAVE GREATER LITTER DECOMPOSITION RATE THAN NATIVES?
Further, I examined whether the different tissue traits are associated with litter decomposition rate and invaders can increase nutrient cycling through faster litter decomposition than natives. I determined decomposition rates of root and leaf litter among 48 and 78 woody species, respectively and revealed no different leaf and root decomposition rates between native and non-native species, suggesting litter decomposition rate is not a pathway that invasive species affect soil nutrient processes in the Eastern U.S. forests (Jo et al. 2016).
Litter bag experiment to determine leaf and root decomposition rates for native and non-native species in Eastern USA. Pompey, NY (USA). Photo by Hana Kim
HOW DO INVADERS ALTER ECOSYSTEM PROCESSES?
Finally, I investigated invader impacts on soil N processes in a monoculture experiment. After two years, invaders had greater above- and below-ground productivity.

Monoculture plots of native and non-native invaders at Syracuse University experimental garden. Syracuse, NY (USA). Photo by Insu Jo

Invaders facilitated N cycling via greater litter N input into the soil that increased soil N availability, and had greater fine root production and SRL that increased plant N uptake. 
Although the greater above-ground production of invaders reduced soil temperature and moisture, which can reduce soil microbial activity, the stimulatory effects of a greater flow of litter N to the soil appeared to overwhelm any negative effects that invaders had on soil microclimate (Jo et al. 2017). 
 
Hypothetical relationships between plant traits and soil attributes that affect soil N cycling (Jo et al. 2017). Illustration by Insu Jo
Taken together, my results suggest that invaders have differed above-and below-ground resource-use strategies and invaders’ greater productivity is one of the major drivers that can significantly change ecosystem processes.