Species diversity and functions in forest ecosystems in a changing climate

US Forest Service provides extensive forest inventory data across the USA through the Forest Inventory & Analysis (FIA) National Program (Figure 1). Using FIA data, I study various topics related to plant invasions and ecosystem functioning in forest ecosystems in the USA.

Recently, I have participated in the project to examine shifts in 86 tree species distributions and factors associated with the range shifts over the past three decades using FIA data (Fei et al., Science Advances, 2017). We found that more tree species have shifted westward than poleward in their abundance, following changes in moisture availability. Our results indicate that changes in moisture availability have stronger near-term impacts on vegetation dynamics than changes in temperature.

Figure 1 Forest inventory (FIA) plot locations in USA

Forest understory invasions mediated by dominant forest mycorrhizal type

Plant-fungal symbiosis plays an important role in plant nutrition and nutrient cycling in forest ecosystems. I explored how dominant forest tree mycorrhizal type (e.g., arbuscular mycorrhizal [AM] or ectomycorrhizal [ECM]) affects understory plant invasions in temperate forests. I found understory invasions were positively associated with AM dominant forests where forest floor thickness and soil C:N ratio were lower than ECM dominant forests (Figure 2). More invasions in AM dominant forests can further change nutrient dynamics in the invaded ecosystems, altering ecosystem structure and functions (Jo et al., Ecology Letters, 2018).

Figure 2 Relationships between dominant tree mycorrhizal type and understory species abundance (Jo et al. 2018).

Expansion of non-native invasive plants in the eastern United States

Expansion of nonnative 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- and below-ground resource-use strategies of understory woody 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 (Figure 3). 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., Biological Invasions, 2015).

Figure 3 Hypothetical relationships between above- and below-ground resource use strategies using plant traits and soil attributes that affect soil N cycling (Jo et al. 2017). Illustration by Insu Jo

Drivers of leaf and root decomposition

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 (Figure 4), 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., New Phytologist, 2016).

Figure 4 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 (Figure 5). After two years, invaders had greater above- and below-ground productivity.

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 (Figure 3). 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., Journal of Ecology, 2017).

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.

Figure 5 Monoculture plots of native and non-native invaders at Syracuse University experimental garden. Syracuse, NY (USA). Photo by Insu Jo

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