Research
Soil effects on plant distributions and migration
Soil properties have important impacts on plant fitness, but we have little knowledge of how soil properties influence large-scale plant distributions and potential migration under climate change. The common view that soil effects operate mostly at local scales could be misleading because soil properties can also vary greatly at large spatial scales. Two studies in my Ph.D. address these questions – one studying soil effects along a latitudinal gradient (Eastern North America, see preprint), and one along an elevational gradient (Mont-Mégantic, Québec). We found that soil properties had substantial effects on plant distributions on both the latitudinal and elevational gradients, and such influences could be more pronouncedly in colder environments. Furthermore, the unsuitable soil environments at high latitudes/elevations may represent a major constraint on plant migration under climate change. I am also exploring the evolutionary mode and tempo of plant soil niches, to ask whether plants are likely to adapt to the high-latitude soils where future climates are likely to become suitable.
The relative importance of climate (grey) and soil (yellow) variables in predicting species distributions of plants in Eastern North America (a), and soil effects for different plant growth forms (c) and habitats (b).
The influence of soil pH and elevation on the abundance of an understory herb Trillium erectum at Mont-Mégantic. The pH effect changes from neutral at low elevation to positive at high elevation.
The relative importance of climate (grey) and soil (yellow) variables in predicting species distributions of plants in Eastern North America (a), and soil effects for different plant growth forms (c) and habitats (b).
Forest dynamics under climate change
Differences between the distributions of juvenile trees and adults in geographic or niche space have been used to infer climate change effects on tree range dynamics. Previous studies have reported narrower latitudinal or climatic niche ranges of juvenile trees compared to adults, concluding that tree ranges are contracting. However, more comprehensive sampling of adult trees than juvenile trees in most regional forest inventories could potentially bias such ontogenetic comparisons. In this first chapter of my Ph.D. thesis, we first reported spatial simulations showing that reduced sampling intensity can result in underestimates of range and niche limits. We then reanalyzed the U.S. Forest Inventory and Analysis data, comparing the range and niche limits between adult trees and saplings of 92 tree species, both using the original data and two resampling procedures. Resampling aimed to reduce sampling biases by controlling for either sampling area or the number of individuals sampled. Overall, these resampling procedures had a major influence on the estimation of range limits, most often by reducing, eliminating, or even reversing the tendency in the original analyses for saplings to have broader distributions than adult trees. More details in the Ni & Vellend, 2021.
Larger sampling areas in local plots can result in larger estimated latitudinal ranges by increasing the probability of detecting occurrences at the range edge.
The previous finding that juveniles’ trees had narrower latitudinal ranges than adult trees (original analysis) were likely caused by sampling biases. After controlling for sampling issues, juvenile trees generally had broader ranges than adult trees.
Larger sampling areas in local plots can result in larger estimated latitudinal ranges by increasing the probability of detecting occurrences at the range edge.
Introduction history, distributions, and invasiveness of alien plants in China
A prominent feature of the Anthropocene is massive species exchanges among regions, and some alien species cause serious environmental impacts to local ecosystems. China, as one of the major emerging economies, is facing rapidly increasing invasion risks. During my M.Sc. research, we used herbarium and literature records to reconstruct the introduction history of alien naturalized plants in China. We found that the temporal dynamics of plant introductions in China were mainly driven by intentional introductions (Ni & Deane, 2022). The history and sizes of botanic gardens determined the spatial pattern of plant introductions, especially for those species introduced for ornamental purposes (Ni & Hulme, 2021). We also found that the determinants of invasiveness vary across invasion stages. Factors associated with evolutionary adaptation and population expansion predict invasion success and extent, while traits related to the competitive ability determine the severity of impacts (Ni et al., 2021). Furthermore, we found that root traits, which are usually neglected in invasion ecology, could play a major role in determining the competitiveness of invasive plants (Ni et al., 2018).
The spatial pattern of alien plant introductions in China, which is mainly driven by the number of species in botanic gardens.
Root growth was measured in the lab, and fast root growth was found to explain the competitiveness of invasive plants.
The spatial pattern of alien plant introductions in China, which is mainly driven by the number of species in botanic gardens.