Time-transgressive niches of plant taxa and biomes in North America

Yue Wang and Jenny L. McGuire, School of Biological Sciences, Georgia Institute of Technology, Atlanta, GA, Jenny L. McGuire, Interdisciplinary Graduate Program in Quantitative Biosciences, Georgia Institute of Technology, Atlanta, GA, Jenny L. McGuire, School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA

Background/Question/Methods
Given how rapidly and dramatically climate is changing, we need effective strategies to predict potential plant distributions under future climate change. These are often predicted based upon the climates where plants live today. However, the realized niches of plants today are, by definition, a subset of their fundamental niches, and these realized niches have been further reduced by anthropogenic landscape transformations. Here we introduce a strategy to use time-transgressive niches to more accurately approximate the fundamental niches of plants and examine its utility across two ecological scales – plant biomes and individual plant taxa. In looking at multiple scales, we can assess how niche dynamics of individual taxa feed into the niche dynamics of the biomes they comprise as they respond to changing climates.
We evaluate the extent to which realized niches have shifted over time, the extent to which the climatic niches of the individual plant taxa contribute to changes in the biome niche, and the utility of a time-transgressive niche for describing niches across ecological scales. We use pollen data from the Neotoma fossil database to examine the climatic niches of nine plant biomes and 15 key plant taxa in North America at three time periods: deglaciation from 18 to 10 ka; Holocene from 10 ka to 1500 AD; and modern after 1500 AD. We examine how realized niches have shifted over these time periods, and compare these to the overall time-transgressive niche for each biome and constituent group of taxa.

Results/Conclusions
The realized niches of plant taxa and biomes have changed significantly through time, yielding a broader time-transgressive niche than that of any single time period. Most plant taxa and biomes had their coldest temperatures during deglaciation and the warmest temperatures during modern times, together with the broadest niche space during deglaciation and the smallest niche space in the Holocene. However, the shift and expansion of biome niches through time are not entirely caused by the niche shift of key plant taxa. The niche shifts calculated by the weighted average of niches from key plants differ significantly from the niche shifts experienced by the biomes. Our work implies that plants respond to climate dynamically and differently across ecological scales, from biomes to individual taxa. To better understand the relationship between plants and climate and predict plant distributions under climate change, ecological research should prioritize studying individual plant taxon dynamics.