Phylogenetic niche conservatism of Picea and Quercus: analysis and implications for palaeoclimate reconstructions

Phylogenetic niche conservatism (PNC) in climate space implies consistent climate responses of plant taxa distributed across different geographical regions. PNC can be considered as an expected consequence of optimizing selection. Optimizing selection favours appropriate combinations of plant traits and maintains these combinations over evolutionary time. It enables species to track their optimal environmental conditions, and governs the climatic tolerances of plant lineages. PNC is implicitly required by pollen-based palaeoclimate reconstruction. As pollen is rarely identifiable to the species level by morphological classification, climatic PNC at higher taxonomic levels can justify the use of geographically extensive data sets of contemporary pollen assemblages in the reconstruction of climates of the geologically recent past.

We set out to evaluate the PNC hypothesis in two genera, Picea and Quercus, that are widely distributed in the Holarctic phytogeographic realm. These genera are characteristic of boreal and temperate forest biomes, respectively. We characterized the realized climatic niches of 29 Picea and 160 Quercus species by their optima (u) and tolerances (t) using Generalized Linear Models (GLMs) and Generalized Additive Models (GAMs) in a three-dimensional climate space defined by a moisture index (MI, representing plant-available moisture), mean temperature of the coldest month (MTCO, representing winter cold) and growing degree days above a base level of 5 ℃ (GDD₅, representing summer warmth). We then used phylogenetic analyses and published phylogenetic data to test whether more closely related species occupy more similar climatic niches. We designed an R function, and developed an index of niche overlap, to test whether the combined climatic ranges of species within each genus are coherent in present-day climate space.

The correlation between climatic niche separation and phylogenetic distance in Picea was found to be weak. This is probably either because (i) parallel evolution leads to similarity among distantly related species; or (ii) analyses on a small phylogenetic scale amplify the divergence among closely related species. Nevertheless, the genus Picea as a whole occupies a coherent climatic niche, consistent with PNC. Quercus showed positive correlations between climatic niche separation and phylogenetic distance. A consistent climatic differentiation between predominantly evergreen versus deciduous clades indicates climatic PNC within major Quercus clades.

These results indicate phylogenetically conserved climatic niches in plant clades with broad geographical distributions, and support the inference of Quaternary climate changes based on pollen assemblages at genus or subgenus levels.