Montane, or temperate, families
Flat, or negative, ERR slopes are also informative if regarded as indicators of ecological strategy (stress-tolerance) or resiliency to variation in environmental conditions. Relatively neutral ERR slopes may also reflect richness that is predominantly distributed mid-gradient, farthest from physical boundaries which may artificially truncate potential range-sizes, or for flora with ranges that are not limited by soft ecological boundaries (e.g. distributions that traverse vegetation zones or habitat types) (Whitman et al. 2021). One of the oldest families in our study, Podocarpaceae, had a shallow ERR slope (0.13), with species’ distributions that are constrained by competitive exclusion within contemporary plant communities, rather than climate, with species that are restricted to habitat refugia in montane areas or soils with extreme nutrient limitation (e.g. ultramafic soils) (Coomes and Bellingham 2011). Upper montane forests are often noted for Gymnosperms from the Southern Hemisphere, with clades that are believed to have evolved and established before the current positioning of the archipelago (Culmsee and Leuschner 2013). Another outlier of our study is Ericaceae, with a flat ERR slope (– 0.001), which we have labeled as “2” on all scatterplots. Counter to most tropical clades, the species-rich genus Rhododendron (Ericaceae) migrated into Malesia from temperate areas of the Himalayas, with Borneo as a distribution hub followed by rapid speciation across New Guinea as recently as 6 million years ago (Webb and Ree 2012, Soza et al. 2022). Rhododendronlikely colonized montane areas first, with downward elevational expansion facilitated by exploitation of novel ecological niches and epiphytic growth forms (Whitman 2018). As a post-hoc analysis (see Supporting information), we found that the Rhododendron had centers of richness at some of the highest elevations (est. 2142 m a.s.l.) and a negative ERR slope (- 0.04). To further test the idea that temperate origins are affiliated with reduced ERR slopes, we ran a secondary post-hoc analysis of families not included in the main study, grouping together species from Hamamelidaceae, Juglandaceae, Magnoliaceae, and Nyssaceae (n = 63) and found that the combined families had a weak ERR slope of 0.17 (adj. R2 = 0.70,P -value < 0.001). Our findings suggest that montane-temperate association or point of origin is a stronger factor for determining ERR slopes than evolutionary age or richness. A broader question worth investigating is whether directionality of expansion (e.g. into the tropics versus out of the tropics) influences how range-sizes shift over a gradient, and whether ecological strategies also remain constant. If montane species can tolerate a wider span of abiotic conditions and habitat types, a different unknown is whether biotic competition is instead the most limiting factor for downward expansion. Rising temperatures, combined with upward migration of competition, may place future constraints on montane species’ range-sizes.