Lowland, consistently tropical, families
For lowland tropical flora, a notable family is Dipterocarpaceae with high endemism to Sundaland and one of the steepest positive ERR slopes (0.53), which we have labeled as “1” on all scatterplots. Dipterocarpaceae are described as having one-seeded fruits with reduced dormancy ability (Ashton 1988), with limited dispersal because of reduced wind flow beneath the forest canopy. The migration ability of Dipterocarpaceae is further limited by intermittent masting, narrow elevational distributions, and exclusive occurrence within mature forests, often with specialized soil associations (Ashton 1988, Davies et al. 2005). The genus Shorea (Dipterocarpaceae) is sometimes compared to Syzgium (Myrtaceae) based on similarities in intraspecific morphological variation and endemism (Ashton and Lee 2022) and both taxonomic groups having peaks in richness within tropical lowland habitats (< 400 m). However, Myrtaceae has higher endemism within Sahul relative to Sundaland, and Syzgium species are regarded as well-dispersed via animals (Ashton and Lee 2022). As a post-hoc analysis of this study (see Supporting information), we found that Shorea has an ERR slope of 0.54 whereas Syzygium had 0.37, and Myrtaceae as a whole 0.38 Despite the tropical lowland affiliation of Myrtaceae, the family has broader latitudinal occurrence, extending southward into temperate latitudes with seasonality (Hawkins et al. 2011).
Our study reinforces the concept that taxa occupying environments with prolonged periods of climate stability are associated with steeper positive ERR slopes. Under a climate change scenario where temperature increases, lowland tropical species have the benefit of being able to shift upwards in elevation. However, migration to maintain a narrow temperature window may be futile if these species lack resilience to increasing variability of habitat conditions, or if they rely on specific secondary factors such as edaphic conditions or essential services provided by another species with limited geographic span. A potential practical application of this finding is that ERR slopes may provide a broad-scale quantitative metric of a taxonomic groups’ potential response to climate change, which could be used to focus limited resources for more intensive investigations on particularly vulnerable species.