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.