Inferences from studies along natural elevational gradients
Despite the contrasting elevational trends along individual transects in
our study (increasing) and the general global trend (decreasing) found
by Galmán et al. (2018), our results confirm that the variation
in insect herbivory with elevation along individual transects cannot be
explained entirely by traditional climate variables (Galmán et al.
2018). Inconsistent relationships between elevation and ecological
characteristics, when comparing multiple individual gradients and
regional scale variation, have been shown for a multitude of above- and
belowground variables (Sundqvist et al. 2013, Read et al. 2014). While
adding ecological context, e.g. field-layer vegetation (Zhao et al.
2018) or grazer presence (Bernes et al. 2015, Vowles et al. 2017), may
sometimes be sufficient to explain unexpected trends, the complex
geometry of mountainous landscapes poses some challenges to the
simplistic, implicit assumption of universality in the relationship
between elevation and abiotic variables, such as temperature (Körner
2007). Further, we often implicitly assume that the driver and response
variable change at similar rates, i.e. are in a steady state, but this
is for instance rarely the case in studies of plant community
compositional responses to climate change along natural gradients
(Damgaard 2019, Hagedorn et al. 2019). Some of the discrepancies in
observed elevation patterns likely reflect difference in variables,
methods and variation in the extent to which confounding factors obscure
shifts solely related to temperature change with elevation. Our study
provides a useful test of the power of elevation gradients by recording
the same suite of variables using the same methods across multiple
gradients within the same ecosystem in a rather constrained geographical
area. While the relationship with elevation was rather consistent across
scales for some variables (leaf N content), we found considerable
variation in elevation trends in others (leaf herbivory level). Yet,
identifying useful moderator variables (e.g. solar insulation) to
account for these scale-dependent differences in trends, might be a way
to allow for quick integration of space-for-time substitution data from
different geographical contexts into ecosystem models. This could
improve understanding of important long-term responses to climate change
(Dunne et al. 2004; Elmendorf et al. 2015), until sufficient
understanding of the underlying mechanisms emerge. Thus, to understand
the effect of elevation at a broader scale, we need both local
elevational gradient data and regional moderator variables. For example,
the contrasting relationship between elevation and BIH in the Subarctic,
shown in our study, and the trend at lower latitudes, may be
incorporated into models by using latitude as a moderator variable. This
approach needs further confirmation, but we show that setting up
multiple elevation gradients in the same biome within a rather
constrained area yield useful data for such assessments.