On the relevance of stemflow: An argument against funneling ratios and
for a return to scaled flux-per-unit-area metrics.
Abstract
From inside the stemflow research community, the past decade’s progress
might look great: 1) the number of papers published on stemflow per year
has doubled; 2) citations of stemflow publications have more than
doubled; and 3) the number of research sites monitoring stemflow is on
the rise. However, from a broader perspective, a brief Web of Science
bibliometric analysis of the past decade reveals issues with these
trends: 1) annual publication numbers have increased year-to-year for
most topics in natural science, but stemflow publication trends are
lower than related and broader disciplines; 2) self-citation is
significantly higher for stemflow research than other disciplines (e.g.,
26% compared to 2% for all hydrology); and, most importantly, 3) we
may have more stemflow data, but we still lack a clear understanding of
stemflow’s mechanistic importance. This creates ambiguities as to
whether and how stemflow processes can be incorporated into hydrological
models and concepts. In this presentation, I argue that we should
progress from using metrics that are exclusively used by those who work
on stemflow (e.g., unitless metrics such as funneling and enrichment
ratios) towards using scaled flux-per-unit-area metrics that may support
better integration into hydrological and ecological models (e.g., water
or chemical yield per unit canopy area). While the magnitudes of
funneling and enrichment ratios from individual plants have effectively
conveyed to broader audiences the possibility for stemflow to play
important roles in ecosystem functioning, I argue that we need to now
move onto mechanistic investigations of stemflow’s impact on specific
processes at ecohydrologically relevant scales. Dimensionless (often
individual plant-scale) funneling-type metrics may not be useful in this
regard, as they tell us nothing about where stemflow goes or what roles
stemflow may play in the ecosystem. They also rely on an estimate of
infiltration area, which has rarely been observed to date. Their use is
further limited to falling liquid-phase rain, which prevents comparison
of stemflow observations/processes under occult precipitation (fog,
condensation) or mixed and solid-phase precipitation (snow, rime, etc).
Please view the “Make Stemflow Unit-ed Again” companion video on
YouTube: https://youtu.be/4vPk9m45V0c