“For decades scientists have searched for a set of simple, easily measured traits that could be used to predict how plants respond to environmental change at any site around the world,” says Associate Professor Jennifer Firn of the Queensland University of Technology.
“These traits have been referred to as the ‘holy grail’, because they could serve as a standardised instrument, a biological barometer, to predict the effects of global change on the earth’s ecosystems.”
However, a unique global experiment using the infrastructure at 27 research sites across four continents, including at TERN’s Great Western Woodlands SuperSite in Western Australia, has challenged this theory by concluding that one of the most commonly used leaf traits is not an appropriate indicator of plant response to anthropogenic changes in grasslands ecosystems.
The Mt Caroline NutNet site at TERN’s Great Western Woodlands SuperSite, in Western Australia showing the control unfenced area and a fenced area where increased nutrient loading (nitrogen, phosphorus and potassium) has been applied (image courtesy of Suzanne Prober)
In the study, Jennifer and her colleagues measured how key leaf traits respond to two of the most prevalent global changes, increased nutrient loading (nitrogen, phosphorus and potassium) and altered grazing rates.
“We found that specific leaf area—a morphological trait commonly used to indicate shifts in plant growth strategy—did not respond to up to four years of soil nutrient additions,” says Jennifer.
“And, we found few significant changes in leaf traits when vertebrate herbivores were excluded in the short-term, which is contrary to expectations from plant-defence theory.”
“Our results show that changes in leaf morphological traits—in the same species or because of species turnover—do not necessarily represent a ‘common currency’ for comparing ecosystem-level responses in grasslands to anthropogenic perturbations.”
Scanned images that illustrate the diversity of leaves sampled in the study across the 27 grassland sites located in Australia, Canada, United Kingdom, United States and Switzerland (graphic courtesy of Evidently So, © 2018-19)
So, if leaf morphological plant traits like specific leaf area aren’t the holy grail of biological barometers, what are? Unfortunately, it’s not that simple and there may not actually be one ultimate barometer, says Jennifer’s fellow researcher Dr Suzanne Prober of CSIRO.
“Leaf nutrient concentrations are useful as barometers of short-term nutrient enrichment, but specific leaf area appeared not to be. Specific leaf area still has its uses, for example as a measure to distinguish plant defence-competition tradeoffs, but it seems that a complete species replacement may be needed over the longer term for it to be used to detect response to fertilisation,” says Suzanne.
This research at 27 global grassland sites, which form part of a globally replicated experiment (the Nutrient Network) being conducted at over 100 sites around the world, has indicated that the characteristics of an ecosystem, rather than traits at the individual-scale, are of greater importance.
“When it comes to dominant plant species, leaf nutrients are responsive to soil nutrient levels, even across sites characterised by very different climatic and soil conditions, and are potentially more consistent plant functional indicators than specific leaf area, particularly in the short term,” says Suzanne.
“Before trait-based ecological studies can scale the responses of leaf traits from individuals to communities and ecosystems, a more definitive understanding of when, where and how to interpret changes in plant trait values is needed. This includes how to match plant traits to appropriate environmental conditions depending on the characteristics of specific ecosystems.”
Such sentiments echo one of TERN’s founding philosophies, that science at the ecosystem scale, across disciplines and including both natural and managed landscapes, is the best way to understand and secure our ecosystems and their services in the face of current and future challenges.
Structural equation model diagram representing connections between leaf traits, experimental nutrient addition treatments, and site-level average climatic and pretreatment edaphic conditions, as well as species turnover - hover over the boxes to animate the graphic (graphic courtesy of Evidently So, © 2018-19)
Published in TERN newsletter June 2019