Scanned images that illustrate the diversity of leaves sampled in this study from 27 grassland sites. These very leaves were collected from sites located in Australia, Canada, United Kingdom, United States and Switzerland and brought together in this collage.
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. These traits have been referred to as the ‘holy grail’ because they could serve as a standardized instrument, a biological barometer, to predict the effects of global change on the earth’s ecosystems.
In a recent Nature Ecology and Evolution paper, co-authored by RCAAE member Dr John Morgan and including data from the Australian Bogong alpine grassland site, we measured how some of these commonly used leaf traits respond to two of the most prevalent global changes, increased nutrient loading and altered grazing rates, across a set of 27 grasslands sites in four countries. These sites are part of a globally replicated experiment, the Nutrient Network, being conducted at over 100 sites around the world.
Leaf nutrient concentrations (i.e. percent nitrogen phosphorus and potassium) increased in fertilized plots, which is consistent with ecological theory. However, this result is at odds with a well-known theory in agriculture called the growth-dilution effect which predicts the increased growth of the plants will outpace nutrient accumulation in the tissue. Contrary to existing ecological theory and expectations from physiological ecology, specific leaf area (SLA), perhaps the most widely-used leaf trait, did not show a consistent response to nutrient addition. We found no consistent changes in any of the leaf traits, which is contrary to expectations from plant-defense theory.
Why are leaf traits so important to plant ecologists?
The environment in which plants grow and the organisms that eat them can sculpt their features or ‘traits’ including the area, weight, and thickness of their leaves, the number of seeds they produce, the height they reach, and the amount and type of roots they grow. These traits, in turn, can determine the effectiveness of each species – and the combination of species in a location – in capturing energy from the sun or nutrients from the soil.
Ecologists use changes in leaf traits to compare plant growth strategies and subsequently infer how ecosystems function. Despite the high diversity of species globally, it is reasonable to expect that there may be consistent trait responses because all plants have much in common including their reliance on three essential resources, light, water and elemental nutrients, which sustain common functions of growth, reproduction, defense, and storage.
Plant ecologists use traits to discover commonalities, but leaf traits are also used more practically to understand the impacts of disturbance and for rebuilding plant communities in restoration efforts. The practical application of leaf traits to infer ‘function’ has been ongoing for decades without a global experimental test of whether leaf traits actually respond in a predictable way to short-term perturbations. Our experimental test helps to isolate cause from correlation in the relationship between plant function and plant traits. Leaf nutrient concentrations are useful as barometers of short-term nutrient enrichment, but not SLA. SLA still has its uses, a measure commonly used to distinguish plant defense-competition tradeoffs. It might be that SLA is less plastic and thus a complete species replacement is needed over the longer term in response to the treatments to detect a change.
The full paper led by Jenn Firn, Leaf nutrients, not specific leaf area, are consistent indicators of elevated nutrient inputs, can be found at http://dx.doi.org/10.1038/s41559-018-0790-1