Editor's Choice

September 2011 (99:5)

The importance of biotic factors as regulators of ecosystem processes, such as decomposition and nutrient cycling, has become recognised only relatively recently. One possible reason for this is that community and ecosystem ecology have, to some extent, developed independently of each other: while community ecology has traditionally focused on understanding how communities are structured by environmental and biotic factors, ecosystem ecology has concentrated on understanding controls on the flow of energy and nutrients within ecosystems. However, this has changed in recent decades, and there is now widespread recognition that biotic factors play a key role in regulating ecosystem processes.

The impact of plant communities on ecosystem processes
The study by Daniel Laughlin (Nitrification is linked to dominant leaf traits rather than functional diversity) is broadly concerned with understanding the mechanisms by which biotic factors regulate ecosystem processes that occur in soil. Specifically, he tackled two contrasting hypotheses that have been put forward to explain how plant communities impact on ecosystem processes. These are: (1) the ‘diversity hypothesis’, which proposes that the diversity of functional types of species affects ecosystem functioning through complementary use of resources, or via a greater array of chemical compounds involved in nutrient cycles; and (2) the ‘mass ratio’ hypothesis, which proposes that species effects on ecosystem processes are in proportion to their relative input to primary production. Here, Laughlin simultaneously evaluated, using structural equation modeling, each of these hypotheses in the context of nitrogen (N) cycling - and especially the microbial process of nitrification - in a ponderosa pine forest in northern Arizona, USA.

Testing the hypotheses
To do this, Laughlin collected data on plant traits and soil properties from a series of plots across the study area. A range of functional traits of the understory herbaceous plant species present were measured, including shoot leaf area (SLA), leaf dry matter content (LDMC), leaf N content, fine root [N], seed mass, specific root length (SRL), canopy height and mean flowering date. To test for the mass ratio hypothesis, these traits of individual species were scaled to the community level by calculating ‘community-weighted mean’ leaf trait axis scores, which are community trait means weighted by the relative abundance of each species. To test the functional diversity hypothesis, a functional diversity index (i.e. quadratic entropy’ (FDQ)) was calculated, which is the sum of the distances between pairs of species in trait space, weighted by the product of their relative abundances. It was then assumed by Laughlin that the mass ratio hypothesis was supported if the community-weighted mean leaf trait axis was significantly related to nitrification, after statistically controlling for the role of abiotic soil factors. Whereas, the diversity hypothesis was supported if functional diversity was found to be a significant predictor of nitrification after accounting for abiotic soil factors.

What Laughlin found was that soil nitrification, a key process in the N cycle, was more strongly linked to dominant leaf traits than to functional diversity, thereby lending more support to the mass ratio hypothesis of Grime (1998). This suggests that herbaceous plant species’ controls on soil nitrification in ponderosa pine forest are in proportion to their relative input to total abundance, and that the diversity of leaf litter quality inputs to soil may not be so important for this aspect of the N cycle. In particular, Laughlin found that the dominance of fast-growing plant species with high leaf N and root N, high SLA and low LDMC are linked with soils of high nitrification potential, this supports previous studies in grassland ecosystems which find positive relationships between plant tissue N and both soil nitrate and net nitrification (Orwin et al. 2010). Although there was no support for a relationship between functional diversity and nitrification, plant functional diversity was positively associated with total herbaceous biomass. As noted by Laughlin, this indicates that there might be positive diversity benefits to community productivity in ponderosa pine forest. Such a response might be due to complementary use of resources (Hopper et al. 2005). But, as highlighted by Laughlin, the possibility of a sampling effect cannot be ruled out (Huston 1997).

Scaling up from plants to ecosystems
This study provides strong evidence that the functional traits of dominant plant species play a key role in driving ecosystem processes, in this case nitrification, and that their influence in proportion to their relative input to primary production. Moreover, the findings add weight to the growing view that that leaf traits of dominant species, and community-weighted trait values, can serve as effective predictors of ecosystem processes (e.g., Garnier et al. 2004, Fortunel et al. 2009). There will of course be situations where this doesn’t apply, such as when sub-dominant species have disproportionate effects on ecosystem processes relative to their contribution to total community biomass (e.g. Wardle and Zackrisson 2005; Peltzer et al. 2009) or when species have strong interactive (non-additive) effects on the ecosystem process (e.g. Ball et al. 2008). However, the results of Laughlin’s study add weight to the growing view that it is possible, in some situations, to scale up from traits of individual plants to process rates in soil at the whole ecosystem-level scale.

Richard Bardgett
Editor, Journal of Ecology


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