Tissue chemistry of biocrust species along an aridity gradient and comparison to vascular plant leaves

1. Biocrusts are an extensive autotrophic groundcover but are regularly excluded from ecosystem models. One rationale for their exclusion is that their contributions to C and N cycles are negligible in comparison to vascular plants. Here we estimated the magnitude of the biocrust contribution to nutrient cycling in functional terms, comparing tissue chemistry and stoichiometry per ground area with vascular plants per leaf area. 

2. We quantified tissue C, N, P and a range of micronutrients in 21 abundant biocrust taxa from an aridity gradient spanning 500 km of Australian drylands. 

3. The C, N and P tissue chemistry of biocrust species and vascular plant leaves occupied a similar range, both as ratios and per area. This implies that in functional terms, biocrusts could be considered as a single layer of leaf area spread out across the ground and added to Leaf Area Index (LAI) measurements accordingly. 

4. Within biocrust species, the C:N ratio significantly declined with increasing aridity, which was entirely driven by carbon. This finding demonstrates a key functional difference between vascular leaves and biocrusts: C:N ratios in vascular plants are lower in drier environments due to elevated levels of N-rich photosynthetic machinery relating to a trade-off in water-use regulation, while C:N ratios in biocrusts decline with aridity most likely due to carbon shortages relating to their inability to regular water loss (poikilohydry). 

5. As global drylands become drier with climate change, our findings suggest that biocrust tissue carbon stocks will decline gradually until the point where carbon budgets become negative and standing biomass is no longer viable. 

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