Nutrient use efficiency (NUE) is defined as the rate of plant productivity per unit of nutrient taken up from soil. NUE couples important elemental cycles such as carbon, nitrogen and phosphorus, and the scale of inquiry ranges from individual organs to entire biomes. Reproduction differs from vegetative production in that the plant strategy is to release carbon and nutrients in propagules during dispersal, which is a strategy that may not promote high NUE. The extent to which reproduction influences NUE across biological scales has yet to be explored despite studies pointing to its importance. For example, photosynthesis in leaves surrounding reproductive tissues is commonly enhanced due to reproductive tissues’ high sink strength, and seed dispersal necessarily results in nutrient loses from plants.
To investigate how reproduction influences NUE at the global scale we analyzed a database of biomass and nutrient contents (N and P) for the litterfall classes of leaves, woody debris, and reproductive tissues. Specifically, we addressed three questions. First, are there systematic differences in biomass and nutrient contents among the three litterfall classes? Second, are there climatic influences on the differences at the global scale? Third, is there a way to quantify how reproductive output influences NUE?
Results/Conclusions
We found that P concentrations in reproductive litter were ~1.5x higher than leaf litter and ~2.3x higher than woody debris (F2,248 = 21.93; p<0.001). Similar, though non-significant, trends between N concentrations of different litter classes were observed (F2,314= 2.62; p=0.075). We did not find a significant correlation between mean annual temperature (MAT) or mean annual precipitation (MAP) and the concentrations of N and P in any litterfall component. We did however find that both leaf litterfall and woody debris biomass production correlate positively with MAT (r=0.56; p<0.001 and r=0.36; p<0.001) and MAP (r=0.49; p<0.001 and r=0.28; p<0.001). Reproductive litterfall biomass was not correlated to MAT or MAP, suggesting reproductive growth is not sensitive to major climate gradients, unlike vegetative growth.
The high P cost of reproduction may explain the lack of correlation between reproductive litterfall biomass and MAT or MAP because P limitation may constrain reproductive output. Because the biomass of reproductive litter appears to respond differently to climate gradients and display elevated P contents relative to vegetative litter, we suggest that steps should be taken to outline and test drivers of how, when, and where reproductive allocation influences NUE.