Plant genetic identity influences litter structure with consequences for litter layer moisture retention
Plant litter quantity, quality, and physical structure are important because they influence plant, microbial, and invertebrate community composition and ecosystem function. The study of plant litter is often conducted in the context of the importance of plant species or functional groups influencing decomposition rates, nutrient cycling, and biota via litter chemistry. The importance of physical litter traits, especially when influenced by plant hybridization and intraspecific variation, has largely been ignored. Using an experimental garden containing trees from the Populus fremontii × P. angustifolia hybrid system, we developed a litter curling index (LCI) from 0 (completely flat) to 1 (highly curled) to quantify the physical trait of litter curling, a trait that can impact habitat complexity and arthropod abundance. We then explored the influence of the Populus hybrid system and intraspecific variation on LCI scores. Additionally, we used a greenhouse study to test the hypothesis that relatively flat litter would retain more moisture over time in its litter layer compared to more curled litter using naturally flat P. fremontii litter, naturally curled P. angustifolia litter, and artificially curled P. fremontii litter, which held all other litter characteristics, including chemistry, constant.
The study on the effects of plant hybridization and intraspecific variation on LCI scores yielded two major results. 1) The cottonwood hybrid system explained 44% of the variation in LCI scores with mean LCI scores of P. angustifolia litter almost doubled that of P. fremontii. F1 hybrid litter had intermediate LCI scores. 2) The degree in which litter was curled was significantly influenced by P. angustifolia genotype. Additionally, the greenhouse study indicated that naturally flat P. fremontii litter contained more moisture in its litter system after two weeks of drying compared to the naturally curled P. angustifolia and artificially curled P. fremontii litter. Our results indicate that the relative shape of litter (relatively flat vs. curled) is sensitive to two scales of plant genetic identity – tree genetic position along a hybrid gradient and plant genotype. Additionally, this genetically-linked trait can strongly influence moisture retention in litter systems, even when litter chemistry is held constant (i.e., naturally flat vs. artificially curled P. fremontii litter treatments). Therefore litter shape may indirectly influence litter and soil biota, decomposition rates, and nutrient cycling via its ability to regulate moisture retention in the O-horizon of soil.