Root functional diversity in C3 and C4 grasslands in Hawaii

Abstract:

Root systems play an integral role in grassland ecosystem functioning due to their resource acquisition and conservation strategies. A considerable gap in our knowledge of C₃ and C₄ grasslands is our understanding of belowground root functional diversity. Our aim was to determine whether root system traits allowed for the identification of functional strategies of grass species and to see if these traits differed along resource gradients (precipitation and nitrogen). The functional root traits (specific root length, diameter, root tissue density, root length density, and % of fine roots) of nine grass species, four C₃ and five C₄, were evaluated from root samples collected from field plots at 100 -150 m intervals along an elevation gradient in Hawaii Volcanoes National Park. An analysis of variance found that there were significant differences in individual root trait values between species (P < 0.000) and photosynthetic pathway (P < 0.025). There was also evidence that the relationships between individual root traits and gradient resources were influenced by photosynthetic pathway (P and r²-values for all interactions were < 0.001 and > 0.625, respectively). A principal component analysis (PCA) found that two components accounted for 86 % of the explainable variation in our data. PCA found that C₄ species had larger root diameters, whereas, C₃ species had higher % of fine roots and specific root lengths. C₃ and C₄ species diverged less in root tissue density. C₃ species had more resource acquisition root traits, while C₄ grasses had a more conservative resource strategy. These results have important implications for how grassland ecosystem dynamics may be altered by shifting patterns of C₃-C₄ grasses with global change.