Ecosystem consequences of plant colonization as an evolutionary force

Background/Question/Methods

As species ranges shift due to natural and anthropogenic causes, plants encounter novel biotic and abiotic conditions that often leads to divergence in plant traits. Genetic change in a population can have community and ecosystem level consequences. To examine if colonization can lead to genetically differentiated populations, we used a series of ~3000 year old forest fragments of Metrosideros polymorpha (Myrtaceae) which are actively colonizing a 158 year old lava flow on the Island of Hawai’i. To determine if the variation in traits in trees between forest fragments (kīpuka) and the surrounding lava matrix (colonizers) are due to selection, cuttings from the field were grown in a common environment for two years to determine the genetic basis of traits. We hypothesized that colonization can lead to evolution on the landscape, selecting for individuals that allocate less into shoot biomass production and have higher leaf pubescence in a common garden. To determine if this variation in plant traits alters ecosystem function, we analyzed field soils for total carbon and nitrogen pools. Further, we measured microbial activity using lab incubations of CO₂ and N mineralization. Finally, we measured soil enzyme activity for β-1,4-glucosidase, α-1,4-glucosidase, β-1,4-N acetylglucosaminidase, acid phosphatase, phenol oxidase and peroxidases.

Results/Conclusions

Significant variations in tree height and leaf pubescence observed in the field were mirrored with observations in the common garden. Individuals from the newly colonized matrix, on average, allocate 46.9% less into shoot biomass production and have 17.3% higher leaf pubescence when grown in a common garden. Field soils were different, with soils collected under trees from the kīpuka having a 7.2% lower pH, a higher total soil N Δ18.2%, and higher total soil C Δ4.0%. Kīpuka soils were also 10.8% slower at mineralizing carbon in the lab. The enzyme activity of phosphatase when standardized by total soil carbon was 47.3% greater in soils collected from under lava matrix plants. Our common garden observations were found to significantly correlate with changes to soil N and soil pH. Further, we were able to correlate tree height to changes in soil carbon cycling and soil phosphatase activities. Our data suggest that divergence can occur on contemporary time scales during colonization events with consequences for soil processes.