Hydrologic gradients play an organizational role in the community assembly of terrestrial plants. Although both flooding and drought can drive species distribution patterns along hydrologic gradients, less is known about the traits promoting plant performance under flooding than those promoting drought tolerance.
We performed a greenhouse experiment to examine the response to flooding of seven willow species with contrasting distributions along hydrologic gradients. Cuttings of the species were partially submerged for six weeks and then allowed to recover for another six weeks. We monitored growth, gas-exchange, leaf water potential, and measured two traits that we hypothesized could enhance flooding, lenticel density and adventitious roots. We also harvested a subset of experimental plants of four species to measure biomass allocation. We aimed to address the following questions:
i. Did the flooding treatment affect growth, biomass allocation, gas-exchange, and leaf water potential of the species?
ii. Did lenticel density and biomass allocation to adventitious roots predict the responses of species to the flooding treatment?
iii. Was species’ response of growth to the flooding treatment correlate with species’ hydrologic niche?
Results/Conclusions
Preliminary analyses yielded three conclusions. First, plants received the flooding treatment showed greater stem growth (measured by basal area and height) than those received the control treatment. Furthermore, there was a significant species by treatment interaction effect on basal area growth during the recovery but not the flooding period. In contrast, total leaf area, gas-exchange rates, and pre-dawn leaf water potential did not show any significant difference between treatments. The four harvested species showed higher total biomass and shoots to roots ratio under the flooding than the control treatment. Together, these results suggest the flooding treatment improved growth of the willow species by enhancing stem growth and increasing shoots to roots ratio.
Second, species were significantly different in their lenticel densities and in the percentage of biomass allocated to adventitious roots. However, variation in the two traits among species was not correlated with the variation in species’ response of basal area growth to the flooding treatment.
Finally, species' response of basal area growth to the flooding treatment was not correlated with species’ natural distribution along hydrologic gradients. Instead, species’ hydrologic niche was significantly correlated with relative stem growth rate. A fast stem growth rate may promote survival in wetlands by allowing plants to avoid submergence of shoots.