Determining the effects of reduced water availability on seed germination of five bottomland hardwood tree species

Globally, floodplain forests are experiencing shifts in species composition associated with drier conditions and disruptions of flood pulse hydrology. The specific processes behind these shifts in composition are not fully understood, but differential effects of drought on regeneration processes such as seed germination may be partially responsible. To determine how reduced water availability impacts seed germination of different floodplain tree species, a lab-controlled germination experiment was conducted. Seeds from tree species common to floodplain forests of the southeastern and southcentral United States whose abundance have been altered by drier hydrogeomorphic conditions were examined. These seeds included desiccation resistant, or orthodox, seeds of sugarberry (Celtis laevigata) and green ash (Fraxinus pennsylvanica), and desiccation sensitive, or recalcitrant, acorns of overcup oak (Quercus lyrata), water oak (Quercus nigra), and willow oak (Quercus phellos). Seeds of each species were incubated with one of eight osmotically adjusted water solutions ranging in water potential from 0.0 MPa to -1.4 MPa. This reduction in water potential decreases the water available to the seeds for germination. After four weeks, seed germination of all species decreased with reduced water potential; however, desiccation tolerance did not correspond with the ability to germinate under lower water potential. Orthodox seeds only germinated in higher water potential treatments. Sugarberry reached 30 % germination in only the control 0.0 MPa treatment while green ash reached 30 % germination in treatments as low as -0.4 MPa. In contrast, recalcitrant acorns continued to germinate under lower water potentials. Water oak maintained 30 % or greater germination under all treatments and willow oak reached 30 % down to -0.8 MPa. Overcup oak was the only species to not respond to water potential treatment. With respect to maximum germination, sugarberry and green ash reached maximum germination an average of 9 days sooner than the oak species. The results of this study agree with others that demonstrate that seed germination success is sensitive to environmental water conditions and that species specific differences in germination traits are linked to broader life history strategies that are adaptive to common environmental conditions in their range.