The combined effect of light availability and drought on survival of two forest understory herb species

Dense forest canopy influences understory microclimate by lowering the air and surface temperatures. It can mitigate the adverse effects of global climate change in forest communities. On the other hand, low light availability in dense forests is the main constraint for the understory species photosynthesis. And the resulting lack of carbohydrates may make herbs more vulnerable to increasing drought induced by climate change. Here we assessed the survival and ecophysiology of two morphologically similar but physiologically rather contrasting perennial herbs typical of temperate forest understories: isohydric Asarum europaeum and relatively anisohydric Hepatica nobilis. We emulated a summer drought period by growing adult plants of these two species in the greenhouse under 10% (simulating sparse forest) and 1% (simulating dense forest canopy) of outdoor light. Half of the individuals were subjected to water stress by withholding the watering until ca 50% mortality occurred. After the drought phase, we fully watered the plants and let them recover for one month.

The lowest predawn water potential measured during the experiment was -3.6 MPa, and the midday water potentials of plants in the sun were -5.4 and -8.1 MPa for Asarum and Hepatica, respectively. Light saturated photosynthesis (A_sat) of fully watered herbs was by more than 50% higher in the Asarum than in Hepatica at the end of the experiment. The two herbs plastically adjusted their A_sat to the light environment, so that A_sat of shaded Hepatica was by 38% and of shaded Asarum by 29% lower than in controls. After the period of drought, A_sat of stressed plants of both species grown in the lighter conditions fully recovered. In the shaded variant, however, only A_sat of Hepatica recovered but that of Asarum did not. Intrinsic water use efficiency (WUE_i) was higher in isohydric Asarum than in Hepatica. WUE_i was also higher in the herbs grown in light than in the shade. Specific dyeing to functional xylem indicated a higher proportion of conductive xylem (which means less damage by embolism) in plants grown in shade than in the light, in both species. To sum up, sufficient light may help some understory species to recover from severe water stress.