In savanna ecosystems, first-year tree seedlings experience strong competition for both light and moisture near the soil surface due to seasonal rainfall patterns as well as competition with the grass canopy. Theory suggests that plant species experiencing highly dynamic resource availability should utilize plasticity in ecophysiological traits, e.g. adjustments that optimize light interception. Studying how first-year tree seedlings respond to multiple resource limitations is key to understanding how life history shapes species’ current distributions.
Under field conditions in Serengeti National Park, Tanzania, we measured soil moisture and light levels associated with seedlings of two dominant east African tree species. Acacia (=Vachellia) robusta is distributed in mesic, productive areas with a tall grass canopy, and A. tortilis occurs in more arid areas where grass cover is highly variable. Then, under glasshouse conditions, we grew both species under combinations of drought and sun exposure. We calculated the silhouette:projected leaf area (SPAR) to assess architectural adjustments, and we used the ratio of height:leaf biomass (HLM) to evaluate resource allocation. The light response of photosynthesis (Amax and Asat) was measured with a Li-6400. At the end of the experiment, water was withheld and we calculated declines in whole-plant hydraulic conductance (Kplant).
Results/Conclusions
At the whole-plant architecture level, SPAR was highly plastic in A. tortilis in response to light level; greater sun exposure caused more self-shading, which is an adaptation that minimizes exposure to full sunlight. Decreasing sun exposure caused A. robusta to shift allocation towards height instead of leaves (HLM) – a strategy for escaping the taller grass canopy it experiences. At the leaf level, A. tortilis also showed more plasticity in the light response of photosynthesis. In response to sun exposure treatments, Amax and Asat were highly plastic in A. tortilis but not in A. robusta. Under well-watered conditions (Ψsoil > -0.1 MPa), Kplant was 4.84±1.3 and 4.82±0.63 mmol H20 m-2 s-1 MPa-1 for A. robusta and A. tortilis, respectively. But during extreme drought (Ψsoil < -2.0 MPa), Kplant declined significantly more in A. robusta than A. tortilis (1.30±0.16 and 3.34±0.39, respectively). Our study demonstrates that these species have contrasting ecophysiological strategies for coping with the unique microclimatic conditions and subsequent differences in resource availability that they experience in the field, and provides insight about the role of plasticity in successful tree seedling establishment in savannas.