The capability of trees to respond to changing environmental conditions could contribute to forest diversity.  For example, both spatial and temporal variation in factors influencing water loss could favor one species over another.  Ecophysiological response to growing conditions, such as vapor pressure deficit (VPD), photosynthetically active radiation (PAR), and soil moisture, could be useful in explaining species coexistence.  In this research, we developed a state-space model relating sap flux, measured with thermal dissipation probes, to stand-level conductance and transpiration for four deciduous tree species in a North Carolina mixed-hardwood forest: Carya tomentosa, Liquidambar styraciflua, Liriodendron tulipifera, and Quercus alba.  Up to four trees for each species were instrumented to measure sap flux in the outer xylem.  Soil moisture in the upper 20 cm was measured in the vicinity of the trees while PAR and VPD were measured above the canopy.  We expected that species differences with regards to conductance and transpiration would be consistent with a trade-off between maximizing conductance and minimizing sensitivity to drought.  We also expected that this trade-off would be complicated by species specific light and soil moisture responses. 

Results/Conclusions

In preliminary analyses, reference conductance (conductance at 1kPa VPD and non-limiting PAR) varied greatly among species, with L. styraciflua (130 mmol m^-2 s^-1) being greatest, C. tomentosa (88 mmol m^-2 s^-1) and L.tulipifera (58 mmol m^-2 s^-1) being intermediate, and Q. alba being low (27 mmol m^-2 s^-1).  For all species, the sensitivity to VPD (rate of decline in conductance with ln[VPD]), was similar to values reported in the literature (~0.6 of the reference conductance).  All species other than Q. alba experience some light limitation of canopy averaged conductance at 1500 μmol m^-2 s^-1 incident PAR.  Soil moisture had little effect on conductance, though the full range of soil moistures observed in the field did not occur during the study period.  Of these species, Q. alba dominates dry, upland forests, whereas moist lowland sites tend to be more diverse.  This could be due to low sensitivity of Q. alba to drought, allowing it to maintain steady rates of transpiration and low rates of water loss.  On wetter sites, faster growing species with higher conductance may more effectively compete for space, light, and moisture.