As drought becomes more frequent, it is important to understand how trees may utilize different strategies to mitigate drought stress. While rooting depth and stomatal controls are well-recognized as important factors constraining plant responses to drought, adaptations of fine-roots in shallow soils may also enable greater exploitation of available soil water, particularly in response to mild drought. However, sap flow measurements commonly used to compare drought sensitivities of different trees are difficult to link belowground traits on a species-specific basis due to the intermingled nature of fine roots in mixed communities. In this study, we monitored sap flow reductions of 7 diverse temperate tree species in response to a 20-day drought period and linked these with measurements of specific root length (SRL, m g-1), standing biomass, and length of absorptive roots. Measurements were made in mature, of mono-dominant stands at The Morton Arboretum (Lisle, IL) allowing us to link aboveground responses to belowground investments on a species-specific basis. We hypothesized that species with higher SRL would show less sensitivity to mild drought. We further hypothesized that increasing fine-root biomass and length would be negatively related to drought sensitivity. Here, sensitivity is defined by reductions in maximum daily sap flow rate.
Results/Conclusions
Among the 7 species, reductions in maximum daily sap flow rate ranged from 11.2 to 74.1%, SRL of pooled absorptive roots ranged from 6.8 to 17.2 m g-1, while estimates of standing biomass and length ranged from 36 to 280 g m-2 and 634 to 2575 m m-2, respectively. Consistent with our first hypothesis, higher absorptive root SRL was related to lower reductions in sap flow (p<0.01, r2=0.72). This suggests that the greater exploration of soil volumes by high SRL species may enable them to acquire more soil water during surface soil drying associated with mild drought. Contrary to our second hypothesis, our results indicated no significant relationships between standing root length or standing biomass (p>0.05, all cases) to tree sensitivity (p>0.10). These results were consistent even after normalizing standing length and biomass based on stand basal area and stem density (p>0.10, all cases). Moreover, though not significant, the general trends indicated that greater root biomass was associated with greater sensitivity, perhaps due to greater rates of soil water depletion. Together, these results suggest that in addition to rooting depth, wood anatomy, and stomatal controls, fine-root strategies in shallow soils may impact whole-tree performance in response to mild summer drought.