The complexity of temporal distribution of rainfall, plant allocation, and carbon sinks

Background/Question/Methods

Climate models predict and observations show that rainfall regimes, in total amount and temporal distribution, are changing significantly in many areas across the globe.  Together with changes in atmospheric concentration of CO2, temperature, and humidity, these changes are creating novel environments of water availability.  We expect these changes to influence productivity of plants as well as their allocation patterns - allocation to basic plant components such as leaves, woody biomass, and fine-roots.  Because of the differential residence time and decomposition rates of these components, allocation patterns can have a significant effect on carbon storage.  The responses give natural landscapes the potential to dampen or accelerate the rate of increasing concentration of CO2 in the atmosphere.  However changing allocation patterns may be complex and depend on mechanisms that are not yet incorporated in global predictions.

Here we bring together a model of competition for water and light within a forest stand (Farrior et al. 2013) with a model of stochastic rainfall influences on soil moisture (Rodriguez-Iturbe et al. 1999) to investigate and understand the influence of differences in rainfall regimes on plant allocation and carbon storage. 

Results/Conclusions

We have found that rainfall patterns influence plant allocation and forest carbon storage in complex ways. On average, increasing rainfall decreases allocation to fine roots and increases allocation to woody biomass and leaves.  However, the influence of timing of that rainfall can be significant.  At low rainfall increasing the frequency of storms, decreases the probability of storms with large amounts of rainfall, decreasing the time plants spend in water saturation.  This has the effect of decreasing allocation to woody biomass and increasing allocation to fine roots. At high rainfall this pattern reverses.  Sites where storms are more frequent have higher water availability than sites where storms are less frequent.  Increasing the frequency of storms decreases the amount of rainfall per storm and decreases the frequency of runoff events.  This has the effect of increasing allocation to woody biomass.

Changes in rainfall and the timing of its distribution also have significant influences on carbon sinks.  Our model predicts carbon sinks should be the highest at relatively high rainfall levels and when that rainfall arrives as frequent, small storms.  Under these conditions, plants experience the greatest increases in time in water-saturation (caused by enhanced water-use efficiency) and the benefits of increased photosynthetic efficiency during water saturation are the greatest.