Do perennial bioenergy crops offer greater resistance to vapor pressure deficit stress than annuals?

Bioenergy from biofuel crops will be an important tool for meeting global renewable energy demands and evolving energy mandates. As ongoing research demonstrates the environmental costs of traditional bioenergy candidates (e.g., maize bioethanol), novel perennial crops have emerged as enticing alternatives. Perennial crops offer numerous benefits over annuals, including more efficient nutrient use and enhanced soil carbon storage. Moreover, the robust rooting structures of perennials are also better equipped to extract water from deeper soils profiles, thereby buffering carbon sequestration potential and leading to more resilient yields under extreme growth conditions including heat and drought.

Though perennial bioenergy feedstocks are valued for their enhanced productivity and stress tolerance, the extent that these benefits will persist in a warming climate is uncertain. Rising temperatures are shifting hydroclimates across United States agricultural lands, uniformly intensifying vapor pressure deficit (VPD), whereas precipitation and soil moisture responses are varying regionally. While the rooting profiles of perennials confer greater advantages over annuals during soil water dry-down, the degree by which these respective life-history strategies confer differential success when faced with elevated VPD remains unresolved.

To shed light on these uncertainties, we perform a statistical decomposition of ecosystem carbon flux observations among co-located annual (e.g., maize) and perennial (e.g., miscanthus & switchgrass) bioenergy crops to evaluate the relative limitations imposed by soil water vs. VPD stress. Preliminary results suggest that gross primary productivity of annual systems is more sensitive to both soil water and VPD stress than their perennial counterparts. However, the greatest productivity declines imposed by elevated VPD on all systems occurred when soil water availability was most abundant. Collectively, these findings highlight that the enhanced carbon mitigation potential of perennial systems will persist in a future characterized by shifting drought regimes and increasingly high VPD.