Carbohydrate Depletion and Vulnerability to Embolism in Ponderosa Pine

Tree drought-induced mortality is increasingly leading to forest loss across the western U.S. as the severity of drought intensifies under global climate change. Under drought, the water column in the xylem – the tree’s water transport system – encounters increasing tension, potentially leading to embolism and interruption of water transport. Repeated embolism can ultimately lead to complete hydraulic failure and eventual drought-induced mortality. Studies in poplar have shown that depletion of stored non-structural carbohydrates (NSC) – which is common under drought – increases their vulnerability to xylem embolism. However, no studies have explored the link between NSCs and xylem vulnerability to embolism in conifer species in naturally occurring field populations. In this study, we tested whether NSC depletion influences vulnerability to embolism in naturally occurring Pinus ponderosa saplings. This study is part of an ongoing experiment to explore the consequences of NSC depletion on ponderosa pine water relations. Specifically, NSC pools were manipulated through experimental shading and drought treatments leading to four treatments: unshaded-ambient water; shaded-ambient water; unshaded-drought; shaded-drought. Embolism vulnerability curves were generated on branches from each treatment using the air injection method: stems were subjected to progressively increasing tensions to induce embolism, and at each tension we measured the hydraulic conductivity. NSC concentration chemical analyses used the enzymatic method. Drought and shade treatments were effective in reducing soil moisture and light availability, respectively. However, we found no evidence of differences in vulnerability to embolism among drought and shade treatments, suggesting that potential treatment-induced NSC depletion does not affect vulnerability to embolism in Pinus ponderosa. This study adds to the knowledge about the role of NSCs in drought responses, thus providing insights to advance modeling and predictions of drought-induced mortality risk in ponderosa pine forests.