Elastic adjustment in a drought-tolerant boreal conifer compared to a broadleaved pioneer

Drought limitation is an increasing concern in boreal forests due to climate change. Explaining the mechanisms behind drought-induced growth limitations and tree mortality has thus become an important topic in plant ecophysiology. Research on drought damage in trees often has a focus on xylem embolism resistance, though drought acclimation by osmotic, apoplastic and elastic adjustment is also important at explaining species-specific differences in drought tolerance. Elastic adjustment has not much been examined in boreal forests in the field, although laboratory studies with seedlings of boreal conifers suggested that elastic adjustment could be significant. Elastic adjustment means the stiffening of cell walls causing reduced cell wall elasticity, which allows the rapid lowering of plant water potentials and thereby ensures continued water uptake from drying soil. Therefore, cell wall elasticity, osmotic potentials and other parameters quantifying tree water relations were analyzed in two subsequent growing seasons in a naturally drought-limited southern boreal forest in Mongolia. These analyses included the highly drought-tolerant late-successional Larix sibirica and the early-successional Betula platyphylla and covered a drought year and a subsequent moist year. L. sibirica acclimated to the dry conditions of the first year by reducing cell wall elasticity and by osmotic adjustment. Cell wall elasticity in L. sibirica decreased with increasing tissue dehydration in the dry year, supporting the reduction of water potentials and thus hydration during drought. By contrast, cell walls in B. platyphylla became more elastic with progressive dehydration, which hampers water uptake under drought. However, L. sibirica lost its ability to reduce cell wall elasticity at strong tissue dehydration in the year after the drought, perhaps increasing the vulnerability of L. sibirica to consecutive droughts. The results suggest that elastic adjustment contributes to the generation of drought tolerance in L. sibirica , but to a lesser degree in B. platyphylla.