Climate change is projected to increase the frequency and severity of droughts in many regions; however, there is not yet a consensus on how forests will respond in part because we lack information about how hydraulic traits change as trees age. In this study, we aimed to determine if and how hydraulic traits differ with age in coast redwoods (Sequoia sempervirens) growing in a secondary forest. The following research questions were addressed: Is there a difference in hydraulic traits, dawn and midday leaf water potential (Yleaf) and leaf turgor loss point (TLP), between coast redwood age groups? Do hydraulic traits differ in coast redwood juveniles growing under nursery trees compared to those growing in more open locations? Using diameter at breast height (DBH) as a proxy for age we separated coast redwoods within Redwood Regional Park in Oakland, California into 5 DBH size classes: 3-13cm, 21-34cm, 76-86cm, 108-115cm, and 140-165cm. To measure the nursery-tree effect on the hydraulic traits of recruits we had two groups within the smallest size class: one with juveniles located within ~0.3m of a more mature coast redwood and another with juveniles growing >1.7m from the nearest mature tree.
Results/Conclusions
We found that dawn Yleaf had no trend across size-classes and ranged from -5.71±0.01 MPa to -6.30±0.01 MPa. In contrast, both midday Yleaf and TLP decreased from -0.94±0.01 to -1.47±0.02 MPa and -1.78±0.01 to -2.14±0.02 MPa, respectively, over the total increase in size class. Additionally, the juvenile trees growing in close proximity to a mature tree had considerable higher midday Yleaf and TLP (-0.94±0.001 and -1.78±0.01 MPa, respectively) compared to the freestanding juvenile trees (-1.69 ±0.09 and -2.06±0.01 MPa, respectively). This indicated that the juveniles growing in close proximity to a more mature tree benefit from a nursery effect. These results provide a physiological basis for explaining how larger, and therefore older, trees within a forest canopy tolerate higher levels of water-stress when compared to subdominant and understory trees. This study also provides a basis for parameterizing ontogenetic plasticity of plant hydraulic traits in dynamic vegetation models, which will improve their predictions of ecosystem responses to climate change.