PS 5-54 - Assessing the functional significance of wood nutrient resorption in a temperate forest

Background/Question/Methods

Tree stem wood represents a major nutrient pool in forest ecosystems across the globe. However, little is known about the capacity for wood to serve as an important store of macronutrients during times of seasonal leaf flush. To investigate this, we took ~10 cm tree cores from 4 tree species (Quercus rubra, Quercus alba, Acer saccharum, and Acer rubrum) pre-leaf flush (April 2021) in 10 plots spanning a natural gradient of nitrogen (N) availability in Manistee Michigan, USA. In each plot, individual trees were fertilized with N, or a combination of all macronutrients (“All”) for >10 years, along with unfertilized (“Control”) trees. Additionally, post-leaf flush cores were collected in June 2021 for Acer species.

Wood nutrient data were used to address two hypotheses: (i) If sapwood N serves as critical store for seasonal leaf flush then there will be a seasonal decline in sapwood N following leaf flush in Acer trees, with a weaker decline for fertilized than control trees. (ii) If nutrient resorption from developing heartwood is important to support leaf flush under low nutrient availability, then differences in heartwood and sapwood nutrient concentrations will increase with decreasing soil fertility and will be higher in control than fertilized trees.

Results/Conclusions

Preliminary data pooling of all sites and treatments showed that sapwood N concentration pre-leaf flush was higher than post-leaf flush N for A. saccharum (P < 0.05) but not for A. rubrum. However, for both A. rubrum and A. saccharum, sapwood N concentration was higher in pre-leaf trees fertilized with N (P < 0.05 for both species). For all pre-leaf flush data, all species displayed significantly higher N concentration in sapwood than in heartwood when comparing average sapwood and heartwood content across treatments (P < 0.05). Furthermore, Quercus species showed higher sapwood-heartwood differences than Acer species (0.13 vs. 0.06, P = 0.01). However, contrary to our hypothesis there is no discernable effect of treatment on intraspecific sapwood-heartwood differences across all species measured.

Altogether, current data provides partial evidence that sapwood Nitrogen content does contribute to seasonal leaf-flush, however, the data shows little support that wood resorption from heartwood to sapwood is primarily driven by nutrient availability, and thus, does not appear to be an actively controlled mechanism for increasing plant nutrient use efficiency in low fertility soils in temperate forests.