Using carbon, nitrogen, and oxygen isotopes to understand why trees respond to forest management
Management activities such as thinning, prescribed burning, or fertilization change resource availability within stands and alter tree growth. However, because these activities often change the availability of moisture, nutrients, and light simultaneously, the physiological or ecological mechanisms responsible for changes in tree growth can be unclear. Within the forestry research community, there has been expanding interest in understanding how management activities affect both resource availability and tree physiology so that more mechanistic prescriptions can be developed to meet objectives such as the optimization of forest productivity or adaptation to climate change. Stable isotopes have become widely used in studies of forest hydrology, tree physiology, and biogeochemistry because they can integrate and trace physical and physiological processes over time and space. More recently, stable isotopes have been adopted to understand how and why trees respond forest management activities around the world. To better understand physiological responses to forest management, we conducted a meta-analysis of the use of the stable isotopes 13C, 18O, and 15N in forestry research.
We found an exponential increase (r2 = 0.86) in the number of studies using stable isotopes for forestry research and identified more than 75 articles describing a wide range of forestry practices, including irrigation, afforestation techniques, residue removal, and prescribed fire. However, most of these articles have focused on fertilization (27 studies), thinning (30 studies), or competition control through the application of herbicides or mechanical treatments (14 studies). Fertilization increased δ13C and δ15N, but did not consistently affect δ18O, indicating increased nitrogen uptake and photosynthesis. Thinning had inconsistent effect on δ13C, but tended to decrease δ18O, indicating increased stomatal conductance and transpiration. Competition control treatments occurred most often shortly after planting and had similar, but weaker, effects on δ18O compared to thinning. However, competition control had a positive effect on δ15N, indicating a stronger influence on plant nutrient availability than plant water relations. Overall, our analysis showed that forest management consistently increased resource availability and plant physiological performance and that stable isotopes can be powerful tools to creating a mechanistic understanding of how plants respond to forest management.