COS 111-1
High mortality contributes to the successional decline of temperate nitrogen-fixing trees
Symbiotic nitrogen (N) fixation is the major N input to many ecosystems. It is also involved in a perplexing paradox. Temperate forests are often limited by soil N supply, suggesting that N-fixing trees should have a selective advantage. However, N-fixing trees are typically excluded during succession in temperate forests. Why do N-fixing trees drop out during succession despite the persistence of N limitation? Several hypotheses exist, largely focusing on physiological costs and tradeoffs. Empirical tests of these hypotheses are scarce, however, due to the difficulty of measuring these costs and tradeoffs. Knowledge of the demographic processes that underlie successional trends would provide a proximate answer to the paradox: Are N fixers growing slower, dying faster, or recruiting less as succession proceeds? Furthermore, knowing the relative importance of these demographic processes would narrow the scope of potential physiological mechanisms. In this study we quantified demographic rates of N-fixing and non-fixing trees across succession using data from the U.S. Forest Inventory and Analysis. We analyzed >125,000 forest plots aged 0-1028 years that are systematically distributed across the U.S., containing >17,000,000 trees. We used maximum likelihood statistics to analyze our data.
Results/Conclusions
Compared to non-fixing trees, N-fixing trees had higher relative growth rates very early in succession, but a significant growth disadvantage in intermediate aged forests. Mortality rates were twice as high for N-fixing compared to non-fixing trees in young stands, and the high mortality of N fixers persists across succession. Overall, our demographic analysis suggests that the decline of N-fixing trees through succession in temperate forests is due largely to higher mortality rates, although growth and recruitment also play roles.