Abstract
Symbiotic nitrogen (N) fixing trees are absent from old-growth temperate and boreal ecosystems, even though many of these are N-limited. To explore mechanisms that could select against N fixation in N-limited, old-growth ecosystems, we developed a simple resource-based evolutionary model of N fixation. When there are no costs of N fixation, increasing amounts of N fixation will be selected for until N no longer limits production. However, tradeoffs between N fixation and plant mortality or turnover, plant uptake of available soil N, or N use efficiency (NUE) can select against N fixation in N-limited ecosystems and can thereby maintain N limitation indefinitely (provided that there are losses of plant-unavailable N). Three key traits influence the threshold that determines how large these tradeoffs must be to select against N fixation. A low NUE, high mortality (or turnover) rate and low losses of plant-unavailable N all increase the likelihood that N fixation will be selected against, and a preliminary examination of published data on these parameters shows that these mechanisms, particularly the tradeoff with NUE, are quite feasible in some systems. Although these results are promising, a better characterization of these parameters in multiple ecosystems is necessary to determine whether these mechanisms explain the lack of symbiotic N fixers-and thus the maintenance of N limitation-in old-growth forests.
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Claims 8
Claim · model-written summary
With no fixation costs, fixation increases until nitrogen ceases to limit production.
Verbatim
e0“When there are no costs of N fixation, increasing amounts of N fixation will be selected for until N no longer limits production”
t0“nitrogen fixation”
e1“Symbiotic nitrogen (N) fixing trees”
Claim · model-written summary
Tradeoffs with mortality or turnover can select against fixation and maintain nitrogen limitation.
Verbatim
e0“tradeoffs between N fixation and plant mortality or turnover, plant uptake of available soil N, or N use efficiency (NUE) can select against N fixation in N-limited ecosystems and can thereby maintain N limitation indefinitely”
t0“nitrogen fixation”
e1“Symbiotic nitrogen (N) fixing trees”
Claim · model-written summary
Tradeoffs with soil nitrogen uptake can select against fixation.
Verbatim
e0“plant uptake of available soil N, or N use efficiency (NUE) can select against N fixation”
t0“nitrogen fixation”
e1“Symbiotic nitrogen (N) fixing trees”
Claim · model-written summary
Tradeoffs with nitrogen use efficiency can select against fixation.
Verbatim
e0“N use efficiency (NUE) can select against N fixation”
t0“nitrogen fixation”
e1“Symbiotic nitrogen (N) fixing trees”
Claim · model-written summary
Low nitrogen use efficiency raises the likelihood of selection against fixation.
Verbatim
e0“A low NUE, high mortality (or turnover) rate and low losses of plant-unavailable N all increase the likelihood that N fixation will be selected against”
t0“nitrogen fixation”
e1“Symbiotic nitrogen (N) fixing trees”
Claim · model-written summary
High mortality raises the likelihood of selection against fixation.
Verbatim
e0“high mortality (or turnover) rate and low losses of plant-unavailable N all increase the likelihood that N fixation will be selected against”
t0“nitrogen fixation”
e1“Symbiotic nitrogen (N) fixing trees”
Claim · model-written summary
Low losses of plant unavailable nitrogen raise the likelihood of selection against fixation.
Verbatim
e0“low losses of plant-unavailable N all increase the likelihood that N fixation will be selected against”
t0“nitrogen fixation”
e1“Symbiotic nitrogen (N) fixing trees”
Claim · model-written summary
Published parameter data suggest the tradeoff mechanisms, especially nitrogen use efficiency, are feasible in some systems.
Verbatim
e0“these mechanisms, particularly the tradeoff with NUE, are quite feasible in some systems”
t0“nitrogen fixation”
e1“Symbiotic nitrogen (N) fixing trees”