In this Appendix I explain how the model parameters for the reference plant were
derived from empirical studies. I considered a hypothetical tree species in the tropics
as the reference plant species. Plant trait and environmental parameter values were
thus obtained mainly from literature data reported from the tropical forest ecosystem.
Plant Reproduction rate (r) and seedling mortality rate (m ) was derived from S
the study of Augspurger (1983) and Bell et al. (2006). Both studies focused on one
dominant tree species, Platypodium elegans and Sebastiana longicuspis, respectively,
and provided data for germinating seedling density and natural mortality. I assumed
the reference plant species to have parameter values between the two plant species, so
that trait values can fall within a realistic range when conducting simulations.
The living plant biomass was reported to be 133 Mg C ha-1 for an Amazonian
forest (Pyle et al., 2008). Under the assumption that at steady state one hectare can support the living of 1000 adult individual, the biomass of adults (BA) was set as
130000 g C for one adult. The biomass of one seedling individual (BS) was assumed
to be 10 g C as Markesteijn & Poorter (2009) reported that seedling biomass after one
growing season is between 1 to 35 g for 62 tree species in an Amazonian forest.
The value of plant C:N ratio is adapted from the study of Cernusak et al. (2010).
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The study showed that leaf N:C ratio lies between 20 to 60 (mg N g-1 C) for 13 plant
species. I assumed a C:N ratio (JP) value of 40 (g C g-1 N), which corresponds to a
value within their reported range.
Seedling biomass growth rate was estimated from aboveground wood
productivity, which is reported to lie between 2.11 to 4.33 Mg C ha-1 year-1 for
tropical forests (Pyle et al., 2008; Malhi et al., 2009). Under the assumed adult biomass and plant C:N ratio, I used 9ͪ10-7 g N-1 m2 day-1 as the growth parameter. For
the adult mortality rate, it was estimated that 0.02 adult individuals dies per year
(King et al., 2006). Therefore, I assumed an adult mortality rate (m ) of 5ͪ10A -5 day-1.
The value of litter decomposition rate is based on the study by Kurokawa &
Nakashizuka (2008), which reported that the litter decomposition rate for 40 tree
species in Malaysian tropical forest lie between 0.67 to 4.85 year-1. I thus assumed a
decomposition rate ( dec ) of 0.006 day-1 for the reference species. Litter production
rate of adults per unit N uptake (l) was estimated from litterfall data from tropical
forests (Clark et al., 2001; Malhi et al., 2009), which estimated that the magnitude of
this flux is between 3.4 to 7.3 Mg C ha-1 year-1. I set the trait value as 0.008 g C g N-1
day-1 ind.-1 such that the modeled flux has the same order of magnitude.
I adapted the value of pathogen infection efficiency (DS) from Augspurger (1983)
and Bell et al. (2006). It was documented that 58.25% of Platypodium elegans
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seedlings died due to pathogen infection within three months, while 50% of
Sebastiana longicuspis seedlings died across five weeks. Similar to previous traits, I
assumed the reference plant species to have trait values between the two plant species.
The pathogens infection efficiency of adult (D ) is assumed to be much smaller A
compared to those of seedlings (i.e. DSͪ10-4). I assumed the pathogen assimilation
ratio (b ) of the reference plant as 0.6 following other theoretical studies (Miki et al., P
2010), which is a reasonable value since my model considered nitrogen flux instead of
carbon. The value of litter return ratio ( f ) is set to 0.35 as I assumed 5% of plant P
nitrogen will directly return to the soil N pool after pathogen infection.
For the pathogen mortality rate (GP), Hancock (1981) observed that the decline
of Pythium ultimum in the field follows exponential decay with a half-life of
approximately 30 days, yielding a decay rate around 0.02 day-1. For the case of
mycorrhiza, an experiment by Staddon et al. (2003) showed that some fraction of
mycorrhizal hyphae have a turnover time up to 30 days (i.e. mortality rate equal to
0.03 day-1). Here, I assumed a slightly smaller pathogen mortality rate with the value
0.01 day-1 to fulfill mathematical conditions from Appendix S3. I assumed the same
value for mycorrhiza mortality rate (G ). M
For the parameter value of carbon transfer ratio (Cmax), Bryla and Eissenstat
(2005) suggested that the total cost of plant-mycorrhiza associations ranges from 3 to
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36% of the carbon fixed by photosynthesis. Whereas among the carbon allocated to
mycorrhizal associations, more than 40% is consumed by respiration (Bryla &
Eissenstat, 2005). Therefore, I set the value of carbon transfer ratio as 0.2 and the
mycorrhiza carbon assimilation ratio (eM) as 0.6.
The value of negative impact coefficient between microbes is calculated from a
study by Sikes et al. (2009), which showed that the percentage colonization of root by
pathogens declined 20 to 40% within five months when growing with mycorrhizas. I
set the negative impact of mycorrhizas on pathogens (EPM) as 0.005 g N-1 m2 day-1 so
that a similar decrease can be realized in the model system. I assumed that the
negative impact of pathogens on mycorrhizas (E ) have the same magnitude. MP
For the environmental parameters, Menge et al. (2009) reported that the
atmospheric N deposition flux is within the order of 10-100 kg N ha-1 year-1 in
polluted ecosystems, while the leaching flux of plant-available inorganic N and
plant-unavailable inorganic N lies between 0.1-10 and 0.2-70 kg N ha-1 year-1,
respectively. I assumed a deposition input (I) equal to 0.005 g N m-2 day-1, which is
adapted from Vitousek & Sanford (1986) from tropical forest studies. This value
corresponds to a flux of 18.25 kg N ha-1 year-1, which is reasonable if I assumed an
ecosystem with low anthropogenic impact. I assumed the leaching rate of inorganic N (Le) and organic N (φ) is 0.0002 day-1 and 0.00008 day-1, respectively. The values of
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these parameters are chosen in a way that the modeled N flux was within the range
reported by Menge et al. (2009).
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