Nemaplot hyperspectral data analysis and population modellingEvaluation reinvented

 

Excursion: the impact of climate change on Heterodera schachtii populations

Damage beet cyst nematode
Field symptoms of H. schachtii
Growth reduction by H. schachtii
Heterodera schachtii Cysts, eggs, Juveniles
Sugar beet canopies with BCN

Simulation results of future decades

The application of the Leslie model on data from 2011 to 2014 (Site Elsdorf, Rhineland, 50°56'21N,6°33'48E) as a tool for the analysis of hyperspectral reflectance data had additionally shown how a) yearly accumulated temperatures (degree days, base 8°C, in short DD8) have increased compared to the temperature records the model has been calibrated to (1971-1992) and b) the multiplication rates of H. schachtii have increased disproportionately high, similar to the population dynamics, which occurred occasionally in earlier times, for example in 1983. That rises the question about the direction of future nematode dynamics. The PIK1, Germany, has provided a weather data set from 1902 to the year 2100 for the given sugar beet production area Elsdorf. We have chosen a moderate scenario on purpose for future scenarios. The status of the underlying nematode model  have been unchanged, considering solely the temperature related changes. The H. schachtii population model run with the temperature data for this 200 years with a population (Pi) of 1000 E&L/100 ml soil, a Pi value, where density effects are of minor concern.

Temperatursummen von 1902 bis 2100
Fig. 1: Degree Days °C (DD8) from 1902 to 2100

Degree Days °C (TS8)

The special feature of the Leslie model is the ability to transform a specific temperature pattern of a season into patterns of nematode dynamics, i.e. the occurrences of certain temperature events in time are of importance. For the demonstration of a long time trend, the use of Degree Days on a base 8 (DD8), as common for H. schachtii,  is more suitable. The daily mean temperatures are added up from the 1st of March until the end of October of each year. The model starts (simulated sugar beet planting) when DD8 exceeds a value of 60°C. Although there is a common trend in the DD data (fig. 1), the following focus is much more important: DD8 are more or less in the range of 1200 to 1600 °C, which correspond to a multiplication rate of 2-5 for given Pi from a nematological point of view. Just for orientation, these reproduction rates yield in equilibrium population densities, where crop losses by H. schachtii are somewhat controlled by 50% sugar beet rotations including intercropping, or 33% sugar beet rotations with two non-hosts (see also fig. 5). Much higher DD8 have been noticed up to year 2000, but less frequent than compared to the current century. In terms of relevance for the nematodes we can distinguish four periods: P1 corresponds to the years 1902-2000; P2 = 2001-2020; P3 = 2021-2050, and finally P4 = 2051-2100;

2. Cysts development of H. schachtii

Cyst dynamics of H. schachtii
Fig. 2a: Simulated cysts dynamics 1902-2000
Cyst dynamics of H. schachtii
Fig. 2b: Simulated cysts dynamics 2001-2020
Cyst dynamics of H. schachtii
Fig. 2c: Simulated cysts dynamics 2021-2050
Cyst dynamics of H. schachtii
fig. 2d: Simulated cysts dynamics 2051-2100

Temperature affects certainly the complete nematode-sugar beet - interaction, but we focus on cyst development, especially at the end of season, as the late season dynamics determine the Pi value at the next sugar beet season.


For more background details about all nematode development stages and the growth processes of the sugar beet with warmish temperatures we refer to an additional related page of the project back

Fig. 2 a-d represent the comparison of expected cysts dynamics for each period (P1-P4) to the overall median of 200 years, as usual inclusive the 95% quantile). The quantile describe the variation on the x- axis (i.e. changes in development time) and y axis (i.e. changes in population size). No matter which period is of interest, the cyst population grows at the end of season, an average of 150-200 cysts/100 ml soil is expected (green chart line). During P1 the begin of cyst development is delayed, the upper extremes are not reached in this period, but occasionally densities of 800 cysts and more are found. P2 represents more or less the overall mean, but the mean cyst number is larger than the overall mean (yellow line, fig. 2b), meaning more extreme temperature events occur during P2. The system "calms down" a little bit in P3, extreme events have still a chance, but are less frequent than in P2. Dynamics fasten in P4 again, cyst appearance is earlier and population size is higher under extreme conditions. The median is also increased (fig. 2d), more or less caused by the frequency of higher DD8.

2. Simulated Pf values of H. schachtii

Pf values of Heterodera schachtii cysts
Fig. 3a: Simulated Pf values (cysts) from 1902-2100 at a given Pi of 1000 E&J
Pf values of Heterodera schachtii E&J
Fig. 3b: Simulated Pf values (E&J) from 1902-2100 at a given Pi of 1000 E&J

Fig. 3 a & b summarize the current findings. The simulated multiplication rates are depicted as number of cysts as well as E&J/100 ml soil. (The differences within the model is just a factor of 24, hence the same shape of the chart, but a different scale on the y-axis). No doubt, extreme multiplication rates has had happened all times and is still happen. While these extremes are rare events in P1, P2 is a period of frequent events of top multiplication rates. The absolute population densities shown in fig. 3 might be seriously too high, nevertheless, a relative ratio over time is more than obvious. By any chance the decrease rates under non - hosts might be temperature dependent as well, hence the reduction rate might be a little bit higher than 43 to 47% per year ( we have not found any evidence about that hypothesis during model development). At this point of development the direction of the population density is effective up with more frequent temperature extremes, approaching rotation specific equilibrium densities far above thresholds levels and with severe limiting effects on future sugar beet production.

3. Reproduction rates of H. schachtii with increasing Degree Days

Pf/Pi ratio cysts
Fig. 4a: Simulated Degree Days based multiplication rates of cysts
Pf/PI Verhältnis E&L
Fig. 4b: Pf/Pi ratio based on Degree Days
Pf/Pi ratio fit
Fig. 4c: Fitted linear - plateau model to Pf/Pi data; parameters: lower plateau = 2.4, slope = 0.1, onset DD8 = 1618 °C

A more than interesting phenomenon is observed in the comparison of (simulated) reproduction rates and the occurrence of DD8. The multiplication rate of 2-5, as mentioned before, corresponds to a DD up to 1500/1600 °C (Fig. 4 a, b). That is the rate which can just be tolerated for a sufficient sugar beet production in long term view. Variance become larger above 1600°C (fig. 4 a, b), the Pf/Pi ratio increases to 7- 10. Above 1700 °C things are getting seriously worse with top end ratios of 20, still higher DD's lead to up to ratios of 40(!). All Pf values, which might be completely unrealistic, but already occurred at the one or other occasion. To determine the findings in terms of statistical parameters a linear-plateau model have been fitted to the data (fig. 4c). The model application provides a quantitative relation in detail: the onset point is in the range of ~1618 °C, below that value an average multiplication rate of 2.4 is found at the given Pi of 1000 E&J/100 ml soil. Above the critical onset point the multiplication rate increases with a rate of 1 for each 10°C added to heat sum. A small side effect of the data of fig. 4: the stochastic potential of the Leslie model looks fantastic and realistic, generated by the transformation of temperature patterns of a season into a nematode dynamics.

4. Forecasting Pi values of H. schachtii at next planting sugar beets with given DD8 and a 33% sugar beet (standard) rotation.

Pf/Pi  Zysten
Fig. 5a: Pi values after 3 years at given Degree Days
Pf/PI Verhältnis E&L
Fig. 5b: Pi values (log-transformation) after 3 years at given Degree Days

It has to be mentioned again, no additional limitations are implemented in the model taking into account potential further restrictions at higher temperatures (for example water stress, or something similar). Hence, doubts about the absolute Pf values are absolutely legitimate. But the proposed multiplication effect after exceeding a critical DD boundary is real and adds up to population densities which can be controlled only by long time breaks in the sugar beets rotation. Fig. 5a (absolute scale) and fig. 5b (logarithmic scale) summarizing the effect of a 33% rotation. The later chart provides a better overview about the Pi values at next planting sugar beets (year 3). With a given Pi of 1000 E&J/100 ml soil the population decreased on average at DD up to 1500 °C, the equilibrium density will be below the 1000 E&J in long term view (green and purple line). In the range from DD 1500 to 1700 °C the Pi will end up at 1200 E&J, the rotation specific equilibrium density will increase in the long term (red line). As usual, more interesting than the mean or median is the variance. Even at this DD values, the Pi can increase up to 5000 E&J in the worse case, but can collapse down to 400 E&J in the best case. Above the critical DD8 of 1700 °C the new Pi values end up at 5000 E&J on average (yellow line). The "traditional" threshold level is exceeded by ten fold. The variation is high in the range of 1400 to 10000 E&J. Even with an acceptable error of 50% of the model output, the system approaches to population densities which will limit the future sugar beet production. Using the common tolerant/resistant sugar beet varieties is the current option, but such densities increases the pressure and will support new H. schachtii races.

Final chapter remarks:

1) The model runs predict  a significant increase of the nematode population with respect to the given temperatures from 2001 to 2020. It is currently not known, if this forecast has become reality. 2) The reason for this chapter is certainly not to queue the endless rows and discussions of climate change warnings. The changes are inevitable.

1 Climate data have been provided by Dr. Thomas Kartschall, Potsdam Institute for climate impact, PIK, Germany

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