Browsing by Person "Tongkoom, Krittiya"

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Reconciling indigenous and scientific ecosystem and soil fertility indicators in swidden systems of Northern Thailand
(2021) Tongkoom, Krittiya; Cadisch, Georg
Crop rotations in today’s swidden systems of Northern Thailand typically include five to ten years of fallow. Regarding ecosystem functions, these systems are relatively close to secondary forests when compared to modern agricultural systems; but they are under pressure for intensification, i.e. shortened fallow periods. In general, criteria are needed to decide whether fallow duration can be reduced, safeguarding ecosystem restoration and provision of food and income for farmers. Acknowledging that a comprehensive assessment would cover multiple aspects, our study focuses on the role of fallow duration on tree community succession and use abundances of tree species considered as soil fertility indicators. We studied recovery indicators of tree communities at two potential broad-leaved forest climax sites that differ in soils, forest type and agricultural intensification: An intensive system of one-year upland rice, then one- to two-year maize cultivation with synthetic inputs followed by six years fallow; and an extensive system with one-year upland rice cultivation without agrochemicals and ten years fallow. In a case study village of extensive site, we investigated in how far abundance of indicator tree species corresponded to measured soil fertility parameters and whether an extended list of indicator species could improve prediction of these soil properties. Contrasting systems were chosen to test the applicability of our indicators, not to compare their management practices. From 2010 to 2011, eight variables related to stand structure and tree diversity and four soil properties were either monitored or surveyed in chronosequence plots representing different fallow ages. For each variable, means per fallow year were compared by least squares means (LS-means), and quadratic regressions from mixed models were fitted. Significant differences between LS-means and optima of regressions served to distinguish fallow stages and served as indicators of recovery and system stability. Stepwise multiple regressions confirmed fallow age as main determinant for most variables. Tree species indicator also identify by the component of multiple linear regressions function of each interested soil properties. Numbers of tree species and diversity index recovered to levels of the previous rotation within the respective fallow time, but in both systems were far from climax communities, probably due to seed-bank depletion and shift toward resprouting species. While species dominance changed over time in the extensive system, the intensive system was dominated by a single species. In the extensive system only tree density passed a peak during the fallow period, while biomass-related variables approached plateaus. In combination with the replacement of early fallow species, this points to the onset of competition and transition between successional stages. For the intensive system, no structural variable passed a maximum. With only one of eight indicators on the extensive site fulfilling the statistical criterion of passing a peak during the prevailing fallow time, reducing fallow periods is not recommended for our cases. Generally, combining LS-means and quadratic regression allowed assessing fallow duration based on distinct successional stages at different sites. The approach should include various relevant site-specific indicators, in our case representing biomass and carbon storage, species and structural diversity, considered crucial for both sites. From interview on the extensive site, farmers listed 11 tree species that relate to certain soil quality related properties. They named indicators of good soils for cropping, inappropriate soils for upland rice cropping and hard soils. Botanical tree inventories on 135 plots of one to ten years fallow age were conducted. Abundances of farmers’ indicator on one hand as well as inventory species on the other were introduced into different regression models to predict soil fertility parameters measured on the same plots. Both models were then compared regarding predictive power. Measured fertility parameters such as soil organic matter (SOM), pH, plant available phosphorus (Pav) - related to farmers’ criteria ‘good soil’ or inappropriate for rice cropping’ - as well as bulk density (BD, for ‘hard soil’), changed significantly during the fallow period, initially towards temporary pessima in years 6 to 7 followed by recovery towards year ten. Most indicator species, like Macaranga denticulata for Pav or Dalbergia cultrata for SOM, were clearly related to the soil quality characteristics attributed to them by farmers. Only in one case a species used as farmer indicator for hard soils was selected by multiple regression as predictor for high Pav. Including all tree species found during inventories into multiple regressions significantly improved predictions of measured soil parameters by AIC > |2|. Ten additional species from the survey model had potential to improve the farmer indicator model. Relative density, i.e. abundance of indicator tree species over abundance of all species, did not always match soil properties dynamics, so that the use of the regressions appears more informative for cropping decisions. Our approach to relate indicator species and measured soil parameters is not site-specific, but parameters are. Applicability of the approach could be extended if further farmer criteria such as weed suppression, represented by tree structure parameters as predictors of adequate fallow age, would complement soil fertility indicators. Based on the development of the multiple indicators of recovery of ecosystem services and soil fertility, it is not recommended to reduce fallow age at the two investigated study sites.