Abstract:
Dillenia suffruticosa (Griffith) Martelli, that spreads fast in low-lying areas in wet zone of Sri Lanka is currently listed as a nationally important Invasive Alien Species that deserves attention in ecological studies. Thus, impact of this woody invader on physical, chemical properties of soil and below and above ground flora was investigated. Five sampling sites were identified along a distance of 46km from Avissawella to Ratnapura. At each site, two adjacent plots [1m x10m each for D. suffruticosa present (D+) and absent (D-)] were outlined. Physical and chemical soil parameters, microbial biomass and number of bacterial colonies in soil were determined using standard procedures and compared between D+ and D- by ANOVA using SPSS. Rate of decomposition of D. suffruticosa leaves was also determined using the litter bag technique at 35% and 50% moisture levels. Above ground plant species richness in sample stands was compared using Jaccard and Sorenson diversity indices. Decomposition of D. suffruticosa leaves was slow, but occurred at a more or less similar rate irrespective of moisture content of soil. Particle size distribution in D+ soil showed a much higher percentage of large soil particles. Higher % porosity in D+ sites was a clear indication that the soil was aerated. The pH was significantly lower for D+ than D- thus developing acidic soils whereas conductivity has been significantly high making soil further stressed. The significant drop in Cation Exchange Capacity (CEC) in D+ soil was a remarkable finding to be concerned with as it correlated with fertility of soil. Significantly higher values of phosphates reported in D+ soil support the idea that plant invaders are capable to increase phosphates in soil.
Higher biomass values recorded for D+ sites together with higher number of bacterial colonies could be related to the unexpectedly recorded higher Organic Carbon. Both the Jaccard and Sorenson indices indicated that D+ and D- sites were dissimilar with respect to above ground plant species richness. Thus, changes in above ground vegetation and soil properties due to the invasion were identified and further studies are needed for determining the degree of soil deterioration due to the invasive behavior of D. suffruticosa.
