A physiological investigation into the invasive behaviour of some plant species in a mid - Country forest reserve in Sri Lanka

Abstract

An introduced and naturalized plant species which increases its population excessively at the expense of other species in a given community is defined as an invasive species. The objective of this study was to determine whether some selected physiological characteristics were responsible for the invasive behaviour of eight plant species in the Udawattakelle forest reserve. These included saplings of three tree species, three shrub species and two herbaceous species. These were divided into three groups as early-successional, late-successional and gap species depending on their habitat within the forest. To compare the physiological measurements, ten non-invasive (i.e. 'standard') species were also selected. Results showed strong evidence that higher leaf net photosynthetic rates (Pn) under both high and low light conditions could be one of the factors responsible for the invasive behaviour of the late successional species Myroxylon balsamum. Among the early successional and gap species, higher Pn under low light conditions could contribute to the invasive behaviour since it could give the invasive species a competitive advantage under the frequently cloudy conditions experienced in the Mid-Country Wet Zone in which Udawattakelle is located. A higher stomatal resistance (R1) was shown to be another factor contributing to the invasive behaviour in all successional groups because it would not only aid in the conservation of water during dry periods, but also give an advantage in the competition for water because of the higher water use efficiency at high R1. In addition, a higher leaf nitrogen content, which is an indicator of higher photosynthetic capacity, was shown to be a factor contributing to the invasive behaviour in all successional groups. A higher leaf potassium content which helps to maintain cellular turgor under high transpiration rates, was shown to contribute to the invasive behaviour of early-successional and gap species which experience high solar irradiance levels, but not in late-successional species.