The influence of soil fertility and light intensity on field layer development in urban secondary woodlands

The study investigated the influence of soil fertility and light climate on field layer development in urban secondary woodlands with an introduced ground flora, with the aim of contributing towards ground flora enhancement methodology and sympathetic secondary woodland management. The investigation was based on a series of complementary field and greenhouse replicated plot, single and multiple factor experiments.  Multivariate analyses of vegetation response and measurements of soil fertility, light climate and other environmental variables were used to examine the influence of soil fertility and light climate in combination and in interaction on field layer community establishment from seed. Light intensity (PAR) was a major determinant of ground flora development and was readily manipulated by canopy thinning.  Optimum dark phase irradiance, for the successful establishment of introduced species, was 5-20% of ambient PAR (photosynthetically active radiation). This was achieved in both field experiments by removing 50% canopy cover from varied canopy starting points (i.e. simple monospecific vs. multi-layered mixed). All introduced species grew well across the full range of soil conditions in the experimental programme.  Soil fertility was a major determinant of field layer development, although its influence was usually secondary to that of light intensity.  The relative importance of different aspects of soil fertility were less predictable than those of the light climate.  Fertilisation enhanced nutrient uptake and ground flora establishment in woodland plots, which were buffered from edge effects, although fertilisation is unlikely to prove viable or ethical in most woodland restoration schemes. The interaction between soil fertility and light intensity enhanced introductions in the field, but reduced their success in the artificial ground flora communities of the polytunnel, encouraging negative competition from arable weeds.