Katherine M. Giljohann

Quantifying Plant Colour and Colour Difference as Perceived by Humans Using Digital Images

Human perception of plant leaf and flower colour can influence species management. Colour and colour contrast may influence the detectability of invasive or rare species during surveys. Quantitative, repeatable measures of plant colour are required for comparison across studies and generalisation across species. We present a standard method for measuring plant leaf and flower colour traits using images taken with digital cameras. We demonstrate the method by quantifying the colour of and colour difference between the flowers of eleven grassland species near Falls Creek, Australia, as part of an invasive species detection experiment. The reliability of the method was tested by measuring the leaf colour of five residential garden shrub species in Ballarat, Australia using five different types of digital camera. Flowers and leaves had overlapping but distinct colour distributions. Calculated colour differences corresponded well with qualitative comparisons. Estimates of proportional cover of yellow flowers identified using colour measurements correlated well with estimates obtained by measuring and counting individual flowers. Digital SLR and mirrorless cameras were superior to phone cameras and point-and-shoot cameras for producing reliable measurements, particularly under variable lighting conditions. The analysis of digital images taken with digital cameras is a practicable method for quantifying plant flower and leaf colour in the field or lab. Quantitative, repeatable measurements allow for comparisons between species and generalisations across species and studies. This allows plant colour to be related to human perception and preferences and, ultimately, species management.

The seed ecology of two invasive Hieracium (Asteraceae) species

Two highly invasive hawkweeds, Hieracium aurantiacum and H. praealtum L., have established in the Australian Alps. Our experiments aimed to provide a better understanding of the seed ecology of these species, essential if successful eradication strategies are to be developed. Results indicated that viable seeds are produced in large numbers. Seeds germinate spontaneously and synchronously with light and moisture and are killed when exposed to temperatures above 110C for 150s. No seedlings ofH. aurantiacumemerged from soil samples taken from a chronosequence of known infestations, but some H. praealtum seedlings emerged from soil sampled from sites where plants flowered the preceding summer.Germinationwasgreatlyreducedforbothspeciesunderdarkness,andfewseedlingsemergedwhenburied.Where herbicideswereappliedatalatestageoffruitdevelopment,hawkweedscontinuedtoproduceviableseeds,albeitinreduced numbers. Overall, our findings indicate that both species produce a large amount of seed, little of which enters the long- term soil seed bank. This, and the limited ability of buried seeds to germinate and/or emerge, means that Hieracium control programs should emphasise short-term (1-2-year) monitoring of treated sites and stress the importance of reducing the seed set.