An Analysis of Temperate Deciduous Shrub Phenology in Downer Woods, University of Wisconsin-Milwaukee, Wisconsin, USA

Shrub species, both native and non-native, are an important component of temperate deciduous forest ecosystems but are an often-overlooked and under-studied functional group. Shrubs tend to leaf-out earlier than trees in spring and retain their leaves later in autumn thus extending the overall growing season and the carbon uptake period of the forest ecosystem. In this study, a range of 5- native and 3- non-native shrub species were identified in a deciduous urban woodlot, and the phenology was monitored over a 3-year period on the University of Wisconsin-Milwaukee campus. The aim of this work was to determine any variation in the timing (DOY) and duration (days) of key spring (bud-open, leaf-out, full-leaf unfolded) and autumn (leaf color, leaf fall) phenophases between native and non-native species. Preliminary results revealed interesting findings with buckthorn Rhamnus cathartica (an alien invasive/non-native species) consistently leafing out later than most native species and taking longer to reach full-leaf unfolded. Additionally, non-native species such as European privet Lingustrum vulgare have a longer growing season than native species ranging from 14 days to 35 days longer in non-native species than native species across the three-year period. This shows how non-native species can lengthen the fall season compared to native species. These results could add to the understanding of how non-native shrub species may gain a competitive advantage over native shrubs and may help inform future conservation management plans

Extracted leaf chlorophyll content with corresponding SPAD values of temperate deciduous native and non-native shrubs, autumn 2018, southern Wisconsin, USA

During the autumn senescence period 2018, we collected Minolta-502 SPAD values spanning a 10-week period (DOY 250 to 312) from 1,044 leaf samples for 5 native (Ribes americanum Wild currant; Prunus virginiana Chokecherry; Viburnum acerifolium Maple leaf viburnum; Viburnum lentago Nannyberry; Cornus alternifolia Pagoda dogwood) and 3 non-native (Lonicera morrowii Honeysuckle; Rhamnus cathartica Common Buckthorn; Ligustrum vulgare European privet) temperate deciduous shrub species growing in Downer Woods - a small urban woodlot on the campus of the University of Wisconsin-Milwaukee, USA. SPAD values were recorded twice weekly from DOY 250 to 296 on all species and subsequently on two occasions (DOY 302 and 312) when leaves were present on non-native species only. On each occasion SPAD readings were recorded at the same location on each of three leaves from three individual plants of each species (nine readings per species). The same leaves (or leaf part) were subsequently removed and placed in individual labelled plastic bags prior to freezing at -28 °C for Chl extraction at a later date. Leaf samples were removed from the freezer and allowed to thaw before discs of 1 cm in diameter were cut from each using a standard borer. Each leaf disc was placed in a 15 ml polyethylene centrifuge tube prior to adding 10 ml dimethyl sulphoxide (DMSO) and the lid secured. The tubes were placed in a water bath and incubated at 65°C (following Hiscox and Israelstam 1979) for 2 hr. Samples were removed from the water bath and a 3 ml extract of each supernatant was pipetted individually into a quartz cuvette for spectrophotometer (Beckman-Coulter DU-640) analysis, using a DMSO blank prior to recording absorbance of all leaf samples at 750, 663, 645, and 480 nm. Total Chl, Chl a and Chl b concentrations were calculated following Arnon's (1949, doi:10.1104/pp.24.1.1) equations while carotenoid concentration was determined following Wellburn (1994, doi:10.1016/S0176-1617(11)81192-2): Total Chl (g/l) = 0.0202 A645 + 0.00802 A663 Chl a (g/l) = 0.0127 A663 + 0.00269 A645 Chl b (g/l) = 0.0229 A645 + 0.00468 A663 Total carotenoids (mg/l) = (1000 A480 - 2.14 Chl a - 70.16 Chl b)/220 Absorbance values were corrected for turbidity using absorbance at 750 nm. Total Chl, Chl a, Chl b and carotenoid contents were calculated on a leaf area basis (mg/cm² leaf area). On visual inspection of the scatter plots of SPAD vs total Chl, Chl a and Chl b it was evident that some of the points may have been outliers. In order to statistically determine if any of the points were true outliers, histograms of the residuals of the regression for SPAD and Chl content for each species were constructed and any point that lay outside the normal distribution was considered an outlier and omitted from further analysis. Based on the distribution plots of the residuals, six data points were identified as outliers, and omitted from further analysis as they lay outside the normal distribution of the data set. The outliers were for two native (Wild Currant and Dogwood) and one non-native (Privet) species and all showed a higher than normal Chl content for a given SPAD value. These are indicated in column 'Comment'