Invasive Typha spp. are appropriate model species for chronosequence development and paleo-dating because of their invasion dynamics, ecological impacts, physical structure, and persistent and readily identifiable wind-distributed pollen. In the Laurentian Great Lakes region, the invasive narrow-leaved cattail (Typha angustifolia L.) and hybrid cattail (Typha × glauca Godr.), a hybrid between native T. latifolia L. and T. angustifolia (Smith, 1987), are dominant and ecologically disruptive species (Mills et al., 1993; Galatowitsch et al., 1999). Recent genetic analyses has revealed that in the upper Great Lakes region, in sites where both parent species are present, F1 hybrids tend to dominate, but backcrossing and advanced generation hybrids also occur (Snow et al., 2010; Travis et al., 2010). Additionally, T. angustifolia and T. × glauca are structurally similar, making differentiation from aerial imagery difficult. Because of physical similarities, habitat overlap, and similarities in invasion dynamics, invasive Typha spp. are commonly undifferentiated in the ecological literature (e.g. Frieswyk & Zedler, 2007; Trebitz & Taylor, 2007; Tulbure et al., 2007; Chun & Choi, 2009; Vaccaro et al., 2009; Mitchell et al., 2011). Therefore, in this study, we considered both T. angustifolia and T. × glauca as ‘invasive Typha’.
Invasive Typha has become abundant in Great Lakes regional wetlands (Mills et al., 1993; Trebitz & Taylor, 2007) because of increased propagule pressure (Zedler & Kercher, 2004; Lockwood et al., 2005), alterations in hydrology (McDonald, 1955; Wilcox et al., 1985; Shay & Shay, 1986; Wilcox & Nichols, 2008; Wilcox et al., 2008; Farrell et al., 2010) and anthropogenic nutrient enrichment (Crosbie & Chow-Fraser, 1999; Trebitz et al., 2007; Trebitz & Taylor, 2007; Morrice et al., 2008). Typha tolerates a wide range of water levels (Harris & Marshall, 1963; Waters & Shay, 1990), and recent historically low water levels have been linked to invasions into Lake Michigan and Lake Huron coastal wetlands (Frieswyk & Zedler, 2007; Tulbure et al., 2007; Lishawa et al., 2010). Climate change is predicted to further reduce water levels over the next 50–100 years (Mortsch & Quinn, 1996; Lofgren et al., 2002; Angel & Kunkel, 2009), likely increasing Typha dominance (Lishawa et al., 2010). Following establishment, Typha can spread rapidly (Tulbure et al., 2007; Boers & Zedler, 2008) and is typically much larger than the native species it replaces (Woo & Zedler, 2002). Because of high rates of primary productivity and slow decomposition (Davis & Van der Valk, 1978; Freyman, 2008), litter accumulates in Typha beds (Vaccaro et al., 2009) eventually excluding other macrophytes (Larkin et al., 2012). In Great Lakes coastal wetlands, T. × glauca dominance reduces plant community diversity (Tuchman et al., 2009; Lishawa et al., 2010) and sediments in T. × glauca stands tend to have unique physical composition, microbial communities (Angeloni et al., 2006) and elevated nutrient concentrations (Tuchman et al., 2009; Farrer & Goldberg, 2009; Lishawa et al., 2010). Typha's great biomass and persistent litter give Typha stands novel structure, allowing for remote sensing demarcation. Historical aerial and satellite imagery have been used successfully to assess Typha spread rates (Boers & Zedler, 2008) and increased dominance through time (Wilcox et al., 2008; Farrell et al., 2010). Invasive Typha are also appropriate species for paleoecological analyses because they have distinct and persistent pollen. Typha latifolia produces tetrad pollen grains, whereas T. angustifolia produces monads and hybrid T. × glauca produces monads, dyads, triads and tetrads. Thus, presence of dyads and triads is indicative of T. × glauca (Finkelstein, 2003), and the ratio of pollen types indicates relative dominance of Typha species. Furthermore, Typha pollen is widely wind-dispersed and persists as a significant extralocal component of the pollen record (Janssen, 1984; Clark & Patterson, 1985; Finkelstein & Davis, 2005), allowing for wetland-scale interpretation of Typha dominance from a single pollen core.
Our goals were to develop replicable methods for accurately reconstructing the spatial-temporal spread of dominant invasive plant species and link aerial photo-interpretation with paleobotanical analyses. First, we mapped invasive Typha's distribution through time in two Great Lakes coastal wetlands to create an invasion chronosequence. Second, we validated our interpretation with paleobotanical sediment core data and explored invasion dynamics with spatial and paleo-data. The results of this research will be used to examine the effects of invasive species residence time on wetland ecosystem structure and function (e.g. Mitchell et al., 2011) and the effectiveness of Typha management treatments.