Invasive plant species, including aquatic plants, are known to alter ecosystem structure and function through high competitiveness for light, water, and space due to an absence of predators and other stressors found in their native habitats (Gérard and Triest 2018; Spencer and Ksander 2006). Significant human resources are required to manage infestations of aquatic weeds that are left unchecked. Unmanaged invasive aquatic plant populations can interfere with human use of water or increase the risk of flooding due to high densities of plant material that impact drainage (Gettys 2020). Nonindigenous species are also detrimental to the economy where approximately 50,000 species cause nearly $125 billion of major environmental damage and economic losses every year (Allendorf and Lundquist 2003). There are several growth forms of nonindigenous plants ranging from emergent to submergent plants; however, free floating plants can be problematic as they are subject to movement by water and wind currents and can completely cover the surface of a water body (Gettys 2020). One such floating plant is Eichhornia crassipes (water hyacinth), which has infested water bodies in multiple states in the U.S. (Gettys 2020).
Eichhornia crassipes is a perennial aquatic plant that is considered the world's worst invasive aquatic plant (Villamagna and Murphy 2010). Originally from South America, E. crassipes is a free-floating vascular plant that causes major ecological and socioeconomic issues by forming dense, interlocking mats, which is done by its complex stolon and root structure, and ability to rapidly reproduce (Villamagna and Murphy 2010). Eichhornia crassipes produces sexually and asexually via clonal growth of stolons, or with seeds germinating within six months of dry conditions (Ueki and Oki 1979). Although E. crassipes primarily reproduces and spreads vegetatively, it is also known to have high seed production (approximately 3,400 seeds m2; Albano et al. 2011). Eichhornia crassipes reproduction and growth are strongly influenced by air and water temperature, where an increase in temperature correlates to more growth (Wilson et al. 2005). Similarly, Shu et al. (2014) found a positive correlation between temperature and E. crassipes root length. Eichhornia crassipes is sensitive to winter frost, which likely suppresses the growth of plants in the northern U.S. and southern Canada from becoming larger infestations commonly observed in the southern U.S. (Kriticos and Brunel 2016).
Oxycaryum cubense is another aquatic invasive plant also from South America and is commonly associated with E. crassipes (Robles et al. 2011; Watson and Madsen 2014). O. cubense has now spread to parts of Africa, Mexico, and the Southeastern United States (Bryson et al. 2008; Carter 2005). Considered a perennial invasive aquatic plant, O. cubense is known to exist as an epiphytic species that forms large floating islands, or tussocks, during initial colonization by utilizing its roots and rhizomes to intertwine with other invasive species and use the host as a raft (Robles et al. 2011; Turnage 2018; Watson and Madsen 2014). Associations include E. crassipes, giant salvinia (Salvinia molesta D.S. Mitch.), water fern (Salvinia minima Baker), hydrilla [Hydrilla verticillata (L. f.) Royle], floating pennywort (Hydrocotyle ranunculoides L. f.), anglestem primrose-willow [Ludwigia leptocarpa (Nutt.) H. Hara], parrotfeather [Myriophyllum aquaticum (Vell.) Verdc.], Eurasian watermilfoil (Myriophyllum spicatum L.), American pondweed (Potamogeton nodosus Poir.), marsh mermaidweed (Proserpinaca palustris L.), and humped bladderwort (Utricularia gibba L.) (Watson and Madsen 2014). Oxycaryum cubense relies on E. crassipes for its upright plant architecture as O. cubense seeds germinate in its leaf axils (Bryson et al. 2008). Once the tussock root/rhizome network of O. cubense assimilates enough sediment and outcompetes its host for nutrients and sunlight, the species is then capable of surviving independently; reproducing via buoyant seeds and vegetative fragments that break off from the tussock (Turnage 2018; Watson and Madsen 2014). Multiple populations of O. cubense are currently invading many multi-use lakes, reservoirs, and flowing waters that provide drinking water, hydro-electric power, aquatic food such as fish, outdoor recreational activities, and navigation for commercial and military vessels (Watson and Madsen 2014). Dense O. cubense growth has also prevented the development of native aquatic plant populations by shading out desirable submersed or emergent vegetation, cascading down the aquatic food chain to economically important fish being negatively affected (Robles et al. 2007; Robles et al. 2011; Watson and Madsen 2014). Additionally, O. cubense has the potential to negatively affect threatened and endangered species because it can shade out all littoral habitat in a waterbody. Similar to E. crassipes and other aquatic plants, temperature is considered a factor for growth, as the species is known to tolerate tropical or subtropical climates and isn't likely to tolerate freezing temperatures (Grippo et al. 2014); however, the direct influence of temperature on growth is relatively unknown.
Temperatures in the U.S. increased 1.8°C from 1895 to 2016 with 38% of that increase (0.7°C) occurring in the last 35 years of that period, suggesting many invasive species (like E. crassipes and O. cubense) can be expected to spread further north (Kriticos and Brunel 2016; Vose et al. 2017). O. cubense has been observed spreading farther northward into the mid-southern regions of the United States in the last 20 years (Bryson et al. 2008; Fernandez 2013; Rahel and Olden 2008; Grippo et al. 2014; Vose et al. 2017). However, there has been little attempt to quantify E. crassipes or O. cubense growth and development in response to environmental factors like air temperature.
Roltsch et al. (1999) found that linear-based degree day models are a good additive and potentially better predictor of phenological development of plants than non-linear models (Roltsch et al. 1999). Accumulated degree-days (ADD) are heat units that are a measure of the time duration at various temperatures used to predict the length of time (i.e., calendar days) it takes to achieve the occurrence of certain life stages (sprouting, peak biomass, flowering, senescence, etc.) in a plant's life cycle (Snyder 1985; Snyder et al. 1999; Spencer et al. 2000). Since ADD is based on temperature accumulation, the predictive capability is somewhat immune to changes in latitude within a region when upper and lower threshold temperatures are incorporated, such as the Single Sine Method (Snyder et al. 1985; Snyder et al. 1999; Spencer et al. 2000). Therefore, calculating ADD needed to complete one life stage at one latitude should be useful at a different latitude; for example, a target species that needs 500 ADD to complete a life stage may do so in 30 days in the northern U.S. and 20 days in the southern U.S. The objective of this study was to model and predict peak biomass development via ADD for E. crassipes and O. cubense