Light alters the impacts of nitrogen and foliar pathogens on the performance of early successional tree seedlings

Introduction

Shade-intolerant species are, by definition, unable to persist in low-light environments. This may occur because shade-intolerant species tend to allocate resources toward rapid growth and limited defense (Walters & Reich, 1999; Myers & Kitajima, 2007; Valladares & Niinemets, 2008). Because this shade-intolerant strategy prioritizes growth of new tissue over defense against consumers, the performance and survival of shade-intolerant plants may decline in environments where growth is slowed by nutrient or light limitation. Importantly, in these resource-limited environments, performance and survival can be further reduced by high consumer pressure (Coley, Bryant & Chapin, 1985; Fine, Mesones & Coley , 2004; Hahn & Maron, 2016). In this way, consumer pressure may interact with light and nutrient availability to drive succession from high-light, early-successional fields to low-light, later-successional forests (Augspurger, 1984b; Coley, Bryant & Chapin, 1985; Myers & Kitajima, 2007; Pasquini, Wright & Santiago, 2015; Griffin et al., 2016).

The impacts of consumers on shade-intolerant species may increase as light and nutrients decline. This is because the fast-growing, poorly defended strategy is only advantageous when resources are ample enough to support high growth rates. Otherwise, these plants cannot maintain their high metabolic rates. By consuming plant tissue, pathogens and herbivores further reduce the ability of these plants to acquire the resources (e.g., light, nutrients, water) necessary for the growth and maintenance of their metabolically costly tissue, potentially driving precipitous declines in performance. Thus, in low-light conditions, shade-intolerant species may be especially susceptible to disease for several reasons. First, shade-intolerant species often exhibit low constitutive defenses (Stamp, 2003). Second, limitation by nutrients and light may prevent plants from constructing some defense compounds (Hanssen et al., 2020; Huang, et al, 2020). Third, light limitation may lead to the down-regulation of both the salicylic and jasmonic acid pathways (De Wit et al., 2013), which are key to responding to attack by pathogens and herbivores (Thaler, Humphrey & Whiteman, 2012; De Wit et al., 2013; Ballaré & Pierik, 2017). Moreover, nutrient limitation may be especially detrimental for shade-intolerant species (e.g., Ward, 2020), because it can drastically reduce their ability to build photosynthetic machinery and some defensive compounds (Coley, Bryant & Chapin, 1985; Stamp, 2003; Hanssen et al., 2020). Thus, susceptibility to pathogens among shade-intolerant species may be particularly high when light and nutrients are limiting (Kitajima & Poorter, 2010; Griffin et al., 2016; Griffin et al., 2017).

Regardless of resource availability, pathogens are critical drivers of dynamics in plant communities (Mordecai, 2011). Throughout forest and grassland systems, pathogens can limit seedling survival (Hersh, Vilgalys & Clark, 2012), ecosystem productivity (Mitchell, 2003; Maron et al., 2011), species’ ranges (Spear, Coley & Kursar, 2015; Bruns, Antonovics & Hood, 2019), and can promote diversity (Bever, Mangan & Alexander, 2015; LaManna et al., 2017). Pathogens can alter community composition through negative density-dependent seedling mortality (Comita et al., 2014; Bayandala, Masaka & Seiwa, 2017; Uricchio et al., 2019; Jia et al, 2020), benefiting more resistant species (Welsh, Cronin & Mitchell, 2016; Cappelli et al., 2020), and those that have escaped their specialist herbivores and pathogens (Heckman, Wright & Mitchell, 2016; Heckman et al., 2017). Because pathogen impacts can also change with light and nutrient availability (e.g., Dordas, 2009; Veresoglou et al., 2013; Heckman, Wright & Mitchell, 2016; Ballaré & Pierik, 2017; Liu et al., 2017; Agrawal, 2020), shade-intolerant species may experience large differences in pathogen impacts across the range of habitats they occupy (Augspurger, 1984a). Thus, pathogens may reinforce shade-tolerance differences among species, promoting niche differentiation (McCarthy-Neumann & Kobe, 2008; Krishnadas & Comita, 2018).

In this study, we assessed the role of light and nitrogen supply in seedling susceptibility to pathogens and how pathogen impacts on seedling performance are mediated by light and nitrogen. We did this using an important pioneer species of old fields and early-successional forests, Liquidambar styraciflua (Oosting, 1942; Wright & Fridley, 2010; Fridley & Wright, 2018). In these early-successional environments, L. styraciflua can experience severe foliar disease (McElrone et al., 2010) and varied nutrient and light conditions. Thus, the interaction between pathogens and resource supply may be important for understanding successional dynamics. To date, this interaction has been addressed in only a few studies in natural systems (e.g., Griffin et al., 2016; Griffin et al., 2017; Ward, 2020). We predicted that:

1. Foliar fungal pathogen damage will be highest when light and nitrogen are both limiting.

2. Pathogen impacts on seedling height, a proxy for plant performance, will be highest when light and nitrogen are both limiting.

Methods

Results

Impacts of light and nitrogen on fungicide efficacy

Averaged across all light and nitrogen treatments, fungicide reduced fungal damage by 83% (Spraying, P < 0.001; Fig. 1A). Light and nitrogen availability jointly altered the effect of fungicide on visible foliar fungal damage (Light × Nitrogen × Fungicide; P = 0.03; Table S2): in high light, spraying did not alter fungal damage in either nitrogen level (Tukey HSD: High nitrogen, P = 0.99; Low nitrogen, P = 0.83); in low light, spraying reduced fungal damage to near zero at both nitrogen levels (Tukey HSD: High nitrogen, P < 0.001; Low nitrogen, P = 0.001).

Impacts of light, nitrogen, and fungicide on seedling height

Although light and nitrogen each had significant main effects on seeding height (Light, P < 0.001; Nitrogen, P = 0.022; Table S4), these two factors interacted to influence seedling height (Nitrogen × Light, P = 0.004; Table S4). In high light, seedlings grew 17% taller in the high nitrogen treatment than the low nitrogen treatment (Tukey HSD: P < 0.001). Height did not differ among nitrogen treatments in low light (Tukey HSD: P = 0.62).

Fungicide did not have a significant main effect on seedling height (P = 0.14). Instead, light availability altered the effect of fungicide on seedling height (Fungicide × Light, P = 0.002; Table S4 ; Fig. 2): in high light, sprayed seedlings grew 14% taller than unsprayed seedlings, but did not differ in low light (Tukey HSD: High light, P = 0.002; Low light, P = 0.22). Thus, although fungicide did not reduce visible damage, fungicide appears to have alleviated the negative impacts of fungal disease on seedling performance. Contrary to expectation, light and nitrogen did not interact with fungicide to influence seedling height (Nitrogen × Fungicide × Light: P = 0.72; Table S4).

Discussion

In this study, light, nitrogen, and pathogens additively and interactively influenced seedling performance. Contrary to our expectation, nitrogen and light availability interacted to alter foliar fungal damage: damage increased with increasing nitrogen availability, but only in low light. This suggests that increased nitrogen availability under low light makes plants more susceptible to enemies (Dordas, 2009; Zhou et al, 2015; Ballaré & Pierik, 2017). Although foliar damage was lower under high light, seedling height showed a different pattern: pathogens impacted seedling height more strongly under high light than low light. This is contrary to other studies showing either greater impacts of pathogens and herbivores in low light or no difference in impacts between light environments (e.g., Augspurger, 1983; Myers & Kitajima, 2007; Bayandala, Masaka & Seiwa, 2017). Together, these results demonstrate the importance of light and nitrogen for modulating pathogen impacts on seedling performance.

Nitrogen and light availability interacted to influence pathogen damage in this study. Specifically, pathogen damage increased with nitrogen availability in the shade, but not in the sun. Because there was not a strong main effect of light, it is unlikely that differences in pathogen inoculum between high and low light environments were entirely responsible for this. Instead, low light may have reduced the ability of seedlings to synthesize defense compounds, either because they lacked the carbon to do so or because the jasmonic and salicylic acid pathways were down-regulated (Stamp, 2003; De Wit et al., 2013; Zhou et al, 2015; Ballaré & Pierik, 2017; Huang, et al, 2020). Either of these mechanisms would reduce the ability of seedlings to resist pathogens, leaving their nitrogen-rich leaves more susceptible to pathogen infection. Together, these possible explanations for the interactive effect of light and nitrogen on pathogen damage have important implications for succession (Griffin et al., 2016; Griffin et al., 2017): high nitrogen supply may reinforce the dominance of shade-intolerant species in early stages of succession (i.e., high-light environments) by contributing to growth without increasing damage, but may hinder performance of shade-intolerant species in later successional stages by increasing damage without increasing growth (Reinhart et al., 2010).

Nitrogen and light availability also interacted to influence seedling height, a key proxy for seedling performance. Here, seedling height increased with nitrogen availability in high light, but not in low light. This indicates that light was more limiting than nitrogen to growth in the forest understory, but that nitrogen was more limiting than light in the old field. Despite this, nitrogen and light did not simultaneously interact with fungicide application to influence seedling height. Instead, spraying increased seedling height only in high light, which was contrary to our prediction that pathogen impacts on seedling performance would be larger under low light (Augspurger, 1983; Stamp, 2003; Myers & Kitajima, 2007). This indicates that fungal pathogens were negatively impacting seedling performance in high light, even without visible differences in fungal damage. Additionally, fungal pathogens may not have impacted seedling height in low light because seedling growth was severely light limited regardless of damage.

Although this study demonstrates important impacts of light, nutrients, and pathogens on L. styraciflua performance, there are several limitations. First, this study was short. While this short duration highlights a critical life stage—survival and growth of establishing tree seedlings (De Steven, 1991; Fridley & Wright, 2018)—we cannot account for differences in overwinter survival or impacts of light, nutrients, and pathogens beyond this window. Moreover, we cannot account for differences in shade tolerance across life stages (Valladares & Niinemets, 2008; Falster, Duursma & FitzJohn, 2018). Second, by growing seedlings alone in pots, we eliminated interspecific competition and may have reduced rain splash dispersal of pathogens. Reduced competition may have been more important in herbaceous-dominated old fields (Flory & Clay, 2010; Fridley & Wright, 2012) than in the more sparsely vegetated forest understory. Thus, growing seedlings in pots versus directly in the ground may have had a larger impact on plant growth in the herbaceous-dominated old field than in the forest understory. Third, we did not compare L. styraciflua to any other shade-intolerant or shade-tolerant species (e.g., Chou, Hedin & Pacala, 2018).

These results suggest that pathogens, nitrogen, and light can be important drivers of succession from old fields to forests (Wright & Fridley, 2010; Fridley & Wright, 2012; Meiners et al., 2015). Under high light, foliar disease reduced the performance of L. styraciflua. This high pathogen impact may ultimately slow the growth and spread of L. styraciflua in old fields, potentially slowing the conversion of herbaceous-dominated old fields to early-successional forests (Gill & Marks, 1991). In closed-canopy forests, pathogens may also be important determinants of species occurrence patterns (LaManna et al., 2017). For instance, seedlings of shade-intolerant species, like L. styraciflua, do not aggregate as often in forest understories as shade-tolerant species, indicating conspecific negative density dependence (Clark, LaDeau & Ibanez, 2004), which may result from foliar (Hersh, Vilgalys & Clark, 2012) or belowground disease. This may prevent shade-intolerant species from maintaining large enough seedling and sapling populations to exploit infrequent tree falls and could result in their exclusion from forest understories (O’Hanlon-Manners & Kotanen, 2004; Wulantuya et al., 2020). Moreover, pathogen impacts on shade-intolerant species can increase in shaded habitats, potentially increasing niche differentiation between shade-tolerant and -intolerant species (McCarthy-Neumann & Kobe, 2008). As in other studies, pathogens in this study were important in low light, where foliar disease increased with increasing nitrogen availability. But increased disease had a limited impact on seedling performance. Instead, light limitation was too severe, even when pathogen pressure and nitrogen limitation were alleviated, to allow rapid seedling growth.

Conclusions

In conclusion, we found that nitrogen and light interact to impact fungal damage, with the highest levels of damage at high nitrogen and low light. Despite this, spraying fungicide impacted seedling height more under high light than low light, indicating that pathogen pressure was higher when light was abundant and that fungal pathogens exerted negative impacts on seedling performance beyond what was visible on leaves. Overall, though, light limitation had the largest influence on seedling performance, overwhelming both nitrogen limitation and pathogen pressure (Ward, 2020). Given this, it appears that the shade-intolerant species L. styraciflua can be excluded from later-successional habitats solely by reducing light availability—high pathogen pressure and nitrogen limitation may further promote this, but are not required.

Supplemental Information