Shade is the most important factor limiting growth of a woody range expander

Abstract

The expansion of woody plants into grasslands and old fields is often ascribed to fire suppression and heavy grazing, especially by domestic livestock. However, it is also recognized that nutrient availability and interspecific competition with grasses and other woody plants play a role in certain habitats. I examined potential factors causing range- and niche expansion by the eastern redcedar Juniperus virginiana, the most widespread conifer in the eastern United States, in multifactorial experiments in a greenhouse. Historical records suggest that the eastern redcedar is a pioneer forest species, and may be replaced as the forest increases in tree density due to shading. Another possible factor that affects its distribution may be nutrient availability, which is higher in old fields and other disturbed lands than in undisturbed habitats. In its historic range, eastern redcedars are particularly abundant on limestone outcrops, often termed ‘cedar barrens’. However, the higher abundance on limestone could be due to reduced interspecific competition rather than a preference for high pH substrates. I manipulated shade, fertilization, lime, and interspecific competition with a common dominant tree, the post oak Quercus stellata. In a separate experiment, I manipulated fire and grass competition. I measured growth rates (height and diameter) and above- and belowground biomass at the end of both experiments. I also measured total non-structural carbohydrates and nitrogen in these plants. Shade was the most important factor limiting the growth rates and biomass of eastern redcedars. I also found that there were significant declines in nitrogen and non-structural carbohydrates when shaded. These results are consistent with the notion that the eastern redcedar is a pioneer forest species, and that shade is the reason that these redcedars are replaced by other tree species. In the second experiment, I found that a single fire had a negative effect on young trees. There was no significant effect of competition with grass, perhaps because the competitive effect was shading by grasses and not nutrient depletion. Overall, the effects of shade were far more apparent than the effects of fire.

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

“…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.…”

“…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 ).…”

“…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:…”

Effects of post oak Quercus Stellata and Smooth Brome Bromus Inermis Competition on Water Uptake and Root Partitioning of Eastern Redcedar Juniperus Virginiana

“…Similar results have been obtained in other studies that determined the effects of competition by grasses on trees 48-51 . Ward (2020, 2021) 29,54 showed that shade is the most important factor affecting the growth rate and biomass of ERC. On the other hand, we found no evidence of the negative effect of post oak competition on ERC performance, likely due to the niche differentiation between the ERC and post oak.…”

“…An important factor that contributes to the improved growing conditions of ERC is reduced plant competition with neighboring plants 28 , such as grasses (Ward, 2020). A major grass competitor is Bromus inermis (hereafter smooth brome).…”