Changes in light and nitrogen availability under pioneer trees may indirectly facilitate tree invasions of grasslands

Abstract

Summary The first trees establishing in grasslands may indirectly favour their seedlings in competition with neighbouring herbaceous vegetation by increasing soil fertility with nitrogen‐rich litter and by reducing light levels under their canopies. It is predicted that increasing soil nitrogen availability will accelerate invasion of trees by stimulating their growth more than that of herbaceous species. Decreasing light availability is predicted to increase tree invasion by limiting the growth of herbaceous vegetation more than that of trees (competitive release). We tested these predictions using Chinese tallow tree (Sapium sebiferum), which is an aggressive alien invader of grasslands in the southern USA, and Hackberry (Celtis laevigata), which is a native tree common in these grasslands. Nitrogen and light were manipulated in two factorial field experiments in grasslands in Texas, USA. In the first, nitrogen was increased and light was decreased in plots with planted Celtis or Sapium seedlings. In the second experiment, light availability to planted Celtis or Sapium seedlings was increased by holding back prairie vegetation. In the first experiment, growth of Celtis and Sapium seedlings increased with nitrogen fertilization while the above‐ground biomass of prairie vegetation did not change. Prairie vegetation biomass decreased and tree seedling growth increased under shading. Sapium's growth increased dramatically in the treatment with combined nitrogen and shade. Sapium survival decreased when shade was applied. In the second experiment, Sapium growth increased with increased light. Thus, increased Sapium growth at low light levels in the shade experiment was probably a consequence of decreased competitive interference from prairie vegetation, rather than better absolute performance of Sapium in low light levels. These results provide evidence for facilitation as a mechanism involved in tree invasions of grasslands. Changes in resource levels, perhaps in combination with other factors, may explain rapid conversion of grassland communities to woodlands after the first pioneer trees are established. The marked response of Sapium to the combination of nitrogen and shade suggests that these positive feedbacks may be particularly strong for this alien plant species.

Rapid adaptation of insect herbivores to an invasive plant

“…In fact, tallow tree had a lower growth rate than natives in the two locations where it has been present for a longer period of time (figure 1a). There are many possible explanations for this pattern, such as differences in soil types (Barrilleaux & Grace 2000), moisture (Burns & Miller 2004;Butterfield et al 2004) or environmental conditions (Rogers & Siemann 2002;Siemann & Rogers 2003c) among sites, but results of herbivory bioassays indicated that pest accumulation likely plays a role in this reversal of tallow tree and native performances in at least the Georgia site. Since seedling survival and growth can determine plant population dynamics (Crawley 1996), these results for seedlings may indicate tallow tree invasion pressure in these locations as well.…”

Long-Term Effects of Adding Nutrients to an Oligotrophic Coastal Environment

“…Disturbance can be broadly defined to include alterations in resources (Pickett and others 1989), such as changes in the availability of limiting nutrients that subsequently alter plant community structure and composition (Fourqurean and others 1995;Siemann and Rogers 2003).…”

The relative importance of seed competition, resource competition and perturbations on community structure

“…being a poorer competitor for light, water or others resources (e.g. Siemann and Rogers, 2003). We calculate the competitive dominance and, respectively, the relative size d i of a PPS by normalising its biomass per m 2 of occupied area BM i with the sum of biomass of all n PPSs:…”

“…This assumption is to a first approximation reasonable, since differences in competitive ability result mainly on differences in size. For instance, the acquisition of light depends on size (Bengtsson et al, 1994) such that, for example smaller plants are over-shaded by larger ones (Siemann and Rogers, 2003). But smaller sized plants could also have higher competitive abilities than bigger ones (Keating and Aarssen, 2009).…”