Fluctuating resources and invasion establishment: Linking demography with environmental drivers
Understanding the mechanisms that drive the success or failure of novel plant populations is central to invasion biology. However, ecologists face important challenges with respect to the idiosyncrasies of specific species and systems and the difficulty in linking environmental drivers to population-level outcomes. In this study we experimentally tested the theory of fluctuating resource availability with an approach that quantitatively integrates environmental, physiological, and demographic datasets using integral projection models (IPMs). The supply of soil moisture in an old-field community was experimentally manipulated in situ at the Boston Area Climate Experiment (BACE; Waltham, MA, USA) with treatments consisting of 50%, 100%, and 150% of ambient precipitation. Within each precipitation treatment we manipulated the potential uptake of soil moisture by manually clipping background vegetation or leaving it intact. We invaded experimental plots in the autumn of both 2012 and 2013 with seeds of Persicaria lapathifolia, an annual herb that is common to the area, but largely absent from the plant communities at BACE. We sampled: complete life-cycle demography for nearly 2,000 P. lapathifolia individuals, environmental variables such as soil moisture and soil inorganic nitrogen, and physiological variables such as leaf chlorophyll and tissue C:N.
Overall, increases in soil resource supply and/or decreases in uptake promoted invasion by P. lapathifolia. Furthermore, plots that did not receive manipulations hypothesized to be favorable to invasion were entirely resistant to invasion in 2013 (i.e. total seed production near zero). This is broadly consistent with the theory of fluctuating resource availability, but amid substantial variability across microsites. Soil moisture manipulations interacted with the clipping treatment to influence soil nitrogen availability, with drought plots exhibiting higher nitrate levels, particularly when background vegetation was clipped. This was associated with variation in tissue C:N and leaf chlorophyll, suggesting direct and indirect effects of interacting soil resource manipulations. Although P. lapathifolia in the drought treatment exhibited higher growth early in the growing season, low soil moisture in the summer was linked to reduced growth and survival that alone reduced annual population growth by roughly 40% compared to the high soil moisture treatment. This shift in demographic rates throughout the growing season suggests that antecedent drought may promote invasion in the spring (through a hypothesized negative legacy on background community resistance), but is later countered by the effects of concurrent drought.