(A) A map of McLaughlin reserve detailing the nested experimental design and structure of a block of plots at a single site [adapted from ref. 16). Experimental sites (blue) were collated in groups of 5 (yellow) and evenly distributed across both halves of the reserve (orange); see Materials and Methods for a detailed description of how the dispersal treatments were imposed. Within a site, shaded plots represent controls that were deemed irrelevant to this current study. Colored arrows correspond with contrast plots used to calculate compositional and diversity differences between the control plot (a proxy for the natural community state) and the dispersal treatment plots, as required to answer the first research question (Fig. 2). Compositional and diversity differences between the 1 m plot and control plot (i.e., 1 m vs. 1 m; purple) were assessed to determine baseline turnover/richness differences at the plot level within a single site. (B) Climate at McLaughlin Reserve during the study period (2014–2016). “Years” are delineated by growing season (i.e., September to August), as this delineation is most appropriate for our research questions. Light blue bars indicate total annual precipitation (mm) received over the growing season, with mean monthly precipitation (mm) and mean monthly temperature (°C) represented by dark blue and red lines, respectively. Figure panel A was designed with help from S. Heredia.
Significance
Dispersal helps maintain diversity by moving propagules over space. However, how dispersal affects diversity in environments that vary in space and time is unclear. We tracked how experimental seed dispersal conducted across a range of spatial scales impacted plant community diversity in serpentine grassland communities over three years with highly divergent rainfall. We show that high dispersal can enhance diversity by supplementing aboveground seed banks so species are present to respond when conditions become favorable. We also found delays in population growth with shifts from dry to wet conditions may help maintain populations of weak competitors. Our work reveals an interplay between dispersal and dormancy that may be important for maintaining diversity in variable climates.
Abstract
The size and composition of local species pools are, in part, determined by past dispersal events. Predicting how communities respond to future disturbances, such as fluctuating environmental conditions, requires knowledge of such histories. We assessed the influence of a historical dispersal event on community assembly by simulating various scales of dispersal for 240 serpentine annual plant communities that experienced a large shift from drought to high rainfall conditions over three years. We collected, aggregated, and redistributed the aboveground seed bank (i.e., all loose seed present on the soil surface before the growing season initiates) from 30 sites across five nested spatial scales (1 m, 5 m, 100 m, 5 km, and 10 km), and annually censused communities to identify change in community structure among dispersal scales and over time. Our one-time dispersal manipulation diversified aboveground seed banks, provided insurance against temporal variability at intermediate and large dispersal distances (100 m), and even supported community reassembly toward a new compositional state at the 10 km scale. We also found evidence that temporal lags directed community responses to environmental fluctuations, preventing rare species extirpations and providing subordinate species discrete windows of time to supplement their seed banks. Our results reveal a joint spatiotemporal equilibrium in this system where dispersal through space interacts with temporal fluctuations in climate to support species’ persistence via aboveground seed banks. This experiment underscores the importance of dispersal for diversity maintenance in this global biodiversity hotspot, where the magnitude and frequency of future climate fluctuations is uncertain.