Seedling responses to soil moisture amount versus pulse frequency in a successfully encroaching semi-arid shrub.

ABSTRACT

Rainfall timing, frequency, and quantity is rapidly changing in dryland regions, altering dryland plant communities. Understanding dryland plant responses to future rainfall scenarios is crucial for implementing proactive management strategies, particularly in light of land cover changes concurrent with climate change. One such change is woody plant encroachment, an increasing abundance of woody plants in areas formerly dominated by grasslands or savannas. Continued woody plant encroachment will depend, in part, on seedling capacity to establish and thrive under future climate conditions. Seedling performance is primarily impacted by soil moisture conditions governed by precipitation amount (quantity) and frequency. We hypothesized that (H1) seedling performance would be enhanced by both greater soil moisture and pulse frequency, such that seedlings with similar mean soil moisture would perform best under high pulse frequency. Alternatively, (H2) mean soil moisture would have greater influence than pulse frequency, such that a given pulse frequency would have little influence on seedling performance. The hypotheses were tested with Prosopis velutina, a shrub native to the United States that has encroached throughout its range and is invasive in other continents. Seedlings were grown in a greenhouse under two soil moisture treatments, each which was maintained by two pulse frequency treatments. Contrary to H1, mean soil moisture had greater impact than pulse frequency on seedling growth, photosynthetic gas exchange, leaf chemistry, and biomass allocation. These results indicate that P. velutina seedlings may be more responsive to rainfall amount than frequency, at least within the conditions seedlings experienced in this experimental manipulation.