Woody plant proliferation has been occurring in grasslands globally. Shrub recruitment is a critical first step in the resulting grassland-to-shrubland transformation process. Shrub recruitment may vary with the timing, frequency, and amount of precipitation (PPT), with activities of seed/seedling predators, and with ungulate grazing intensity. Predicting shrub recruitment therefore requires knowledge of the relative strength of these factors and their interactions. Here, we ask, “How do PPT, ant/rodent herbivory, and large ungulate grazing interact to influence velvet mesquite (Prosopis velutina) recruitment in Sonoran Desert grasslands?” An Automated Rainfall Manipulation System was used to manipulate PPT, wherein plots received -65% ambient PPT (= ‘drought’), +65% ambient PPT (= ‘wet’), and ambient PPT (n= 10/PPT treatment). Half of each plot was clipped to 15 cm simulating heavy grazing; the remaining half was unclipped. Exclusion treatments (None, Rodents, Ants, Rodents+Ants) were installed within each PPT x Clipping combination. Scarified mesquite seeds were placed within each treatment combination at the start of the monsoon in July 2017, 2018 and 2019.
Results/Conclusions
First year recruitment of the 2017, 2018 and 2019 seedling cohorts was consistently higher in wet and ambient conditions compared to drought, with the highest in 2017 (47 to 58% across all PPT and grass defoliation treatments), lowest in 2019 (6 to 10%), and intermediate in 2018 (1-22%) (χ2 = 277.8, p < 0.001). These interannual differences in 1st year recruitment occurred despite all years having a comparable number of rain days (30, 31 and 33 in 2017, 2018 and 2019, respectively) and total monsoon season precipitation (230, 136 and 224 mm; χ2 = 0.12, p = 0.94). However, the maximum number of consecutive days of rainfall in the high recruitment year of 2017 (16 days totaling 153 mm PPT) exceeded that in 2018 (9 days totaling 38 mm) and 2019 (6 days totaling 67 mm). Recruitment was reduced in areas accessible to both rodents and ants (2017: 44.5 ± 2.1% vs. 56.8 ± 1.9% when protected); 2018: 11.9 ± 2.5% vs. 18.1 ± 2.7%; 2019: 4.7 ± 1.4% vs. 10.1 ± 2.4%) (χ2 = 46.1, p < 0.001). Contrary to expectations, grass defoliation (none vs. clipped) had no effect in any year (χ2 = 0.05, p = 0.82). Our data suggest shrub recruitment in Sonoran grasslands is highly sensitive to intra-seasonal rainfall patterns (ηp2 = 0.59), modestly sensitive to rodent/ant herbivory (ηp2 = 0.03), and insensitive to herbaceous cover (ηp2 < 0.001)