The Tamaulipan thornscrub forest once covered the Lower Rio Grande Valley and is characterized by a subtropical climate and high biodiversity. The habitat of over 35 endangered species now faces conservation challenges due to population growth. To promote biodiversity conservation, 3.2 million seedlings have been planted in the region since 1995. However, assessment of factors influencing reforestation success is limited, such as the role of herbaceous weeds. At La Sal del Rey National Wildlife Refuge, we conducted a manipulative experiment to discern if weeds are facilitative, competitive, or have no impact on native species of contrasting growth habits. We hypothesize weed presence will be facilitative, particularly for fast-growing early-successional species, due to the shade and shelter provided during stressful (hot + dry) conditions. In June 2019 we selected twenty individuals of eight species and assigned half to each of two treatments: weed exclusion (n=10) and control (n=10) for a total of 160 seedlings. We conducted monthly demographic surveys to measure plant vigor, height, branching, and animal damage-- indicators of overall seedling health and survivorship. We also quantified seedling light-saturated net photosynthetic rate (Asat) to determine physiological impacts of weeds, as well as the abundance of beneficial and herbivorous insects.
Results/Conclusions
Overall, weed presence did not significantly affect seedling mortality or net plant height growth after 6 months of weed exclusion, even when considering contrasting growth habits (fast vs. slow). However, mammalian herbivory was prevalent in many seedlings. When we examined the response of seedlings without significant damage, we found evidence that fast-growing species are in fact aided by weeds. Our analysis shows that the facilitative effect of weeds on non-animal damaged seedlings was during periods of intense abiotic stress (August-September). However, the facilitative effect of weeds is lost during the non-stressful times of the year (October-December). Asat measurements indicated no significant difference between treatments, suggesting that the positive effect of weed exclusion on fast-growing plants is likely due to improved microclimatic conditions rather than changes in inherent photosynthetic capacity of seedlings. Taken together, these results suggest that while weeds do facilitate certain species during stressful conditions in accordance with theory and expectations, they may not necessarily shield seedlings from herbivores. Future work should explore the drivers of animal damage and the effects of seedling palatability on the success of restoration efforts.