Theses and Dissertations

Date of Award


Document Type


Degree Name

Master of Science (MS)



First Advisor

Dr. Bradley Christoffersen

Second Advisor

Dr. Rupesh Kariyat

Third Advisor

Dr. Teresa Patricia Feria Arroyo


Dryland ecosystems need effective restoration strategies to address severe degradation. It is often assumed that voluntary forb weeds either compete with or have no effect on native seedlings. In contrast, theory and empirical work suggest a potential facilitative role for such forb weeds. We conducted a manipulative weed exclusion experiment at a semi-arid site in South Texas targeted for large scale forest restoration and subsequently dominated by early successional forb weeds to discern the net effect of these weeds on the growth and survival of target thornscrub tree and shrub seedlings. Overall, weed presence did not significantly affect seedling mortality or net plant height growth after 6 months of weed exclusion, even when considering contrasting seedling growth habits. However, mammalian herbivory was prevalent in many seedlings. Excluding seedlings with significant animal damage, we found that during periods of significant abiotic stress (hot and dry), weed presence significantly improved native seedling growth rates, in accord with plant facilitation theory. This facilitative effect was driven primarily by species with a fast growth habit. During hot and dry conditions, light-saturated photosynthetic capacity (Asat) and air temperature on seedlings adjacent to weeds were not significantly different from that of weed-excluded seedlings, but afternoon light levels were reduced by ~50%, possibly indicative of lower leaf temperatures and improved microclimatic conditions. Our results offer sufficient evidence to warrant further research into how best to exploit plant-plant facilitation in dryland forest restoration and management. Future work should consider the ecological strategies of pioneer weed species by strategically planting clustered, multispecies restoration pockets in harsh environments to ameliorate microclimatic conditions and minimize effects of herbivory.


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