UC Santa Cruz

This dissertation examines the effects of nutrient limitation on plant physiology and community assembly through the lens of Mendocino’s iconic pygmy forest, a plant community in California, USA, that is severely stunted by acidic, low-nutrient, and high aluminum soils, but that experiences negligible water stress year-round.

Chapter 1 examines physiological mechanisms that lead to stunted growth by comparing 12 leaf functional traits between conspecific plants growing in the pygmy forest and nearby tall conifer forests. Pygmy plants did not have depressed leaf photosynthetic rates, which I had hypothesized were the mechanism of stunting. Instead, they grew fewer leaves, but their leaves were thicker and tougher, indicating they allocate fewer resources toward new leaves and growth, but instead increase investment in defense from mechanical damage and herbivory. This resource-conservative strategy increases their survival rate but decreases their growth rate.

Chapter 2 examines xylem physiology by comparing 13 anatomical and hydraulic traits between pygmy and conspecific control plants. Pygmy plants had smaller conduits, but could transport more water to each leaf than controls, due to their reduced leaf production. Pygmy plants had divergent responses in cavitation resistance, a critical predictor for drought survival: in a vessel-bearing angiosperm, pygmy plants were more vulnerable to cavitation, while in a conifer, pygmy plants were more resistant. Overall, changes in water transport due to nutrient limitation were profound, despite the absence of water stress.

Chapter 3 examines community assembly in the pygmy forest using a series of 100 m2 plots. Pygmy forest plots had more species than adjacent tall conifer forests, but these species were more closely related to each other, indicating that fewer taxonomic groups can survive in the pygmy forest. Within the pygmy forest, higher soil aluminum levels, which stunt root growth, predicted lower phylogenetic diversity. Taxa with high water transport efficiency, a resource-acquisitive strategy, were rare at these low diversity sites, indicating that water transport is a key predictor for which species survive on the most stressful soils.

Overall, this dissertation reveals the critical role resource conservation plays in plant response to nutrient limitation across scales.