Dr. Holl's Current Research Projects
Recovery of Tropical Forest in Abandoned Pasture
For a number of years I have studied factors limiting recovery and strategies for facilitating recovery in abandoned pasture in Costa Rica. My research shows that lack of seed dispersal and competition with pasture grasses are the most important factors limiting pasture recovery at the site I studied (Holl 1999). I have tested different strategies for facilitating recovery, including bird perching structures (Holl 1998) and planting native tree seedlings (Holl et al. 2000; Holl and Quiros-Nietzen 1999). I have also studied the effect of early-successional shrubs on long-term forest recovery (Loik and Holl 2001; Holl 2002). Starting in 2004, I established a large-scale tropical forest restoration project in southern Costa Rica in collaboration with Dr. Rakan Zahawi (Organization for Tropical Studies) and with funding from the Earthwatch and the National Science Foundation. We are testing questions about “applied nucleation” by planting islands of native tree seedlings to facilitate recovery and studying the effect of the amount of surrounding forest cover on ecosystem recovery. We are collecting extensive data on seed dispersal, seed fate, vegetation establishment, and seedling dynamics. We are also collaborating with other investigators who are studying birds, bats, plant physiology and nutrient cycling. Dr. Rebecca Cole recently completed her Ph.D. studying seed dispersal, predation, and germination in this system, and Leighton Reid is studying seed dispersal by bats and birds on these plots.
Disturbance Effects on Vegetation Community Dynamics in California Coastal Prairie
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Since 1998, former graduate student, Grey Hayes, and I have investigated the effects of cattle grazing and mowing on endangered annual herbs in California coastal prairie, using both large scale surveys and manipulative studies at multiple sites. Our results suggest that when native species are present, grazing favors native annual forbs, but decreases the cover of native perennial forbs (Hayes and Holl 2003a). However, in the absence of a native seed bank clipping serves primarily to shift the vegetation community composition from exotic annual grasses to exotic annual forbs (Holl and Hayes 2003b). We have studied the effect of reintroduction and disturbance regimes on the endangered annual Santa Cruz tarplant (Holocarpha macradenia) (Holl and Hayes 2006). We now have 10 years of vegetation data from our manipulative experiments that we are analyzing. My current graduate student, Mike Vasey, and I are conducting monitoring of grassland on the UCSC campus with a focus on impacts on the endangered Ohlone Tiger Beetle, and graduate student Lewis Reed is studying grassland and coastal sage scrub restoration.
Large-scale Riparian Restoration along the Sacramento River
Large-scale riparian restoration along the Sacramento River - I am studying the spatial scale at which ecosystem recovery is regulated in restored riparian forests along the Sacramento River. The Nature Conservancy and other government and non-profit agencies are working to restore nearly 5000 acres of riparian forest along 100 river miles. Primarily, I am conducting fieldwork to assess the importance of landscape vs. local site factors on the colonization and establishment of native understory plant species in restored riparian forest. Our past research suggests that establishment of native understory plants is primarily influenced by local factors and much less so by the surrounding landscape (Holl & Crone 2004). Prairie Johnston (master’s student at California State University Chico) and I recently completed an experimental study testing the relative importance of of overstory cover, exotic understory cover, and distance to forest on native seedling establishment and survival. During spring 2007, in collaboration with master’s student, Charles McClain, I repeated my earlier vegetation survey in order to assess how understory plant communities in restored sites have changed over time, which soon will be published in Restoration Ecology. I have also collaborated with social science graduate students and faculty at UCSC studying ecological and management feedbacks between restored sites and agricultural land in this landscape. I anticipate focusing my future research efforts in California more in the Santa Cruz area and am interested in talking to prospective graduate students who have research questions related to restoration of vegetational communities along the central coast of California at multiple spatial scales.