Invasive Pine Tree Effects on Northern Coastal Scrub Structure and Composition

Abstract Vegetation that becomes overtopped usually experiences a decrease in abundance or species richness. When an overtopping plant alters the physiognomy of the existing vegetation (e.g., trees invading a shrubland), ecosystem processes can also be dramatically altered. Worldwide, Monterey pine (Pinus radiata) cultivars have been planted in Mediterranean-like climates and are known to invade surrounding natural communities. Ecological impacts resulting from these invasions have been widely investigated; however, the effects from solitary pine trees on the vegetation they overtop are lacking. Furthermore, studies on the impact of P. radiata cultivars from the California floristic province, where P. radiata is native, do not exist. In coastal California, north of the present-day range of native P. radiata stands, cultivars of this species have invaded northern coastal scrub vegetation. To determine the impact of pine invasion on species richness and structure in this habitat, floristic surveys were conducted in 20 blocks that consisted of invaded and uninvaded plots. An invaded plot contained two subplots located under the canopy of an isolated pine tree, whereas a paired, uninvaded plot contained two subplots located in coastal scrub adjacent to each pine. Pine trees selected ranged in size from 2.8 to 119 cm (1.1 to 46.9 in) basal diameter. Our results demonstrate that understory native cover and species richness are negatively correlated with tree size. Understory exotic plant cover and richness of species other than P. radiata did not show any correlation with tree size, mainly because exotic plants had a very low abundance overall. Nomenclature: Monterey pine, Pinus radiata D. Don Management Implications: It is not surprising that large, invading trees would affect or alter the vegetation of treeless communities. Understanding what types of changes are taking place is critical for determining how and when invasive tree removal or other types of management practices should be implemented. Based on our results, to minimize the negative effects of invading trees on native plant cover and species richness in coastal scrub habitat, invasive trees should be removed as early in their stage of growth as possible. Furthermore, removing invasive trees before they become reproductively mature can also help control their spread, although this was not a focus of our study. In our study, we demonstrate how the effect of an invasive tree species can be understood from just one point in time by sampling the understory of several different-sized trees. Pinus radiata is one of the most widely planted trees in the world, so these findings and methods may be helpful to a broad audience and applicable to other invaded habitat types, although the ecological impacts of this species could be different in other parts of the world. These methods may also be useful for studying the effects of invasive tree species in general. In our study area, one of the management objectives is to preserve the mosaic of coastal scrub and grasslands that are habitat for numerous special-status species. Therefore, understanding how the primary vegetation type (coastal scrub) is altered by an exotic tree can provide justification for spending resources on control methods. Although there is compelling evidence that the invasive pines in our study area are derived from cultivars bred for commercial purposes, additional genetic work from the study area should be done to confirm that assessment. We argue that cultivated P. radiata with an unknown origin would be ill-suited for in situ conservation of native P. radiata or for use as neonative populations. There also could be significant differences in how cultivars vs. native strains of P. radiata behave (growth rate, fitness, ability to spread with and without fire cues, among others), which could affect how they are managed.