Interference of bull thistle (Cirsiumvulgare) with growth of ponderosa pine (Pinusponderosa) seedlings in a forest plantation

Abstract

The biennial thistle Cirsiumvulgare (Savi) Tenore significantly reduced ponderosa pine (Pinusponderosa Dougl. ex Laws.) seedling growth during the second year of infestation but had insignificant effects in the first year when all thistles were in the rosette stage. Pine stem diameter relative growth rate was significantly negatively correlated with four different indices of thistle interference and with visual estimates of thistle cover. Total thistle density (adults + rosettes) within 2.0 m of target seedlings best explained differences in stem relative growth rate, but density of adults alone and visual estimates of thistle cover were nearly as good. Simple regressions indicated that soil moisture and pine predawn leaf water potential were significantly negatively correlated with thistle density and significantly positively correlated with stem relative growth rate, but multiple regressions and path analyses indicated that their effects on seedling growth were negligible relative to the other (unexplained) effects of thistle density. Foliar nitrogen, phosphorus, and potassium concentrations were not significantly correlated with thistle density and failed to explain differences in seedling growth. Although it remains unclear how thistles suppressed pine seedling growth, if these results hold true at other sites, plantation managers will have at their disposal relatively easy methods for assessing thistle interference.