Ecosystem changes following selective mortality of individual trees can be difficult to predict and depend on the response of surviving trees in the ecosystem. Insect pests and pathogens can remove one species leaving behind competing individuals. In the Southeastern United States, southern pine beetle (Dendroctonus frontalis Zimmermann) selectively attacks and kills southern pines, particularly loblolly pine (Pinus taeda L.) which is prevalent throughout the region. Loss of loblolly pine should alter water and nutrient cycling unless co-occurring species are able to compensate for this lost functioning. The goals of this study were to determine changes in water and nutrient cycling of surviving trees experiencing mortality of nearby individuals. We selectively girdled overstory loblolly pines to simulate southern pine beetle attack and measured the physiological functioning of mid-canopy sweetgum (Liquidambar styraciflua L.) trees that were growing near girdled or control pines. Sapflow rates and environmental parameters including soil moisture, vapor pressure deficit (VPD) and photosynthetically active radiation (PAR) were measured continuously over a three year period that included pine mortality. Photosynthetic capacity was compared in sweetgums near girdled or control pines. Sweetgum leaf nitrogen content and litterfall nitrogen content were compared between the pine girdling year and the pine mortality year.
Results/Conclusions
We found that sweetgums near girdled pines used significantly less water compared with sweetgums near control pines during the pine girdling year, used similar amounts of water during the following pine mortality year and tended to use more water during the post-mortality year. Relationships between water use and environmental parameters followed these yearly trends with sweetgums near girdled pines exhibiting significantly lower slope terms compared to sweetgums near control pines in the relationship between daily sapflow and either PAR or VPD during the girdling year, exhibiting similar slope terms in the mortality year and significantly higher slope terms in the post-mortality year. Most photosynthetic parameters were similar between sweetgums in differing locations except for maximum Rubisco carboxylation rate (Vcmax) which was significantly larger in sweetgums near girdled pines during the mortality year. Across sweetgum trees, both leaf and litterfall nitrogen content were significantly higher in the pine mortality year compared with the previous pine girdling year. Together, these results suggest that surviving trees can compensate for extra nutrients available to the ecosystem during mortality but that compensation in water use shows a lagged response with sweetgums being negatively affected while pines undergo mortality but exhibit increased water use following mortality.