Rapid tree mortality following experimental drought

Background/Question/Methods

Drought induced tree mortality has become conspicuous in mid-latitude regions of the Earth. But, the causes and consequences of this phenomenon remain largely unknown. Several interrelated factors have been proposed to explain tree mortality following drought e.g. failure to water transport, carbon starvation, bark beetle attack, fungal infestations, but empirical evidence to support individual and connected mechanisms inducing drought is scarce. To test the causes of tree mortality and/or survival, we are conducting a rainfall manipulation in a piñon-juniper woodland at the Sevilleta LTER in central New Mexico, USA. We are investigating how Pinus edulis and Juniperus monosperma respond to rainout treatments in large (1600 m2) replicated (n=3) plots. We are asking two broad questions: 1) what, if any, are the hydraulic mechanisms controlling tree response to chronic drought? 2) what are the physiological, ecological and environmental interrelations among the different factors causing tree mortality? Theoretical developments enriched with results from the first year of this study provide insights on the mechanisms underlying tree mortality and its consequences. Results/Conclusions Experimental drought was immediately effective as indicated by decreased soil and plant water potentials, and down regulated water fluxes in treated plots with respect to controls. In fact, ten months after the drought imposition, 7 out of 19 of the P. edulis trees growing in drought plots (36 %) showed rapid signs of mortality by turning their crowns brown within a three week timespan. During the ten month period, plant water potentials were kept below limits that allowed transpiration in this species and likely precluded trees from gaining carbon through photosynthesis. Physiological down regulation during the mortality event was accompanied by bark beetle (Ips confusus) and associated ophiostomatoid fungi infestation in the sapwood of dying trees. Although the relative importance of all these interrelated factors is not yet discerned, we believe that a predisposition to tree mortality is mediated by plant hydraulic limits during drought.