Resource allocation trade-offs in a mast-seeding conifer: pinon pine prioritizes reproduction over defence

The cost of plant reproduction or defence at the expense of other fitness traits is a central component of life history theory. Yet the three central resource allocation pathways of growth, reproduction and defence have rarely been assessed simultaneously nor across individual to landscape scales. This information is critical towards identifying the physiological, environmental and genetic mechanisms underpinning resource allocation. This study assessed trade-offs in resource allocation between tree growth, defence and reproduction across scales among pinon pine (Pinus edulis), a widespread mast-seeding conifer of the southwestern USA. Time series (2004-16) of tree growth (radial and primary shoot growth), defence (resin duct production; a key constitutive defence for this species) and cone production among individual trees from populations across a broad environmental gradient were used to investigate these trade-offs in resource allocation across three scales: individual, population and landscape. We found evidence for a defence-reproduction trade-off among individuals whereby total resin duct area in annual xylem rings was lower during years of above-average cone production. Despite variability in cone and resin duct production across trees within a population and across populations, there was no association between these fitness traits at either of those scales. There was no evidence of trade-offs between cone production and growth at any scales measured, whereas resin duct production and growth were positively related at all scales. Our study suggests that a strategic trade-off occurs whereby investment into defence is temporarily curtailed to favour reproduction, despite increased risk of exposure to natural enemies and the ability of pinon pine to simultaneously allocate carbon to growth and defence. Our study provides new insights into physiological expressions of growth, defence and reproduction over time in this long-lived masting conifer and indicates the presence of trade-offs with direct importance for individual fitness and population dynamics under global change.