Understanding how plants respond to their environment is a major uncertainty in the global carbon cycle. Temperatures are expected to rise almost universally across the planet while rainfall and humidity have less consistent trends with deluge and drought (e.g., extreme events) becoming more common. Increasing Vapor Pressure Deficit (VPD) is expected to change physiological performance and species persistence under novel climates. Conifer trees of the genus Pinus represent a large portion of terrestrial carbon flux and have relatively simple leaf hydraulic structure creating approachable and opportune system for investigating plant hydraulic and physiological function in changing environments.
Using three Pinus species, virginiana, echinata, and strobus we measured a suite of leaf and stem anatomical and physiologic traits. We grew seedlings of each species in growth chambers under ideal conditions (25°C daytime temperatures, 415PPM CO2, 800 µmol PPFD) to promote growth of new adult foliage representative of the controlled environment. A novel VPD response technique was used to quantify changes of gas exchange and photosynthetic performance by independently changing temperature and humidity. Using structural equation modeling, we analyzed the hydraulic pathway of water within the leaf considering all measured parameters.
Results/Conclusions
The different species had a wide range of anatomical traits including number of needles per fascicle, mesophyll thickness, and vein diameters. Physiological measures of Amax, gmax, turgor loss points, minimum conductance, leaf hydraulic conductance and xylem vulnerability were also variable. P. virginiana had the highest measures of Amax, gmax and leaf hydraulic conductance possibly resulting in larger decreases in A and gs under differing VPD regimes. Our model results show that mesophyll thickness and leaf hydraulic conductance were major influences on the VPD response of both A and gs. Our findings can be used by plant physiologists to predict response to VPD across the whole genus and inform investigations across other taxa. Existing plant trait databases provide the opportunity to investigate the extensibility within Pinus, and more broadly, conifer trees. Future investigations will continue this research under soil drought and among more species within the genus.