Write 2-3 sentences about why VPD may be important for budburst and/or leafout

[AileneKane]

@mrjohnston write 2-3 sentences about how VPD, RH or something similar may explain physiologically how and why soil moisture affects budburst (and perhaps leafout? phenology). Include references.

[mrjohnston]

As plants are the central component of the soil-plant-atmosphere continuum, plant water status is a function of both water supply in the soil and water demand by the atmosphere. [The combined influence of water supply and demand on plant water status has recently been highlighted in the context of drought, as increased temperatures associated with global change conditions are predicted to intensify the effects of precipitation deficit (e.g. Griffin and Anchukaitis 2014, Williams et al. 2015).]

For both water supply and demand to play a role in budburst and/or leafout phenology, the following conditions would be required:

(1) plant water status influences budburst and/or leafout phenology (2) plants are connected to a water supply (the soil) - via roots (3) plants are connected to a water-demanding reservoir (the atmosphere) - typically via stomata, but potentially also via lenticels and/or cuticularly.

This paper aims to address (1), and item (2) is certainly true when soils are unfrozen and moist enough that there is root-soil contact. There has been some suggestion that (3) is also true, even pre-budburst: Wolter and Kozlowski (1964) found that dormant buds of all forest species examined transpire as a function of vapor pressure deficit. Therefore, if water status influences phenological timing, we expect that atmospheric water demand may play a role, in addition to soil moisture.

Griffin and Anchukaitis 2014. How unusual is the 2012-2014 California drought? Geophysical Research Letters. 41: 9017-9023. Williams et al. 2015. Contribution of anthropogenic warming to California drought during 2012-2014. Geophysical Research Letters 42: 6819-6828. Wolter and Kozlowski 1964. Transpiration capacity of dormant buds of forest trees. Botanical Gazette 125: 207-211.

[mrjohnston]

Ailene -- I'm happy to develop whatever part of this seems relevant, if any.

To be honest, I expected to find that atmospheric demand wouldn't play a role pre-budburst, so I was surprised to find the Wolter & Kozlowski paper. WolterandKozlowski.pdf

[mrjohnston]

Also this one! This gives a better sense for how much water loss pre-budbreak we might be talking about: 4-5% of annual water loss and ~8% of daily stand transpiration at maximum LAI (measurements of Q. alba & A. rubrum trees at Duke Forest).

"In the 2 weeks prior to bud break, a low yet detectable flux of water was found in many individuals (Fig. 1). Al- though the flow may provide water used in bud swell be- fore bud burst (Sperry et al. 1994), and for refilling the winter-embolized vessels, the linear response of the flux to [VPD] (Fig. 4), and the large quantity of water (ca 32 kg for an average-size tree over the 2 weeks) indicate that much of the water is lost to the atmosphere through the bark."

OrenandPataki2001.pdf