Define big-picture take-aways, potentially adjust organization

[teixeirak]

Here's what I currently see as the big-picture take-aways, with potential rearrangement:

describing the gradients

biophysical

• canopy leaves are exposed to more solar radiation and higher evaporative demands
• vertical gradients are particularly pronounced in forests with dense canopies

leaf temperature

• the biophysical gradient would tend to make sun leaf temperatures elevated above air temperature, particularly when stomatal and /or boundary layer conductance is low (because of drought or low wind)
• we expect leaves near the top of the profile to experience greater max temperatures/ greater temperature variability

leaf traits

• dramatic gradients in leaf traits represent adaptations to deal with the thermal/hydraulic stress described--> canopy trees are adapted to hotter and more variable leaf temperatures

leaf metabolism

• while it's very well established that leaf metabolism generally increases with height across the vertical gradient, we have very little knowledge about differences in thermal sensitivity, optimal temperatures, and thermal damage thresholds
• if we take geographic gradients as a reliable proxy, we'd expect temperature sensitivities to reflect the environment to which the trees are adapted/ acclimatized

size-structuring of whole-tree and ecosystem functioning

• individual performance responses (growth, mort) are mixed: top of canopy is bad place to be during drought, but understory can be more stressed by high temperatures under humid conditions(?)
• partitioning of ET, GPP, etc

implications

global change implications

• Will climate change dampen or amplify gradients?

  warming

• canopy trees in worse position during drought,
• but understory may experience more stress from chronic warming because of fewer adaptations

  canopy disturbance

• loss of canopy trees through natural or human disturbance --> breaking of canopy buffering. This may be advantageous in some situations , but in others expose understory species to conditions to to which they are not adapted.

  projecting in time and space

  how to connect to remote sensing, scale across the landscape

  how to model

[teixeirak]

This potential rearrangement switches the order of current sections 2-3, and places greater relative influence on some of the implications, which we discussed at the end of the call. It would be good to hear what others think: @NidhiVinod @SlotMartijn @mcgregorian1 , @eoway , @m-n-smith , @tyeentaylor , (and Lawren's not in this repo...just sent invite)

[tyeentaylor]

What if you change the name of "Leaf Hydraulics and Temperature" to "Leaf Temperature", and focus there on the drivers of leaf temperature that are not temperature-regulating adaptations. These would include the biophysical drivers mentioned in the previous section, and other "incidentals" such as hydraulics (including tree-height constraints on maximum transpiration rates) and leaf toughness (as an adaptation to withstand hydraulic strain and UV exposure etc., which incidentally affects the leaf thermal time constant). So it's kind of like this section describes the vertical variation in leaf temperature that would be expected if there were no thermoregulatory adaptations of leaf traits. Then the next section says "leaves deviate from these expected leaf temperatures through thermoregulatory adaptations". You can then also describe how those adaptations are coordinated with other limitations like hydraulic path length with height.

The bullet point under Leaf metabolism suggests that there are no consequences to these vertical gradients, which seems to undermine the impact of the paper. But the writing in the draft indicates that the vertical gradient in temperature responses is basically insufficiently explored to say much. If it's insufficiently explored, then it may be best to fill this section with theory and observations from other contexts (such as elevation and latitudinal gradients) that do demonstrate metabolic adaptations to temperature across thermal gradients, and the consequences of temperature change (e.g., Duque PNAS on thermophilization of Andean tree communities via disproportionate death of less warm-adapted species).

For implications, a similar logic could be followed, i.e. warming will disproportionately affect the less warm-adapted functional types within the forest canopy, essentially creating holes in the canopy that will take a long time to refill (especially in slow-growing tropical sub-canopy specialists), as is observed with Andean thermophilization (death is faster than recruitment and growth). Differential mortality within the canopy structure will alter the profile of metabolism and hence emergent forest function. Such alteration to canopy structure may be detectable by lidar and thermal remote sensing. If we know how metabolism maps to the thermal profile, then the observed forest structural changes can inform prediction of forest function.

[teixeirak]

Thanks, @tyeentaylor ! A few responses inline:

What if you change the name of "Leaf Hydraulics and Temperature" to "Leaf Temperature", and focus there on the drivers of leaf temperature that are not temperature-regulating adaptations. These would include the biophysical drivers mentioned in the previous section, and other "incidentals" such as hydraulics (including tree-height constraints on maximum transpiration rates) and leaf toughness (as an adaptation to withstand hydraulic strain and UV exposure etc., which incidentally affects the leaf thermal time constant). So it's kind of like this section describes the vertical variation in leaf temperature that would be expected if there were no thermoregulatory adaptations of leaf traits. Then the next section says "leaves deviate from these expected leaf temperatures through thermoregulatory adaptations". You can then also describe how those adaptations are coordinated with other limitations like hydraulic path length with height.

Yes, I reached the same conclusion re section header and changed it in the draft already (and now above). I think I'd prefer to keep all the traits in the following section, though, as I think it's hard to separate thermoregulatory adaptations from others. What we've done in the draft is to describe the biophysics of how environment, transpiration, and leaf size affect leaf T, and then observed gradients.

The bullet point under Leaf metabolism suggests that there are no consequences to these vertical gradients, which seems to undermine the impact of the paper. But the writing in the draft indicates that the vertical gradient in temperature responses is basically insufficiently explored to say much. If it's insufficiently explored, then it may be best to fill this section with theory and observations from other contexts (such as elevation and latitudinal gradients) that do demonstrate metabolic adaptations to temperature across thermal gradients, and the consequences of temperature change (e.g., Duque PNAS on thermophilization of Andean tree communities via disproportionate death of less warm-adapted species).

Good point. I've noted this in the draft.

For implications, a similar logic could be followed, i.e. warming will disproportionately affect the less warm-adapted functional types within the forest canopy, essentially creating holes in the canopy that will take a long time to refill (especially in slow-growing tropical sub-canopy specialists), as is observed with Andean thermophilization (death is faster than recruitment and growth). Differential mortality within the canopy structure will alter the profile of metabolism and hence emergent forest function. Such alteration to canopy structure may be detectable by lidar and thermal remote sensing. If we know how metabolism maps to the thermal profile, then the observed forest structural changes can inform prediction of forest function.

Noted in the draft (which still remains poorly developed in terms of implications).

[NidhiVinod]

@teixeirak, should the big picture take-away for this section remain the same?

leaf temperature

• the biophysical gradient would tend to make sun leaf temperatures elevated above air temperature, particularly when stomatal and /or boundary layer conductance is low (because of drought or low wind)

• we expect leaves near the top of the profile to experience greater max temperatures/ greater temperature variability

From our chat, here is what we were thinking:

Tleaf section:

-Tleaf generally tracks Tair, with some modification according to the energy balance principles.

In situ section:

• variation across forest types is good. It's also a good point that the leaves have very different Ts than stems (in large part because of traits and gs).

[teixeirak]

I think this Tleaf content looks good, but it will be best to go over this in person (#48 ).

[teixeirak]

I think we can close this.