Species Diversity and the Relationship to the Hydraulic Survivablity of Forests in Drought. [View all]

[NNadir]

The scientific paper I will discuss in this post is this one: Hydraulic diversity of forests regulates ecosystem resilience during drought (Anderegg et al. Nature volume 561, pages 538–541 (2018).

Chronic international indifference to climate change - we are all involved, left and right, and in my opinion, all indifferent, all contemptuous of science that we don't intuitively like, based not on the facts of science and on the result of experiment, but on bias - has cause a remarkable rise in temperatures and macroscopic destabilization of the frequency of major weather extremes.

Among those extremes one that bothers me personally quite a bit - I'm an arbophile (if that's a word), a lover of trees - is drought. One of the things I love about the State of New Jersey, where I live, is its forests. As I watch trees die - something going on with increasing frequency - a little piece of me rots with the wood.

I am not a biologist or plant physiologist, but here and there, when time allows, I try to sneak in a few views of papers relating to the physiology of trees.

Some years back, on another site (where I was banned for telling the truth, not that telling the truth is something I would change about myself, even in the era of the revived Big Lie, for mere convenience or popularity) I wrote about one of these readings: Nitrogen, Climate Change, Drought, and Tree Physiology.

The Gods cursed Cassandra, but in all mythology, she is my moral favorite. The insistence among human beings to hear only what they want to hear is ageless.

Things have gotten much worse since I wrote that bit in 2010; when I wrote that piece the concentration of the dangerous fossil fuel waste in the planetary atmosphere was roughly 385 ppm. This month - near the annual minimum - the concentration is roughly 406 ppm.

Whatever it is we think we're doing isn't working and so long as we lie to ourselves by embracing popular nonsense thinking the acceleration of things getting worse will continue.

Anyway, brief excerpts from the Nature paper and some pictures:

Water, carbon and energy exchanges from the land surface strongly influence the atmosphere and climate; these exchanges are dominated by plants in most ecosystems1. Plant physiological responses to water stress influence these fluxes5,6, and the resulting land-surface feedback effects influence local weather as well as the regional atmospheric circulation7. Furthermore, changes in vegetation physiology and cover can drive shifts in sensible and latent heat fluxes that intensify droughts2,3,4,8. Anthropogenic climate change is expected to intensify the hydrological cycle globally, leading to more frequent and more severe droughts in many regions9. Therefore, understanding the drivers of land–atmosphere feedback effects during drought and simulating them in Earth system models is critical for robust future projections and assessment of climate change impacts.

Seminal work has shown that grassland plots with more species exhibit smaller declines in productivity during drought and recover productivity much faster following drought10, indicating that plant biodiversity—particularly functional diversity—may be important for capturing how the land surface interacts with the atmosphere during extreme events. Indeed, it is well-established that—just as a diversified stock portfolio is more likely to survive market turbulence11—diversity can stabilize community function through multiple mechanisms12. First, diverse communities are more likely to contain species with different traits that dictate how they respond to disturbances13. As a result, at least some species are likely to persist through any given disturbance14. Second, diverse communities are more likely to contain competitors that exhibit compensatory dynamics: when drought causes one species to decline in function, its competitor may increase in function and stabilize community function12. Critically, diversity–stability effects are mostly absent in most global land-surface models, most of which represent each biome or plant functional type with a single set of functional traits15, partly owing to a lack of understanding of which functional traits are the most important at ecosystem scales.

In their study of forests, the authors consider a number of factors, the age of the forest, leaf area, and several factors related to the biophysics of water transport, the point at which 50% of the xylem water conductivity is lost, and a factor they call "HSM," the "hydraulic safety margin," by considering the species which make up 80% of the biomass of the forest, and physical traits like wood density.

This graphic gives an idea of their focus:



The caption:

The percentage of explained variance (R2) in an ordinary least-squares linear regression of community-weighted plant traits (dark red, hydraulic traits; red, other traits) and site metrics (black, above the dashed line) in explaining cross-site patterns in the coupling (a) and sensitivity (b) of latent energy exchange variation in response to drought variables. Site metrics include: forest age (Age), species richness (nspp) and fraction of forest composition that is gymnosperm species (Gfrac). Traits include the community-weighted mean and standard deviation (s.d.) of: wood density (WD), specific leaf area (SLA), Amax, P50, HSM and Psimin. For samples sizes for each trait, see Supplementary Table 3. Asterisks indicate statistically significant regressions (P?=?0.001 (a) and P?=?0.02 (b) after correction for multiple hypothesis testing).

Based on statistical inference, they conclude that the HSM factor is important.

They write:

All else being equal, forest communities with higher hydraulic diversity (defined as a higher standard deviation in HSM) experienced smaller variation in latent energy fluxes explained by VPD and soil moisture. This pattern was generally robust, especially for the drought coupling metric, to the method of data processing...

Another graphic on the topic:



The caption:

a, Drought coupling is expressed as the percentage of explained variation (R2). b, Drought sensitivity is expressed as the summed absolute values of standardized coefficients of drought variables regressed against latent energy (LE) exchange as a function of daily VPD, soil moisture (SM) and their interaction (regression: LE?=?f(VPD, SM, VPD?×?SM). Hydraulic variation is expressed as the community-weighted standard deviation in the HSM of each species. Colours indicate biomes of deciduous broadleaf (green) and needleleaf (red) forests. The size of the dot indicates the number of days included for each flux site. The solid black line is the best-fit ordinary least-squares linear regression and dashed lines are the 95% confidence interval of the regression fit (n?=?23 independent sites).

They do some work that they define as "preliminary" using satellite data to study the water health of forests, focusing in particular on the United States - a prominent North American country ruled by an ignorant racist with a fondness for sexual assault and his equally mindless enabling cranks - but also encompassing global "estimates."

They produce this graphic:



The caption:

a, Drought coupling as the percentage of explained variation (R2) in an ordinary least-squares linear regression by drought variables on an index of above ground plant water content variation at midday (regression: vegetation optical depth at midday (VODmidday) = f(VODnight, VPD). b, Native plant species richness (percentage of the maximum). Data available from http://ecotope.org/anthromes/biodiversity/plants/data/. c–h, Regressions between these two variables for six major biomes. c, Tropical and subtropical moist broadleaf forests. n?=?1,380 grid cells. d, Tropical and subtropical dry broadleaf forests. n?=?241 grid cells. e, Temperate broadleaf and mixed forests. n?=?1,289 grid cells. f, Temperate coniferous forests. n?=?318 grid cells. g, Boreal forests. n?=?1,784 grid cells. h, Mediterranean-type forests, woodlands and shrubs. n?=?319 grid cells. Each point represents an individual grid cell from the map and redder colours indicate a higher density of points. Red lines show ordinary least-squares regression lines of best fit. Numbers in the upper right of panels indicate the linear generalized least-squares regression R2 and P values indicate statistical significance of that regression after accounting for spatial autocorrelation.

They find, unsurprisingly but nevertheless disturbing in the age of monoculture, that the best resilience in forests facing drought is species diversity. (I'm not sure, however, that adding diversity is a good idea, since the effects of introduced species both intentional and unintentional has greatly impacted the natural biosphere.)

They have a nice and detailed description of their methods in the paper. In conclusion to their discussion they write:

Our initial analysis suggests that satellite measurements of canopy water content may be promising for overcoming some of the scarcity barrier of limited eddy covariance sites for scaling and testing drought responses at continental scales. Our results provide evidence that hydraulic diversity is a critical element of biodiversity for next-generation land-surface models to include to improve simulations of carbon, water and energy fluxes in a rapidly changing climate.

A nice read.

If, by the way, you are interested in supporting and learning science, it does appear that Nature is offering a significant discount on a subscription making it quite affordable. My wife bought me one for my birthday, which saves my fat ass from traveling to scientific libraries a bit.

I love growing old with her, but as I near the end of my life, which I have loved so much, I am pained at what we are leaving behind.

I trust you're having a great autumn weekend.