Not a Great Day to be a Plant?

Effects of dry soils on plants can help droughts persist

Some Good Things Happen On a Drizzly Day

What Makes A Good Day For Plants?

"It's a great day to be a plant," a fellow graduate student would intone as we faced another dreary, misty, drizzly day in coastal Connecticut.  As a budding plant physiologist, he would go on to say that partial sun, high humidity and a constant drizzling source of water for the soil were perfect for most plants.  Indeed, it was the hot, dry sunny days we humans longed for that put the most stress on plants.

Photosynthesis for most plants reaches a maximum well before full sunlight intensity.  Excess sunlight increases leaf temperature which can decrease net photosynthesis and increase demand for water, enhancing the likelihood of drought stress.  Extreme heat and drought have been known to actually kill tree leaves, even here in usually humid New England.

There have not been many good days for plants here in the seacoast of New Hampshire this summer.  July featured 7 days over 90 degrees (F) where a "normal" year would have maybe 6 such days in a full summer.  Rainfall was less than half of the average. 

And it wasn't just here or just July.  Maps from the Northeast Regional Climate Center show above average temperatures for all of May through July and well-below normal rainfall across much of the northeast.

Tourists and resort owners love this weather - as long as water supplies hold out - plants not so much.  Some towns in my area have banned all outdoor watering.  Lawns are brown and garden plants are suffering as well.  Red maple foliage is wilting on some trees - something that I don't remember seeing before. 

Temperature, Rainfall And Drought Stress

How do we describe or classify stages of drought and drought stress?  From previous essays you may know of my fondness for simple, understandable indexes of the state of the weather/climate system.  Many of these have been around for many years, and here is one that fits this description and relates to our topic:  The Palmer Drought Index.  First presented by Wayne Palmer in 1965, this is essentially a simple water balance model that uses precipitation inputs, soil water storage capacity, and the power of the atmosphere to evaporate water based on temperature and humidity, to estimate a cumulative surplus or deficit of soil moisture over time. 

The U.S. Weather service maps this index nationally and updates it frequently.  From the temperature and precipitation maps above, this current map of the Palmer index for the northeast is not a surprise.  Drought stress is particularly acute in coastal Massachusetts and New Hampshire. 

Droughts and associated fires have been big news across the western and southwestern U.S. for a number of years, and the current national map of the Palmer index reflects this.

NOAA's Climate Prediction Center, which posts this map and many others, is well worth a visit, and there are regional climate centers that cover all parts of the U.S.

You may have noticed the letters S and L on these maps - what do they mean?  They distinguish impacts on plants from those on water supplies.  S - short-term - means that the top layers of soil, those accessed by plant roots, are dry.  This kind of drought can develop quickly - over a couple of weeks - and can also be reversed quickly.  L - long-term - refers to more protracted drought of the kind that can affect water supplies by reducing water tables, deep soil water storage, and reservoirs.  S and L can occur together, as seen here for much of Texas.

So far, our New England drought is classified as short-term.  Still not good for plants, but just a couple of storms - or maybe even a single hurricane - could be enough to provide relief!  The fact that some towns here are calling for reduced water use reflects how strongly this area relies on streamflow and shallow wells for its water supply.

In these essays, I try to put each topic into a historical context by asking if something similar has happened before, and what is predicted for the future.

There have been worse times, drought-wise.  The "Dust Bowl" years of the mid-1930s recorded some of the most extensive and severe Palmer Drought Index numbers ever.  So we may not be in uncharted territory yet.

But that may well change by the end of the century.  A study released in 2010 used projected changes in climate to predict the number of extreme drought months across the U.S. by the end of the century.  Unless you live in Boston or Seattle, it is not a pretty picture.  Away from these northern coastal areas, the number of extreme Palmer Drought index months is projected to increase from less than 1 in 40, to as many of 1 in 7 to 9 (from 2.5% of months to as many as 10-15%). 

I tend to think of extreme heat and drought in tandem.  Hot, sunny days seem to both occur during and help to prolong droughts.  There is a connection - and a feedback.

In humid regions, plants play a large role in determining how much water vapor is in the air (how humid it is) by turning liquid water in soils, into water vapor (humidity) in the air.  They do this through a process called transpiration. 

Leaves need to be open to the atmosphere in order to take in carbon dioxide for photosynthesis.  This happens through pores in the leaves called stomates (or stomata)

The cell surfaces inside leaves need to be moist in order to absorb the carbon dioxide that comes in through those pores.  This means that water evaporates from those surfaces and then migrates out through those same pores and into the atmosphere.

Basically, the loss of water from leaves (transpiration) is the unavoidable consequence of taking in carbon dioxide for photosynthesis.  This is why plants require so much water - not primarily for the chemical reactions of photosynthesis, but to replace the water lost through transpiration while photosynthesis is occurring, and in order to avoid wilting. 

Just how much water is lost for each molecule of carbon dioxide gained during photosynthesis is called water use efficiency and is actually one of the key ratios determining plant growth in both native and agricultural ecosystems.

The amount of water thrown into the atmosphere by transpiration is enough to increase humidity.  I have a colleague who studies forest-atmosphere interactions and he said he could detect the beginning of the growing season, when leaves became active and photosynthesis began, by the change in humidity.

Let's think about water and forests and drought.  When soils are moist, photosynthesis and transpiration are not limited by the amount of water available.  Carbon gain will proceed based on light and temperature, drawing moisture from the soil and losing it to the atmosphere, increasing humidity.  Increased humidity can increase the chances of summer thunderstorms, replenishing, in part, the lost soil moisture.

You may have read of the value of trees in urban settings or around homes in providing cooling on hot days.  This is often expressed in terms of air conditioner equivalents!  There are two parts to the cooling effect.  The easiest to visualize is shade.  The tree leaves absorb or reflect sunlight that then does not reach the house roof or windows.

The second is the cooling effect of transpiration.  Evaporation in any form requires a lot of energy, so when those leaves are losing water to the atmosphere (as vapor) the conversion from liquid water to water vapor at the leaf surfaces absorbs energy and provides cooling.  It's like splashing your face with water.  The water itself might cool you some, but the evaporation of that water is even more effective.

So we have a functioning forest taking up carbon dioxide and giving off water vapor, cooling and humidifying the atmosphere.

 And then the rains stop. 

Plants draw on moisture stored in the soil and continue to photosynthesize.  As transpiration proceeds, water is drawn from the soil and soil moisture decreases.  As soil moisture becomes harder to draw and water stress develops in leaves, the pores or stomates will begin to close, shutting down both photosynthesis and transpiration - and preventing wilting as well. 

This reduces carbon gain for the trees and also reduces the contribution to humidity of the air, and the cooling effect of transpiration. 

Lower humidity can reduce the chance of those summer thunderstorms, but can also decrease cloud cover, increasing direct sunlight and driving higher temperatures.  Drier air will also both increase and decrease in temperature more rapidly, so more sunlight and drier air could push daytime maximum temperatures higher still. 

Forests, Water Loss And Feedbacks To Rainfall

All of this is a long way around to the question of feedbacks among temperature, humidity, rainfall, and the persistence of drought.

While larger weather and climate patterns trigger and relieve drought, can plants accentuate the stress?  This is a big topic, but consider these two facts from recent studies. 

The first, from a full review across many different ecosystem types, reports that, for forests with full canopies (high leaf area) transpiration (evaporation from inside the leaves) is about 4 times higher than evaporation outside of leaves (for example from soil surfaces).  This means that about 80% of precipitation returned to the atmosphere is by transpiration. 

The second, reporting on work in the Amazon, says that at least 20% of rainfall across that vast forested region has been "recycled" at least once - meaning that water provided to the atmosphere through transpiration by trees has come down again in the same area as rainfall, and been transpired again.  The title of this paper emphasizes the role of this recycled transpiration in buffering the impacts of drought.

These two suggest that the absence of transpiration during periods of drought accentuates the dryness of the air, and can help to perpetuate the absence of clouds and rainfall, contributing to the persistence of drought.  The absence of the cooling effect of transpiration can also accentuate high temperatures, contributing to higher demand for evaporation, and greater water stress for plants.

Your perception, then, that excessive heat and drought tend to go together makes sense.  That droughts tend to persist and are hard to break results in part from the fact that an important source of moisture to the regional atmosphere is shut off, as is a process for cooling the air, when plants are under severe water stress and transpiration is shut down.

So drought leads to more extreme heat on hot days, and water stress on plants reduces an important source of humidity that might help break the drought.

Long-term changes in weather patterns are the final source of drought relief, but plants and soils can play an important role in determining the severity and persistence of droughts through these feedbacks to the climate system.

Sources

The site for the Northeast Regional Climate Center is here:

http://www.nrcc.cornell.edu/regional/monthly/monthly.html

and the site with links to all regional centers is:

https://www.ncei.noaa.gov/regional/regional-climate-centers

Sources on the Palmer Drought Index include:

https://en.wikipedia.org/wiki/Palmer_drought_index

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/wrcr.20342

http://danida.vnu.edu.vn/cpis/files/Refs/Drought/Drought%20Indices.pdf

The drought monitor maps are from NOAA's Climate Prediction Center:

https://www.cpc.ncep.noaa.gov/

The 2010 study on future Palmer index values is described here:

https://globalchange.mit.edu/sites/default/files/MITJPSPGC_Reprint_10-14.pdf

The figure on the two-way exchange of carbon dioxide and water over leaves is from:

https://earthobservatory.nasa.gov/features/LAI/LAI2.php

The review paper on the transpiration as a fraction of total water return to the atmosphere is here (see in particular the top two graphs in figure 1):

https://scholarworks.iupui.edu/bitstream/handle/1805/14816/Wei_2017_revisiting.pdf;sequence=1

The article on rainfall "recycling" in the Amazon is here:

https://www.nature.com/articles/s41558-018-0177-y