Dry times & Drought resilience
Big picture
Limited water has all kinds harmful implications for trees. An obvious culprit is reduced rainfall. But how much water falls - whether through rain or irrigation - is only one factor determining how much water is plant-available, so simply adding water in dry times is only a partial solution. General soil conditions and irrigation practices have a large impact on how much water a soil can hold, exactly how soil holds the water it has, and how root systems develop - all significant determinants of how much water a tree can actually take up.
​
In this post
How limited available water affects trees
Where roots live
How water moves through soil / how soils hold moisture
What makes soil lousy
How to protect soil from being damaged
How to know if soil is damaged
How to repair damaged soils
How limited water affects trees​
We've got a general sense that dought = bad. But why exactly?
​
-
90% of water taken up by a tree is evaporated during photosynthesis. For the most part trees are kind of like rivers - water comes in a goes out. This flow-through is key to making the simple sugars that fuel a tree's activities and become the physical building blocks for new growth. Not enough water means less photosynthesis and less raw material for trees to work with.
-
Evaporating all that water has a cooling effect for trees and the surrounding environment. Less water means a hotter microclimate in and around a tree.
-
Drought stress can stunt tissue development, affecting the tree for the remainder of the season or even for years.
-
Because nutrients are dissolved in water, reduced water uptake also means reduced nutrient uptake.
-
Less water in tissues means higher concentrations of nutrients. It's like making a reduction in cooking. These higher nutrient concentrations make great eating for some types of pests.
-
Less photosynthesis means less material for building defensive compounds to resist pests.​
How water moves through soil​
There are various kinds of water in the soil. There's water that moves through and water that stays. There's water that's easily available and water that's bound up tight. This is determined by they types of soil particles, how they're arranged, and the types of spaces left in between them. Those gaps are referred to as pores, and collectively represent a soils pore space.
​​
As water fills pores, it displaces air. Much of the water then drains out of the larger pores - the macropores. This water that drains out is referred to as gravitational water. As gravitational water flows away, fresh air is drawn into the soil. Once gravitational water has drained the soil still has quite a bit moisture and is said to be at "field capacity". At field capacity a soil is filled with a mix of fresh air and water.
The image below illustrates a soil with different sized pores at field capacity. Some of the pores are large enough that water has drained through and filled with fresh air while other pore spaces are small enough to have retained water, which is then plant available over an extended period.
​
​The good news is that we can have tremendous control over soil and water conditions. We can't do much about temperature or light or storm intensity or air polution but we can do a lot to improve and protect soil conditions, and soil conditions have implications for drought tolerance. It's trite, but true:
For healthy plants grow your soil.
​
You can grow your soil by:
-
preventing and reversing compaction
-
ensuring enough organic matter
-
adjusting soil chemistry (like pH) if needed
-
periodic deep watering during drought
​
That might sound like a lot of work, but the first three bullet points should be rare interventions. Once soil has good fundamentals, keeping it that way is mostly a matter of irrigating as needed. If you're the DIY type, you can do a web search for your local agricultural extension soils lab. There, you'll likely find information about how to collect and submit soil samples for analysis for nutrients, organic matter content and pH, along with recommendations. For ​​​a deeper explanation of soils, root systems and drought resilience strategies, read on.
​
​
​
It's easy to forget how much of a tree is below ground and out of sight. Even as professionals we some times look at the above-ground part as though we're seeing the whole story rather than a fraction of it. A tree's root system represents something in the neighborhood of a third of its biomass. If it's surprising just how much of a tree is below ground, it's equally surprising just how different this part of the tree looks.
Below ground, trees radiate outward far more than they extend down. Root systems are very shallow relative to their height. Even the roots of a 100 foot tall tree will still be limited to the top foot or two of soil.
​
Of the water that's left in a soil after gravitational water has drained, some is easily available to plants and other water is bound so tightly that it is effectively unavailable. Imagine a sponge under the tap. At a certain point water will drain out of the sponge - gravitational water. Turn off that tap and let all the gravitational water drain out of the sponge there will still plenty in the sponge. If you were to squeeze the sponge water is easily available. But keep squeezing and at a certain point you'll no longer be able to get water out of the sponge. Nonetheless, the sponge is damp and still contains water. This is like the difference between soil with high water potential (lots of readily available water) and soil with low water potential (water still in the soil but not neccesarily plant available). For more on this, try a web search for adhesion vs cohesion water.
It seems like the soil available to trees should be almost limitless, as though roots should be able to plumb the earth's depth endlessly. But roots have a specific set of requirements and those conditions exist very close to the surface. The part of the earth that plants rely on is like an impossibly thin skin over the planet.
If you dig a hole in a place that's relatively undesturbed you'll notice distinct layers. The top layer is organic matter like leaves and sticks in a forest environment, or dead grass in a prairie. Below the organic layer is dark, loose soil. Further down, the soil tends to get tighter, more dense, and often lighter in color - often yielding to clay and eventually stone. These different bands are referred to as horizons.
In the soil profile pictured above, the upper horizon is dark and crumbly, like chocolate cake. There will be lots of different sized voids that water can run through and then refill with air. The void is also where microbes live and space for roots to move into. The darkness comes from broken down organic matter. The organic matter feeds the microbes which in turn help build and maintain the crumbly, loose structure. Deeper in the soil profile there is less organic matter and less pore space. The soil is denser. Water and air don't move as freely. There is less air, less opportunity for water flow, less life. These deeper horizons have few roots, fewer fungi, fewer worms, amoeba, bacteria, nematodes, etc. Soil transitions from teeming with life - like 1,000,000,000 bacteria per gram in the upper horizons - to inhospitable very quickly. This is why the vast majority of a tree's roots tend to be found in the top foot or two of soil.
​
The shallowness of usable soil is both bad and good news. The bad news is that because most roots are very close to the surface, they're easily damaged. And because the top foot or two of soil represents the vast majority of plant-usable soil volume, it's very easy to mess that up too. Messing it up is a simple as driving over soil once. That's right, driving over soil one time can squish the majority of pore space. Imagine stepping on a sponge and having it not pop back up afterwards. Walking repeatedly in the same area or even just leaving soil exposed to the elements with no cover of mulch or ground cover can also result in compaction. The good news is that improving soil conditions involves altering a relatively shallow area, so it's not a herculean task. Furthermore, it's almost always a matter of augmenting what's already there, rather than entirely removing and replacing tons of material. Case in point: 5% organic matter by volume is considered a healthy amount. That dark, crumbly upper horizon in the photo above doesn't actually have that much organic matter in it. Organic matter is likened to yeast in bread making; the vast majority of dough is just flour, and a little yeast goes a long way. Similarly, if a soil is low in organic matter, getting it back up around the 5% mark shouldn't require a huge overhaul. Rather, it just requires augmenting what's already there with a bit of organic material.