The drought is over, but how did it impact your trees?

Aside from depriving plants of vital water, droughts are also known to exert profound effects on the chemical balance of soils, in addition to their physical structures, and microbial communities. All of which being critical factors which supportt a trees longterm health status.

So, in order to act preventatively and proactively, a homeowner should ideally be somewhat knowlegable of a few key points regarding florida droughts and their potential impacts on soil health.

pH Balance

The first point I would like to address is soil pH balance, because the soils pH balance can be a primary determining factor to consider when deciding how to respond with corrective action or mitigate drought stress.

For example, the watering process implemented during a drought should be handled differently depending on a soils pH balance.

For alkaline soils, the water should be applied lightly but frequently.

For Acidic soils, water should be applied more deeply but less often.

As moisture is lost in soil, dissolved salts and base cations (such as calcium, sodium, magnesium, ect)tend to become more concentrated in the remaining soil solution, which can ultimately alter the pH balance and indirectly change the availability of various nutrients for plants to absorb.

This “evapoconcentration” can sometimes alter a soils nutrient availabilty in ways that are harmful to a trees longevity, especially in soils with low buffering capacity( which are very common in floridas sandy soils).

So a few weeks or months of drought can potentially push soil pH balace into unsurvivable ranges for many species that are already living in high pH soils

Vital plant nutrients such as (iron, manganese, zinc, ect). can all become completely unavailable to plants when soil pH ranges reach to a certain height. So, with high alkalinity soils already being a common issue in Florida soils, this recent drought has the potential of causing much more damage to Florida trees than just the harm that water restriction can bring.

Also, drier soils tend to have lower CO₂ diffusion aswell, so naturally, less carbonic acid forms—another mechanism pushing pH higher.

Under moist conditions, soil CO₂ dissolves in water to form carbonic acid (H₂CO₃), which helps soils pH lean toward the more acidic range.

Additionally, because drought stress curtails root exudation and microbial metabolism, their decreased presence is another factor to increase pH even more than it already is because all microbes and plant roots are known to exude organic acids (e.g., citric, oxalic acids)]

Which means that homeowners who already know that they have especially high soil pH, should remain extra vigilant regarding the health of their trees during a drought,

Microbial Communities

Since water is an essential for nutrient diffusion and cell turgor, an extended drought can shrink microbial populations by 30–80%, depending on intensity and duration.

With the various types of microbes tolerating draught adversities in their own way, naturally there is bound to be a shift in the composition and ratios of certain micro-organism, compared to others.

For example: Fungi (especially filamentous species) tend to be more drought-tolerant than bacteria because their hyphae can bridge air-filled pores.

Within bacteria, spore-formers (e.g., many Actinobacteria) and those with very robust cell walls often survive better, whereas fast-growing Gram-negative bacteria decline.

With each bacterial and fungal type producing their own unique phytohormones and signalling molecules(which can greatly impact the acivity of those around them), these communal change can often create a snowball like effect which can favor the replication of certain microbes over others.

Among the various other metabolic and functional changes that drought can have on microbial populations; Enzyme activity (e.g., cellulases, phosphatases) is a major factor that can decrease rapidly in overly dry soils(which is mainlt detrimental because these enzymes are partially responsible for the breakdown of organic matter and the release of nutrients for plant absorption).

This means that nitrogen cycling is especially impacted as Nitrification (ammonium → nitrate) slows or stalls without sufficient moisture and denitrification (nitrate → N₂/N₂O) largely ceases in aerobic dry soils, but can “flash” under brief rewetting events.

So, with overall soil biomass decreasing and the ratios of good vs bad bacteria quickly changing, I think its not difficult to imagine why a trees susceptability to disease might inceease greatly during or after a drought.

Counter measures?

Ofcourse the best defense against a drought is the provision of adequate water before any dehydration occur, although its often already too late by the time people even realize that they are experiencing a drought.

So here a few potential ways to mitigate the damage:

Sulfur:

For those who have performed a soil test and learned that their pH balance is already much too high, an application of sulfur might be helpful in order to decrease the pH and save the tree from further immediate damage.

Elemental sulfur being best for long term gradual pH decreases and Aluminum Sulfate being best for more abrupt and pronounced pH changes.

Mulching

Mulching is a great way to help decrease evaporartion and retain moisture/co2 and nutrients. (acidic mulches such as pine bark or peat are especially beneficial)