What Are Phloem Signals?
The phloem is part of the tree's vascular system and primarily responsible for transporting sugars, nutrients, and chemical signals. These signals are essential for:
1. Defense: Alerting the tree and nearby trees to threats like pests or pathogens.
2. Resource Allocation: Directing nutrients to areas in need, such as damaged branches.
3. Environmental Responses: Reacting to drought, temperature changes, or mechanical damage.

How Phloem Signals Work
1. Chemical Messaging:
When a tree is stressed (e.g., a pest infestation), it produces signalling compounds like jasmonic acid, salicylic acid, and ethylene.
These chemicals travel through the phloem to activate defense mechanisms in other parts of the tree.

2. Electric Signalling:
Some studies suggest trees send rapid electrical impulsions through the phloem, akin to how nerves work in animals.
This happens when a tree experiences physical damage or sudden stress, such as wounding or drought.

3. Volatile Organic Compounds (VOCS):
Trees also release VOCs into the air as warnings to neighboring trees, but the phloem handles more localized, internal signals.

Phloem Signalling in Pitch Pine (and Similar Ontario Species)
Pitch Pine:
Known for its resilience and ability to produce resin in response to rests, pitch pines (found more commonly in the northearster U.S.) rely on phloem signals to trigger resin flow to seal wounds and trap invaders like bark beetles.

Ontario Species with Similar Mechanisms:
a. White Pine: Similar to pitch pine, white pines use phloem signalling to mobilize resin against pests like the white pine weevil.
b. Red Maple: Can signal internally to isolate and respond to fungal infections.
c. Black Walnut: Produces juglone, a chemical that inhibits nearby plant growth (allelopathy). This triggered partly through root signalling but also involves phloem communication.
d. Spruces (e.g., White and Black Spruce): Use phloem signals to ramp up production of terpene-based chemicals when under attack by spruce budwords or bark beetles.

Ontario-Specific Examples of Phloem-Based Defense
1. Emerald Ash Borer (EAB) and Ash Trees:
Native ash trees in Ontario attempt to defend against EAB by sending phloem signals to produce callus tissue around larvae galleries, although often unsuccessful.
2 Sugar Maple and Freeze-Thaw Cycles:
During spring, sugar maples use phloem signalling to coordinate sap flow based on temperature fluctuations.
3. Eastern Hemlock and Woolly Adegid: Hemlocks in Ontario signal internally to increase secondary metabolites in response to adelgid infestations.

Why Is This Important?
Phloem signalling underpins a tree's ability to survive and adapt. Ontario's diverse tree species rely on these mechanisms to deal with challenges like pests, pathogens, and environment stressors. Understanding this helps in forestry management, pest control, and conservation efforts.