BEAVERS - direct damage to aspen-ash-others by chewing and felling, indirect damage through flooding, Michigan has thousands of acres of beaver floodings

L-flood killed trees by beaver
R-beaver chewed tree

BURLS - odd growths that can lead to rotting, often creates grain pattern deformations popular for novelty wood crafts, indication of some sort of stress, occurs on many species

L- sugar maple burl
R- yellow birch burl

DEER - overbrowsing can eliminate understory plant species as well as tree regeneration, can create simplified ecological states, can diminish habitat quality, protection against deer is expensive, controversial management issue

L-depopulated northern hardwood stand
C-cedar browse line
R-browsed red-osier dogwood

INVASIVE SPECIES - in the UP - garlic mustard and buckthorn are serious forest invasive species, sometimes native species can become invasive when ecological dynamics are altered (e.g. deer, Pennsylvania sedge, ironwood, et al.)

L&L - garlic mustard
R&R - glossy buckthorn

MICE - will chew bark from stems, can cause high mortality among younger trees

L-girdled aspen
R- girdled pine

NEMATODES - tiny worm-like animals that feed on roots, can cause mortality, can associate with fungi and viruses, difficult to diagnose, symptoms similar to other problems

L-flowering dogwood roots
R-pine wilt nematode

PILEATED WOODPECKERS - these large birds excavate cavities for nesting and to reach insect larvae, cavities are important for other wildlife, other woodpeckers damage bark in the search for insect larvae, damage usually a sign of more significant tree health problems

L-adult bird
C-cavities on dead maple
R-cavities on live cedar

PORCUPINES - chews off bark, girdling will kill trees or portions of trees, special problem in conifer plantations

L-damaged tamarak
C-note tooth marks on yellow birch
R-entrance to den tree
   

SAPSUCKERS - birds that peck holes through bark and then consume the sap, holes are usually in rows and columns

L-on mountain ash
R-on hemlock

TREE AGE - not a direct damaging agent but older, overmature trees become more susceptible to nearly all other damaging agents, old trees are not necessarily large trees, forests with disproportionate numbers of overmature trees are vulnerable to rapid change (disturbance)

L-overmature jack pine
R-hemlock stand entering breakup phase

HERBICIDE DAMAGE - herbicide on the wrong species can lead to damage or mortality, evidenced by brown and/or curled foliage, similar symptoms to many other damages

L-damage from underspraying
M- damage from overspraying
R-hardwood leaf damage

HYDROLOGICAL CHANGE - poorly constructed roads block movement of water that can flood one side of the road and dry-up the other side of the road, water level changes can kill wetland forest types, ditches and drains have altered millions of wetland acres

L-cedar killed
R-emplaced fire control ditch

SALT INJURY - browning foliage, usually conifers, is often visible along highways, usually due to absorption of road salts

L-salt damge to fir
R-damage to white pine

DROUGHT - arguably one of the more important factors that pre-dispose forests to a wide range of pests, prolonged droughts increase risk factors, drought sometimes kills outright, crown dieback is a common symptom, younger and older trees more vulnerable    

L-killed paper birch
R-understory balsam fir killed
     

LIGHTNING - long spiral cracks on tall trees can be from lightning, white pine canopy emergents especially susceptible, trees can explode under the correct conditions

L-exploded tree
R-split tree

MECHANICAL DAMAGE- any wound resulting from breakage or bark removal, can be natural or human-caused in origin, structural damage to limbs or trunk, trees wounds never heal and can only be grown over   

L-"catface"
C-ligature wound  
R-deformity

SCORCH - occurs when water loss exceeds water availability, especially common with oaks and maples, can also be caused by a bacterium, leaf edges turn brown, can happen during extended periods of high heat, can be associated with soil issues (e.g. compaction)

L-aspen scorch
R-sugar maple scorch     

SNOW, HAIL, & ICE - heavy weight loads can cause branches to break, small trees can be bent resulting in lack of access to light, effects are worse when combined with wind, hail can strip leaves, flowers, and fruits  

L-jack pine breakage
R-ice load breakage  

WILDFIRE - heat damage or outright combustion kills living tissues under the bark, fire can be a regenerative force, some forest types are naturally adapted to wildfire (e.g. jack pine, paper birch), distinct from prescribed burning

L-aftermath in spruce-fir
R-burn on upper half of image

WIND - breakage can occur at places weakened by insects, disease, or malformation; major wind events cause considerable damage but can be a regenerative force in the landscape

L-split at V-fork
C-roots sprung by wind
R-tornado path          

WINTER INJURY - under certain conditions, above-ground tissues can dry out to the point of death, freezing tissue is not usually the cause, dead needles and twigs are not usually noticable until spring or early summer, most noticable on conifers

L-Christmas tree injury
C-damage to red pine
R-damage to white pine