Drs. David Dickens and David Clabo, UGA Warnell School of Forestry…
In recent month, you may have noticed an unusually high number of dead or dying trees in your longleaf pine stands. Stands with mortality problems in recent months were often established (at least partially) through Conservation Reserve Program (CRP) cost share funds about 15-20+ years ago. These stands often share a few characteristics which have influenced the mortality levels observd by landowners this fall and winter. These characteristics include unthinned stands, stands that are 15-20+ years old, stands established on old-field sites, and stands with regular pine straw raking (Figure 1). Several factors and their combined effects have lead to mortality issues in these stands.
First, is the competition among the trees themselves. Competition for growing space, soil water, and soil nutrients increases as trees grow larger. As tree crowns and root systems begin to compete for resources more intensely, weaker and smaller trees begin to die in a process called self-thinning. Secondly, longleaf pine established through the CRP program was often planted on old-field sites to meet wildlife habitat and soil conservation goals of the program. Former old-field sites often have much greater soil fertility, improved soil tilth, and essentially no hardwood competition compared to sites that have had trees growing on them for several decades. Longleaf pine is adapted to low soil fertility levels commonly associated with sandy textured soils in the Coastal Plain. When longleaf pine is planted on old-field sites with greater fertility, the trees often grow more quickly than normal and may need to be thinned at a younger age than they would if they were planted on a similar site that was previously forested.
A second factor affecting these stands is an unusual sequence of weather and climatic trends that started during the later half of 2018. The above average precipitation amounts during winter of 2018/2019 followed by two periods of hot temperatures and dry conditions prevalent throughout south Georgia during the late spring and late summer/early fall of 2019 stressed all types of vegetation including trees. The winter of 2018/2019 was unusually wet, and was followed during 2019 with high temperatures during the two drought periods that occasionally exceeded 100° F. Drought conditions reached a peak during the second to third week of June in the southern half of Georgia, and again during the middle of October. In longleaf pine stands where mortality issues are occurring, atypical weather trends combined with competition among trees in unthinned stands is resulting in greater individual tree stress.
A third factor influencing longleaf pine stand mortality issues is pine straw raking operations that occur in many stands in that age range. Pine straw raking when done more than once per year or when carried out using mechanical balers often removes a thin organic soil layer comprised mostly of decomposed pine needles that is important for maintaining soil moisture levels, especially during drought conditions. Past studies have illustrated that trees become more stressed when this organic soil layer is removed by raking operations. It is increasingly important to leave this layer in stands that are close to needing thinning. Encourage straw rakers to leave as much of this organic soil layer as possible, and do not rake stands more than once per year.
Together these factors are leading to longleaf pine stands that are experiencing mortality from primary stressors (e.g. drought), and secondary stressors (e.g. pine bark beetles). Pine bark beetles such as the three species of Ips beetle common to the Coastal Plain and black turpentine beetle are the main culprits for tree mortality related to secondary stressors. Ips and black turpentine are attracted to stressed trees. Usually they will attack one to a few trees (infestations can be as large as an acre or two) that appear to be randomly scattered throughout a stand (Figure 2). They can both attack the same individual trees. Look for pitch tubes (sticky and yellowish-white substance that often looks like oozing popcorn) on the bark plates or in valleys between the bark plates of infested trees (Figure 3). If the pitch tubes are large (slightly larger than a US quarter coin, Figure 4) and are found in the first eight feet of the tree stem, then this is evidence of black turpentine beetle activity. Trees that fade from green to yellow to red from the top down are sure signs of bark beetle attack. In addition, you can peel back bark on recently dead or dying trees and see beetle galleries where they feed. Another sign of beetle activity is when you see sawdust (called frass) at the base of the pines in trees that have fading needle color. These beetles (especially Ips) tend to proliferate in hot, dry weather. Removing dead trees from the stand does not help with managing populations of these beetles, and we generally recommend against using insecticides which requires spraying healthy looking pines to or try to save pine trees grown for forest management objectives. The current insecticides used for beetle control have to sprayed on the sections of tree stems where the beetles are likely to occur, which can be difficult for some Ips species that are found from mid-stem height into the stem crown area. The hot and droughty conditons of 2019, along with no thinning and intensive pine straw raking in many old-field post-CRP longleaf pine stands created a near perfect suite of environmental conditions for increased bark beetle activity throughout the Coastal Plain of Georgia.
Stand Evaluation and Options
Deciding on what to do with longleaf pine stands experiencing mortality can be daunting for landowners. A relatively simple evaluation of several rows of ten trees scattered throughout a stand can help with the decision process. Walk a row of ten trees and note whether or not the trees appear dead or alive. For living trees, note if they have any obvious defects in the first 33-40 ft such as a fork, ramicorn branch(s) (sharp angled branch that is about 1/3 of the tree’s diameter at the point it originates at), cankers (hollowed out wounds on the stem or branches with copious resin flow), and broken tops, or excessive sweep ( greater than 3 inches of stem curve in a 16 foot run. Examine five to seven rows of ten trees per ten acres in stands with mortality problems. If average survival is below 50-60% it may be time to start thinking about clear cutting the stand and salvaging what timber you can. If survival rates are greater than 60% and you wish to continue managing the stand, you should consider if there would be enough quality trees remaining to carry on the rotation for a future harvest. Of the surviving trees you have, you would want to have a minimum of 50-75 trees/ac (110-175+ per acre preferred) that could be carried out (good form trees) to a second thinning or final harvest. Stands can also be raked for pine straw longer than they might be otherwise if they are thinned (typically a 4th or 5th row thinning with removal of small or defective trees on leave rows) and thinning is not especially heavy (residual basal area ≈80 ft2/ac). Residual trees in properly thinned stands can respond with increased crown growth over the couple of years following thinning resulting in greater needle production. Understory vegetation management is important in stands that have been thinned if raking is to continue.
Outlook
Bark beetle populations during 2020 will depend largely on climatic conditions throughout south Georgia. If hot and dry conditions persist during the spring and summer months, high bark beetle populations may carry over from 2019. It is recommended that you take action swiftly by contacting a professional forester if you suspect that pine mortality and/or bark beetles are becoming an issue in any pine stand you own or manage, and visit your stand(s) regularly (every 2-3 weeks) looking for beetle activity signs.
Additional Resources
Thinning
https://bugwoodcloud.org/bugwood/productivity/pdfs/Guide_thinning_pine_plantations.pdf
Pine Straw
https://www.bugwood.org/productivity/topic.cfm?id=4
Pine Bark Beetles
https://www.gfc.state.ga.us/forest-management/forest-health/pine-bark-beetles/PineBarkBeetlesBrochure.pdf
https://www.gatrees.org/forest-management/forest-health/pine-bark-beetles/Pine%20Bark%20Beetles%20in%20Urban%20Areas%20070918.pdf
Drought Conditions Information
https://droughtmonitor.unl.edu/
