Mountain pine beetle (MPB) (Dendroctonus ponderosae Hopkins) is a bark beetle that lays eggs within the vascular system of the mature trees of lodgepole pine (Pinus
contorta). First known epidemic infestation was recorded in 1994 in the forests of British Columbia (BC). Prior to 1994, the infestation was negligible, owing to the fact that extreme low temperatures of winter killed the beetle, thereby curbing the problem. In the past two decades, however, due to global warming the winter temperatures are not low enough to kill the beetle. This has escalated the problem of deforestation and thereby has unbalanced the hydrological cycle. The severity of the infection is attributed not only to the increase in winter temperatures, but also to higher proportions of mature trees present in BC forests that form the perfect hosts for the MPB. These high proportions of mature trees were accomplished as a part of the forest fire regulation policy implemented many decades ago. (Hélie et al 1).
The BC Ministry of Forest and Range have estimated that by the year 2018, 78% of the mature trees of the BC forest area would be killed by this infestation and 34% of the total forest area would be gone (van de Vosse 1). Another disturbing fact is that the MPB works in association with blue stain fungi and together they have found a new host population in the younger pine trees as well. This is believed to be an adaptation to proliferate in case no mature pine trees are available as hosts. (Dhar and Hawkins 2). The alarming rate of the infestation and possible detrimental ecological and economical outcomes have pushed the scientists to look for quick and effective solutions to save this ecosystem from further harm. This essay will discuss the delicate balance of the hydrological cycle sustained by these forests and the detrimental effects that the MPB is imposing on the forest and the ecosystem. The paper will also try to provide plausible solutions that could put an end to this epidemic.
The hydrological cycle
The BD province accounts for 60 million hectares of forested land, of which, the economically viable pine grows on 14 million hectares of the forested land. Lodgepole pine, western white, white bark pine, limber pine and ponderosa are some of the common species of pine found in the forest. One of the primary concerns of the MPB infestation is its impact on the hydrological cycle. The hydrological cycle is also called as the water cycle, which is the process of evaporation of water on the land surface to form rain clouds and ultimately cause precipitation as rain. The presence of dense forest or a dense canopy will catch most of the rain and snow. This interception of the rain and snow by the plants and canopies results in high evaporation and thus, more rainfall. (Hélie et al 1).
The MPB and the pine tree infestation
The MPB infestation causes less than 2% death of the pine trees and is classified as being endemic. However, with current projections, the infestation is an epidemic. The beetle digs through the bark and reaches the inner phloem. Phloem is responsible for water transportation from the roots to the leaves. The beetles dig the bark and forms channels in which the females lay eggs. While digging through the bark, they allow the entry of the blue stain fungi into the vascular system. The fungi feeds on the tissue and disrupts the water intake. The larvae that hatch out of the eggs take their nutrients from the phloem and sapwood. This phenomenon disrupts water transport to the crown where the pine needles grow. The infestation lasts for 1 year during which period the larvae deprive the tree of its water and nutrients and gradually kill the needles. This changes the color of the needles from green to yellow to red and finally to a dying gray. Earlier, the progression of the infestation used to be slow or stopped during winters when the temperatures hit -37 to -18°C. However, in the recent decade the winter temperatures are not low enough to stop or slow down the progression. This condition is attributable to global warming (Hélie et al 7).
MPB infestation and its influence on the hydrology of the forest
It is not possible to assess the impact of MPB over a large area such as the BD pine forest. Thus, small areas such as stand-scale and watershed scale are studied for impact and then extrapolated.
Stand scale impacts
Stand-scale can be defined as a homogenous area of an ecosystem concerning topography, climate, ecological disturbance, vegetation, species and soil so that the region can be considered a single unit. Such areas do not exceed a hectare.
Snowfall accumulation and ablation
Snowfall, snow ablation and rain are components of the BC forest hydrological cycle. It has been observed that due to the destruction of the canopy, large amounts of snow (43%) reaches the forestland and the ablation (evaporation) of the same during spring and summer is only 29%. Snows that fall on north facing slopes melt much slowly due to cooler temperatures and cause more water absorption into the soil. This is a classic example of high intake and low expenditure. That is, the earth receives high rainfall, but gives back very less, thereby unbalancing the hydrological cycle. Since the MPB attacks the mature trees, a study was done to look for rates of snow and rainwater accumulation and ablation in regions with younger trees. It was found that younger trees and clear-cut lands have similar accumulation and ablation profiles. This indicates that mature trees play a very vital role in the water cycle of the pine forests (Redding et al. 36).
Nearly 60 to 70% of the rain that falls over the BC forests is intercepted by the canopy and is evaporated even before reaching the ground. This is the major input of water back into the cycle. Less than 40% of the water reaches the ground to form streams and ground water in mature pine tree areas. The risks associated with wetlands is the poor regrowth of the forest, hindrances in the silviculture practices, poor drainage of water resulting in sensitive loose soils. (Redding et al. 37).
Watershed scale impacts
Watershed scale refers to that homogenous area of land where all the precipitates and ablated snow water drains into a single stream. Such an area is usually 10 hectares or so. Studying an area at a watershed scale can throw light on the effect of MPB on the geology, stream flow and related data.
Increased peak flow and annual water yield
The infestation of the pine trees has a domino effect on the forestland, the water table, the drainage, increased peak flow (rate of stream discharge) and the water yield (amount of water released from drainage area annually) during spring and summer. Studies conducted on Okanagan basin and Colorado forests have indicated that salvage harvesting (removal of infested trees by mechanical means) have resulted in increased annual water yield due to drainage from precipitation. 30 to 35% loss of foliage resulted in an increase of water yield by 15 to 16%. The increase in water yield is an important characteristic because it dictates the speed, volume and content of the water flowing downhill. (Redding et al. 39).
Impact on aquatic ecosystem
The drained water, naturally, moves into aquatic bodies, which affects the aquatic life. As the MPB infested trees die, they leave large amounts of pine needles and foliage on the ground. The salvage harvesting process also leaves large amounts of large woody debris (LWD), which eventually is washed into the lakes and other water bodies into which the precipitated water drains. Unaffected mature trees also produce LWD, but the output by infected trees and mechanized activities produce alarmingly high quantities of LWD. The fish habitat depends on LWD, but an excess of the debris could change the concentration of the fish. This could indirectly affect the fisheries. The impact extends to riparian zone (riverbank ecosystem) where it could result in soil erosion and high sediment concentration in the water bodies. (Redding et al. 40).
In Colorado, there have been evidences of change in water quality after incidents of MPB infestation. The debris that wash into the streams add to the nitrogen and organic content. Thus, the surface water was noted to have high nitrates and high levels of sediments. The nitrogen cycle and carbon cycle are indirectly affected by the infestation as the dissolved nitrogen and carbon cannot be sequestered by the terrestrial plants. The soil nutrient might shift towards higher carbon and nitrogen content primarily because there are fewer plants to take up the nutrient and even more nutrients are added to the soil from the dead trees and pine needles. Constant salvage harvesting and loss of riparian canopy due to MPB increases exposure to sunlight and elevates the temperature of the stream. Such an increase in temperature could prove pernicious to aquatic organisms that do not have the necessary adaptations. High temperatures could promote the formation of algae, which could decrease the dissolved oxygen content for the fish. This could lead to death of fish and affect the fisheries. (Redding et al. 41).
MPB infestation and its influence on the forestland
Watering up of the forestland means that the soils would be too loose to sustain heavy machinery that are routinely for forestry operations such as harvesting. This necessitates the need for lightweight machines to replace older robust machines. This is not only a case of inconvenience, but also an unwanted financial burden. (van de Vosse 5).
During spring, there is a higher rate of snow ablation as most of the snow is on the forest floor. This raises the level of water in the streams. To add to this problem, the harvest is usually carried out during spring-summer and this requires road. Laying roads not only aids the down flow of snow and water through a clean slope of road, but also disturbs the already fragile subsurface water system. All this could cause flooding and create havoc locally. (van de Vosse 5).
Managing the MPB infestation
Earlier, when the infestation was considered an endemic problem, the affected trees were cut down and burned before they could lose all of their economic value. However, with the current rate of infestation, this approach of salvage harvesting (salvaging the economic value of the wood) by cutting it down or burning might only add more to the existing misery. Therefore, scientists are looking at alternate approaches to remediate this problem.
Currently, many simulation models are being examined to assess the extent of impact that the MPB epidemic could cause over next 10 or 20 years. The reason for using such mathematical and computational model is the difficulty that arises from studying vast areas of land. The study itself is physically difficult and may not be accurate. That is why such models rely on information from stand-scale and watershed scale studies. Some methods of management are discussed below.
Direct control treatments
Direct control methods are the traditional methods that can be used for low-level infestations. These typically include felling the trees from root and burning, pheromone baiting the trees, single infested tree removal and debarking using monosodium methanearsonate (MSMA) (Nelson et al 21).
MSMA is essentially a pesticide that is applied to the tree base. This is the location where the majority of the infestation starts and thus, killing the beetles at this point is the best strategy to eliminate the entire colony of the MPBs. The pesticide enters into the tissue system and can kill the MPBs present under the bark and inside the tree. However, this technique will only work if the infestation is sited within 24 hours of the initial attack. (Nelson et al 23).
The pheromone baiting uses repellant pheromones that send signals to the MPB that the tree is full and that the newer MPB need to find a new tree. Another variant of this technique is to use pheromones such as trans verbenone and exo brevicomin that attract the beetles to a single tree. Once the beetles infest the tree, the bait tree is destroyed before the female lay eggs and the next generation is born. Not many scientists agree with the pheromone technique citing the reason that this does not eliminate the problem and only leads to destruction of healthy trees as bait. (Nelson et al 23).
Feel and burn has been previously discussed. Methods such as small patch harvesting is used when very few numbers of trees have been infested and are uprooted from the spot along with nearby susceptible trees. This technique is commonly used in combination with above-mentioned methods.
Long-term control inherently means preventive measures. The strategy in long-term control should aim at preventing the infestation altogether or at least control any outbreak from spreading to other trees. The techniques used should address susceptibility and use a rating system to prioritize the severity of treatment required. The recurrence of the beetle attacks is attributable to the symptomatic treatment of the infection. This idea needs to be changed using simulation models and field techniques.
Using computational hydrological models
Since the paper is about MPB’s impact on the hydrology of the BC forests, the use of hydrological models for impact assessment is a logical step. Water resource evaluation of non-point silviculture sources (WRENSS) hydrological model was used by a group of scientists in calculating the impact of spruce bark beetle on the forest of Colorado and Wyoming. The model is a low-complexity model that requires very little input data to form a prediction. University of British Columbia has developed a hydrological model called UBC watershed model that can predict post-infestation hydrograph by applying simple energy balance data (Unnila, Guy and Pike 5).
Growing a mosaic of pines that differ in species and age can lead to the growth of a stand that is diverse and paves way for quick removal of infestation as the beetle will not have many host trees to attack. Such a type conservation can be achieved using clear-cutting small areas and reforesting with different varieties of pines. Such a silviculture techniques require constant monitoring before the ecosystem can sustain itself.
Selective removal of pines once they hit the age of 15 years might prove beneficial with respect to stand vigor and longevity. This is typically done with a spacing of 10X10 feet around each tree.
Calculating susceptibility index, beetle pressure index and risk index
The first step towards prevention would be to rate the areas and trees based on their age and therefore their susceptibility. The susceptibility index is directly proportional to the risk of infestation and is calculated using the formula
Susceptibility index (S) = P x A x D x L
Where, P = Percent of susceptible pine basal area, A = Age factor, D = Stand density factor, and L = Location factor.
Once the risk factor has been assessed, the likelihood of beetle attack must be measured using the beetle pressure index, which logically states that the proximity of a beetle population to a susceptible mature stand will increase the likelihood. From calculating these two factors, the risk index is calculated using the formula,
R = 2.74 [S1.77e-0.0177S][B2.78e -2.78B]
Where, e = base of natural logarithm = 2.718, B = Beetle pressure index, S =
Higher the risk index number, more susceptible would be the pine population. Following this method will help the forest managers prioritize the treatment regime (Gibson 6).
Dhar, Amalesh, and Chris DB Hawkins. "Regeneration and growth following mountain pine beetle attack: A synthesis of knowledge." Journal of Ecosystems and Management 12.2 (2011).
Gibson, Kenneth. USDA Forest Service. Management Guide for Mountain Pine Beetle: Dendroctonus ponderosae Hopkins. 2010. Web.
Hélie, J. F., et al. "Review and Synthesis of Potential Hydrologic Impacts of Mountain Pine Beetle and Related Harvesting Activities in British Columbia." 2005.
Nelson, Trisalyn, et al. "The impact of treatment on mountain pine beetle infestation rates." Journal of Ecosystems and Management 7.2 (2006).
Redding, Todd. "Mountain pine beetle and watershed hydrology." BC Journal of Ecosystems and Management. 9.3, 2008: 33-50. Web.
Uunila, Lars, Brian Guy, and Robin Pike. "Hydrologic effects of mountain pine beetle in the interior pine forests of British Columbia: Key questions and current knowledge." Journal of Ecosystems and Management 7.2, 2006. Web.
van de Vosse, Hanna. Ministry of Environment Mountain Pine Beetle Action Team. Ministry of Environment. Mountain pine beetle infestation: hydrological impacts. Prince George, BC, 2008. Web.