Pine Mortality Research Articles

Little recovery of the residual stand after mountain pine beetle disturbance in old stands in the northern Rocky Mountains, Alberta, Canada.

In early 2000s, long-distance wind dispersal of mountain pine beetle (MPB; Dendroctonus ponderosae Hopkins) resulted in massive outbreaks in the northern Rocky Mountains, Alberta, Canada, outside of the beetle’s natural range. We analyzed data from permanent plots measured for up to 15 years after MPB outbreaks in fire-origin, mature lodgepole pine (Pinus contorta var. latifolia Engelm.)-dominated stands scattered over an area of ∼ 75,000 km2. We evaluated stand level stocking, regeneration, and ingrowth of three species groups (pine, shade-tolerant conifers, and broadleaves) in stands where > 50 % of pine basal area was killed. Using multiple linear regression analysis, we examined the relationship between stand and climatic covariates and basal area growth, density of regeneration, and ingrowth into the sapling size class at 9–15 years post-outbreak. Results showed that total live basal area for all species combined in trees with height ≥ 1.3 m occupies on average < 50 % of the pre-MPB levels and is unchanged between both post-MPB measurements at 19.5 m2 ha−1. Ongoing pine mortality led to negative net growth, which exceeded the positive net growth in other conifers and resulted in zero net change for the whole stand. Post-MPB ingrowth and regeneration rates were close to zero for pine but increased slightly in other conifers and broadleaf species. Regression analysis revealed a negative relation between post-MPB growth and quadratic mean diameter of the reconstructed pre-MPB total basal area, while initial basal area and composition post-MPB varied among species. Our findings support the hypothesis that, post-MPB, mature pine dominated stands stagnate due to ongoing pine mortality, the maturity of remnant overstory, and a lack of adequate understory and regeneration that can accumulate sufficient growth at short-to mid-term post-outbreak. Our conclusions contrast other research, suggesting that outbreaks leave degraded residual stands with declining pine overstory and that transition to vigorous productive mixed stands is impossible in the absence of stand-replacing disturbance or silvicultural investment.

A Climate-Sensitive Mixed-Effects Individual Tree Mortality Model for Masson Pine in Hunan Province, South–Central China

Accurately assessing tree mortality probability in the context of global climate changes is important for formulating scientific and reasonable forest management scenarios. In this study, we developed a climate-sensitive individual tree mortality model for Masson pine using data from the seventh (2004), eighth (2009), and ninth (2014) Chinese National Forest Inventory (CNFI) in Hunan Province, South–Central China. A generalized linear mixed-effects model with plots as random effects based on logistic regression was applied. Additionally, a hierarchical partitioning analysis was used to disentangle the relative contributions of the variables. Among the various candidate predictors, the diameter (DBH), Gini coefficient (GC), sum of basal area for all trees larger than the subject tree (BAL), mean coldest monthly temperature (MCMT), and mean summer (May–September) precipitation (MSP) contributed significantly to changes in Masson pine mortality. The relative contribution of climate variables (MCMT and MSP) was 44.78%, larger than tree size (DBH, 32.74%), competition (BAL, 16.09%), and structure variables (GC, 6.39%). The model validation results based on independent data showed that the model performed well and suggested an influencing mechanism of tree mortality, which could improve the accuracy of forest management decisions under a changing climate.

Competition between resprouting chaparral and the recruits of a serotinous conifer following stand-replacing fire

In western North America, fire regimes are shifting towards more frequent, larger, and more severe wildfire. There is concern that this will shift the vegetation type in many areas, especially on the lower, drier slopes. In northern California, mature serotinous conifers and resprouting shrub species easily regenerate in severe patches of any size. There is no consensus, however, regarding the effects of shrub competition on conifer recruitment; conifer response to shade varies with shade tolerance and abiotic factors. Many conifers and almost all chaparral shrubs are shade intolerant, and we expect shading to be the main driver of the inter-species competition between these taxa on dry low-elevation, slopes We chose to examine early post-fire regeneration of knobcone pine (Pinus attenuata), a shade intolerant serotinous conifer, because (a) as a serotinous species we could be assured of a high initial density of recruits, and (b) it is mainly found on lower-elevation slopes in a matrix of chaparral. We examined the competitive interactions of the pine and shrubs within the 2018 Carr and Delta fires at the third and fourth post-fire years, as well as at the 2008 Motion Fire at the 14th post-fire year, focusing on two measurements of shrub shading: inter-shrub porosity (% shrub cover) and intra-shrub porosity (species-specific ground-level light availability). Our response variables included recruitment success (recruits per ovulate cone) and growth (height). We only chose stands where knobcone pine was a minor pre-fire component to ensure a high density of vigorously resprouting shrubs. We found (1) there were significantly fewer pine recruits under shrubs, with the bulk of the shrub-induced mortality of knobcone pine occurring before the third growing season; (2) knobcone pine averaged about six established recruits per burned parent tree by the third year following fire; and (3), extrapolating from height reconstruction of post-fire knobcone pine regeneration from the 2008 Motion Fire, the remaining tree recruits are expected to persist and dominate the stand within a decade of the fire. We conclude that competition with shrubs on low elevation sites in northern California does have a negative effect on knobcone pine density but is insufficient to seriously impede a dramatic post-fire increase in conifer density when conifer regeneration arrives promptly following fire.

Siberian Pine and Larch Response to Warming-Drying Climate in the Southern Boundary of Their Range

Trees’ growth and areal responses to changing climate are primarily expected within the edges of the species range. Here, we compared the responses of Siberian pine (Pinus sibirica Du Tour), a moisture-sensitive species, and drought-resistant larch (Larix sibirica Ledeb.) at the southern part of their ranges in the Siberian Mountains (the Tannu-Ola Ridge). We study the species’ growth and proportion in the forests from forest-steppe to treeline ecotone along the elevation gradient. These studies are based on radial growth index (GI) analysis and GI dependence on the climate variables. We used satellite time series to detect the land cover changes (areas of larch and Siberian pine, as well as shrubs and birch). We compared trees’ GI before and after warming “restart” in the late 1990s. Generally, GI dependence on the air temperature was negative at elevations below c. 1600 m a.s.l., whereas GI dependence on the moisture variables (precipitation, vapor pressure deficit, and soil moisture) was positive for both species. Above 1600 m, increasing air temperatures stimulated species growth, whereas the influence of moisture variables was negative (for larch) or neutral (for Siberian pine). After the warming restart, the GI of both conifers increased in moisture-sufficient high elevations and treeline ecotone, whereas within low elevations (&lt;1300 m), the GI was stagnant or suppressed. Both species’, especially Siberian pine, negative growth dependence on air temperature and positive dependence on the moisture variables strongly increased since the warming restart. We found a risen growth dependence of both species on the soil-stored water during the previous year (September–October), which smoothed moisture stress at the beginning of the growing season. Yet both species’ growth also suffered as a result of early spring warms. We found that larch is migrating in both uphill and downhill directions, while Siberian pine is migrating uphill only. Forests loss occurred at low elevations (&lt;1300 m), whereas forest and shrub gain occurred at high (&gt;2000 m) ones. The upper boundary of the forests and shrubs is migrating uphill at rates of about 0.8 and 0.3 m/y, respectively. We observed a decrease in Siberian pine proportion in the forests, whereas areas of larch and birch strongly increased (by 150% and 100%, respectively), which indicates the retreat of Siberian pine from its southern habitat. We suggested afforestation of the areas of Siberian pine mortality by the drought-tolerant larch species.

Oaks enhance early life stage longleaf pine growth and density in a subtropical xeric savanna.

The interplay of positive and negative species interactions controls species assembly in communities. Dryland plant communities, such as savannas, are important to global biodiversity and ecosystem functioning. Sandhill oaks in xeric savannas of the southeastern United States can facilitate longleaf pine by enhancing seedling survival, but the effects of oaks on recruitment and growth of longleaf pine have not been examined. We censused, mapped, and monitored nine contiguous hectares of longleaf pine in a xeric savanna to quantify oak-pine facilitation, and to examine other factors impacting recruitment, such as vegetation cover and longleaf pine tree density.We found that newly recruited seedlings and grass stage longleaf pines were more abundant in oak-dominated areas where densities were 230% (newly recruited seedlings) and 360% (grass stage) greater from lowest to highest oak neighborhood densities. Longleaf pine also grew faster under higher oak density. Longleaf pine recruitment was lowest under longleaf pine canopies. Mortality of grass stage and bolt stage longleaf pine was low (~1.0%yr-1) in the census interval without fire. Overall, our findings highlight the complex interactions between pines and oaks-two economically and ecologically important genera globally. Xeric oaks should be incorporated as a management option for conservation and restoration of longleaf pine ecosystems.

The metabolic fingerprint of Scots pine-root and needle metabolites show different patterns in dying trees.

The loss of leaves and needles in tree crowns and tree mortality are increasing worldwide, mostly as a result of more frequent and severe drought stress. Scots pine (Pinus sylvestris L.) is a tree species that is strongly affected by these developments in many regions of Europe and Asia. So far, changes in metabolic pathways and metabolite profiles in needles and roots on the trajectory toward mortality are unknown, although they could contribute to a better understanding of the mortality mechanisms. Therefore, we linked long-term observations of canopy defoliation and tree mortality with the characterization of the primary metabolite profile in needles and fine roots of Scots pines from a forest site in the Swiss Rhone valley. Our results show that Scots pines are able to maintain metabolic homeostasis in needles over a wide range of canopy defoliation levels. However, there is a metabolic tipping point at around 80-85% needle loss. Above this threshold, many stress-related metabolites (particularly osmoprotectants, defense compounds and antioxidants) increase in the needles, whereas they decrease in the fine roots. If this defoliation tipping point is exceeded, the trees are very likely to die within a few years. The different patterns between needles and roots indicate that mainly belowground carbon starvation impairs key functions for tree survival and suggest that this is an important factor explaining the increasing mortality of Scots pines.

Assessing the effect of invasive organisms on forests under information uncertainty: The case of pine wood nematode in continental Europe

Forests worldwide are experiencing increasingly intense biotic disturbances; however, assessing impacts of these disturbances is challenging due to the diverse range of organisms involved and the complex interactions among them. This particularly applies to invasive species, which can greatly alter ecological processes in their invaded territories. Here we focus on the pine wood nematode (PWN, Bursaphelenchus xylophilus), an invasive pathogen that has caused extensive mortality of pines in East Asia and more recently has invaded southern Europe. It is expected to expand its range into continental Europe with heavy impacts possible.Given the unknown dynamics of PWN in continental Europe, we reviewed laboratory and field experiments conducted in Asia and southern Europe to parameterize the main components of PWN biology and host-pathogen interactions in the Biotic Disturbance Engine (BITE), a model designed to implement a variety of forest biotic agents, from fungi to large herbivores. To simulate dynamically changing host availability and conditions, BITE was coupled with the forest landscape model iLand. The potential impacts of introducing PWN were assessed in a Central European forest landscape (40,928 ​ha), likely within PWN’s reach in future decades.A parameter sensitivity analysis indicated a substantial influence of factors related to dispersal, colonization, and vegetation impact, whereas parameters related to population growth manifested a minor effect. Selection of different assumptions about biological processes resulted in differential timing and size of the main mortality wave, eliminating 40%–95% of pine trees within 100 years post-introduction, with a maximum annual carbon loss between 1.3% and 4.2%. PWN-induced tree mortality reduced the Gross Primary Productivity, increased heterotrophic respiration, and generated a distinct legacy sink effect in the recovery period. This assessment has corroborated the ecological plausibility of the simulated dynamics and highlighted the need for new strategies to navigate the substantial uncertainty in the agent’s biology and population dynamics.

Mountain Taiga in a Warming Climate: Contrast of Siberian Pine Growth along an Elevation Gradient

The growth and survival of trees in the Siberian Mountains are experiencing a strong influence on climate warming. We analyzed Siberian pine (SP, Pinus sibirica) growth within the treeline ecotone in high (&gt;1000 m) and low (&lt;900 m) lands. We used ground surveys, dendrochronology, and climate variable data analysis. We found a contrasting response of SP growth with increasing air temperature and moisture parameters along the elevation gradient. In the treeline ecotone and highlands, the tree’s growth has been increasing since warming onset in the 1970s, whereas in the lowlands, the initial growth increase switched to a growth drop since the beginning of the 2000s, with a consequent partial mortality of the Siberian pine forest caused by warming-driven water stress in combination with bark borers’ attacks. This mortality suggests the retraction of the Siberian pine range in the lowlands of the Siberian Mountains. The projected drought increase will likely lead to the substitution of Siberian pine with drought-tolerant species. The tree’s growth index (GI) dependence on air temperature and moisture variables includes two phases. In the first phase (since the warming onset in the 1970s), the trees’ GI was positively correlated with elevated temperature, whereas correlations with precipitation and soil moisture were negative. During the second phase (since the increase in warming in the 2000s), negative correlations between the GI and moisture variables switched to positive ones. The correlations of the GI with air temperature switched from positive to mostly insignificant. The wind’s influence on the trees’ growth changed from negative to insignificant since the 2000s within all elevation belts. Afforestation within the areas of Siberian pine mortality should not be based on the planting of Siberian pine but on drought-tolerant species such as larch (Larix sibirica) and Scots pine (Pinus sylvestris).

Heterobasidion annosum and Armillaria ostoyae significantly reduce radial growth during three years before stem death in infected Pinus pinaster

Butt and root-rot diseases caused by Armillaria sp. and Heterobasidion sp. are highly destructive to conifers worldwide. These diseases lead to loss of growth, wood degradation, and tree mortality, often necessitating premature logging. However, little is known about the effect of infection on the growth and mortality of adult pines during forest development. In this study, mature maritime pines were sampled from two forest sites in south-west France. In 2013, the Sanguinet site provided both healthy (asymptomatic) and Heterobasidion annosum-infected pines, while in 2019 the Mano site also provided Armillaria ostoyae-infected pines. Radial growth dynamics were compared between healthy and infected pines using two complementary approaches taking into account the diameter of each tree: 1. Compensation over 10 years and year by year comparisons of mean (dendrochronogram) between groups of pines with same Last Year of Radial Growth (LYRG); 2. Analysis of covariance by modeling the relationship between basal area increment and basal area in healthy or infected pines for each LYRG. Our results indicate that symptomatic trees infected by Heterobasidion annosum are characterised by a rapid reduction in radial growth across the entire tree circumference. The covariance method was more efficient to show that the radial growth of infected trees was significantly lower (between 25% and 72% loss) in the three years preceding the year of growth arrest, on both sites, i.e., for two different periods of years. For Armillaria ostoyae, growth data shows the same trends than for H. annosum, with a sharp decline over the last 2–3 years of growth. Covariance results for LYRG 2018 and 2019 reveal that infection symptoms significantly reduces the radial growth of infected trees compared to asymptomatic trees over the last three years of growth. These results show that infection of maritime pine by butt and root-rot fungi can lead to a rapid decrease in radial growth, and this decrease is independent of the calendar year of cambial death at breast height.

Climate Change-Driven Cumulative Mountain Pine Beetle-Caused Whitebark Pine Mortality in the Greater Yellowstone Ecosystem

An aerial survey method called the Landscape Assessment System (LAS) was used to assess mountain pine beetle (Dendroctonus ponderosae)-caused mortality of whitebark pine (Pinus albicaulis) across the Greater Yellowstone Ecosystem (59,000 km2; GYE). This consisted of 11,942 km of flightlines, along which 4434 geo-tagged, oblique aerial photos were captured and processed. A mortality rating of none to severe (0–4.0 recent attack or 5.0–5.4 old attack) was assigned to each photo based on the amount of red (recent attack) and gray (old attack) trees visible. The method produced a photo inventory of 74 percent of the GYE whitebark pine distribution. For the remaining 26 percent of the distribution, mortality levels were estimated based on an interpolated mortality surface. Catchment-level results combining the photo-inventoried and interpolated mortality indicated that 44 percent of the GYE whitebark pine distribution showed severe old attack mortality (5.3–5.4 rating), 37 percent showed moderate old attack mortality (5.2–5.29 rating), 19 percent showed low old attack mortality (5.1–5.19 rating) and less than 1 percent showed trace levels of old attack mortality (5.0–5.09). No catchments were classified as recent attacks indicating that the outbreak of the early 2000’s has ended. However, mortality continues to occur as chronic sub-outbreak-level mortality. Ground verification using field plots indicates that higher LAS mortality values are moderately correlated with a higher percentage of mortality on the ground.

Diplodia sapinea as a contributing factor in the crown dieback of Scots pine (Pinus sylvestris) after a severe drought

The frequency and impact of drought on global ecosystems have increased within the last century, while drought has affected tree health in many regions. Diplodia sapinea is a widespread, opportunistic pathogen infecting most conifers, causing Diplodia tip blight, thriving on hosts impacted by stress such as drought, heat, or mechanical wounding. In summer of 2018, a large-scale drought was recorded all over Europe. In late summer, pine trees all over Gotland showed crown dieback, where necrotic twigs and needles were found, especially in the upper part of the crowns. Symptoms were consistent with a potential outbreak of D. sapinea. Effects of the combination of drought and Diplodia tip blight on mortality or recovery of Scots pine in Nordic conditions are unknown. This study confirmed the presence and potential contribution of D. sapinea in the observed damages of Scots pine. Shoot blight and drought led to crown defoliation which was observed one year post-drought, while trees showed a clear recovery of newly grown shoots within the second year. Severely affected pines (>70% of the upper third of the crown with shoot blight) showed increased mortality. Recovery of the surviving trees was independent of previous dieback levels. Diplodia sapinea was most abundant in twigs with shoot blight of the symptomatic trees compared to healthy-looking twigs from the same trees and asymptomatic trees in affected and healthy pine stands. Sampling on affected and healthy sites showed possible endophytic infections with low abundance within healthy-looking twigs. Spore deposition of D. sapinea was monitored on healthy and affected sites for two consecutive years after crown damages occurred to confirm the presence of the opportunistic pathogen in the affected region. Spore deposition was observed during all seasons and correlated with high precipitation during sampling. Our observations provide insights into the emergence of Diplodia tip blight in the Northern countries and underline the potential impact of D. sapinea on tree health in the course of a changing climate.

Factors associated with bark beetle infestations of Colorado Plateau ponderosa pine using repeatedly-measured field plots

Decades of research have brought increasing focus to the biotic and abiotic factors associated with bark beetle infestation of North American conifers. Information is lacking, however, regarding beetle-caused mortality for ponderosa pine of the Colorado Plateau. From 1995 to 2019, we installed and periodically remeasured permanent plots in this forest type with a goal of identifying factors related to infestation probability including tree and stand characters, and abiotic factors such as temperature and precipitation. Forty-eight sites were distributed throughout the Colorado Plateau, each with two transects of 10 contiguous ∼ 0.04 ha circular plots. Stand and tree conditions were measured or interpolated for each year from plot establishment in 1995 until the final measurements in 2019. Temperature, precipitation, and other abiotic conditions were estimated from Daymet interpolations. We used a generalized linear mixed model to identify significant explanatory variables resulting in the best accuracy of predicting stem-level bark beetle-caused pine mortality. Tree and stand conditions were found related to the probability of infestation. Pines >15.2 cm dbh were more likely to become infested compared to pines below that threshold; no relationship was found above that breakpoint. Similarly, stems 80–180 years old had elevated probability of infestation compared to stems < 80 years; stems > 180 years were intermediate. Pines with poor crown vigor (low crown volume, suppressed or intermediate canopy position) were more likely to become infested compared to pines with good crown vigor (high crown volume, dominant or open grown canopy position). Pine basal area (BA), a surrogate for intra-species competition, was positively related to probability of infestation. Pine BA infested the previous year, a surrogate for beetle population size, was also positively related to probability of infestation. Several drought- and precipitation-related variables resulted in similar model accuracy. Best model fit among these, by a slight margin, was contemporaneous climate water deficit (CWD), a measure of plant water stress, summed from April-August. Increasing CWD was associated with increased probability of infestation. Finally, counts of days above 16 °C the previous year were negatively related to infestation probability. That is, relatively warm weather during the previous year was associated with reduced beetle success, possibly because it can result in fractional beetle life cycles that disrupt synchronized attacks on new hosts or leave overwintering brood in less cold hardy life stages.

Features of Scots Pine Mortality Due to Incursion of Pine Bark Beetles in Symbiosis with Ophiostomatoid Fungi in the Forest-Steppe of Central Siberia

Forest decline is a significant issue affecting critical ecosystem processes worldwide. Here, we describe mortality in Pinus sylvestris L. monitored trees caused by the inhabitation of pine bark beetles (Tomicus minor Hart.) in symbiosis with ophiostomatoid fungi (Ophiostoma piceae (Munch) H. et P. Sydow) infection in the forest-steppe of central Siberia. Stem sap flow (Q) and stem diameter fluctuations (dRc) were monitored in eight pine trees during seven consecutive growing seasons (2015–2021). In addition, microcore sampling every ten days allowed the determination of stem wood formation in monitored trees in the 2021 growing season. During 2020 and 2021, two cases of Q termination were recorded among the monitored trees, with microcores revealing no cambium formation. Thus, the seasonal Q onset matches the beginning of the beetle dispersal period when they attack and inhabit tree stems. The decline of circumferential stem size began 10–12 days after Q onset, during the massive inhabitation of beetles into the stems. The disturbance of Q in trees occurred in 21–23 days, and total cessation of Q was observed 23–26 days after the Q onset at the beetle’s egg development phase. The timing of dRc disturbance and Q cessation observed directly coincides with the beetle life cycle. Thus, the phenology of pine trees and T. minor beetles is driven by seasonal weather conditions, particularly the cumulative air temperature (&gt;0 °C).

Western pine beetle voltinism in a changing California climate

Abstract Recent hot droughts in California resulted in ponderosa pine (Pinus ponderosa) mortality attributed to drought and western pine beetle (WPB, Dendroctonus brevicomis). While drought alone can cause tree death, direct warming effects on WPB are a contributing factor. Research on WPB generation timing (voltinism), however, is lacking. We monitored WPB tree attacks and adult emergence timing at two California sites and developed a degree‐day model from field‐observed data. Historical, contemporary, and future temperatures for several California sites were used with the model to examine trends in WPB voltinism. Field data showed a single summer and an overwinter generation at a northern California site. As summer temperatures increased beyond 1900–1980 averages, the predicted number of full and partial WPB generations by 2021 had increased from ~2 annual (one summer and one overwinter) generations historically to ~2.3 at two northern California sites and from ~2.3 to ~3.2 at two warmer California sites. Historical and contemporary data suggest winter warming was not sufficient for an additional generation overwinter. Instead, increases in generations were driven by summer and fall temperatures. Unconstrained increases in the number of future annual generations will be limited by complex, but not well understood, WPB thermal adaptations. Increased knowledge of temperature‐driven WPB population growth will improve forest vegetation models aimed at predicting ponderosa pine mortality in a changing climate.

Factors Affecting the Flight Capacity of Two Woodwasp Species, Sirex noctilio F. (Hymenoptera: Siricidae) and Sirex nitobei M. (Hymenoptera: Siricidae).

Sirex noctilio F. is an invasive woodwasp that causes pine mortality in plantations in China. Sirex nitobei M. is a native woodwasp in large areas of China. In this study, the flight capacity of the two woodwasps was studied and compared using a tethered-flight mill system to find individual factors affecting the flight capacity. After flight bioassays, woodwasps were dissected to determine nematode infestation. Post-eclosion-day (PED) age significantly influenced the flight capacity of S. noctilio females and males; as woodwasps become older, their flight capacity decreased. For S. nitobei, PED age did not significantly affect their flight capacity. In general, the flight capacity of S. noctilio was greater than that of S. nitobei. Females flew further and for longer than males for both Sirex species. The Deladenus spp. parasitism status of the two Sirex species did not significantly affect their flight performance parameters. PED age and body mass were key individual factors significantly affecting the flight capacity of the two Sirex species. In this study, detailed and accurate tethered-flight parameters of S. noctilio and S. nitobei were obtained. Although this is different from natural flight, it also provides us substantial laboratory data on their flight capacity, and facilitates risk analysis of the two woodwasp species.

Modeling slash pine mortality rates incorporating responses to silvicultural treatments and fusiform rust infection rates

The evaluation of stand survival at any age is an essential task that allows foresters to estimate stand dynamics and ultimately, the value of a forest stand. Therefore, any growth and yield system developed with the aim of predicting or projecting standing value requires precise estimates of the number of trees surviving at any point in time. When describing survival through modeling the mortality rate, differential equations using height increments rather than time increments have been shown to improve the overall fit of survival models. Using a long-term data set of slash pine plantations (Pinus elliottii Engelm.) in which silvicultural treatments (i.e., bedding and complete vegetation control) were applied during the establishment phase, and fusiform rust (Cronartium quercuum Berk.) infection rates were recorded, a survival/mortality model of this nature was constructed. Height increments were taken from a proposed dominant height model that included explicit treatment effects. In addition, the proportion of trees infected with fusiform rust at an age of 5 years was added as a predictor describing the mortality rate. Our results show that stand survival is better described by a model in which time increments are used rather than height increments, and although silvicultural treatments were essential for modeling dominant height, mortality was not greatly affected by these treatments and therefore, no additional parameter modifiers associated to these treatments were needed. On the other hand, the inclusion of average fusiform rust infections was essential to describe mortality rates in these stands, with higher infection rates associated to higher mortality rates.

Impact of stand- and landscape-level variables on pine wilt disease-caused tree mortality in pine forests.

Pine wilt disease (PWD) outbreaks have affected extensive areas of South China's forests, but the factors explaining landscape patterns of pine mortality are poorly understood. The objective of this study was to determine the relative importance of stand structure, topography, landscape context, and beetle pressure in explaining PWD severity. During 2020-2021, we identified 66 plots based on mapped PWD infestation severity. We built PWD infestation maps for 2019-2021 through field surveys. Stand structure and topography were obtained from Forest Resources Management 'One Map' and elevation raster data. We then used 'One Map' and PWD infestation maps to determine landscape context and beetle pressure variables at different spatial scales. The relative importance of 12 explanatory variables was analyzed using multi-model inference. In this study, we show that: (i) 1 km was the best spatial scale related to pine mortality, and (ii) models including landscape context and beetle pressure were much better at predicting pine mortality than models using only stand-level variables. Landscape-level variables, particularly beetle pressure, were the most consistent predictors of subsequent pine mortality within susceptible stands. These results may help forest managers identify locations vulnerable to PWD and improve existing strategies for outbreak control. © 2023 Society of Chemical Industry.

Temperature is positively associated with tree mortality in California subalpine forests containing whitebark pine

AbstractWhitebark pine (Pinus albicaulis Engelm.) is a keystone high‐elevation tree species occurring across much of western North America, yet it has been listed as threatened under the US Endangered Species Act due to rapid population declines and extensive ongoing pressures from white pine blister rust (Cronartium ribicola), mountain pine beetle (Dendroctonus ponderosae), and increasing temperature and aridity associated with climate change. Past research has shown that whitebark pine mortality is more likely in hotter and drier sites, but no broad‐extent analyses spanning multiple ecoregions have directly quantified the relationship between mortality probability and its environmental drivers at a fine spatial scale. To address this gap, we used spatially continuous high‐resolution (30 m) maps of mortality in California produced by a remote sensing‐based change detection algorithm, and we developed generalized additive models to explain this mortality variability using relevant biophysical variables. We found that maximum daily temperature was strongly positively associated with mortality intensity and that mortality was also generally more intense in sites with greater solar exposure (e.g., south‐facing slopes). Our results support the interpretation that as climate warms and aridity increases, populations in “trailing edge” (i.e., hot, dry) sites are most vulnerable. The environmental relationships we quantify will aid in identifying the locations and environments with the greatest risk of future mortality as well as those likely to serve as refugia.

Ponderosa pine introduction methods following a high-severity stand-replacing fire to promote forest regeneration

BackgroundIn July 2012, a lightning strike ignited the Arapaho Fire in the Laramie Mountains of Wyoming and burned approximately 39,700 ha. This high-severity fire resulted in 95% mortality of ponderosa pine (Pinus ponderosa P. & C. Lawson) at the University of Wyoming’s Rogers Research Site. Ponderosa pine recruitment post-high-severity wildfire is limited in semi-arid and mid-elevation forests in the Rocky Mountain region due to the reduction of seed supplies from living trees, warm temperatures, and limited precipitation. We used an experimental block design to determine management treatments that would increase ponderosa pine abundance, and we measured the impacts to the vegetation community, ground cover, and bare ground following a high-severity wildfire. Treatments included a combination of one pine introduction treatment (natural regeneration, broadcast seeding, and planted seedlings), one logging treatment (no logging, bole only removal, whole tree removal), and erosion control seeding (no erosion seeding and seeding with a native grass mix) in each plot within a block.ResultsOur results indicate that the pine introduction treatment “planted seedlings” was the most effective restoration treatment in semi-arid, mid-elevation sites, although the overall survival rate of seedlings from initial planting in 2015 to 2017 was only 6%. “Whole tree removal” had a weak positive effect on the “planted seedlings” ponderosa pine abundance. The estimated mean percent moss cover was higher in the “no logging” treatment, and this treatment resulted in a lower mean percent bare ground. Overall, 2 years after implementation, the management treatments did not result in different vegetation communities.ConclusionsNo difference in vegetation functional group cover among the pine introduction and logging treatments at the RRS is likely due to the large landscape heterogeneity with differing slopes and two different aspects coupled with the short time frame since the implementation of the treatments at the site. The direct implications of these findings suggest that hand planting ponderosa pine seedlings is an effective way for managers to reintroduce ponderosa pine 3 years following a high-severity wildfire in semi-arid and mid-elevation sites in the northern Rocky Mountains.

Tree resistance to drought and bark beetle-associated mortality following thinning and prescribed fire treatments

Long-term trends show increased tree mortality over the last several decades, coinciding with above-average temperatures, high climatic water deficits, and bark beetle outbreaks. California’s recent unprecedented drought (2012–2016) highlights the need to evaluate whether thinning and prescribed fire can improve individual tree drought resistance and reduce bark beetle-associated mortality. Using a thinning and prescribed fire study on the Stanislaus-Tuolumne Experimental Forest in the central Sierra Nevada implemented prior to the drought (2011–2013), we used dendrochronological methods to estimate metrics of tree vigor (i.e., growth and resin ducts) of sugar pine (Pinus lambertiana Douglas) and white fir (Abies lowiana [Gordon & Glend.] A. Murray bis) among treatments, as well as between trees that died from bark beetle-associated mortality and their paired counterparts that survived. We used tree vigor to estimate drought resistance as the ratio between growth during drought (2012–2016) and pre-drought (2007–2011) for both species. For sugar pine, we also created analogous ratios for multiple resin duct characteristics to evaluate defense during drought. Our findings indicate that lower competition increased growth resistance of white fir, while prescribed fire had negligible impacts on growth. This translated to lower mortality, with live white fir showing higher growth resistance than those that died. While competition did not strongly affect sugar pine growth, greater growth resistance was noted for trees that lived than trees that died. However, reduced competition and prescribed fire increased defense resistance and resin duct density and relative resin duct area were negatively associated with sugar pine mortality. Live sugar pine showed greater defense resistance than dead counterparts particularly under higher levels of competition. These findings suggest thinning can promote or maintain growth during severe drought conditions and prescribed fire can be applied with negligible costs to tree growth while also producing the additional benefit of stimulating defense systems in sugar pine, which may enable them to better survive bark beetle outbreaks. Therefore, susceptibility to bark beetle-associated mortality may be ameliorated through increasing tree vigor with a combination of forest thinning and prescribed fire.