Ponderosa Pine Plantation Research Articles

Early treatment effects on plantation growth and biodiversity in mature ponderosa pine forest

We address the concerns that managed forest plantations may cause reductions of diversity of understory vegetation. We performed inventories of two mature ponderosa pine plantations in which multiple treatments were applied during plantation establishment. At stand age 35, we measured tree heights and diameters, understory plant cover and diversity, soil nutrients and chemical properties, and soil microbial biomass and diversity with phospholipid fatty acid. We found a significant, positive effect from both herbicide (H) and fertilizer (F) applications on subsequent overstory tree growth and development ( P < 0.05); insecticide (I) effect was minimal. We observed negative effects on understory plant diversity decades later when herbicide was applied during stand establishment. However, lower plant diversity and ground cover appeared to have been caused primarily from overstory canopy closure, supported by the increased understory cover and diversity observed in the HI and HFI plots that had been thinned at age 12. Similarly, while fertilizer increased tree growth at both sites, it only negatively affected understory plants at the higher quality site. We did not find significant influence of treatment effects on soil nutrients and microbial communities. Therefore, to mitigate the potential loss of understory biodiversity in plantations, foresters can manage overstory trees with traditional pre-commercial thinning techniques and early tending.

Can the inter-rotational productivity in ponderosa pine plantations be sustained in the face of climatic change?

Inter-rotational productivity (i.e., productivity of successive rotations) of ponderosa pine plantation was measured on experimentally treated plots with a common design at three diverse sites in northern California. The objectives were to examine the carryover effects of fertilization (F) and understory control by herbicide (H) applied in the first rotation of plantation, on growth in the second rotation and to determine the sustainability of plantation productivity between rotations. Carryover effects varied among the three sites. There was a significant and positive H and F effect (P < 0.03) on tree height and aboveground biomass at Whitmore, a site of intermediate quality; only an H effect at Elkhorn (P < 0.01), the site of low quality; and an F effect at Feather Falls (P = 0.05), the site of high quality. Genetically improved seeds resulted in slightly taller height than the original seed sources, but the difference was not statistically significant. While annual precipitation was similar, growing season precipitation was 6–42 % lesser and air temperature was about 0.9–1.4 °C higher during the second rotation than in the first rotation. Due to the warmer air temperature, the sites likely endured drier air, greater evapotranspiration, and lesser soil water availability. These environmental changes appear to have caused the observed growth reduction in these ponderosa pine plantations. This was despite the management techniques we applied for improved pine growth during the second rotation: genetically improved seeds and control of competing vegetation. It is believed that these are the first reported observations of growth decline in managed ponderosa pine plantations associated with warming climate. If these results are representative of a larger region and a future trend, use of ponderosa pine plantation forestry as a method of increasing forest carbon sequestration, may be overestimated for future rotations.

Acumulación de biomasa en juveniles de arbustos nativos expuestos a distintos niveles de luz asociados a esquemas de manejo forestal de coníferas exóticas

Pinus ponderosa plantations established in the Patagonian steppe put ecosystem functions at risk if they are not properly managed. The response of shrubs to light variations caused by forest plantations and forestry interventions may influence their ecological role. In this work we evaluated the total biomass accumulation dynamics of calafate (Berberis microphylla) and mamuel choique (Adesmia volckmannii) at 20, 60 and 100% irradiance, and their response to the light increase that would occur after interventions such as thinning and harvesting. For each species we evaluated 1) the relationship between biomass and diameter at the base of the stem, 2) the biomass dynamics during two growing seasons in the three irradiance conditions, and 3) the early response to light increase in plants subjected to irradiances of 20 and 60%. In addition, 4) we compared the levels of irradiance that the proposed forest management schemes for the region would suppose with those evaluated here. For both species, biomass and diameter were linearly related and biomass accumulation was lower at 20% irradiances. Calafate showed a tendency to accumulate more biomass at 100%, and mamuel choique, at 60%, although no species showed differences between these irradiances. Whereas calafate from 20% irradiance only increased its biomass by increasing irradiance to 100%, mamuel choique did so by increasing it to 60%. These results suggest that the two species, at least in the juvenile phase, may acclimate to shade, and that mamuel choique would respond better to partial increases in light, as in the case of thinning. The response of calafate suggests that the proposed management, which irradiance levels would be lower than those evaluated here, would not be optimal for its development.

Planted seedling survival in a post-wildfire landscape: From experimental planting to predictive probabilistic surfaces

Across the southwestern United States, high-severity wildfire is causing increasingly large areas of tree mortality and removing the seed sources required for the natural regeneration of these formerly conifer-dominated landscapes. Planting tree seedlings can accelerate reforestation, but in the semi-arid southwestern US, the survival of planted conifer seedlings is typically low. Our research examined how post-fire planting success is influenced by climate, topographic, biotic, and microclimate factors. We present data on a seedling planting experiment within the footprint of the 2011 Las Conchas Fire in northern New Mexico. We planted 768 tree seedlings of four species: ponderosa pine (Pinus ponderosa), piñon (P. edulis), southwestern white pine (P. strobiformis), and Douglas-fir (Pseudotsuga menziesii) in an area burned at high-severity and densely populated by New Mexico locust (Robinia neomexicana) and other shrub species. We established experimental plots stratified by aspect, either northerly or southerly, and cover, either planted under New Mexico locust or in the open, and collected topographical and microclimate measurements over three years. Mean survival over the 3-year period for all species in all treatments was 20%. Using microclimate variables and site-level topographical indices in a piecewise exponential model, we first determined how these factors influenced seedling survival of four conifer tree species, finding that topographical indices describing incoming solar radiation, variables related to vapor pressure deficit, i.e., seasonal maximums, and presence of New Mexico locust increased hazard for all species while variables related to shade, moisture availability, and slope reduced hazard. Following our planting experiment, an additional 1145 ponderosa pine seedlings, planted by USFS, were surveyed for survival and the topographical indices at their locations were recorded. Adding these seedlings' survival data to our experimentally planted ponderosa pine seedlings, we used boosted regression tree models to determine survival probability for ponderosa pine as a product of topographical indices, then extrapolated these findings to the wider landscape, producing a spatial projection of probabilistic seedling survival as a function of readily available topographical data, with a mean AUC value of 0.63. Both sets of analyses highlight the importance of landscape heterogeneity in shaping microclimatic environments and affecting seedling survival in a predictable manner, with areas receiving less solar radiation and more moisture promoting seedling survival. Our results demonstrate that by using fine-scale microtopography that shapes microclimatic environments, future plantings can increase conifer tree seedling survival in high-severity post-wildfire landscapes.

Site quality and intensive early stand management practices affect growth dominance, structural complexity, and tree growth in ponderosa pine plantations

During stand initiation, the influences of silvicultural practices such as competing vegetation control and fertilization have been primarily examined in terms of stand-level growth rather than structural complexity. Understanding how intensive silvicultural treatments alter the long-term trajectory of stand development may be critical to meeting management objectives, such as habitat restoration, minimizing ladder fuels, or carbon sequestration. Previous studies have found mixed evidence for an effect of vegetation control and fertilization on structural complexity. Investigating how early pulse treatments drive tree size differentiation in young stands, as indicated by growth dominance (GD), could potentially allow managers to anticipate or design treatments to alter the development of structural complexity later in stand development. We reanalyzed data from the Garden of Eden experiment, a full-factorial study investigating how competing vegetation and nutrients impact ponderosa pine growth and yield over a gradient of site quality in northern California, USA. Our goals were to determine whether these silvicultural practices alter GD and structural complexity, and whether GD and structural complexity created potential feedbacks to alter stand productivity. We examined data for the first 20 years of stand development across 6 study sites. For each site and observation period, we calculated the growth dominance coefficient of total cubic volume to represent GD and the Gini coefficient of inequality in basal area distribution (GC) to represent structural complexity. We created a series of competing generalized additive mixed models (GAMMs) to disentangle the roles of site, stand age, and treatment on GD and GC. We created a set of competing individual-tree GAMMs at age 20 to isolate the influences of GC and GD on volume increment, controlling for the effects of competition, tree size, site, and treatment influences. In agreement with previous research, we found that GD deviated little from size-symmetry (tree growth being roughly proportional to tree size) and did not vary consistently with treatment. Competing vegetation control and higher site quality promoted lower GC, indicative of lower structural complexity. Fertilization increased structural complexity relative to other treatments on two sites in particular, which may reflect unintentional increases in drought stress from fertilization on sites with metasedimentary soils. While GD had limited influence on individual-tree growth, GC interacted with tree size such that small-medium sized trees grew more rapidly on structurally simple plots. Our results have implications for balancing objectives such as fire risk, carbon sequestration, and habitat during reforestation projects.

Trade-off between growth rate and water use efficiency in southwestern ponderosa pine provenances

We investigated growth and a suite of physiological and structural traits of ponderosa pine (Pinus ponderosa Lawson & C. Lawson var. scopulorum Engelm.) seedlings from different provenances using a field common garden study. Twenty-one provenances from a range of elevations across Arizona and New Mexico were planted in 2018 at a field site in the core of the species range in northern Arizona. We measured stem growth rate for three years (2019–2021), leaf carbon isotope discrimination (Δ13C), and leaf nitrogen concentration in 2020 on all provenances, and leaf-level gas exchange, instantaneous water use efficiency, predawn and midday water potentials, soil-to-leaf hydraulic conductance and specific leaf area on nine provenances in 2020. Provenances differed significantly in stem growth rate, Δ13C, and specific leaf area, and several traits were correlated with provenance environmental and climatic characteristics. Seedlings with higher growth rate had greater Δ13C suggesting a trade-off between growth rate and water use efficiency. Provenances from warmer sites had lower growth rate and Δ13C (higher water use efficiency) than provenances from cooler sites during the driest year. Growth rate of provenances in the driest year was positively associated with stomatal conductance, net photosynthetic rate, and soil-to-leaf hydraulic conductance and negatively associated with instantaneous water use efficiency indicating the importance of maintaining water uptake and gas exchange to growth during drought under dry conditions. Our results enhance understanding of physiological mechanisms of establishment for planted ponderosa pine seedlings and show a trade-off between growth rate and water use efficiency of provenances from different thermal environments that should be useful in selection of seed sources for reforestation.

A novel smartphone-based activity recognition modeling method for tracked equipment in forest operations.

Activity recognition modelling using smartphone Inertial Measurement Units (IMUs) is an underutilized resource defining and assessing work efficiency for a wide range of natural resource management tasks. This study focused on the initial development and validation of a smartphone-based activity recognition system for excavator-based mastication equipment working in Ponderosa pine (Pinus ponderosa) plantations in North Idaho, USA. During mastication treatments, sensor data from smartphone gyroscopes, accelerometers, and sound pressure meters (decibel meters) were collected at three sampling frequencies (10, 20, and 50 hertz (Hz)). These data were then separated into 9 time domain features using 4 sliding window widths (1, 5, 7.5 and 10 seconds) and two levels of window overlap (50% and 90%). Random forest machine learning algorithms were trained and evaluated for 40 combinations of model parameters to determine the best combination of parameters. 5 work elements (masticate, clear, move, travel, and delay) were classified with the performance metrics for individual elements of the best model (50 Hz, 10 second window, 90% window overlap) falling within the following ranges: area under the curve (AUC) (95.0% - 99.9%); sensitivity (74.9% - 95.6%); specificity (90.8% - 99.9%); precision (81.1% - 98.3%); F1-score (81.9% - 96.9%); balanced accuracy (87.4% - 97.7%). Smartphone sensors effectively characterized individual work elements of mechanical fuel treatments. This study is the first example of developing a smartphone-based activity recognition model for ground-based forest equipment. The continued development and dissemination of smartphone-based activity recognition models may assist land managers and operators with ubiquitous, manufacturer-independent systems for continuous and automated time study and production analysis for mechanized forest operations.

Not Only Severe Events: Moderate Dry Periods Impact the Hydraulic Functioning and Survival of Planted Ponderosa Pine Seedlings

Juvenile tree survival will increasingly shape the persistence of ponderosa pine forests in the western United States. In contrast to severe pulse disturbances that induce widespread adult and juvenile tree mortality, moderate periods of low rainfall and warm temperatures may reduce forest persistence by killing juvenile trees at the seedling stage. Intensification of these periods in a changing climate could therefore increasingly restrict both natural regeneration and artificial regeneration of planted seedlings. We conducted a controlled field experiment at a single site in the Front Range of Colorado, USA, to determine the responses and survival of 3 Colorado subpopulations of &lt;1 year old potted ponderosa pines to moderately dry conditions, variation in small rainfall events based on observed patterns, and shaded and unshaded microsite environments. Near surface soil moisture increased slightly following small rainfall events, but declined over the 45-day experimental period. Seedling transpiration and associated canopy cooling declined after ∼13 days, and further declines in transpiration and canopy cooling suggest that the majority of trees in lower rainfall treatments experienced hydraulic dysfunction between days ∼20–30. After 45 days, mortality across all subpopulations and treatments, inferred by relative water loss, exceeded 90–95%. Despite some uncertainty pertaining to the stress tolerance of nursery grown versus naturally germinated conifers, our results show that planted ponderosa pine seedlings &lt;1 year old are unlikely to survive moderate dry periods of 20+ days relying on small rainfall events. Although microsite conditions and soil moisture availability shaped tree hydraulic functioning early in the experiment (days 1–13), later functioning was shaped predominately by the legacy of rainfall treatments. Our results illustrate the importance of moderate dry events that occur consistently as part of seasonal variation in climate, and show how their intensification may constitute a sustained press that limits opportunities for natural and artificial regeneration.

Growth Response of Ponderosa Pine to Intensive Cultural Treatments Varies with Site Quality and Plantation Age

Abstract Long-term forest experiments provide valuable knowledge in managing forests for productivity and other ecosystem services. California’s “Garden of Eden” experiment was established to determine growth potential of Pinus ponderosa plantations in response to intensive cultural treatments. We examined the 20-year growth-response of tree volume to intensive cultural treatments consisting of combinations of herbicide on competing vegetation (H), fertilization (F), and insecticide (I). We found that both H and F treatments synergistically increased tree growth at intermediate and lower-quality sites. Growth increased by 1.5–2.1 times with F, 2.1–2.5 times with H, and 2.3–3.8 times with HF treatments versus control (range = 39.3–109.2 m3 ha−1). Across the highest productive site, H and F effects on volume seemed additive at younger ages, but largely dissipated by age 20, with volume increasing by 1.6, 1.2, and 1.6 times relative to control in F, H, and HF treatments, respectively. However, 20-year mean annual increment was 21.7 m3 ha−1 yr−1 for the F treatment, the highest volume reported for ponderosa pine in California. The results underscore how site-specific cultural treatments, especially H and F, may widely enhance plantation productivity and boost stand development. Study Implications In Mediterranean climates, competing vegetation control is essential for ponderosa pine planation establishment and early growth, especially at intermediate and poor-quality sites. At higher-productivity sites, fertilization enhances stand growth and development, although fertilization’s beneficial effects on growth do not appear until canopy closure. Precommercial thinning in herbicide and fertilization treatments will not reduce overall stand growth 10 years postthinning. Intensively managed plantations appear to have a higher maximum stand density index compared to natural stands or unmanaged plantations. Therefore, these cultural treatments can be used to rapidly reforest areas after disturbances and subsequently promote larger trees on reforested landscapes.

Relationships among forest structure, solar radiation, and plant community in ponderosa pine plantations in the Patagonian steppe

In Patagonia, Argentina, forest management in plantations with exotic pines usually focus on timber production, without considering the plant community of their understory. However, there is growing interest in conserving the biodiversity in these productive systems, due to the important role of biodiversity in the stability and functions of newly created ecosystems. Since solar radiation is one of the main limiting factors for vegetation development in forest plantations established in treeless steppes, we carried out this study with the objectives of analyzing and establishing relationships among variables of the forest structure, the solar radiation transmitted through different forest structures, and attributes of the plant community in ponderosa pine plantations in northwestern Patagonia. We selected three plantations located in the steppe that were managed traditionally and presented a wide range of forest structures. We assessed relationships between variables of the forest structure and variables of the light environment, as well as the relationship of variables of the forest structure and the light environment with the richness and cover of the plant community, using generalized linear mixed-effects models. We also assessed the relationship of diffuse radiation with the presence and cover of species grouped by their growth-form, nativity, and the most frequent species, using generalized linear mixed-effects models and Kendall correlations. Canopy cover and basal area were the forest structural variables that explained most of the diffuse radiation variability, which was the best variable characterizing the light environment. Diffuse radiation decreased linearly as canopy cover and basal area increased, and the understory plant community responded to that gradient, decreasing both total species richness and cover. Besides, different responses of the presence and cover of the most frequent species and species grouped by their growth-form and nativity in response to the available solar radiation in the understory were detected. Whereas some species or groups did not show a significant response, those that did were, in general, positively related to diffuse radiation, although to a different extent. These results indicate that ponderosa pine plantations in Patagonia affect their understory plant community. However, they also suggest that promoting forest management guidelines that lower basal area, raises canopy height and manages residual slash would enhance plant community attributes, balancing timber production and biodiversity conservation.

Allometry of tree biomass and carbon partitioning in ponderosa pine plantations grown under diverse conditions

Forests not only provide forest products to meet people’s demands, but also sequester enough carbon to offset more than 11% of anthropogenic CO2 release in the United States. An accurate estimate of forest carbon sequestration over different forest and forest product combinations requires the species-specific and site-specific allometric biomass equations for trees and understory vegetation at particular forest developmental stages. Here, we constructed the above- and below-ground allometric equations by harvesting 108 20-year-old ponderosa pine (Pinus ponderosa) trees grown under four different silvicultural treatments (four combinations of with and without competing vegetation control (H) and fertilization (F): C = control with no vegetation control and no fertilization, H, F, and HF) at three diverse site conditions in northern California. We found that the logarithmic regressions provided superior estimations of biomass for total, aboveground, bole, coarse roots, branches, and foliage compared to simple or scaled power-law fits. Treatment effect in allometric equations was generally lacking but site effect was significant, suggesting site specific allometric relations exist. With the best allometric equations, we estimated the biomass and carbon for each plot among treatments and sites. In addition, understory woody plant biomass was also measured with fixed area subplots. Results show significantly more total carbon sequestration at the high-quality sites with 113.1–162.5 Mg ha−1 than the lower ones with 85.8–105.9 Mg ha−1 at the intermediate site and 29.3–70.4 Mg ha−1 at the poorest site, respectively. The range of carbon stocks indicates the treatment differences at each site, with the largest effect (240%) at the poorest site and similar at other two sites (~130%). If planted trees only were considered, a trend among sites exists with 93.4 – 149.3 Mg ha−1 at the richest site, 26.2–88.1 Mg ha−1 at the intermediate site, and 17.1–64.2 Mg ha−1 at the poorest site, respectively. Therefore, if we manage forests for carbon or forest products, intensive management practices will substantially enhance plantation productivity and stand development while site quality cannot be changed.

Effect of Silviculture on Carbon Pools during Development of a Ponderosa Pine Plantation

Forest stands can be considered as dynamic carbon pools throughout their developmental stages. Silvicultural thinning and initial planting densities for reforestation not only manipulate the structure or composition of vegetation, but also disturb forest floor and soils, which, in turn, influences the dynamics of carbon pools. Understanding these carbon pools both spatially and temporally can provide useful information for land managers to achieve their management goals. Here, we estimated five major carbon pools in experimental ponderosa pine (Pinus ponderosa) plots that were planted to three levels of spacing and where competing vegetation was either controlled (VC) or not controlled (NVC). The objectives were to determine how an early competing vegetation control influences the long-term carbon dynamics and how stand density affects the maximum carbon (C) sequestration for these plantations. We found that planting density did not affect total ecosystem C at either sampling age 28 or 54. Because of competing vegetation ingrowth, the NVC (85 ± 14 Mg ha−1) accumulated greater C than the VC (61 ± 6 Mg ha−1) at age 28. By age 54, the differences between treatments narrow with the NVC (114 ± 11 Mg ha−1) and the VC (106 ± 11 Mg ha−1) as the pines continue to grow relatively faster in the VC when compared to NVC and C of ingrowth vegetation decreased in NVC, presumably due to shading by the overstory pines. The detritus was not significantly different among treatments in either years, although the mean forest floor and soil C was slightly greater in NVC. While NVC appears to sequester more C early on, the differences from the VC were rather subtle. Clearly, as the stands continue to grow, the C of the larger pines of the VC may overtake the total C of the NVC. We conclude that, to manage forests for carbon, we must pay more attention to promoting growth of overstory trees by controlling competing vegetation early, which will provide more opportunities for foresters to create resilient forests to disturbances and store C longer in a changing climate.

Aboveground Biomass Response to Release Treatments in a Young Ponderosa Pine Plantation

Controlling competing vegetation is vital for early plantation establishment and growth. Aboveground biomass (AGB) response to manual grubbing release from shrub competition was compared with no release control in a twelve-year-old ponderosa pine (Pinus ponderosa Lawson &amp; C. Lawson) plantation established after a wildfire in northeastern California. In addition, response to chemical release followed by precommercial thinning in an adjacent plantation was also examined as a growth potential from a more intensively managed regime, where shrub competition was virtually eliminated. We measured AGB in both planted trees and competing woody shrubs to partition the biomass pools in the plantation. The results showed a significant grubbing treatment effect on basal diameter (BD) at 10 cm aboveground (p = 0.02), but not on tree height (p = 0.055). Height and BD were 2.0 m and 7.4 cm in the manual release, respectively, compared to 1.7 m and 5.6 cm in the control. However, chemical release produced much greater rates of tree growth with a height of 3.6 m and BD of 14.7 cm, respectively. Tree AGB was 60% higher with the manual release of shrubs (1.2 Mg ha−1) than with control (0.7 Mg ha−1) (p &lt; 0.05). The planted area without shrub competition yielded a much higher green tree biomass (16.0 Mg ha−1). When woody shrub biomass was included, the total AGB (trees and woody shrubs) appeared slightly higher, but non-significant in the no release control (13.3 Mg ha−1) than in the manual release (11.9 Mg ha−1) (p = 0.66); the chemical release had 17.1 Mg ha−1. Clearly, shrub biomass dominated this young plantation when understory shrubs were not completely controlled. Although the manual release did increase targeted tree growth to some degree, the cost may limit this practice to a smaller scale and the remaining shrub dominance may create long-term reductions in growth and a persistent fuels problem in these fire-prone ecosystems.

Diversity of exotic ectomycorrhizal Rhizopogon from pine plantations in Patagonia

ABSTRACTWe present an account of Rhizopogon introduced from plantings of exotic pine plantations in Argentine Patagonia. Nuc rDNA internal transcribed spacer ITS1-5.8S-ITS2 (ITS) and nuc 28S rDNA (28S) sequences were used to identify specimens from Argentina and examine their relationships with geographically different Rhizopogon species. Based on phylogenetic analyses, we confirm that four species of Rhizopogon occur in pine plantations across Patagonia. Several Rhizopogon collections from Pinus ponderosa plantations across different provinces cluster with R. arctostaphyli, a species within R. subg. Amylopogon. The majority of Patagonian Rhizopogon, however, form three different lineages in R. subg. Roseoli. The first of these, R. roseolus sensu Trappe, includes numerous collections from Pinus ponderosa, P. contorta, and P. radiata stands of North American affiliation. The second, R. roseolus sensu Martin and Garcia from P. ponderosa plantations, clusters in clade IIIa of the R. roseolus complex, which also includes the holotype collection of R. mohelnensis from the Czech Republic. The third species in R. subg. Roseoli, and fourth species overall from Patagonia, is R. granuloflavus from Pinus ponderosa plantations. Multiplex polymerase chain reaction (PCR) of numerous Roseoli samples failed to produce an amplicon indicative of either Japanese or New Zealand shoro.

Two Decades of Experimental Manipulation Reveal Potential for Enhanced Biomass Accumulation and Water Use Efficiency in Ponderosa Pine Plantations Across Climate Gradients

AbstractIn this study, we report 20 years of data from three ponderosa pine plantations in northern California. Our sites span a natural gradient of forest productivity where climate variability and edaphic conditions delineate marked differences in baseline productivity (approximately threefold). Experimental herbicide application and fertilization significantly reduced competition and improved tree growth by 1.4‐ to 2.2‐fold across sites. At the site of lowest productivity, where soils are poorly developed and water limiting, tree growth increased strongly in response to understory suppression. Small but significant improvements in tree growth were observed in response to understory suppression at the moderate‐productivity site. At the site of highest productivity, where climate is favorable and soils well developed, fertilization increased productivity to a greater extent than did understory suppression. In most cases, the effect of understory suppression and fertilization caused an unexpected growth release, exceeding the anticipated maximum productivity by &gt;5 m of additional height and 60–100% more basal area. At the site of highest productivity, however, understory suppression caused a weak increase on late‐season growth compared to fertilization alone, suggesting a beneficial effect of understory vegetation on long‐term growth at that site. Tree ring cellulose carbon isotopes indicate a negative relationship between intrinsic water use efficiency (iWUE) and tree growth in control stands, which shifted to a positive relationship as both iWUE and tree growth increased in response to management. Cellulose oxygen isotope ratios (δ18O) were positively correlated with iWUE and negatively correlated with vapor pressure deficit across sites, but δ18O was not a strong predictor of tree growth.

Climate Effect on Ponderosa Pine Radial Growth Varies with Tree Density and Shrub Removal

With increasing temperatures and projected changes in moisture availability for the Mediterranean climate of northern California, empirical evidence of the long-term responses of forests to climate are important for managing these ecosystems. We can assess forest treatment strategies to improve climate resilience by examining past responses to climate for both managed and unmanaged plantations. Using an experimental, long-term density and shrub removal study of ponderosa pine (Pinus ponderosa Lawson &amp; C. Lawson) on a poor-quality site with low water-holding capacity and high runoff of the North Coastal mountain range in California, we examined the relationships between radial growth and climate for these trees over a common interval of 1977–2011. Resistance indices, defined here as the ratio between current year radial growth and the performance of the four previous years, were correlated to climatic variables during the same years. We found that all treatments’ radial growth benefited from seasonal spring moisture availability during the current growing year. Conversely, high spring and early summer temperatures had detrimental effects on growth. High-density treatments with manzanita understories were sensitive to summer droughts while lower densities and treatments with full shrub removal were not. The explanatory power of the climate regression models was generally more consistent for the same shrub treatments across the four different densities. The resistance indices for the lower density and complete shrub removal treatment groups were less dependent on previous years’ climatic conditions. We conclude that, for ponderosa pine plantations with significant manzanita encroachment, understory removal and heavy thinning treatments increase subsequent growth for remaining trees and decrease sensitivity to climate.

Thirteen-year growth response of ponderosa pine plantations to dominant shrubs (Arctostaphylos and Ceanothus)

Thirteen-year growth response of ponderosa pine to various manipulations of understory vegetation was studied to determine if a threshold of understory cover can be established for plantation productivity and whether nitrogen-fixing Ceanothus species benefit plantation growth compared to non N-fixing Arctostaphylos species, given their ability to improve site fertility. Results showed that completely controlling competing vegetation increased plantation growth, as was universally expected, but the effects of partial shrub control on plantation performance was inconsistent. We did not find a benefit of nitrogen-fixing shrubs on tree growth, as young plantation growth was slightly better in the non N-fixing plots compared to the N-fixing plots due to differences in shrub cover. An understory cover threshold for tree growth was not observed. Due to the overriding effect of shrub competition, controlling herbaceous species had little effect on plantation growth even though it benefited tree seedling survival. A long-term value of N-fixing shrubs at these sites, if there is one, must be balanced by successful plantation establishment and rapid early growth.

Resilience of a ponderosa pine plantation to a backfiring operation during a mid-summer wildfire

The Mill Fire, which burned in north-western California during the summer of 2012, provided a unique research opportunity when firefighters implemented a backfiring operation to limit wildfire growth. This backfire was ignited and burned through research plots from a long-term study designed to determine the effects of tree density manipulation and shrub control on the growth and stand development of a ponderosa pine plantation. The objectives of this study were to examine the response of these 53-year-old trees to the backfire and to determine how the fire effects differed with plantation structure and composition. Measurements made 4 years post-fire showed that mortality rate was highly variable (from 0 to 100%) and did not relate to tree density, height of live crown, total basal area or shrub cover. Bole char height explained 65% of the variation in mortality rate. Fire appeared to spread primarily through the surface litter and killed a substantial proportion of the shrubs competing with the trees for water and nutrients. Importantly, post-fire tree growth was not significantly affected relative to pre-fire growth. A lack of negative effects of the fire on radial growth was possibly a result of release from inter-tree and shrub competition, which balanced any declines that might have been expected from bole injury or crown loss. Results from the present study demonstrate that ponderosa pine plantations could potentially be treated with managed fire (e.g. prescribed fire) without pretreatment (i.e. thinning, mastication), and still achieve good survival and improved resilience to wildfires burning under uncontrolled conditions.

Effect of thinning and soil treatments on Pinus ponderosa plantations: 15-year results

Thinning with removal of whole trees in a plantation or natural forest stand raises two main concerns – soil compaction from the ground-based machinery and nutrient depletion particularly with whole tree harvest as is often practiced for attendant fuels reduction. To address these concerns, two sets of experimental treatments were imposed in young ponderosa pine plantations. In the first set, we applied four treatments to test the effects of thinning with biomass removal using progressively more soil manipulations: (I) control, (II) thinning only with all biomass removed, (III) same as (II) but followed by sub-soiling in traffic lanes, and (IV) same as (III) but with nitrogen and phosphorus fertilization within traffic lanes prior to sub-soiling. In the second experiment set we applied four combination treatments to test the further effects of soil manipulations with wood chips and fertilizer on traffic lanes. In thinned stands: (i) the harvested trees were chipped, and spread onto traffic lanes followed by sub-soiling and rototilling, (ii) same as (i) but traffic lanes also received N and P with the chips prior to the sub-soiling, (iii) traffic lanes were sub-soiled, then thinning chips were returned to just the surface of traffic lanes, and (iv) same as (iii) but traffic lanes also received N and P fertilizer with the chips. Tree height and diameter were measured three times, starting immediately following treatments and again at 5 and 15years post-treatment. In addition, soil bulk density was measured at 6years and soil chemistry (C, N, and P) was measured at 6 and 16years. Our results indicate: (1) thinning by itself with no subsoiling did not compact the soils, but increased growth rate of residual trees, although the periodic annual increment of basal area and volume was still higher in the control than other main-plot treatments; (2) neither subsoiling nor rototilling, both of which might mitigate soil compaction, enhanced tree growth; (3) short-term plantation growth was not improved with chip returns or chips with fertilization; (4) since thinning and soil treatments showed more insect damage and higher mortality, any management operations that involve cutting or damaging trees or roots should be avoided during active periods of bark beetle flight; (5) both thinning and soil treatments did not reduce carbon sequestration in the mineral soils. A lack of growth benefits from returning thinning chips, rototilling, and direct fertilization for a longer period appeals to further study.

Effect of overstorey trees on understorey vegetation in California (USA) ponderosa pine plantations

Effect of overstorey trees on understorey vegetation in California (USA) ponderosa pine plantations