Longleaf Pine Seedlings Research Articles

Plant growth forms influence sandhill longleaf pine regeneration

Longleaf pine researchers have quantified many aspects of regeneration, growth and yield, but have focused on productive sites such as pine flatwoods and upland pine. Sandhill longleaf pine woodlands represent >40 % of remnant stands of longleaf pine ecosystems; most have no history of agriculture due to poor site productivity, yet many have been degraded by resin collection, grazing and fire exclusion. Restoration of structure and function of sandhill longleaf pine depends on understanding early stand-development processes, which are governed by fire, propagule availability, and competition from a range of overstory tree species not limited to longleaf pine. We analyzed a 16-year monitoring dataset from Eglin Air Force Base in northwest Florida, USA, focusing on how four tree growth forms influenced longleaf pine juvenile stages of seedling and sapling numbers and ingrowth to the subadult size class. Analyses were done with generalized linear mixed models using Poisson, normal, or negative binomial error distributions according to the response variable.Restoration activities were effective at reducing numbers of sapling evergreen hardwoods, and basal area of overstory sand pine and other non-longleaf conifers was reduced by 35 % over the 16-year study. We observed threshold values for adult longleaf pine effects on seedlings and ingrowth; seedling numbers were maximized at 7.8 m2/ha, and ingrowth was maximum at 5.1 m2/ha. Seedling density was strongly governed by mast year. Sand pine, an invasive native pine, was detrimental to longleaf pine at all juvenile life stages even at low basal area. Deciduous oaks, which have been observed to facilitate longleaf pine seedling establishment, instead facilitated ingrowth to the subadult size class. Evergreen hardwoods, which may dampen fire behavior, only decreased longleaf pine density at the sapling stage. Ongoing silvicultural restoration activities should continue to focus on frequent fire and sand pine removal; deciduous oaks should be retained for their facilitative value; and removal of evergreen hardwoods should not be a high priority as long as prescribed fire can propagate in these lower productivity sites.

Xerophytic hardwood retention promotes competition over facilitation in longleaf pine woodlands in the absence of fire

Recent reports of xerophytic hardwood facilitation of longleaf pine seedlings challenge the traditional paradigm that co-occurring hardwood tree species constrain longleaf pine (Pinus palustris Mill.) regeneration. To better understand this dynamic, we established an experiment examining the effects of hardwood retention duration (no retention, one-year retention, four-year retention), overstory basal area (2-to-41 m2 ha−1), and understory vegetation cover on microclimatic conditions and longleaf pine seedling survival, development, physiological performance, and brownspot needle blight (Mycosphaerella dearnessii) infection in the Sandhills Ecoregion of North Carolina, USA. Light availability (μmol m−2 s−1) at the forest floor (∼20 cm) was reduced by 27% in the four-year retention treatment which significantly exceeded reductions found in the one-year (<1%) and no retention (<1%) treatments, respectively, post midstory removal. However, midstory treatment had little effect on soil moisture and temperature at 15 and 30 cm depth. Longleaf pine seedling survival (85%), aboveground biomass (7.51 g dry weight), and root collar size (1.53 cm) were significantly highest in the no retention treatment compared to the four-year retention treatment. Between treatments with midstory retention (one-year vs. four-year retention), longleaf pine seedlings were statistically larger in the one-year retention treatment (5.37 g dry weight) compared to the four-year retention treatment (4.84 g dry weight). Seedling brownspot infection did not statistically vary among midstory retention treatments. Across all midstory treatments, overstory basal area had a stronger negative effect on longleaf pine seedling aboveground biomass development, root collar development and brownspot infection compared to midstory hardwood stem density or understory vegetation cover. Moreover, for most measured metrics, overstory basal area had a stronger negative effect on longleaf pine physiological performance compared to midstory hardwood stem density. Collectively, these results indicate that hardwood retention did not facilitate longleaf pine survival and development in the grass stage and that asymmetric competition from the overstory represents a stronger impediment to longleaf pine sapling recruitment than midstory hardwoods.

Longleaf Pine Seedlings Are Extremely Resilient to the Combined Effects of Experimental Fire and Drought

The longleaf pine ecosystem is dependent on frequent fire. Climate change is expected to influence moisture availability and it is unclear how drought conditions may interact with prescribed fire to influence management objectives associated with maintaining longleaf pine ecosystems. This study aimed to understand the impacts of drought, fire intensity and their interaction on P. palustris grass-stage seedlings. We used droughted and well-watered P. palustris seedlings burned at two different fire intensity levels at an indoor combustion facility. Needle fuel moisture content of burned seedlings was not different between droughted and well-watered groups. Mortality and resprouting only occurred at fire intensity levels exceeding 3.5 MJ m−2 in combination with drought that resulted in predawn water potentials more negative than −1.7 MPa. Our observations of minimal mortality after exposing P. palustris seedlings to a range of fire intensities in a burn lab contrast the higher mortality observed in field studies for the species. Compared to seedlings and saplings of Western US Pinus species, this study demonstrates that P. palustris is considerably more resistant to the combined effects of high surface fire intensity and drought.

Longleaf pine restoration on hydric sites: Understorey plant community responses to site preparation through 15 years

AbstractQuestionLongleaf pine (Pinus palustris) restoration is an important management objective throughout the southeastern United States. Site preparation prior to planting longleaf pine seedlings is often required to overcome challenges to seedling establishment on hydric sites. We know site preparation improves growth and survival of planted longleaf pine on hydric sites, but what are the impacts of site preparation on the understorey plant community?LocationMarine Corps Base Camp Lejeune in Onslow County, NC, USA.MethodsThis study tested eight site preparation treatments, including an untreated control, six combinations of two vegetation control treatments (chopping or herbicide) with three soil manipulation treatments (mounding, bedding, or flat‐planting [no treatment]), and a chopping–herbicide–bedding treatment. We collected data on understorey plant abundance, diversity, and composition through the first three and at 15 years after plantation establishment. We used ANOVA procedures and multivariate ordination to test for differences in understorey responses among the study treatments and through time.ResultsSite preparation had lasting impacts on the understorey plant community. The chop‐only treatment was the only treatment that resulted in dominance of herbaceous vegetation and was among the treatments with the highest diversity and greatest abundance of Aristida stricta (wiregrass). Herbicide produced lasting reductions in Aristida stricta abundance and apparent shifts in community composition. Reduced graminoid abundance was particularly pronounced when herbicide was combined with soil manipulation treatments.ConclusionsUnderstorey responses to site preparation through 3 years differed from responses observed in year 15, highlighting the importance of long‐term monitoring. For example, herbicides controlled shrubs early, but by year 15, herbicide treatments were dominated by shrubs. Restoration scenarios often must balance ecological objectives (e.g., maintenance of a diverse understorey community) with forestry objectives (e.g., timely establishment of overstorey trees), and our findings can be used as a guide to balance these trade‐offs.

Longleaf pine seedling growth and survival: Effects of season and intensity of simulated prescribed burning

Restoration of longleaf pine (Pinus palustris Mill.) forests is an important land management goal in the southern United States. To achieve this goal, longleaf pine is increasingly being planted across its range. Fire is an important ecological factor that has historically shaped and sustained longleaf pine forests. However, we have limited information on how fire impacts longleaf pine seedlings in newly established plantations. We conducted this study in Northwest Florida to examine the effects of fire intensity (low intensity ((LI), ∼100 °C), medium intensity ((MI), ∼300 °C), high intensity ((HI), ∼500 °C), and an untreated control) on survival, growth (root collar diameter (RCD) and height), physiological processes (net assimilation (A), stomatal conductance (gsw), and intrinsic water use efficiency (iWUE)), and biomass allocation of longleaf pine seedlings over a period of six months after fire was applied in four seasons (summer, fall, winter, spring). Within a given season, the survival of seedlings did not significantly differ at different fire intensities, except in the summer (α = 0.05). For the summer application, survival was lower (18%) in the HI treatment compared to the MI (98%), LI (93%), and control (93%) (p = 0.003). RCD growth was significantly affected by fire intensity treatment only for the summer application. For the summer application, RCD growth six months after treatment (MAT) was highest (7.3 mm) in the control compared to the LI (2.7 mm), MI (2.7 mm), and HI (2.0 mm) (p = 0.021). The height growth of seedlings did not significantly differ at different fire intensities, though their values were greater at 6 months compared to 3 months. The effect of fire temperature on percent biomass allocation was only significant for the fall foliage (p = 0.004). Neither root, stem, nor total shoot percent biomass significantly differed for any other season. Largely, A, gsw, and iWUE were not significantly affected by fire temperature. iWUE did differ for MAT (p = 0.003), however, no pattern could be discerned. Overall, our observations suggest that longleaf pine seedlings are highly resistant and resilient to fire intensity in different seasons, with some vulnerability to high-intensity summer fires which can adversely affect their survival and growth. Summer burning is still a viable option for seedling survival provided conditions on the ground favor low/medium intensity fire.

Grass(stage)root movement to ensure future resilience of longleaf pine ecosystems

Recent interest in restoring longleaf pine ecosystems highlights the need for an improved understanding, appreciation, and consideration of longleaf pine’s unique life history compared to other southern pines. Longleaf pine ecosystems are considered more resilient than other southern pine ecosystems and the attributes that confer this resilience may be related to belowground carbon allocation patterns that occur during the seedling stage of its life history where it exists for years in a grass stage. The long residence time spent in the grass stage has impeded adoption of longleaf pine by landowners for decades, which has resulted in efforts to expedite emergence from the grass stage through silviculture, nursery production, and selection breeding techniques; however, expedited emergence may come at the expense of root and belowground carbohydrate storage reserve development—traits that purportedly enhance longleaf pine tree and ecosystem resilience relative to other southern pine species. Here, I caution that efforts to expedite longleaf pine emergence from the grass stage also consider the potential implications on belowground carbon allocation and identify research priorities to help facilitate the improvement of longleaf pine seedling silviculture, nursery production, and breeding in a way that ensures the maintenance of attributes linked to resilience.

Occurrence, density, and distribution of longleaf pine regeneration in southeastern forests: An assessment by forest type, disturbance and site quality

Regenerating longleaf pine is essential for the long-term sustainability of longleaf pine forest ecosystems and the important services those forests provide, particularly for endangered organisms. The objective of this study was to investigate the occurrence, density, and distribution of longleaf regeneration across a range of site quality, forest type, and disturbance regimes within the longleaf pine range from 1995 to 2019. We used 6,128 plots from multiple inventory cycles of the USDA Forest Service Forest Inventory and Analysis (FIA) dataset covering the entire longleaf pine range. The methods of analysis included logistic regression and the Inverse Distance Weighted (IDW) interpolation method. Two sets of measurements of 2,092 plots were used for change detection analysis. Longleaf regeneration was abundant in longleaf and loblolly pine-dominated stands. For a one-unit increase in the basal area ratio of longleaf pine trees in the plots, the probability of occurrence of longleaf pine seedlings and saplings increased up to 239% and 362%, respectively. The density and occurrence of longleaf regeneration were significantly greater on low-quality sites than on high-quality sites. Longleaf seedlings occurred with less frequency on disturbed sites, particularly by insects, diseases, and animals, than on undisturbed sites. Similarly, longleaf saplings were found with less frequency on sites disturbed by harvesting, weather, insects, diseases, and animals. We found a declining geographic range of longleaf pine trees, saplings, and seedlings during the 1995–2019 period. The density and ratio of longleaf seedlings decreased in about 25.2% and 14.0%, and increased in around 13.5% and 6.2% of the study area, respectively. Likewise, the density and ratio of longleaf saplings decreased in nearly 36.1% and 24.8% and increased in approximately 14.4% and 11.5% of the study area, respectively. Understanding the regeneration dynamics of longleaf pine is essential to restore and effectively regenerate longleaf pine forest ecosystems.

Fire Survival of Longleaf Pine (Pinus palustris) Grass Stage Seedlings: The Role of Seedling Size, Root Collar Position, and Resprouting

Longleaf pine (Pinus palustris Mill.) forest is a well-known fire-dependent ecosystem. The historical dominance of longleaf pine in the southeast United States has been attributed to its adaptation known as the grass stage, which allows longleaf pine seedlings to survive under a frequent surface fire regime. However, factors affecting post-fire survival of grass stage seedlings are not well understood. In this study, we measured live and dead longleaf pine grass stage seedlings to quantify the role of seedling size, root collar position, and sprouting in seedling survival following a wildfire in the sandhills of South Carolina. We found that fire resulted in almost 50% mortality for longleaf pine grass stage seedlings. Fire survival rate increased with seedling size, but a size threshold for fire tolerance was not supported. Fire survival depended on the position of root collar relative to the mineral soil. Seedlings with protected root collars (i.e., buried in or at the level of mineral soil) experienced &lt;21%, while seedlings with exposed root collars (i.e., elevated above mineral soil) suffered &gt;90% post-fire mortality. Ability to resprout contributed to 45.6% of the total fire survival, with the small seedlings (root collar diameter (RCD) &lt; 7.6 mm) almost exclusively depending on resprouting. Our findings had significant implications for fire management in longleaf pine ecosystems, and the current frequency of prescribed fire in sandhills might need to be lengthened to facilitate longleaf pine natural regeneration.

Patterns of Longleaf Pine (Pinus palustris) Establishment in Wiregrass (Aristida beyrichiana) Understories

Ecosystem community structure and function is shaped in part by intra- and inter-specific interactions among plants. Facilitative interactions, wherein one plant benefits another's fitness, can strongly influence plant community dynamics. We investigated the potential of an endemic, perennial bunchgrass, wiregrass (Aristida beyrichiana), to function as a nurse plant for longleaf pine (Pinus palustris) seedlings in fire-maintained pine savannas of the southeastern U.S.A. We documented significantly more pine seedlings growing close to established wiregrass bunchgrasses in a site burned one year prior to sampling. Pine seedlings growing close to wiregrass were also significantly taller than those growing further away. This positive spatial association between wiregrass and pine seedlings suggests that wiregrass facilitates early longleaf pine establishment in flatwoods environments, at least within the first year after fire.

The Persistence of Container Nursery Treatments on the Field Performance and Root System Morphology of Longleaf Pine Seedlings

In recent decades, container stock has become the preferred plant material to regenerate longleaf pine (Pinus palustris Mill.) forests in the southeastern United States. We evaluated the effects of container nursery treatments on early and long-term field performance in central Louisiana. Seedlings were grown in four cavity volumes (60–336 mL) with or without copper oxychloride root pruning (Cu or no-Cu) and fertilized at three nitrogen (N) rates. Across treatments, 91% of the seedlings emerged from the grass stage by the second field season, and 88% of the seedlings survived eight years after outplanting (Year 8). Seedlings grown in the largest cavities had greater total heights and stem diameters than those cultured in the 60- and 95-mL cavities through Year 8. Seedlings receiving the least amount of N in the nursery were consistently smaller in stature through Year 8 than seedlings receiving more N. Field growth was unaffected by copper root pruning through Year 8. Foliar mineral nutrient concentrations and seedling nutrient contents of Year 2 seedlings did not respond to nursery treatments. Independent of nursery treatments, seedlings excavated in Year 2 had at least 60% of their first-order lateral roots (FOLRs) originating from the top 4.0 cm of the taproots. The Cu-root-pruned seedlings had twofold the percentage of FOLRs egressed from the top 8.0 cm of the root plug when compared with the no-Cu seedlings. Moreover, the Cu root pruning treatment decreased the percentage of root plug biomass allocated to FOLRs, total within root plug FOLR lengths, and FOLR deformity index. The effects of increasing cavity volume or N rate on the root plug FOLR variables were opposite those of the Cu root pruning treatment. Our results suggest that a tradeoff may exist between seedling stature and a more natural FOLR morphology in outplanted container longleaf pine seedlings.

Seed depredation negates the benefits of midstory hardwood removal on longleaf pine seedling establishment

Midstory hardwoods are traditionally removed to restore longleaf pine on fire‐excluded savannas. However, recent evidence demonstrating midstory hardwood facilitation on longleaf pine seedling survival has brought this practice into question on xeric sites. Also, midstory hardwoods could facilitate longleaf pine seedling establishment, as hardwood litter may conceal seeds from seed predators or improve micro‐environmental conditions for seedling establishment. However, little is known about these potential mechanisms. In this study, we tracked longleaf pine seed depredation and germination in artificially seeded plots (11 seeds/m2) in a factorial design fully crossing hardwood retention or removal with vertebrate seed predator access or exclusion in the Sandhills Ecoregion of North Carolina, U.S.A. Seed depredation averaged 78% across treatments and was greatest in unexcluded plots. Hardwood retention did not affect seed depredation. Longleaf pine averaged 3.6 germinants/4 m2 across treatments, and was six times more abundant where vertebrates had been excluded. Hardwood removal had a strong positive effect on seedling germination, likely due to the removal of litter, but only when vertebrates were excluded. Our results indicated midstory hardwoods are not facilitating longleaf pine seedling establishment. Nevertheless, our results indicated that hardwood removal may not increase longleaf pine seedling establishment, as seed depredation diminished the effectiveness of hardwood removal under mast seed availability. Collectively, these results demonstrate the underlying complexity of the longleaf pine ecosystem, and suggest that planting may need to be part of the restoration strategy on sites where seed depredation limits longleaf pine natural regeneration.

Rarity of shortleaf, slash, and longleaf pine seedlings in oak-pine forest types: An assessment of associated environmental, stand, site, and disturbance factors

Determining the factors that influence the regeneration of pine in oak-pine forests is crucial for understanding natural regeneration dynamics and for planning forest management strategies. We investigated the vegetation structure and seedling dynamics of shortleaf, slash, and longleaf pine in oak-pine forest types of the southeastern United States. We used logistic regression with penalized maximum likelihood estimation because of the rarity of the regeneration from these pines. We performed canonical correlation, analysis of variance, and community structure analysis to produce multivariate relationships and statistical inferences. The classification accuracy of the logistic regression models for shortleaf, slash, and shortleaf pine were 96.0%, 96.7%, and 97.7%, respectively. The models indicated that environmental, climatic, stand, site productivity, and disturbance factors, or their combinations, had a significant influence on seedling occurrence. The probability of occurrence of seedlings of the three pine species increased with the increase in the overstory basal area from these species: one unit increase in overstory purity ratio of shortleaf, slash, and longleaf pine increased the odds of occurrence of their seedlings by approximately 51, 24, and 7 times, respectively.The probability of occurrence of shortleaf pine seedlings increased with increasing canopy openness in stands with a substantial proportion of shortleaf pine in the overstory. The increase in mean annual temperature and overstory basal area of slash pine increased the probability of occurrence of slash pine seedlings on wetter sites that had disturbance through harvesting and silvicultural treatments. The chances of occurrence of longleaf pine seedlings increased with increasing overstory basal area of longleaf pine, species diversity, and mean annual temperature in low density stands on low productive sites. The findings can help forest managers in designing and implementing silvicultural treatments to promote the natural regeneration of the three pines in oak-pine forests.

Fire effects on a fire-adapted species: response of grass stage longleaf pine seedlings to experimental burning

BackgroundLongleaf pine (Pinus palustris Mill.) seedlings have a morphological “grass stage” that is considered to be an adaptation to frequent surface fire regimes. However, fire can kill longleaf pine seedlings and thus may play an important role in longleaf pine regeneration dynamics. We used a prescribed burn simulation tool designed to treat individual grass stage longleaf pine seedlings with controlled delivery of fire treatments and then measured survival and growth responses through two growing seasons. Naturally regenerated grass stage longleaf pine seedlings were randomly selected from three size classes and each assigned one of four treatments (Control, no treatment; Clip, mechanical needle removal; LB, a low-temperature burn treatment; or HB, a high-temperature burn treatment) in both the dormant season (January) and the growing season (May).ResultsSeedlings greater than 15 mm root collar diameter had greater than 0.5 probability of survival after the first growing season in the HB treatment, regardless of the season of treatment application, and seedlings across all sizes had greater than 0.6 probability of survival in the LB treatment after the first growing season. The growing season treatment application resulted in additional mortality during the second growing season, across all seedling size classes, which was not observed in the dormant season application. Burning reduced root collar growth through two growing seasons, likely due to needle mortality and the subsequent prioritization of growth to needle production rather than to root or stem growth.ConclusionsOur results suggest that the interplay between seedling size and fire intensity likely contributes to the success of longleaf pine natural regeneration and that seedling size should be considered when scheduling the first burn following planting of longleaf pine seedlings.

Foliage re-establishment of Pinus palustris Mill. saplings after spring or fall prescribed fire

Repeated fire is key to the viability of longleaf pine (Pinus palustris Mill.) ecosystems, but its acceptance as a management tool may depend on satisfactory longleaf pine growth. This is because longleaf pine establishment often has the dual-purpose of ecosystem restoration and stemwood production. Timely recovery of scorched foliage among longleaf pine seedlings and saplings supports maximum juvenile growth. We identified two means of foliage re-establishment in the growing season after prescribed fire regardless of the season of fire application. New foliage growth after spring or fall fire was correlated with un-scorched foliage biomass and the presence of lateral branches. After prescribed fire in spring, foliage biomass recovery also appeared to benefit from the mobilization of starch. The high carbon demand of foliage recovery after fall prescribed fire was associated with interruption of seasonal starch accumulation in the stem and taproot. The implication of low starch accumulation in stem and taproot tissues during the growing season after fall prescribed fire is unknown and warrants further investigation. Our results demonstrate a positive influence of residual foliage, lateral branches, and stored starch on timely foliage recovery of young longleaf pines after fire. Together with knowledge of longleaf pine development and fuel and climate conditions at the time of prescribed fire, this information will aid prescribed fire practitioners charged with maintaining longleaf pine stands of high vigor.

Responses to Prescribed Fire at Big Thicket National Preserve, Texas, USA

US Federal land managers have utilized hand ignited prescribed fire at Big Thicket National Preserve in efforts to restore the structure and diversity of the longleaf pine (Pinus palustris) forest. A fire ecology study was initiated by Rice University in the early 1990’s and the National Park Service has continued monitoring the plots. Ordination was applied to species abundance data to examine changes in vegetation communities from a variety of prescribed fire treatments and controls. The vegetation data was separated by size class to include overstory, small tree, large sapling and seedling data. Across the size classes and treatments, the sandhill and wetland savanna vegetation types remained less effected by fire treatments and only the upland pine responded to changes in the overstory. Upon reviewing fire return interval histories, it became evident that prescribed fire alone was not changing vegetation communities. Most of the plots did not have longleaf pine trees or seedlings present and only two plots that were mechanical treated showed distinction among other treatment regimes. Restoration treatments including the mechanical and chemical application and seedling plantings are necessary to ensure restoration of the longleaf pine forest structure and diverse understory vegetation.

Effects of basal area on survival and growth of longleaf pine when practicing selection silviculture

Aim of study: Uneven-aged (UEA) management systems can achieve multiple-use objectives, however, use of UEA techniques to manage longleaf pine (Pinus palustris Mill.) forests are still open to question, because of the species’ intolerance of competition. It was our aim to examine the influence of different levels (9.2, 13.8 and 18.4 m2 ha-1) of residual basal area (RBA) on longleaf pine seedling survival and growth following three growing seasons.Area of study: This study was conducted at the Escambia Experimental Forest, located on the Southern Coastal Plain of Alabama, in the southeastern United States.Material and Methods: Selection silviculture was implemented with the Proportional-Basal Area (Pro-B) method. Prescribed burning was conducted before seed dispersal and in the second year after germination. Photosynthetically active radiation (PAR) was measured under the canopy in the study plots. Survival and growth of longleaf pine seedlings were observed for three growing seasons.Main results: An inverse relationship was found between the number of germinants and RBA, but the mortality of germinants and planted seedlings was not affected by RBA. At age three, an inverse relationship was observed between root-collar diameter (RCD) growth of the germinants and RBA, but RCD growth of planted seedlings was not affected by RBA. Most of the study plots contained more than the projected number of seedlings needed to sustain the target diameter structure.Research highlights: Long-term continuous monitoring of seedling development and recruitment into canopy is required to determine the efficacy of UEA management. However, current data suggest that UEA methods may be a viable alternative to the use of even-aged (EA) methods in longleaf ecosystems.

Using silvicultural practices to regulate competition, resource availability, and growing conditions for Pinus palustris seedlings underplanted in Pinus taeda forests

In the southeastern United States, many forest managers are interested in restoring longleaf pine (Pinus palustris Mill.) to upland sites that currently support loblolly pine (Pinus taeda L.). We quantified the effects of four canopy treatments (uncut Control; MedBA, harvest to 9 m2·ha−1; LowBA, harvest to 5 m2·ha−1; and Clearcut) and three cultural treatments (NT, no treatment; H, herbicide release of longleaf pine seedlings; and H+F, herbicide release plus fertilization) on resource availability and growing conditions in relation to longleaf pine seedling response for 3 years. Harvesting treatments reduced competition from canopy trees but resulted in greater abundance of understory vegetation. Harvesting shifted the interception of light from the canopy to the subcanopy vegetation layer; however, total light availability at the forest floor increased with the intensity of canopy removal. Soil moisture was not affected by harvesting or by the cultural treatments. Foliar nutrient concentrations (N, P, and K) of longleaf pine seedlings generally increased with the intensity of the harvest treatment. Of the plant resources measured, we found that light was most strongly correlated with longleaf pine seedling growth and that incorporating the interception of light by subcanopy vegetation improved the relationship over that of canopy light transmittance alone.

The path back: oaks (Quercusspp.) facilitate longleaf pine (Pinus palustris) seedling establishment in xeric sites

AbstractUnderstanding plant–plant facilitation is critical for predicting how plant community function will respond to changing disturbance and climate. In longleaf pine (Pinus palustrisMill.) ecosystems of the southeastern United States, understanding processes that affect pine reproduction is imperative for conservation efforts that aim to maintain ecosystem resilience across its wide geographic range and edaphic gradients. Variation in wildland fire and plant–plant interactions may be overlooked in “coarse filter” restoration management, where actions are often prescribed over a variety of ecological conditions with an assumed outcome. For example, hardwood reduction techniques are commonly deemed necessary for ecological restoration of longleaf pine ecosystems, as hardwoods are presumed competitors with longleaf pine seedlings. Natural regeneration dynamics are difficult to test experimentally given the infrequent and irregular mast seed events of the longleaf pine. Using a long‐term, large‐scale restoration experiment and a long‐term monitoring data site at Eglin Air Force Base, Florida (USA), this study explores the influence of native fire‐intolerant oaks on longleaf regeneration. We test for historical observations of hardwood facilitation against the null hypothesis of competitive exclusion. Our results provide evidence of hardwood facilitation on newly germinated longleaf pine seedlings (&lt;2 yr old) after two mast seeding events (1996, 2011). Using regression‐tree and Kaplan–Meier survival analyses, we found that deciduous oak midstory density was the most significant variable associated with longleaf pine seedling survival rates in the first 2 yr after germination. We found that as few as 43 oak midstory stems ha−1were sufficient to facilitate seedling survival, but as many as 1400 stems ha−1maintained facilitation without competitive exclusion of seedlings. We found that 1.5‐yr‐old pine seedlings were more moisture stressed under more open canopy conditions when compared to those immediately adjacent to a midstory oak canopy. Recognition that deciduous oaks are important facilitators of longleaf seedling establishment on xeric sites represents a significant departure from conventional wisdom and current management practices that has largely focused on competitive exclusion. This points to a critical role of a deciduous oak midstory of moderate densities for long‐term ecosystem resilience in xeric longleaf pine ecosystems in light of climate uncertainty.

Silvicultural treatments for converting loblolly pine to longleaf pine dominance: effects on ground layer and midstorey vegetation

AbstractQuestionsHow do management practices used to enhance longleaf pine (Pinus palustris) seedling survival and growth under a loblolly pine (Pinus taeda) canopy alter the structure of midstorey and ground layer vegetation? Do management treatments achieve general restoration targets for longleaf pine ecosystem structure?LocationMarine Corps Base Camp Lejeune, NC, USA within the Middle Atlantic Coastal Plain Ecoregion.MethodsFour levels of timber harvest were applied to loblolly pine stands: Control (uncut, basal area ~16.2 m2·ha−1), MedBA (residual basal area ~9.0 m2·ha−1), LowBA (residual basal area ~6.4 m2·ha−1) and Clearcut (residual basal area of 0 m2·ha−1). Within each canopy treatment, we applied three cultural treatments selected to facilitate longleaf pine seedling success: NT (untreated), H (chemical control of woody vegetation) and H + F (chemical control plus fertilization). Vegetation responses, including the abundance (cover) of ground layer vegetation and midstorey stem densities, were reported for three growing seasons (2008–2010) following canopy removal.ResultsThe ground layer was dominated by woody vegetation, and total vegetation cover generally increased with increasing canopy removal. Canopy treatment effects varied through time. Clearcut plots had higher total herbaceous and graminoid cover than MedBA and Control plots in 2008, while woody cover was significantly lower on Control plots than on LowBA and Clearcut plots in 2009. Clearcut plots had higher densities of loblolly pines than Control plots in 2009 and 2010. The herbicide treatment reduced hardwood densities, but increased loblolly pine densities, especially in 2010.ConclusionsSuccessful restoration prescriptions are often site‐specific because of different land‐use history, climate, site characteristics and starting conditions. To achieve the restoration objective of creating an open midstorey with an herbaceous‐dominated ground layer when converting loblolly pine stands to longleaf pine dominance on relatively productive sites with abundant hardwoods and aggressive loblolly pine natural regeneration, canopy retention can slow the rate of development of loblolly pine regeneration and herbicides reduce hardwood stem densities. Frequent, repeated burning would likely be required to further reduce woody vegetation and increase the relative abundance of herbaceous vegetation.

Impacts of logging and prescribed burning in longleaf pine forests managed under uneven-aged silviculture

The longleaf pine (Pinus palustris Mill.) ecosystem has historically been very important in the southeastern United States due to its extensive area and high biodiversity. Successful regeneration of longleaf pine forests requires an adequate number of well-distributed seedlings. Thus, mortality of longleaf pine seedlings during logging operations and prescribed burning appears to be important. Longleaf forests have been commonly managed using even-aged silvicultural methods, but, recently, interest in the use of uneven-aged (UEA) methods has increased in these forests. Research on the impact of UEA logging in longleaf forests is limited. In addition, the influence of overstory density on the impact of prescribed burning under UEA management has not been sufficiently studied in longleaf pine forests. In this study, impacts of UEA logging and prescribed burning (both growing and dormant seasons) on longleaf pine seedlings were observed. In addition, the impact of logging and burning on hardwood seedlings, which are detrimental for longleaf pine seedlings' survival, was also monitored. Damage of logging on longleaf pine seedlings was less than that on hardwood sprouts. A growing-season burning conducted in September 2011 killed most of the hardwood seedlings; however, most of the advance longleaf pine seedlings survived with an average survival rate of 91%. Impact of a dormant-season burning (conducted in January and February 2014) on the survival of 2-year-old longleaf seedlings increased with increasing stand density. This study shows that prescribed burning may be responsible for longleaf pine seedlings' mortality in some cases. It also suggests that damage of UEA logging on longleaf pine seedlings may be negligible.