Timber harvest and drought interact to impact oak seedling growth and survival in the Central Hardwood Forest

Soil moisture and water redistribution patterns in white oak (Quercus alba) saplings and trees in fragmented urban woodlands

In the midwestern United States, models predict extended summer heatwaves and increasingly frequent and prolonged drought conditions. In the Chicago region, the potential for large-scale mortality of white oak trees (Quercus alba) coupled with the ongoing decline of white oak sapling recruitment are major concerns for researchers and practitioners. In this study, we determined the sources of water used by mature white oak trees and saplings in three qualitatively different sites within a remnant oak forest in Chicago during the 2021 drought. We investigated soil moisture dynamics (volumetric water content, VWC) and water isotope composition of leaf tissues (δD, δ18O), rainwater, and groundwater. These data were linked to sapling height (proxy for biomass) and ectomycorrhizal (ECM) functional types. We predicted that: (i) mature oak trees use deeper water sources and conducted hydraulic redistribution (HR), and (ii) mature trees shared water with saplings during dry periods via long-distance ECM functional types. Soil moisture decreased progressively from June to October (spring to fall), with August and September having the lowest moisture (<20 % VWC). Following rainfall recharge, temporal patterns of soil moisture showed gravity drainage and then ongoing stair-stepwise drawdown consistent with plant evapotranspiration. Leaf δD and δ18O values in mature trees and saplings were consistent with water uptake from rainfall and subsequent enrichment via evapotranspiration. In two sites, mature trees and saplings demonstrated distinct δD: δ18O slopes, with mature trees more enriched than saplings. In the third site, mature trees and saplings δD: δ18O slopes overlapped but here, the ECM community was dominated by contact-type ECM and sapling height increased with distance from the mature oak. Our findings indicate that HR was not a component of site ecohydrology, and future climate conditions may present increasing challenges for white oak recruitment as both mature trees and saplings compete for limited rainfall-derived soil moisture.

Development of DNA methylation-based epigenetic age predictors in loblolly pine (Pinus taeda)

Biological ageing is connected to life history variation across ecological scales and informs a basic understanding of age-related declines in organismal function. Altered DNA methylation dynamics are a conserved aspect of biological ageing and have recently been modelled to predict chronological age among vertebrate species. In addition to their utility in estimating individual age, differences between chronological and predicted ages arise due to acceleration or deceleration of epigenetic ageing, and these discrepancies are linked to disease risk and multiple life history traits. Although evidence suggests that patterns of DNA methylation can describe ageing in plants, predictions with epigenetic clocks have yet to be performed. Here, we resolve the DNA methylome across CpG, CHG, and CHH-methylation contexts in the loblolly pine tree (Pinus taeda) and construct epigenetic clocks capable of predicting ages in this species within 6% of its maximum lifespan. Although patterns of CHH-methylation showed little association with age, both CpG and CHG-methylation contexts were strongly associated with ageing, largely becoming hypomethylated with age. Among age-associated loci were those in close proximity to malate dehydrogenase, NADH dehydrogenase, and 18S and 26S ribosomal RNA genes. This study reports one of the first epigenetic clocks in plants and demonstrates the universality of age-associated DNA methylation dynamics which can inform conservation and management practices, as well as our ecological and evolutionary understanding of biological ageing in plants.

Responses of Korean Pine to Proactive Managements under Climate Change

Proactive managements, such as the resistant and the adaptive treatments, have been proposed to cope with the uncertainties of future climates. However, quantifying the uncertainties of forest response to proactive managements is challenging. Korean pine is an ecologically and economically important tree species in the temperate forests of Northeast China. Its dominance has evidently decreased due to excessive harvesting in the past decades. Understanding the responses of Korean pine to proactive managements under the future climates is important. In this study, we evaluated the range of responses of Korean pine to proactive managements under Representative Concentration Pathway (RCP) 8.5 scenarios from four General Circulation Models (GCMs). We coupled an ecosystem process-based model, LINKAGES, and a forest landscape model, LANDIS PRO, to simulate scenarios of management and climate change combinations. Our results showed that the resistant and the adaptive treatment scenarios increased Korean pine importance (by 14.2% and 42.9% in importance value), dominance (biomass increased by 9.2% and 25.5%), and regeneration (abundance <10 years old increased by 286.6% and 841.2%) throughout the simulation. Results indicated that proactive managements promoted the adaptability of Korean pine to climate change. Our results showed that the variations of Korean pine response to climate change increased (ranging from 0% to 5.8% for importance value, 0% to 4.3% for biomass, and 0% to 85.4% for abundance) throughout the simulation across management scenarios. Our result showed that regeneration dictated the uncertainties of Korean pine response to climate change with a lag effect. We found that the effects of proactive managements were site-specific, which was probably influenced by the competition between Korean pine and the rare and protected broadleaf tree species. We also found that the adaptive treatment was more likely to prompt Korean pine to migrate into its suitable habitats and promoted it to better cope with climate change. Thus, the adaptive treatment is proposed for Korean pine restoration under future climates.

Rodent population density and survival respond to disturbance induced by timber harvest

Many small mammal populations respond quickly to timber harvest aimed at oak (Quercus) regeneration, which alters microhabitat. We used mark-release–recapture data collected 6–8 years postharvest from the Hardwood Ecosystem Experiment in southern Indiana, United States, to model density and apparent survival of eastern chipmunks (Tamias striatus) and white-footed mice (Peromyscus leucopus) as a function of timber harvest treatments (shelterwood, clearcut, patch cut, and unharvested control). Density, estimated using spatial capture–recapture, increased for chipmunks in all types of harvest openings, but survival was unaffected by harvest. Chipmunk densities in unharvested forest matrix habitat averaged 58% and 71% lower relative to harvest openings and opening edges, respectively. White-footed mouse density was less responsive to timber harvest, but monthly survival rates were reduced by 13% in shelterwoods and 17% in patch cuts relative to control sites. Both rodent species tended to exhibit distance-dependent responses, with higher density of home-range centers near harvest boundaries relative to forest matrix. Structural complexity created at the edges of timber harvest openings can benefit rodents associated with edge habitat 6–8 years after harvest, presumably due to improved foraging efficiency and resource diversity. Cascading effects of rodent demographic responses are likely to affect predation and seed dispersal, which are critical trophic interactions in oak forest ecosystems.

Persistence of balsam fir and black spruce populations in the mixedwood and coniferous bioclimatic domain of eastern North America

The boreal ecocline (ca 49°N) between the southern mixedwood (dominated by balsam fir) and the northern coniferous bioclimatic domain (dominated by black spruce) may be explained by a northward decrease of balsam fir regeneration, explaining the gradual shift to black spruce dominance. 7,010 sample plots, with absence of major disturbances, were provided by the Quebec Ministry of Forest, Fauna, and Parks. The regeneration (sapling abundance) of balsam fir and black spruce were compared within and between the two bioclimatic domains, accounting for parental trees, main soil type (clay and till) and climate conditions, reflected by summer growing degree-days above 5°C (GDD_5), total summer precipitation (May-August; PP_MA). Parental trees and soil type determined balsam fir and black spruce regeneration. Balsam fir and black spruce, respectively, showed higher regeneration in the mixedwood and the coniferous bioclimatic domains. Overall, higher regeneration was obtained on till for balsam fir, and on clay soils for black spruce. GDD_5 and PP_MA were beneficial for balsam fir regeneration on clay and till soils, respectively, while they were detrimental for black spruce regeneration. At a population level, balsam fir required at least 28% of parental tree basal area in the mixedwood, and 38% in the coniferous bioclimatic domains to maintain a regeneration at least equal to the mean regeneration of the whole study area. However, black spruce required 82% and 79% of parental trees basal area in the mixedwood and the coniferous domains, respectively. The northern limit of the mixedwood bioclimatic domain was attributed to a gradual decrease toward the north of balsam fir regeneration most likely due to cooler temperatures, shorter growing seasons, and decrease of the parental trees further north of this northern limit. However, balsam fir still persists above this northern limit, owing to a patchy occurrence of small parental trees populations, and good establishment substrates.

Ten-Year Responses of Underplanted Northern Red Oak to Silvicultural Treatments, Herbivore Exclusion, and Fertilization

Establishing adequate advanced oak reproduction prior to final overstory removal is crucial for regenerating oak forests in the eastern U.S. Many management approaches exist to this end, but benefits associated with any individual technique can depend on the suite of techniques employed and the geographic location. At four mixed-hardwood upland forest sites in central and southern Indiana, we tested factorial combinations of deer fencing, controlled-release fertilization, and various silvicultural techniques (midstory removal, crown thinning, and a shelterwood establishment cut) for promoting the growth and survival of underplanted red oak seedlings. Crown thinning resulted in slow growth and low survival. Midstory removal and the shelterwood establishment cut were nearly equally effective for promoting seedling growth. Seedling survival was strongly influenced by fencing, and differences in survival between silvicultural treatments were minimal when fencing was employed. Fertilization had minimal effects overall, only increasing the probability that unfenced seedlings were in competitive positions relative to surrounding vegetation. We suggest that underplanting oak seedlings can augment natural reproduction, but the practice should be accompanied by a combination of midstory removal and fencing, at a minimum, for adequate growth and survival.

Simulation of oak early life history and interactions with disturbance via an individual-based model, SOEL

Early tree life history and demography are driven by interactions with the environment such as seed predation, herbivory, light availability, and drought. For oak (Quercus) in the eastern United States, these interactions may contribute to oak regeneration failure. Numerous studies have examined the impact of individual factors (like seed predation) on the oak regeneration process, but less information is available on the relative and combined impacts of multiple intrinsic and extrinsic factors on early oak life history. We developed an individual-based, spatially explicit model to Simulate Oak Early Life history (SOEL). The model connects acorn survival, acorn dispersal, germination, seedling growth, and seedling survival submodels based on empirical data with an existing gap model (JABOWA). Using SOEL, we assessed the sensitivity of several metrics of oak regeneration to different parameters associated with early oak life history. We also applied the model in three individual case studies to assess: (1) how variable acorn production interacts with timing of timber harvest; (2) the effect of shelterwood harvest-induced differences on seed predation; and (3) the consequences of interactions between drought, seedling growth and survival, and timber harvest. We found that oak regeneration metrics including percent emergence, seedling density, and sapling density were most sensitive to the amount of acorn production, acorn caching probability by scatterhoarders, and seedling growth rates. In the case studies, we found that timing harvest to follow large acorn crops can increase the density of oak regeneration in the short term following harvest, at least under some conditions. Following midstory removal, lower weevil infestation probability and lower post-dispersal acorn survival resulted in a modest decline in seedling density, but the decline did not persist to the sapling life stage class. Drought frequency had a powerful negative impact on both growth and survival for individual seedlings, which resulted in large reductions in both seedling and sapling density. The case studies presented here represent only a few examples of what could be accomplished within the SOEL modeling framework. Further studies could focus on different early life history parameters, or connect the parameter values to different predictor variables based on field data.