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We propose a model of 'premature tree decline' whereby an absence of fire hastens the mortality of overstorey eucalypts in some forests. This model is relevant to some temperate Australian forests in which fire regimes have shifted from relatively frequent before European settlement to infrequent following settlement. The increased development of midstorey vegetation and litter accumulation has occurred since European settlement in some specific examples of Australian forests and woodlands. Our model proposes that in the long absence of fire: 1. midstorey vegetation reduces the availability of soil water for eucalypts and; 2. Eucalypts have less access to P and/or cations as these elements become locked up in soil, litter and midstorey biomass. We highlight important knowledge gaps and argue that research into ecological burning, for eucalypt health and other values such as biodiversity, is urgently required.

Gross rates of N mineralization, immobilization, and nitrification were measured by ^1^5N isotope dilution in a 10-yr-old conifer plantation and in a mature conifer forest. Gross rates revealed nutrient cycling characteristics that differ from expectations based on more common measures of net rates. Although net mineralization rates were somewhat higher in the young forest than in the old forest, gross mineralization rates in the old forest were 2-3 times as high as gross mineralization rates in the young forest, indicating more rapid turnover of inorganic-N pools in the old forest. Net mineralization rates were <14% of gross mineralization rates. Smaller NO\"3^- pool size and lower net nitrification rates in the old forest than the young forest might lead to the conclusion that the old forest is a non-nitrifying ecosystem and that nitrate is important only in the N cycle of the young forest. However, gross nitrification rates were similar in both young and old forests. Microbial assimilation of NO\"3^- was also significant in both forests, indicating a rapid turnover of a small but important NO\"3^- pool. Microbial assimilation may be an important pathway for NO\"3^- retention in forest ecosystems.

Temperature sensitivity of community-level physiological profiles (CLPPs) was examined for two semiarid soils from the southwestern United States using five different C-substrate profile microtiter plates (Biolog GN2, GP2, ECO, SFN2, and SFP2) incubated at five different temperature regimes. The CLPPs produced from all plate types were relatively unaffected by these contrasting incubation temperature regimes. Our results demonstrate the ability to detect CLPP differences between similar soils with differing physiological parameters, and these differences are relatively insensitive to incubation temperature. Our study also highlights the importance of using both bacterial and fungal plate types when investigating microbial community differences by CLPP. Nevertheless, it is unclear whether or not the differences in CLPPs generated using these plates reflect actual functional differences in the microbial communities from these soils in situ.

Root distributions are typically based on root mass per soil volume. This plant‐focused approach masks the biogeochemical influence of fine roots, which weigh little. We assert that centimeter‐scale root presence‐absence data from soil profiles provide a more soil‐focused approach for probing depth distributions of root‐regolith interfaces, where microsite‐scale processes drive whole‐ecosystem functioning. In 75 soil pits across the continental USA, Puerto Rico, and the Alps, we quantified fine and coarse root presence as deep as 2 m. In 70 of these pits we estimated root mass and created standardized metrics of both data sets to compare their depth distributions. We addressed whether: (a) depth distributions of root presence‐absence data differ from root mass data, thus implying different degrees of root‐regolith interactions with depth; and (b) if root presence or any depth‐dependent differences between these data sets vary predictably with environmental conditions. Presence of fine roots exhibited diverse depth‐dependent patterns; root mass generally declined with depth. In B and C horizons, standardized root presence was greater than standardized root mass; random forest analyses suggest these discrepancies are greater in B horizons with increasing mean annual precipitation and in C horizons with increasing mean annual temperature. Our work suggests that deep in the subsurface, biogeochemical and reactive transport processes result from more numerous root‐regolith interfaces than mass data suggest. We present a new paradigm for discerning patterns in depth distributions of root‐regolith interfaces across multiple biomes and land uses that promotes understanding of the roles of those interfaces in driving key critical zone processes. Plain Language Summary Understanding how plant roots are distributed throughout soil layers is important for predicting where plants promote transformations of soil carbon, generate soil and redistribute nutrients, and modify water flows. All these processes affect climate by regulating how well plants take in atmospheric CO2. In 75 soil pits across the US and the Alps, we quantified fine and coarse root presence as deep as 2 m. In 70 of these we estimated root mass, permitting comparison of these data sets' variation with depth. Fine root presence exhibited especially great variation with depth compared to root mass. Differences between the data sets were greatest in B and C horizons and reflect numerous fine roots that weigh little. Discrepancies between total root presence and mass were linked to mean annual precipitation (MAP) in B horizons and mean annual temperature (MAT) in C horizons, with the root presence metric increasingly dwarfing root mass in these horizons as MAP and MAT increases. We illuminate how, deep in the subsurface, plant‐mediated carbon, water, and nutrient transformations emerge from more numerous root‐soil interfaces than mass data suggest. Our work presents a new paradigm for discerning depth‐dependent patterns of the root‐soil interactions that drive the ecosystem functions that sustain life. Key Points Fine and coarse root presence or absence measurements in 75 deep soil profiles open an illuminating way to characterize root distributions Root presence depth distributions contrast with root mass, underscoring distinct hydrologic and biogeochemical roles of fine and coarse roots Discrepancies in depth profiles of root presence vs. root mass are driven by distinct ecosystem features at different depths

Southwestern ponderosa pine forests were dramatically altered by fire regime disruption that accompanied Euro-American settlement in the 1800s. Major changes include increased tree density, diminished herbaceous cover, and a shift from a frequent low-intensity fire regime to a stand-replacing fire regime. Ecological restoration via thinning and prescribed burning is being widely applied to return forests to the pre-settlement condition, but the effects of restoration on ecosystem function are unknown. We measured carbon (C), nitrogen (N), and phosphorus (P) fluxes during the first two years after the implementation of a replicated field experiment comparing thinning and composite (thinning, forest floor fuel reduction, and prescribed burning) restoration treatments to untreated controls in a ponderosa pine forest in northern Arizona, USA. Total net primary productivity ($260 g C\\cdot m^{-2}\\cdot yr^{-1}$) was similar among treatments because a 30-50% decrease in pine foliage and fine-root production in restored ecosystems was balanced by greater wood, coarse root, and herbaceous production. Herbaceous plants accounted for <20% of total plant C, N, and P uptake in the controls but from 25% to 70% in restored plots. Total plant N uptake was$\\sim 3 g N\\cdot m^{-2}\\cdot yr^{-1}$in all treatments, but net N mineralization was just one-half and two-thirds of this value in the control and composite restoration, respectively. Element flux rates in controls generally declined more in a drought year than rates in restoration treatments. In this ponderosa pine forest, ecological restoration that emulated pre-settlement stand structure and fire characteristics had a small effect on plant C, N, and P fluxes at the whole ecosystem level because lower pine foliage and fine-root fluxes in treated plots (compared to controls) were approximately balanced by higher fluxes in wood and herbaceous plants.

The well-known deceleration of nitrogen (N) cycling in the soil resulting from addition of large amounts of foliar condensed tannins may require increased fine-root growth in order to meet plant demands for N. We examined correlations between fine-root production, plant genetics, and leaf secondary compounds in Populus angustifolia, P. fremontii, and their hybrids. We measured fine-root (<2mm) production and leaf chemistry along an experimental genetic gradient where leaf litter tannin concentrations are genetically based and exert strong control on net N mineralization in the soil. Fine-root production was highly correlated with leaf tannins and individual tree genetic composition based upon genetic marker estimates, suggesting potential genetic control of compensatory root growth in response to accumulation of foliar secondary compounds in soils. We suggest, based on previous studies in our system and the current study, that genes for tannin production could link foliar chemistry and root growth, which may provide a powerful setting for external feedbacks between above- and belowground processes.

Forest management, climatic change, and atmospheric N deposition can affect soil biogeochemistry, but their combined effects are not well understood. We examined the effects of water and N amendments and forest thinning and burning on soil N pools and fluxes in ponderosa pine forests near Flagstaff, Arizona (USA). Using a ¹⁵N-depleted fertilizer, we also documented the distribution of added N. into soil N pools. Because thinning and burning can increase soil water content and N availability, we hypothesized that these changes would alleviate water and N limitation of soil processes, causing smaller responses to added N and water in the restored stand. We found little support for this hypothesis. Responses of fine root biomass, potential net N mineralization, and the soil microbial N to water and N amendments were mostly unaffected by stand management. Most of the soil processes we examined were limited by N and water, and the increased N and soil water availability caused by forest restoration was insufficient to alleviate these limitations. For example, N addition caused a larger increase in potential net nitrification in the restored stand, and at a given level of soil N availability, N addition had a larger effect on soil microbial N in the restored stand. Possibly, forest restoration increased the availability of some other limiting resource, amplifying responses to added N and water. Tracer N recoveries in roots and in the forest floor were lower in the restored stand. Natural abundance δ¹⁵N of labile soil N pools were higher in the restored stand, consistent with a more open N cycle. We conclude that thinning and burning open up the N cycle, at least in the short term, and that these changes are amplified by enhanced precipitation and N additions. Our results suggest that thinning and burning in ponderosa pine forests will not increase their resistance to changes in soil N dynamics resulting from increased atmospheric N deposition or increased precipitation due to climatic change. Restoration plans should consider the potential impact on long-term forest productivity of greater N losses from a more open N cycle, especially during the period immediately after thinning and burning.

1. Plant roots and their mycorrhizal symbionts are critical components of forest ecosystems, being largely responsible for soil resource acquisition by plants and the maintenance of soil structure, as well as influencing soil nutrient cycling. Silvicultural treatments should be guided by knowledge of how these below-ground components respond to different forest management practices. 2. We examined the cumulative effects of 20 years of prescribed burning at 2-year intervals. We measured fine root length density and fine root and mycorrhizal root biomass in the upper 15 cm of mineral soil in a south-western ponderosa pine forest over a complete burn cycle. 3. Repeated burning reduced fine root length, fine root biomass and mycorrhizal root biomass, as well as the amount of nitrogen and phosphorus stored in these below-ground pools. 4. Estimates of fine root production, fine root decomposition and nutrient dynamics were similar in burned and control plots. 5. Synthesis and applications. Although repeated-prescribed fire may be an effective, low-cost approach for reducing fuel loads and lessening the chance of a catastrophic wildfire in ponderosa pine forests, our results suggest that this strategy may negatively affect below-ground biomass pools and nutrient cycling processes in the long term. We recommend that mechanical reductions in fuel loads be conducted in these and similar forests that have not experienced fire for decades, before fire is reintroduced as a management tool.

Changes in vegetation due to drought-influenced herbivory may influence microclimate in ecosystems. In combination with studies of insect resistant and susceptible trees, we used long-term herbivore removal experiments with two herbivores of pinon (Pinus edulis Endelm.) to test the general hypothesis that herbivore alteration of plant architecture affects soil microclimate, a major driver of ecosystem-level processes. The pinon needle scale (Matsucoccus acalyptus, Herbert) attacks needles of juvenile trees causing them to develop an open crown. In contrast, the stem-boring moth (Dioryctria albovittella Hulst.) kills the terminal shoots of mature trees, causing the crown to develop a dense form. Our studies focused on how the microclimate effects of these architectural changes are likely to accumulate over time. Three patterns emerged: (i) scale herbivory reduced leaf area index (LAI) of susceptible trees by 39%, whereas moths had no effect on LAI; (ii) scale herbivory increased soil moisture and temperature beneath susceptible trees by 35 and 26%, respectively, whereas moths had no effect; and (iii) scale and moth herbivory decreased crown interception of precipitation by 51 and 29%, respectively. From these results, we conclude: (1) the magnitude of scale effects on soil moisture and temperature is large, similar to global change scenarios, and sufficient to drive changes in ecosystem processes. (2) The larger sizes of moth-susceptible trees apparently buffered them from most microclimate effects of herbivory, despite marked changes in crown architecture. (3) The phenotypic expression of susceptibility or resistance to scale insects extends beyond plant-herbivore interactions to the physical environment.

Ponderosa pine-bunchgrass ecosystems of the western United States were altered following Euro-American settlement as grazing and fire suppression facilitated pine invasion of grassy openings. Pine invasion changed stand structure and fire regimes, motivating restoration through forest thinning and prescribed burning. To determine effects of restoration on soil nitrogen (N) transformations, we replicated (0.25-ha plots) the following experimental restoration treatments within a ponderosa pine-bunchgrass community near Flagstaff, Arizona: (1) partial restoration-thinning to presettlement conditions, (2) complete restoration-removal of trees and forest floor to presettlement conditions, native grass litter addition, and a prescribed burn, and (3) control. Within treatments, we stratified sampling to assess effects of canopy cover on N transformations. Forest floor net N mineralization and nitrification were similar among treatments on an areal basis, but higher in restoration treatments on a mass basis. In the mineral soil (0-15 cm), restoration treatments had 2-3 times greater annual net N mineralization and 3-5 times greater annual net nitrification than the control. Gross N transformation measurements indicate that elevated net N mineralization may be due to increased gross N mineralization, while elevated net nitrification may be due to decreased microbial immobilization of nitrate. Net N transformation rates beneath relict grassy openings were twice those beneath postsettlement pines. These short-term (1 yr) results suggest that ecological restoration increases N transformation rates and that prescribed burning may not be necessary to restore N cycling processes.