Historical forest conditions are often used to inform contemporary management goals because historical forests are considered to be resilient to ecological disturbances. The General Land Office (GLO) ...surveys of the late 19th and early 20th centuries provide regionally quasi-contiguous data sets of historical forests across much of the Western United States. Multiple methods exist for estimating tree density from point-based sampling such as the GLO surveys, including distance-based and area-based approaches. Area-based approaches have been applied in California mixed-conifer forests but their estimates have not been validated. To assess the accuracy and precision of plotless density estimators with potential for application to GLO data in this region, we imposed a GLO sampling scheme on six mapped forest stands of known densities (159–784 trees/ha) in the Sierra Nevada in California, USA, and Baja California Norte, Mexico. We compared three distance-based plotless density estimators (Cottam, Pollard, and Morisita) as well as two Voronoi area (VA) estimators, the Delincé and mean harmonic Voronoi density (MHVD), to the true densities. We simulated sampling schemes of increasing intensity to assess sampling error. The relative error (RE) of density estimates for the GLO sampling scheme ranged from 0.36 to 4.78. The least biased estimate of tree density in every stand was obtained with the Morisita estimator and the most biased was obtained with the MHVD estimator. The MHVD estimates of tree density were 1.2–3.8 times larger than the true densities and performed best in stands subject to fire exclusion for 100 yr. The Delincé approach obtained accurate estimates of density, implying that the Voronoi approach is theoretically sound but that its application in the MHVD was flawed. The misapplication was attributed to two causes: (1) the use of a crown scaling factor that does not correct for the number of trees sampled and (2) the persistent underestimate of the true VA due to a weak relationship between tree size and VA. The magnitude of differences between true densities and MHVD estimates suggest caution in using results based on the MHVD to inform management and restoration practices in the conifer forests of the American West.
•Water runoff will change the most out of five services under future climates.•One-third of high biodiversity areas are threatened by high change in CWD.•Carbon storage in southern California has an ...estimated value of $7.5 billion.•Quantification of services under future climates can inform climate-smart strategies.
Climate change is projected to impact ecosystem functioning, however its effect on the provision of ecosystem services is uncertain. This is particularly relevant on federal lands which harbor extensive tracts of natural vegetation. We assessed change in four ecosystem services (water runoff, groundwater recharge, carbon storage, and biodiversity) and one disservice (sediment export) in southern California between current and end-of-century (2070–2099). We used five general circulation models ranging from warmer wetter (CNRM-CM5, CCSM4) to hotter, marginally drier (IPSL-CM5A-LR) to hotter drier (FGOALS-g2, MIROC-ESM) under RCP8.5. We found greatest projected change in water runoff, from an increase of 127% under a warmer wetter GCM to a decrease of −60% under a hotter drier future. Carbon storage is projected to change the least, from an increase of 52% to a decrease of −31% across GCMs. We also determined that one-third of high biodiversity areas are threatened by high change in climatic water deficit. We estimated the current monetized annual value of sediment removal costs to be $172 million per year and the economic value of carbon storage as $7.5 billion. Understanding the impacts of climate change on ecosystem services can help develop climate-smart strategies for the sustainable management of natural resources.
Species distribution model (SDM) projections under future climate scenarios are increasingly being used to inform resource management and conservation strategies. A critical assumption for projecting ...climate change responses is that SDMs are transferable through time, an assumption that is largely untested because investigators often lack temporally independent data for assessing transferability. Further, understanding how the ecology of species influences temporal transferability is critical yet almost wholly lacking. This raises two questions. (1) Are SDM projections transferable in time? (2) Does temporal transferability relate to species ecological traits? To address these questions we developed SDMs for 133 vascular plant species using data from the mountain ranges of California (USA) from two time periods: the 1930s and the present day. We forecast historical models over 75 years of measured climate change and assessed their projections against current distributions. Similarly, we hindcast contemporary models and compared their projections to historical data. We quantified transferability and related it to species ecological traits including physiognomy, endemism, dispersal capacity, fire adaptation, and commonness. We found that non-endemic species with greater dispersal capacity, intermediate levels of prevalence, and little fire adaptation had higher transferability than endemic species with limited dispersal capacity that rely on fire for reproduction. We demonstrate that variability in model performance was driven principally by differences among species as compared to model algorithms or time period of model calibration. Further, our results suggest that the traits correlated with prediction accuracy in a single time period may not be related to transferability between time periods. Our findings provide a priori guidance for the suitability of SDM as an approach for forecasting climate change responses for certain taxa.
California fire regimes have been altered from those that occurred prior to Euro-American settlement, and are predicted to continue to change as global climates warm. Inclusion of fire as a ...landscape-level process is considered essential to successful ecological restoration in many ecosystems, and presettlement fire regimes provide foundational information for restoration or “realignment” of ecosystems as climate change and land use changes progress. The objective of our study was to provide an up-to-date, comprehensive summary of presettlement fire frequency estimates for California ecosystems dominated by woody plants, and to supply the basis for fire return interval departure (FRID) mapping and analysis in California. Using the LANDFIRE Biophysical Settings (BpS) vegetation-fire regime types as a framework, we used literature review and the outcomes of regional expert workshops to develop twenty-eight presettlement fire regime (PFR) groups based on similarity of their relationships with fire. We then conducted an exhaustive review of the published and unpublished literature pertaining to fire return intervals (FRIs) observed prior to significant Euro-American settlement in the twenty-eight PFRs, and summarized the values to provide a single estimate of the mean, median, mean minimum, and mean maximum FRI for each PFR.
Water availability is one of the most critical issues facing southern California. Consequently, the role and management of intact watersheds on public lands that supply water are paramount. We ...undertake the first regional study of climate impacts on hydrological services (runoff, recharge, and climatic water deficit) across the four national forests of southern California-the Angeles, Los Padres, Cleveland and San Bernardino. We assess the exposure, sensitivity, and vulnerability of water resources by comparing current conditions (1981-2010) to mid-century (2040-2069) and end-of-century (2070-2099) using three general circulation models (GCMs) under RCP8.5. Half of the study area is projected to exceed 2015's drought conditions in 10%-30% of the years between now and end-of-century under the moderate GCM (CCSM4), and one-third of the area is projected to exceed 2015 in 50% of the years under the hotter, drier projection (MIROC-ESM). Under a moderate projection, mean runoff increased by 1.2× by the end-of-century for three of the national forests, while mean recharge decreased by 0.9× across all forests. Projected end-of-century climatic water deficit increased on average 1.1× across the four forests. We assessed the vulnerability of watersheds by comparing the projected mean change between current and future climates with the current inter-annual variability using three categories of vulnerability. Under the moderate projection, one-third of the 385 watersheds were moderately vulnerable to changes in runoff and recharge (+ −0.2 to + −1× the standard deviation of current inter-annual variability) and ∼12 watersheds were highly vulnerable, suggesting an era of new hydrological conditions by the end-of-century. Half of the Forest Service's priority management watersheds had moderate or high vulnerability for runoff and recharge. Spatial data on hydrological services and their vulnerability can directly assist in climate-smart planning, allowing tradeoffs to be assessed between proposed management actions and their effect on hydrological services.
Fire was the dominant ecological process controlling forest structure and succession in western North American conifer forests for thousands of years. Because fires are now suppressed, and because ...widespread logging has greatly altered vegetation structure, land managers often use estimates of pre-Euro-American settlement forest conditions to help guide restoration actions. It follows that it is important to fully understand the characteristics of pre-Euro-American settlement fire regimes. Percentages of high-severity fire, fire rotation period, and proportions of early, mid, and late developmental forests are intricately and inextricably linked. Early twentieth century vegetation maps that appear to reflect aspects of pre-settlement forest conditions demonstrate that large patches of montane chaparral, which is often an early seral community in yellow pine-mixed conifer forests, were mostly restricted to steep south-facing canyon slopes at higher elevations associated with fir or subalpine forests. When resampled to a 16 ha minimum mapping unit, we found that an early twentieth century vegetation map shows montane chaparral occupied only about five percent of the landscape outside of areas that were highly impacted by gold rush era mining. We found that successional modeling suggests that similar levels (7 %) of high severity per fire in pre-settlement yellow pine-mixed conifer forests in the Sierra Nevada would permit about half of the forested area to reach late development state, consistent with other estimated amounts of pre-settlement old-growth forests. In contrast, we found that high levels (30 %) of high severity per fire—the current average—would result in only about 13 % of forests in a late development state. We do not see evidence that large proportions of high-severity fire were typical during pre-settlement fires in yellow pine-mixed conifer forests in our study area.
Managed relocation is defined as the movement of species, populations, or genotypes to places outside the areas of their historical distributions to maintain biological diversity or ecosystem ...functioning with changing climate. It has been claimed that a major extinction event is under way and that climate change is increasing its severity. Projections indicating that climate change may drive substantial losses of biodiversity have compelled some scientists to suggest that traditional management strategies are insufficient. The managed relocation of species is a controversial management response to climate change. The published literature has emphasized biological concerns over difficult ethical, legal, and policy issues. Furthermore, ongoing managed relocation actions lack scientific and societal engagement. Our interdisciplinary team considered ethics, law, policy, ecology, and natural resources management in order to identify the key issues of managed relocation relevant for developing sound policies that support decisions for resource management. We recommend that government agencies develop and adopt best practices for managed relocation.
At small scales, areas with high native diversity are often resistant to invasion, while at large scales, areas with more native species harbor more exotic species, suggesting that different ...processes control the relationship between native and exotic species diversity at different spatial scales. Although the small-scale negative relationship between native and exotic diversity has a satisfactory explanation, we lack a mechanistic explanation for the change in relationship to positive at large scales. We investigated the native–exotic diversity relationship at three scales (range: 1–4000 km2) in California serpentine, a system with a wide range in the productivity of sites from harsh to lush. Native and exotic diversity were positively correlated at all three scales; it is rarer to detect a positive relationship at the small scales within which interactions between individuals occur. However, although positively correlated on average, the small-scale relationship between native and exotic diversity was positive at low-productivity sites and negative at high-productivity sites. Thus, the change in the relationship between native and exotic diversity does not depend on spatial scale per se, but occurs whenever environmental conditions change to promote species coexistence rather than competitive exclusion. This occurred within a single spatial scale when the environment shifted from being locally unproductive to productive.
Yellow pine (
Pinus
spp. L.) and mixed conifer (YPMC) forests of California, USA (Alta California), have been negatively affected since Euro-American settlement by a century or more of logging, fire ...exclusion, and other human activities. The YPMC forests in northwestern Mexico (northern Baja California) are found in the same climate zone as those of Alta California and support mostly the same dominant species, yet they are much less degraded, having suffered little logging and only 30 years of fire suppression. As such, the Baja California forests are believed to more closely approximate pre-Euro-American settlement conditions, and they have been proposed as reference ecosystems for restoration and management of Alta California forests. We studied fire severity trends in the Sierra de San Pedro Mártir National Park (SSPMNP), which supports the largest area of YPMC forest in Baja California, to determine whether fire severity is rising over the last three decades in the same manner that it is rising in the Sierra Nevada of Alta California. We used LANDSAT data to identify 32 fires that burned 26 529 ha in the Sierra de San Pedro Mártir National Park in the period 1984 to 2010. Of this, 1993 ha burned in YPMC forest types in 17 fires. We found no temporal trends in forest burned area or in the proportion of high severity fire, but we did find that the mean size of high severity patches within fires is rising. In the SSPMNP, the overall proportion of fire area burned at high severity averaged 3 % in both yellow pine and mixed conifer forests. We found no significant autoregressive effects of year in any of our analyses, but the year with the most burned area occurred after drier-than-average periods. In the SSPMNP data, there was no correlation between burned area and proportion of high severity fire; we interpreted this to mean that differences in fuels in SSPMNP were more important to fire behavior than weather conditions. The SSPMNP continues to burn at very low severities, even after 30 years of effective suppression of lightning-ignited fires. This is in stark contrast to similar forests in Alta California, which are experiencing fires of sizes and severities that fall far outside the historical range of variation. Current fire severities in the SSPMNP are very similar to the levels of severity described for Alta California YPMC forests before Euro-American settlement. Nonetheless, fire suppression policies in Mexican national parks in northern Baja California are causing increases in forest fuels and may be the cause of recent increases in high severity patch size. Current wildfire trends in YPMC forests in Alta California should serve as a warning to Mexican managers that continued fire exclusion in the Baja California YPMC forests is a recipe for ecological disaster in these unique and important ecosystems.
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