Forest structure, stand composition, and climate-growth response in montane forests of Jiuzhaigou National Nature Reserve, China.

Montane forests of western China provide an opportunity to establish baseline studies for climate change. The region is being impacted by climate change, air pollution, and significant human impacts from tourism. We analyzed forest stand structure and climate-growth relationships from Jiuzhaigou National Nature Reserve in northwestern Sichuan province, along the eastern edge of the Tibetan plateau. We conducted a survey to characterize forest stand diversity and structure in plots occurring between 2050 and 3350 m in elevation. We also evaluated seedling and sapling recruitment and tree-ring data from four conifer species to assess: 1) whether the forest appears in transition toward increased hardwood composition; 2) if conifers appear stressed by recent climate change relative to hardwoods; and 3) how growth of four dominant species responds to recent climate. Our study is complicated by clear evidence of 20(th) century timber extraction. Focusing on regions lacking evidence of logging, we found a diverse suite of conifers (Pinus, Abies, Juniperus, Picea, and Larix) strongly dominate the forest overstory. We found population size structures for most conifer tree species to be consistent with self-replacement and not providing evidence of shifting composition toward hardwoods. Climate-growth analyses indicate increased growth with cool temperatures in summer and fall. Warmer temperatures during the growing season could negatively impact conifer growth, indicating possible seasonal climate water deficit as a constraint on growth. In contrast, however, we found little relationship to seasonal precipitation. Projected warming does not yet have a discernible signal on trends in tree growth rates, but slower growth with warmer growing season climates suggests reduced potential future forest growth

Changing forest structure across the landscape of the Sierra Nevada, CA, USA, since the 1930s

Understanding the dynamics of forest structure aids inference regarding future forests and their distributions around the world. Over the last few decades, several papers have addressed changing forest structure in the Sierra Nevada, CA, USA, but these studies were limited in scope. We carried out a broad comparison of forest density and composition in the 1930s versus the 2000s for the west slope of the central and northern Sierra Nevada, using the two most extensive data sets available. Forests in this region have endured a long, complex history of human disturbance, and are now experiencing climatic shifts. We subdivided the landscape into elevation and latitude zones and compared historical and modern tree densities within each zone. We compared densities in historical plots to burned and unburned modern plots, as well as densities of individual tree species in historical vs. modern plots for their entire elevational distribution. Density of small trees (10.2-30.4 cm dbh) was significantly higher in the modern data set for all elevations and all latitudes, ranging from 20 to 148% higher. However, density of large trees (61.0 cm) was lower in the modern data set for most elevations and latitudes, ranging from 41% to 60% lower in most zones. Density difference of mid-sized trees (30.5-60.9 cm) was mixed, but was generally higher in modern plots. The pattern of more small trees but fewer large trees held for most individual species as well, but with notable exceptions. Our comparison of burned and unburned plots strongly implicates fire suppression as a driver of increased density of small trees in low- to mid-elevation forests. However, modern high-elevation (.2500 m) forests, where fire suppression impacts should be minimal, were also significantly denser than historical plots. Changing climatic conditions may be driving increased densities of small trees in high elevations, as well as decreased densities of large trees across the region

Climate Change Impacts on California Vegetation: Physiology, Life History, and Ecosystem Change

Dominant plant species mediate many ecosystem services, including carbon storage, soil retention, and water cycling. One of the uncertainties with climate change effects on terrestrial ecosystems is understanding where transitions in dominant vegetation, often termed state change, will occur. The complex nature of state change requires multiple lines of evidence. Here, we present four lines of inquiry into climate change effects on dominant vegetation, focusing on the likelihood and nature of climate change–driven state change. This study combined physiological measurements, geographic models, historical documented cases of state change, and statewide plot sampling networks together with interpolated climate grids. Together these approaches suggest that the vulnerability to state change will be driven by the proximity of climatic conditions to biological thresholds for dominant species. The sensitivity of the dominant species is a much greater driver of climate vulnerability compared to the degree of climate change seen by a particular place (Section 1). Furthermore, in some cases, physiological measurements on those species can inform the nature of these thresholds (Section 3). The study team’s review of past state change events suggests connections between particular state changes (e.g., forest to shrubland) and particular triggers (e.g., fire; Section 2). The effect of fire is particularly important, as it will likely interact with climatic change with implications for the success of different life history strategies among woody plants (Section 4). Our work suggests that the biological thresholds of dominant species will play a crucial role in the vulnerability of California terrestrial ecosystems. Understanding where climate change will push dominant species past these thresholds should be a major focus of future research

Forest structure, stand composition, and climate-growth response in montane forests of Jiuzhaigou National Nature Reserve, China.

Montane forests of western China provide an opportunity to establish baseline studies for climate change. The region is being impacted by climate change, air pollution, and significant human impacts from tourism. We analyzed forest stand structure and climate-growth relationships from Jiuzhaigou National Nature Reserve in northwestern Sichuan province, along the eastern edge of the Tibetan plateau. We conducted a survey to characterize forest stand diversity and structure in plots occurring between 2050 and 3350 m in elevation. We also evaluated seedling and sapling recruitment and tree-ring data from four conifer species to assess: 1) whether the forest appears in transition toward increased hardwood composition; 2) if conifers appear stressed by recent climate change relative to hardwoods; and 3) how growth of four dominant species responds to recent climate. Our study is complicated by clear evidence of 20(th) century timber extraction. Focusing on regions lacking evidence of logging, we found a diverse suite of conifers (Pinus, Abies, Juniperus, Picea, and Larix) strongly dominate the forest overstory. We found population size structures for most conifer tree species to be consistent with self-replacement and not providing evidence of shifting composition toward hardwoods. Climate-growth analyses indicate increased growth with cool temperatures in summer and fall. Warmer temperatures during the growing season could negatively impact conifer growth, indicating possible seasonal climate water deficit as a constraint on growth. In contrast, however, we found little relationship to seasonal precipitation. Projected warming does not yet have a discernible signal on trends in tree growth rates, but slower growth with warmer growing season climates suggests reduced potential future forest growth

Forest structure, stand composition, and climate-growth response in montane forests of Jiuzhaigou National Nature Reserve, China.

Montane forests of western China provide an opportunity to establish baseline studies for climate change. The region is being impacted by climate change, air pollution, and significant human impacts from tourism. We analyzed forest stand structure and climate-growth relationships from Jiuzhaigou National Nature Reserve in northwestern Sichuan province, along the eastern edge of the Tibetan plateau. We conducted a survey to characterize forest stand diversity and structure in plots occurring between 2050 and 3350 m in elevation. We also evaluated seedling and sapling recruitment and tree-ring data from four conifer species to assess: 1) whether the forest appears in transition toward increased hardwood composition; 2) if conifers appear stressed by recent climate change relative to hardwoods; and 3) how growth of four dominant species responds to recent climate. Our study is complicated by clear evidence of 20(th) century timber extraction. Focusing on regions lacking evidence of logging, we found a diverse suite of conifers (Pinus, Abies, Juniperus, Picea, and Larix) strongly dominate the forest overstory. We found population size structures for most conifer tree species to be consistent with self-replacement and not providing evidence of shifting composition toward hardwoods. Climate-growth analyses indicate increased growth with cool temperatures in summer and fall. Warmer temperatures during the growing season could negatively impact conifer growth, indicating possible seasonal climate water deficit as a constraint on growth. In contrast, however, we found little relationship to seasonal precipitation. Projected warming does not yet have a discernible signal on trends in tree growth rates, but slower growth with warmer growing season climates suggests reduced potential future forest growth

Twentieth-century shifts in forest structure in California: Denser forests, smaller trees, and increased dominance of oaks.

We document changes in forest structure between historical (1930s) and contemporary (2000s) surveys of California vegetation through comparisons of tree abundance and size across the state and within several ecoregions. Across California, tree density in forested regions increased by 30% between the two time periods, whereas forest biomass in the same regions declined, as indicated by a 19% reduction in basal area. These changes reflect a demographic shift in forest structure: larger trees (>61 cm diameter at breast height) have declined, whereas smaller trees (<30 cm) have increased. Large tree declines were found in all surveyed regions of California, whereas small tree increases were found in every region except the south and central coast. Large tree declines were more severe in areas experiencing greater increases in climatic water deficit since the 1930s, based on a hydrologic model of water balance for historical climates through the 20th century. Forest composition in California in the last century has also shifted toward increased dominance by oaks relative to pines, a pattern consistent with warming and increased water stress, and also with paleohistoric shifts in vegetation in California over the last 150,000 y

A regional climate summary for Jiuzhaigou National Nature Reserve.

<p>Annual (A) and monthly (B) climate summaries from the Songpan meteorological station (32.65N 103.56E, 2852 m elevation). This station is the longest-running station near Jiuzhaigou National Nature Reserve (JNNR). Mean temperature (dots; left axis) and precipitation (grey bars; right axis), 1951–2009. Data are from the Chinese Meteorological Association <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0071559#pone.0071559-Cook2" target="_blank">[37]</a>.</p

Twentieth-century shifts in forest structure in California: Denser forests, smaller trees, and increased dominance of oaks

We document changes in forest structure between historical (1930s) and contemporary (2000s) surveys of California vegetation through comparisons of tree abundance and size across the state and within several ecoregions. Across California, tree density in forested regions increased by 30% between the two time periods, whereas forest biomass in the same regions declined, as indicated by a 19% reduction in basal area. These changes reflect a demographic shift in forest structure: larger trees (>61 cm diameter at breast height) have declined, whereas smaller trees (<30 cm) have increased. Large tree declines were found in all surveyed regions of California, whereas small tree increases were found in every region except the south and central coast. Large tree declines were more severe in areas experiencing greater increases in climatic water deficit since the 1930s, based on a hydrologic model of water balance for historical climates through the 20th century. Forest composition in California in the last century has also shifted toward increased dominance by oaks relative to pines, a pattern consistent with warming and increased water stress, and also with paleohistoric shifts in vegetation in California over the last 150,000 y

All woody taxa exceeding 2 cm dbh sampled in 28 circular 15 m radius forest plots of Jiuzhaigou National Nature Reserve (Sichuan Province, China).

*<p>seedlings and saplings not distinguished within genera and for the purpose of this table lumped into the most abundant taxa identified within the genus.</p><p>Plots sampled cover a total of 1.98 sampled hectares. Total stem count, total basal area sampled, and the frequency of species encountered in plots are presented along with Importance Value (IV300: sum of relative stem density, relative basal area and relative frequency). Species are ordered by IV 300 within classes of : (a) conifers; (b) hardwoods that somewhere exceed 10 cm dbh; and (c) hardwoods that never exceed 10 cm dbh in our sample (including species that primarily grow as shrubs and vines). Seedling and sapling counts are from sub-samples within forest plots. Saplings were counted in one quadrant of the plot (a randomly chosen 90° quarter circle, 177 m²). Seedlings were counted in eight 0.5 m² subplots within each plot. Parenthetical numbers following generic designations indicated the number of recognized species found within JNNR <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0071559#pone.0071559-Bobbink1" target="_blank">[18]</a>.</p