Bibliography - Forest

In coming decades, global climate changes are expected to produce large shifts in vegetation distributions at unprecedented rates. These shifts are expected to be most rapid and extreme at ecotones, the boundaries between ecosystems, particularly those in semiarid landscapes. However, current models do not adequately provide for such rapid effects-particularly those caused by mortality-largely because of the lack of data from field studies. Here we report the most rapid landscape-scale shift of a woody ecotone ever documented: in northern New Mexico in the 1950s, the ecotone between semiarid ponderosa pine forest and piñon-juniper woodland shifted extensively (2 km or more) and rapidly ( <5 years) through mortality of ponderosa pines in response to a severe drought. This shift has persisted for 40 years. Forest patches within the shift zone became much more fragmented, and soil erosion greatly accelerated. The rapidity and the complex dynamics of the persistent shift point to the need to represent more accurately these dynamics, especially the mortality factor, in assessments of the effects of climate change.

The paper initially outlines selected uncertainties influencing climate change and their linkages with hydrology which have led to only a small section of the hydrological community (divided into 2 groups) being pro-active. Due to the foregoing uncertainties, the strategy adopted in this paper will be to focus on the principal conclusions from controlled experimental catchment studies and related process hydrology connected with land-use change arising from anthropogenic influences. The underlying philosophy is that even major natural disruptions to climate cause ecohydrological shifts in the response of landscapes and such changes may be indicated from recent hydrology research evaluating man-made impacts. The paper assesses the existing conclusions from hydrological work undertaken in both the closed forests of the humid tropics and the open forests of the tropical semi-arid regions based mostly from experimentation in headwater catchments. Such studies are concerned with the hydrological responses to the impacts of forest conversion on the change in total water yield and, in turn, the processes connected with dry weather flow (delayed flow) and storm runoff (quickflow). By taking the above approach, possible hydrological changes to climate change will be inferred, including some consideration given the outputs from atmospheric General Circulation Models (GCMs) using the Amazon basin as an example.

Juniper and pinon woodlands have been expanding throughout the Intermountain West, USA since the late 1800s. Although causal factors attributed to woodland expansion have been documented, data are lacking that describe the influence of topographic features on rates of development and structural attributes of expanding woodlands. Our primary objective was to determine the relationship between stand-level developmental and structural attributes of four expanding western juniper (Juniperus occidentalis) woodlands with two topographic features commonly important to forest vegetation patterns, site exposure (an index of insolation exposure based on slope and aspect) and elevation. To accomplish this we measured tree density and age across four western juniper woodlands in Oregon and Idaho. Site exposure and elevation were consistently correlated with spatial and temporal components of woodland development and structure. Holding slope constant, a shift from a north to south aspect resulted in a 4.9 trees/ha/year decrease in tree establishment at a similar elevation. A 200 m rise in elevation was associated with a 1.8 trees/ha/year increase in tree establishment with similar exposure. A 100 m rise in elevation was associated with a 5-year decrease in the time required for a stand to reach dominance and a 22% increase in dominant tree density among stands with similar site exposure. Although significant, site exposure and elevation only explained a portion of the process of woodland dynamics. However, our data suggest exposure and elevation are contributing variables influencing woodland expansion across landscapes, resulting in spatial and temporal heterogeneity in stand structure and development. Models of woodland expansion incorporating landscape topographic features may be practical for identifying windows of opportunity for less costly restoration.

Four independent studies of conifer growth between 1880 and 2002 in upper elevation forests of the central Sierra Nevada, California, U.S.A., showed correlated multidecadal and century-long responses associated with climate. Using tree-ring and ecological plot analysis, we studied annual branch growth of krummholz Pinus albicaulis; invasion by P. albicaulis and Pinus monticola into formerly persistent snowfields; dates of vertical branch emergence in krummholz P. albicaulis; and invasion by Pinus contorta into subalpine meadows. Mean annual branch growth at six treeline sites increased significantly over the 20th century (range 130-400%), with significant accelerations in rate from 1920 to 1945 and after 1980. Growth stabilized from 1945 to 1980. Similarly, invasion of six snowfield slopes began in the early 1900s and continued into snowfield centers throughout the 20th century, with significantly accelerated mean invasion from 1925 to 1940 and after 1980. Rate of snowfield invasion decreased between 1950 and 1975. Meadow invasion and vertical leader emergence showed synchronous, episodic responses. Pinus contorta invaded each of ten subalpine meadows in a distinct multidecadal pulse between 1945 and 1976 (87% of all trees) and vertical release in five krummholz P. albicaulis sites also occurred in one pulse between 1945 and 1976 (86% of all branches). These synchronies and lack of effect of local environments implicate regional climate control. Composite weather records indicated significant century-long increases in minimum monthly temperature and multidecadal variability in minimum temperature and precipitation. All ecological responses were significantly correlated with minimum temperature. Significant interactions among temperature, precipitation, Pacific Decadal Oscillation (PDO) indices, and multiyear variability in moisture availability further explained episodic ecological responses. Four multidecadal periods of the 20th century that are defined by ecological response ( <1925; 1925-1944; 1945-1976; >1976) correlate with positive and negative PDO phases, as well as with steps in the rate of temperature increase. These diverse factors in spatially distributed upper-montane and treeline ecosystems respond directionally to century-long climate trends, and also exhibit abrupt and reversible effects as a consequence of interdecadal climate variability and complex interactions of temperature and moisture.

Approximately half of the tropical biome is in some stage of recovery from past human disturbance, most of which is in secondary forests growing on abandoned agricultural lands and pastures. Reforestation of these abandoned lands, both natural and managed, has been proposed as a means to help offset increasing carbon emissions to the atmosphere. In this paper we discuss the potential of these forests to serve as sinks for atmospheric carbon dioxide in aboveground biomass and soils. A review of literature data shows that aboveground biomass increases at a rate of 6.2 Mg ha^-1 yr ^-1 over the first 80 years of regrowth. During the first 20 years of regrowth, forests in wet life zones have the fastest rate of aboveground carbon accumulation with reforestation, followed by dry and moist forests. Soil carbon accumulated at a rate of 0.41 Mg ha^-1 yr^-1 over a 100-year period, and at faster rates during the first 20 years (1.30 Mg carbon ha^-1 yr^-1). Past land use affects the rate of both above- and belowground carbon sequestration. Forests growing on abandoned agricultural land accumulate biomass faster than other past land uses, while soil carbon accumulates faster on sites that were cleared but not developed, and on pasture sites. Our results indicate that tropical reforestation has the potential to serve as a carbon offset mechanism both above- and belowground for at least 40 to 80 years, and possibly much longer. More research is needed to determine the potential for longer-term carbon sequestration for mitigation of atmospheric CO2 emissions.

An assessment of the vulnerability of forest ecosystems in Mexico to climate change is carried out on the basis of the scenarios projected by 3 climate models. A vegetation classification was performed according to 2 models, the Holdridge Life Zone Classification and the so-called Mexican Classification (a climate-vegetation classification based on typologies developed for Mexico). Projections of climate models were based on a doubled CO2 concentration condition. The models used were: the CCCM, which estimates an average increase in temperature for the country of 2.8"C and a decrease in annual precipitation of 7 %; the GFDL-R30, which estimates an increase in both parameters by 3.2"C and 20% respectively; and a sensitivity model in which a homogeneous increase of 2°C in temperature and a 10% decrease in precipitation are applied throughout the country. In general, the cool temperate and warm temperate ecosystems were the most affected and tended to disappear under the conditions of the 3 scenarios In contrast, the dry and very dry tropical forests and the warm thorn woodlands tended to occupy larger areas than at present, particularly under the conditions projected by the CCCM model. However, under the GFDL-derived scenario an increase in the distribution of moist and wet forests, which would be favoured by an increase in precipitation, was predicted.