Christiane W. Runyan

Bistable dynamics between forest removal and landslide occurrence

It is well documented that deforestation results in an increase in landslide frequency due to the control that forest roots have on slope stability. The loss of forest vegetation leads to a reduction in soil cohesion and a decrease in the shear strength of the soil profile. As a result, the slope becomes more susceptible to landsliding and the return time of landslides decreases. When a landslide removes the soil profile, there may not be adequate time for seedlings to grow and enhance soil stability. In this study, we investigate whether bistable dynamics emerge from the interaction of forest vegetation with the formation and accumulation of colluvial deposits in soil-mantled landscapes. To that end, we develop deterministic and stochastic models of landslide occurrence with a dynamic vegetation component. Results show that bistability exists for the deterministic case for both steep and shallow hollows under event and supply limited conditions. However, for the stochastic case, the randomness of landslide occurrence largely changed the states of the system such that the system only exhibited one stable state, which was the fully vegetated condition. Examining different management practices under stochastic conditions showed that the system eventually recovered; however, management practices influenced the recovery time of the forest. Thus, different management practices could render the land in a state of low vegetation over economically significant time periods.

Hydrological and Climatic Impacts

Introduction Forest ecosystems strongly affect the water cycle through their impact on evapotranspiration, precipitation, infiltration, runoff, and, consequently, soil erosion and stream-flow (see following sections). The removal of forest vegetation leads to an increase in water yields (e.g., Bosch and Hewlett, 1982; Section 2.4) and a shift in the predominant mechanism of runoff generation (Dunne and Black, 1970a; Dunne, 1978; Section 2.3). It also enhances snowpack accumulation and shortens the snowmelt season (Section 2.4). Moreover, in deforested watersheds, evapotranspiration is strongly reduced (Section 2.7). The impact on precipitation is more complex: Large-scale (i.e., >10 5 km 2 ) deforestation is expected to reduce regional precipitation (e.g., Bonan, 2008a), though the effect of forest removal also depends on synoptic patterns of atmospheric circulation and geographic setting (e.g., latitude, location with respect to mountain ranges and oceans). The deforestation of small watersheds ( 2 ) is not expected to have a substantial impact on precipitation, whereas the clearing of intermediate sized areas (15,000–50,000 km 2 [Lawrence and Vandecar, 2015]) could increase local precipitation (see also Chapter 4). Deforestation can also affect the hydrologic conditions and microclimate of nearby (downwind) ecosystems (Ray et al., 2006). Landmasses receive water as precipitation and lose it either as water vapor fluxes into the atmosphere (evapotranspiration) or as surface and subsurface flows in the liquid phase (runoff). In recent years, these two fluxes have been named green water and blue water flows, respectively, to stress the fact that evapotranspiration receives a strong contribution from vegetation (transpiration) (Falkenmark and Rockstrom, 2004; Figure 2.1). As explained in the following sections, the overall effect of deforestation on the hydrologic cycle is a decrease in water vapor fluxes and increase in runoff. Thus, green water flows decrease and blue water flows increase. This means that more water is likely to become available for societal withdrawals (but also for environmental uses) in areas located downstream from the cleared watershed. In turn, forest management can strongly impact the water resources of a watershed, and sometimes the thinning of woody vegetation has been proposed as an option to increase water availability in semiarid areas (e.g., Ingebo, 1971; Griffin and McCarl, 1989). Such an approach, however, has only limited applicability because forest removal and the consequent increase in overland flow have the effect of increasing sediment yields and soil erosion rates, thereby damaging the landscape, often irreversibly within human timescales.

Synthesis and Future Impacts of Deforestation

Benefits of Preserving Forests Forests provide an expansive range of environmental benefits (Table 5.2) that have local to regional (e.g., flood control) and global (e.g., carbon sequestration) relevance (Foley et al., 2007). Hydrological benefits of forests include regulating water supply and river discharge (i.e., moderating high and low flows) by increased transpiration, water storage beneath the forest, and increased travel time for water to reach streams/rivers. Climate benefits of forests include maintaining available precipitation via precipitation recycling (in areas where this feedback exists) and regulating local and global temperature both directly – by reducing diurnal sensible heat fluxes and nocturnal radiative cooling – and indirectly − by taking up atmospheric CO 2 during photosynthesis. For instance, a review by Lawrence and Vandecar (2015) highlighted that complete deforestation of the tropics would lead to a 0.1–1.3°C increase in temperature across the tropics and drying of approximately –270 mm yr –1 (or up to 10%–15% decrease of annual rainfall). The presence of forests can also affect edaphic processes by reducing soil erosion and enhancing soil formation. Biogeochemical benefits of forests include enhancing nutrient availability and reducing nutrient losses, thereby increasing the amount of nutrients available for plant uptake and aiding in sustaining forest growth. Forests also provide many ecological services such as maintaining biodiversity and regulating a range of dynamical trophic relationships. As discussed in Chapter 4, forests can be important for maintaining their own habitat, possibly reducing the occurrence of disturbances such as fire or landsliding, and, in turn, maintaining the wide span of benefits described previously. Forests provide many economic benefits to societies from nontimber forest products (NTFPs) that are harvested from them. These NTFPs provide food (e.g., nuts, honey, bush meat, and fruits), medicine, construction materials (e.g., rubber), bioprospecting (i.e., value for new pharmaceutical products), and agricultural products such as fodder for livestock. Forests also have recreation, cultural, intellectual, aesthetic, and spiritual values that are important to society. Thus, deforestation strongly affects the environment and society. In the following sections we review and summarize some of its major impacts. Ecohydrological and Climate Impacts of Deforestation Most of the past research on the hydrological impacts of deforestation has focused on changes in water yields and flow regulation. The removal of forest vegetation causes a reduction in soil infiltration and evapotranspiration and, consequently, an increase in infiltration-excess runoff and soil erosion.

Economic Impacts and Drivers of Deforestation

Background The classic economic issue surrounding deforestation is whether and how forest resources are allocated optimally in order to maximize net benefits to people (Sills and Pattanayak, 2006). Net benefits are the value of goods and services (peoples willingness to pay in terms of money or some other valuable resource) minus their opportunity costs (what people have to pay or what resources they have available to invest to obtain the goods and services) (Sills and Pattanayak, 2006). The opportunity costs of forested land can vary considerably (Chomitz et al., 2006; Table 5.1). For instance, in Brazils cerrado region, converting native woodlands to soy results in land worth more than

Phosphorus input through fog deposition in a dry tropical forest

3,000 per hectare, whereas land in the Atlantic forest of Bahia, Brazil, is worth just

Global desertification: Drivers and feedbacks

400 per hectare. As discussed by Sills and Pattanayak (2006), net benefits can be calculated from two perspectives. Private benefits and costs directly affect the families or companies making decisions about how resources are allocated. The private net benefits of deforestation are equal to the value of returns from agriculture minus the value of forgone future forest production. Future forest production includes timber that could be sold, as well as goods consumed directly from forest products (i.e., nontimber forest products). However, these private benefits and costs do not include adverse environmental effects, termed “environmental externalities,” that result from deforestation (e.g., see Table 5.2). The other perspective on net benefits takes into account the cost of externalities to calculate social or public benefits and costs. Many socially valuable goods and services do not have prices because they are public goods and are not traded in markets. Public goods and services refer to those whose benefits cannot be denied to anyone and cannot be divided up and sold. For example, biodiversity and carbon sequestration are public goods provided by forests. When forest is initially cleared, there are both immediate and future costs and benefits of the harvested timber. The future stream of net benefits from land uses such as agriculture and ranching is equal to the value of outputs minus the cost of inputs. To compare costs and benefits at their equivalent present value, future values are adjusted by a discount rate. The discount rate can vary considerably in time and with location (e.g., Weitzman, 1994). For instance, interest rates in Madagascar have varied between 5% and 22.8% (Kremen et al., 2000).

Physical and biological feedbacks of deforestation

In many tropical forests, where phosphorus (P) is considered a limiting nutrient, atmospheric deposition can contribute significantly to available P. Previous studies have shown that P inputs from atmospheric deposition are enhanced by plant canopies. This effect is explained as the result of increased deposition of P-rich aerosol particles (dry deposition) and fog droplets (fog or “occult” deposition) onto leaf surfaces. Here we studied the importance of fog as a source of P to a P-limited dry tropical forest. Throughout an 80 day period during the dry season when fog is most common, we sampled fog water and bulk precipitation in a clearing and measured leaf wetness and throughfall in an adjacent secondary and mature forest stand. During the study period, total P (PT) concentrations in fog water ranged from 0.15 to 6.40 mg/L, on average fourteenfold greater than PT concentrations in bulk precipitation (0.011 to 0.451 mg/L), and sixfold and sevenfold greater than throughfall PT concentrations in the secondary and mature forest stands, respectively (0.007 to 1.319 mg/L; 0.009 to 0.443 mg/L). Based on leaf area index, the frequency of fog deposition, and amount of water deposited per fog event, we estimate that fog delivers a maximum of 1.01 kg/ha/yr to secondary forest stands and 1.75 kg/ha/yr to mature forest stands, compared to 0.88 kg/ha/yr to secondary forest stands and 1.98 kg/ha/yr to mature forest stands via throughfall (wet + dry deposition) and stemflow. Thus, fog deposition may contribute substantially to available P in tropical dry forests.