**1. Introduction**

Forests cover thirty-one percent of the total global land area and nearly half of these forests are relatively intact with more than one-third remaining as naturally regenerated forests of native species (primary forests), with no visible indications of disturbances. The total area under forests across the globe is estimated to be 4.06 billion hectares, and this ecosystem provides habitat for a vast variety of terrestrial flora and faunal species [1]. Besides biodiversity, forests provide a plethora of ecosystem services ranging from sociocultural benefits, to nature experiences and climate regulation. Unfortunately, these pristine natural resources are under tremendous threat both from natural and anthropogenic stresses.

The conventional forest management targeted only a minor subset of the accrued benefits from forests and specifically concentrated on harnessing their potential for timber production or recreation [2, 3]. The structure and composition of a large extend of the world's present forest systems are a direct or indirect result of such manipulations. With time, there was an increasing realization that forests provide

services much beyond their traditional uses and have to be viewed from multiple dimensions to realize their full potential. One of the most important among these benefits is that forests are the most viable option for combating global climate change.

Forests exert regulations on the global climate in at least two ways: (i) forest ecosystems remove approximately 3 Pg C annually emitted to the atmosphere by anthropogenic activities and (ii) act as the major terrestrial sink holding more than two times the carbon in the atmosphere [4]. Though the climate mitigation role of forests is beyond doubt, a complex situation emerges on a reverse evaluation of the influence of climate on forests and their concomitant effects on the carbon cycle. Hence, evaluating the cause-effect relationship of forests and climate solely on carbon stocks and sequestration potential may not be sufficient. Forest structure, cover, compositional changes, and biophysical shifts (water and energy balances) play determinable roles in the forest-climate loop. Forest fires, pest outbreaks, invasive alien species, pollution, forest fragmentation, and soil erosion are degradative factors that hamper the functions and productivity of forests, thereby adding to atmospheric carbon. In the present chapter, we intend to analyze (i) biophysical responses of forests to climate change and (ii) the climate-change-induced impacts on forests.

## **2. Defining forest degradation**

The concept of health applied to individual trees would appear quite simple and easily quantifiable by measures such as absence of disease or physiological stress. However, shifts in the monitoring scales from trees to forest as a system make the assessment very complex. Stand productivity is commonly treated as a measure of the health of natural and planted forests. Though this provides a good proximate estimation for utilitarian purposes, it neglects several other important features of forest ecosystems such as vegetation structure, species assemblage, carbon storage, nutrient cycling, and hydrological functions. This deficiency necessitates a more holistic definition of forest health with easily quantifiable attributes. Though giant leaps have been made in technologies and scientific techniques in forestry studies, researchers have struggled for decades with operational definitions of ecosystem degradation. Lanly and Jean-Paul [5] state that "The situation with respect to forest degradation is unsatisfactory particularly because of the imprecision and multiple, and very often subjective, interpretations of the term." Lund [6] could identify more than 50 definitions for forest degradation showcasing the inconsistencies and vagueness associated with the concept.

Ghazoul et al. [7] derived a concept of forest degradation by combining the theories and analogies of resilience and basins of attraction. The proposed basin of attraction represented the different ecosystem states, which continuously modify toward a single or multiple stable steady states. It should be noted that the term steady state does not mean a static state per se, but would be highly dynamic and with interactions between the abiotic and biotic elements that would continuously produce small changes in structure and composition at local levels, even without any disturbance. Even slight disturbances, however, would displace the ecosystems from their present stable steady state to an unstable state and will initiate changes that would enable the system to either return to their earlier state or achieve another stable state. The level of displacement of the system depends on the type, intensity, scale, and frequency of disturbance [8]. The system's ability to return to the earlier stable state (i.e., resilience) would depend on the intensity, frequency, and novelty of the disturbance causing degradation.

#### *Perspective Chapter: Forest Degradation under Global Climate Change DOI: http://dx.doi.org/10.5772/intechopen.106992*

A robust approach to assess forest health has been put forth by FAO (The Food and Agriculture Organization of the United Nations), which combines the perspectives of "forest health and vitality" by considering biotic (e.g., weeds, insects and pathogens) and abiotic (e.g., pollution and drought) stresses and their effects on tree growth, survival, wood yield, wildlife habitat, non-timber forest products, cultural, esthetic, and recreation values. As such, a healthy forest system should be a balanced ecosystem that sustains complexity while providing ecosystem services. Such healthy systems would be recalcitrant to change and exhibit an excellent ability to recover from natural and anthropogenic stressors. Declining forest health or degradation is a global issue, but there exist problems in making realistic assessments because of difficulties in fixing appropriate timescales, reference states, thresholds, and ecosystem values.

The importance of having a proper definition for forest degradation would be crucial in properly defining the type of disturbance (natural or anthropogenic) that has led to a diminished status of the system. Forests are very complex and dynamic systems that constantly shift their structure and composition. Disturbance regimes and their subsequent interactions change forest structure and composition and are usually more expressed at the local to regional scales [9]. However, disturbances by climate change usually have wider ramifications that reverberate across the entire planet. Paleontological records of charcoal and pollen have attributed the rises in fire frequency and resultant degradation in boreal and temperate forest systems to changes in human management and climate [9].
