**3. Biophysical responses of forests to climate change**

There has been an increasing concern on the potential impacts of climate change on forests [10–15]. Due to the comparatively larger size and life span of forests with respect to most agricultural systems, the former systems respond relatively slowly to changes compared with agroecosystems. In agriculture there remains an option to breed climate-resilient varieties or to shift entirely to a new landscape within a few decades. However, such quick fix solutions may not be available for most forested systems.

#### **3.1 Climate tolerance by forest systems**

Tree species in the forests have life spans of more than 200 years and, in most cases, would be able to tolerate a reasonable range of weather fluctuations [16]. However, extreme climatic events such as floods, drought, and wind throws can result in widespread tree mortality and species decline. Tree ring data have been used extensively to quantify the degree to which the growth was suppressed before the tree survival gets affected and to establish the relationship between tree growth decline and climate [17]. Loehle [16] reported that trees in Pacific Northwest are aged between 400 and 500 years, hence would have germinated alongside the Little Ice Age (approximately 550 years ago) and survived a complete cycle of cooling and warming. Studies by Stahle and Cleaveland [18] on Taxodium distichum (bald cypress press) and Scuderi [19] in Pinus balfouriana (foxtail pine) in North America also show that over the life span (several hundred years) of many trees, multiple cycles of weather extremes would have been a rule rather than an exception, and it can be safely expected that trees, to a large extend, can tolerate such fluctuations.

#### **3.2 Plasticity response by trees to climate change**

Trees adopt a conservative growth strategy by adjusting their growth rates with weather. Frequent adverse weather conditions projected in most climate models may not always lead to complete tree mortality, but may rather try to balance with the developing average climatic conditions over a period. This means that trees with long life spans would adopt conservative strategies to avoid rapid growth in short runs of good conditions and thereby death in adverse conditions [16]. Such strategies, however, have been less reported in short-lived and early successional species.

Physiological and morphological acclimation responses by plants also help them to adapt to changing climate. Herbaceous plants have been known to alter their shoot-root ratio to acclimate to growing conditions. However, there are serious gaps in research data with respect to such responses in larger trees. Drought and high intensity rains eroding the fertile topsoil are two prominent effects predicted in most climate change assessments. Trees have been reported to reduce shoot-root ratio and energy demands to decrease stresses due to water deficiency during drought [20, 21]. Such size and biomass adjustments are usually developed at young ages. On the other hand, mature trees usually get locked into the already established biomass partitioning patterns that they would have developed at young age. Such developed functionalities restrict their ability to alter resource demands, hence more likely to succumb to rapidly changing climatic conditions [22]. Under persisting severe climate conditions, trees that die are usually replaced by the same species (i.e., no change in forest composition), but with a more adaptive body organization to tolerate the changing climate situations. However, such adaptations would affect aboveground productivity and would have serious ramifications for the functioning of ecosystem. Species replacement in forest systems would occur only if better adapted species to new and changing climate conditions are present in the regeneration pool and if they are competent enough to outpace the inferior species [15].

#### **3.3 Genetic responses by trees to climate change**

Tree species have physiological ecotypes, each with very distinct limits of tolerance and adaptation to variation in climate. In other words, within each species there would be a broad range of climate optima facilitating wider adaptations to changing climate. Bonan and Sirois [23] observed that the eco-physiological responses did not vary much under a wide range of temperatures.

#### **3.4 Forest soils under climate change**

Soil provides the base matrix for ecosystems to grow and flourish providing all essential growth inputs. The vegetation that develops on a soil in turn supports the base matrix by way of nourishing, protecting, and cycling the resources. A disruption in this cycle would eventually add to the disruptive forces accentuating degradation. The degradation of surface soils of forests by natural or anthropogenic stressors can accelerate soil nutrient leaching especially in tropical humid regions [24, 25]. Thick root mats on sandy soils in the Amazon tropical rainforest have been found to retain up to 99% of the available nutrients, thereby preventing nutrient leaching losses [26, 27]. In forests affected by fire, concentrations of most primary (P, K) and secondary nutrients (Ca, Mg and S) increase rapidly immediately after burning, which promote root development,

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

chlorophyll content, and reproduction in surviving plants [28]. However, most of these nutrients are lost within a short time through leaching and runoff, thereby furthering degradation and supporting ecosystems with lesser diversity and ecological potentials [29–31]. The degradative effects will be faster in coarse texture soils than in clayey soils due to the higher negative charges in clay that strongly bind and retain the nutrients from leaching losses [32] Topography also has a profound influence on soil nutrients with chances of nutrient losses higher on the upper slopes under high-intensity rains [30].

Trees exert direct influence on the availability and quality of water in forest ecosystems [33–35] and play an important role in regulating soil erosion and runoff rates, which gets offset by degradation. Accordingly, climate factors that affect degradation and vegetative cover modification will have substantial impacts on the water-related provisioning services. Interaction of forest systems with water and energy cycles provides the basic foundation for water resource distribution, carbon storage, and terrestrial temperature balances. Studies by Rodrigues et al. [35] showed that climate change could lead to a 24–46% rise in annual soil erosion in managed forest systems and that the losses were consistently higher in systems with lower site qualities. The projected increments in the mean annual temperatures and annual rainfalls under the different climate change scenarios are expected to aggravate erosion risks in forest systems. Apart from high-intensity rains, increased fire incidences by enhanced drought conditions would also open up canopy leading to soil erosion, thereby furthering forest degradation.

Besides a store house of nutrients, forest soil also contains a complex microbial community with diverse metabolic capabilities that regulate all biogeochemical transformations in these systems. These biogeochemical cycles control the nutrient transformation and maintain fertility of soil [36, 37]. Though degradation causes significant decline in beneficial microbial population, studies have also shown that the diversity of certain adaptable microbial groups may increase in some forest systems [38, 39]. Theoretical models predict that soil fungal communities may be more resistant to forest degradation than bacteria in tropical forests [40, 41].

### **3.5 Fire responses of forests under climate change**

Forests are highly vulnerable to wildfires and such fire-induced changes are considered an important part of the landscape dynamics. Wildfires are responsible for degradation in about 4.8 million hectares of forest worldwide, and this accounts for nearly 23% of forest degradation by all factors [42]. Low, moderate, and high-intensity fires occur under a wide set of environmental and climatic variables such as very low humidity, high wind speeds, high temperature, and high dry biomass (fuel) load, all of which are accentuated under changing climate. Once initiated the propagation of forest fire usually follows three mechanisms, which degrade the existing forest systems:


Forest fires, besides effecting the vegetation dynamics, cause prominent disturbances in the system and act as an agent of environmental change with local to regional impacts on land use, productivity, carrying capacity, biodiversity, and regional to global impacts on hydrological, biogeochemical, and atmospheric processes. About 55% of the forest systems in India are affected annually by fires, which result in degradation and exacerbation of carbon dioxide levels in the atmosphere. Besides flora, forest fires affect the forest habitat and population and distribution of short range faunal species. Tropical wild fires produce high organic carbon emissions, trace gases, black carbon, and release almost 100 million tonnes of smoke aerosols into the atmosphere. These submicrometer smoke aerosols emitted in large quantities to the atmosphere play a major role on the radiation balance of the earth-atmospheric system. Further, fires also destroy the organic matter, which would be needed to maintain an optimum level of humus in forest soils. Frequent fires may also decrease the growth of grasses, herbs, and shrubs, which may result in increased soil erosion.

#### **3.6 Biological invasions in forests under climate change**

Invasive alien species (IAS) negatively impact forest ecosystems by extirpating native species, altering ecosystem functions, changing species composition, reducing food and cover for wildlife, and posing threats to biodiversity [43, 44]. Invasive alien plants can affect water availability to native species, increase forest fire hazards, and affect productivity of natural and planted forests. Although a comparison of the number of IAS and their extent of invasion in tropical and temperate forests is impeded due to data deficiency from the former, available information indicates that the number of IAS is increasing rapidly in temperate forests, and this may increase as high latitude areas get warmer with climate change [45].

Climate change (changes in global temperatures, precipitation, fire regimes, and occurrence of climate extremes) is reported to be a major driver of biological invasions by increasing susceptibility of all ecosystems to IAS [46, 47]. Climate change may also increase chances of introduction of new alien species, promote their spread by altering historical biogeographic limits, and enhancing probability of establishment and colonization [48]. If unsuitability of climate had previously prevented a species from establishing in a new biome, a change in climate may help its establishment, invasion, and spread [46, 49].

Climate change is thus a major driver of forest degradation since it promotes invasion by alien species in these ecosystems and causes a multitude of negative impacts. The damages due to climate-change-induced biological invasions can be more severe in forests, which are already degraded and fragmented due to human interventions or other causes. The interactions between climate change, land-use change, forest disturbance, and invasive alien plants may facilitate evolution of novel communities (assemblages of native and alien species) in natural forests [50]. Such interactions can also create new pathways of invasion and enhance vector efficacy [46]. On the other hand, deforestation, which leads to a local rise in atmospheric temperature, light availability, and increased supply of soil nutrients, can promote invasion by alien plants affecting natural regeneration of native tree species in forest ecosystems [51]. In short, the consequences of climate change on invasion by alien species and their impacts on forests are complex.

Intensity and frequency of fire in forests are facilitated by highly inflammable invasive alien plants especially where climate change causes extreme drought and rise in atmospheric temperature [52]. A shift in fire regimes may result from such

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

fire events. And, forest fires may help quick regeneration of alien plants compared with native species and may alter carbon cycling in these ecosystems. Climate change is predicted to intensify the fire regimes in the future, which will help spread and establishment of invasive alien plants [53].

It is reported that warmer and extended autumns promote plant invasions in the understory of boreal forests and a fast-growing invasive herb interrupt regeneration of fir trees in forest gaps in balsam-fir-dominated boreal forests in Canada [54, 55]. Both are examples of climate-change-induced alien plant invasions in boreal forests. Climate warming is reported to promote the rapid spread of alien plants into higher altitude areas where they failed to colonize earlier due to unsuitable cold conditions [56]. This is an impending threat to mountain forests. Extreme climate events such as heavy winds, hurricanes, storms, and floods can help long-distance spread especially of propagules of alien plants, pests, and pathogens [57] causing forest health issues even in continents far-off from the source.

It is predicted that the current sources of IAS may change in the future due to shifts in geospatial matched climates worldwide. This will further impede management measures of IAS including preparedness and prevention. Also, there are indications that the distribution of invasive alien plants in terrestrial ecosystems may expand in the future under different climatic scenarios. The hotspots of these invasions are predicted to be located in South America, Europe, New Zealand, and northern and southern Africa [58]. Chances of invasion of woody and herbaceous alien plants in endangered ecoregions in these invasion hotspots in the changing climate scenarios are also projected.

It is well known that climate change may increase the susceptibility of forest flora and fauna to invasion by alien pests and pathogens by enhancing host susceptibility and range expansion [59]. All these factors will contribute to forest degradation significantly. With tree campaigns promoted across the world, there is a serious risk of fast-growing alien tree species, which are planted to sequester carbon in order to mitigate impacts of climate change, becoming invasive [60]. Adequate attention is yet to be paid on this emerging threat. In the near future, one of the major challenges for land managers and conservationists is that climate-change-induced impacts on forest and other ecosystems will make management actions against IAS a greater challenge [46].

## **4. Climate-degradation interactions in forest ecosystems**

The effect of climate on forest systems can be both negative and positive. Climate change could force forests to periodic stresses such as droughts, intense rains, fires, and wind throws, which would adversely affect the tree resilience in these systems. Such stressors in an intense or frequent form can lead to large-scale mortality in susceptible forest systems and produce patches of dried forest. On the other hand, such climate-mediated stressors can facilitate a wide range of essential ecological process such as nutrient cycling, regeneration, and subsequently creation of new habitats at larger spatial scales. In short, climate stressors could modify the forest landscapes to encompass a diversity of mosaic of successional patches representing different stages of disturbance, recovery, biogeochemical cycles, stand structures, and new habitat niche for fauna [61–63].

Studies by Baccini et al. [64] showed that forest disturbance and degradation accounted for 46, 81, and 70%, of carbon losses from tropical forests in Asia, Africa, and America, respectively. The enhanced emissions from forest disturbance and

degradation initiate a feeder breeder reaction wherein greenhouse gas content in the atmosphere gets accentuated, and this in turn would increase the extend and severity of degradation in terrestrial systems. On the other hand, increased CO2 in the atmosphere is also increasingly viewed as a factor for improving net primary productivity of forests. Elevated atmospheric CO2 content has been reported to increase photosynthesis, a process known as CO2 fertilization [65]. The magnitude of the CO2 fertilization effect would however depend on the carboxylation efficiency and leaf area index of plants to rising atmospheric CO2 concentrations [66]. The forest responses to changing climate would also depend in part on whether the plant photosynthesis from elevated CO**2** content in the atmospheric compensate for enhanced physiological stresses arising from higher air temperatures. Sperry et al. [67] studied competing responses of optimization theory and mechanistic model of tree water transport and photosynthesis and observed that without acclimation, elevated CO2 content could compromise the net primary productivity (NPP) of monoculture stands by way of increased temperature-driven vascular failures resulting in stress and mortality.

Under a changing climate scenario, the capacity of forests to sustain its productivity and ward of degradation would depend on whether the ratio of elevated CO2 (∆Ca) to enhanced temperature (∆T) exceeds the physiological thresholds. In a changing climate, plants would be forced to acclimatize over time, especially in plastic traits (e.g., leaf area), photosynthetic capacity (carboxylation capacity and electron transport capacity), and leaf responses to ΔCa and ΔT [68–70]. The tree density (basal area per ground area), forest leaf area per ground area, and rooting depth in combination would determine the competition for resources and ecosystems response to the ΔCa or ΔT. For forests that do not acclimate, the threshold ∆Ca/∆T must be >89 ppm°C − 1 and for systems that adapt to changing climate, a threshold ∆Ca/∆T of >67 ppm °C − 1 would be required to avoid chronic stress and subsequent degradation. A lower minimum of 55% of existing forest systems without acclimation and 71% of the forests with acclimation are expected to meet the physiological thresholds and negate degradation.

Extreme events associated with climate change can also put enormous pressures on forested systems, degrade them with respect to their ecological functions, and more importantly reduce their capacity to store carbon. Ciais et al., [71] estimated ca. 30% reduction in the gross primary productivity of forests and a strong anomalous net source of carbon dioxide amounting to 0.5 Pg C yr−1 in Europe following heat waves. The increase in future drought could turn forest systems into carbon sources accentuating carbon-climate feedbacks, particularly in the tropics and at higher latitudes [72, 73]. Several studies have reported forest degradation [74, 75] as a major factor responsible for the upward trend in carbon concentrations in the atmosphere. However, quantifying carbon losses due to forest degradation as opposed to estimating carbon stored in a stand replacing the lost forest is often challenging, hence ignored in most global estimation of carbon emissions.

In general, climate change enhances forest degradation by way of enhanced physiological stresses (e.g., under floods and droughts), incidence and spread of invasive alien species, insect pests and pathogens, increased fire incidences, and related degradation. Stressors for degradation can be placed under the broad categories of climate, biotic, and anthropogenic, which interact to produce combined effects on forest systems. For example, climate regulates the intensity and spread of forest fires even if they result from human activities. Similarly, plants stressed by extreme drought conditions would have lesser resources to resist disease or insect outbreak. Such instances would invariably decline the forest health, open up canopy, and give way for invasion by alien plants.

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

Projected climate changes in all likelihoods have been predicted to have profound influences on disturbance regimes and ensuing species demography [76]. The tolerance ability of species to variation in moisture and temperature regimes will be undoubtedly confounded by the alterations caused by disturbances [76]. For instance, climate-change-induced drought enhanced widespread insect pest outbreak leading to massive die-back of different tree species in the Colorado Plateau [77].

Besides the effect of climate on ecosystems, forests also regulate a large array of biophysical properties, which have a direct or indirect impact on the global climate change. Forests absorb a large proportion of sunlight incident on them, thereby having an effectively lower surface albedo than other land uses [78]. In regions with boreal forests, this could produce a cooling effect on deforestation, a reverse of that in the tropical regions. Forests also directly regulate climate by modifying the surface roughness and evapotranspiration. These forest-regulated biophysical factors in turn influence the exchanges of mass, energy, and water between the atmosphere and land surface [79]. These forest-determined surface changes can counteract perceived climate benefits from forest carbon sequestration and may have a negative pressure on the carbon stocks [80, 81].
