Jose Kallarackal

Responses of trees to elevated carbon dioxide and climate change

The enhancement in photosynthesis at elevated concentration of carbon dioxide level than the ambient level existing in the atmosphere is widely known. However, many of the earlier studies were based on instantaneous responses of plants grown in pots. The availability of field chambers for growing trees, and long-term exposure studies of tree species to elevated carbon dioxide, has changed much of our views on carbon dioxide acting as a fertiliser. Several tree species showed acclimation or even down-regulation of photosynthetic responses while a few of them showed higher photosynthesis and better growth responses. Whether elevated levels of carbon dioxide can serve as a fertilizer in a changed climate scenario still remains an unresolved question. Forest-Air-Carbon dioxide-Enrichment (FACE) sites monitored at several locations have shown lately, that the acclimation or down regulation as reported in chamber studies is not as wide-spread as originally thought. FACE studies predict that there could be an increase of 23–28% productivity of trees at least till 2050. However, the increase in global temperature could also lead to increased respiration, and limitation of minerals in the soil could lead to reduced responses in growth. Elevated carbon dioxide induces partial closure of leaf stomata, which could lead to reduced transpiration and more economical use of water by the trees. Even if the carbon dioxide acts as a fertilizer, the responses are more pronounced only in young trees. And if there are variations in species responses to growth due to elevated carbon dioxide, only some species are going to dominate the natural vegetation. This will have serious implications on the biodiversity and the structure of the ecosystems. This paper reviews the research done on trees using elevated CO2 and tries to draw conclusions based on different methods used for the study. It also discusses the possible functional variations in some tree species due to climate change.

Responses of Fruit Trees to Global Climate Change

Increased temperature, aberrant precipitation, and a host of other related factors are expected to cause a global climate change that would adversely affect life on this planet. Fruit trees growing in a changed climate have to cope with rising CO2 atmosphere, phenological changes occurring as a result of increased temperature, lower chilling hours (especially in the temperate regions), impact of aberrant precipitation, and the spread of new diseases. Fruit trees have ecophysiological adaptations for thriving under specific environmental conditions. Compared to natural vegetation, studies of elevated CO2 impacts on fruit trees are limited. Global warming has caused temperate fruit tree phenology to change in various parts of the world. The chilling hours, which is a major determinant in tree phenology in temperate regions, have come down, causing considerable reduction in yield in several species. In the tropics, precipitation is a major factor regulating the phenology and yield in fruit trees. There is a need to develop phenological models in order to estimate the impact of climate change on plant development in different regions of the world. More research is also called for to develop adaptation strategies to circumvent the negative impacts of climate change. This book addresses the impact of climate change on fruit trees and the response of the fruit trees to a changing environment.

Current Trends in Forest Tree Biotechnology

Modern tools of biotechnology offer a variety of options through which it is possible to match the strides made in crop improvement in agriculture and horticulture. Current trends in forest tree biotechnology indicate that this is indeed happening and that some of the hurdles of conventional forest tree improvement are no longer a serious bottleneck. The progress made in in vitro culture of forest trees and the current status of application of the technology is discussed. The trends in use of molecular tools particularly the wide variety of DNA markers available and the identification of novel genes controlling traits of interest are examined. The current status of the technology in genetic transformation of forest trees is also reviewed. The bio-safety issues in forest biotechnology especially those relating to transgenic trees are presented without bias to either side of the ongoing debate.

The Effect of Increasing Temperature on Phenology

The word phenology emanates from the Greek word faino, meaning ‘I reveal’. Phenology is the study of periodic biological events, such as bud break, flushing, flowering and fruit development, closely regulated by climate and seasonal changes, which affect fruit trees among other plants (Cautin and Agusti 2005). Higher temperatures generated as a consequence of global warming are responsible for a reduction or increase in phenological cycles in trees (Fig. 4.1).

Climate Change and Chilling Requirements

Climate change has affected the rates of chilling and heat accumulation, which are vital for flowering and production, in temperate fruit trees (Guo et al. 2014). All economically important fruit and nut tree species that originated from temperate and cool subtropical regions have chilling requirements that need to be fulfilled each winter to ensure homogeneous flowering and fruit set, and generate economically sufficient yields (Westwood 1993; Luedeling et al. 2009a; Luedeling and Brown 2011). Reduced winter chill is likely to have the most severe consequences for fruit production (Luedeling et al. 2011; Darbyshire et al. 2013). This chronic and steady reduction in winter chilling is expected to have deleterious economic impact on fruit and nut production in California, USA by the end of the 21st Century.

Cool, Warm Temperatures and Tree Pollination

In the following section, we provide a detailed description of the flower and its parts because it is relevant to the study of pollination (see Fig. 2.1 for floral parts). Flowers are structures, which consist of an array of parts (organs) borne on a central axis called the receptacle (Rudall 2007). The whole floral structure is suspended by the peduncle, a stalk that attaches the flower to the plant (Glimn-Lacy and Kaufman 2006). The flower might be supported by leaf-like structures called bracts, which are absent or present depending on plant species (Rudall 2007). The typical flower is composed of the internal sexual parts, namely organs, which are covered by sepals and petals (Abrol 2012). The floral perianth or the outer structure of the flower is composed of the sepals referred to as the first whorl or calyx and the petals, which comprises the second whorl or corolla (Rudall 2007). Sepals have a protective function during floral development, are green but can also attain color in some plant species (Glimn-Lacy and Kaufman 2006). Petals are colored parts that function as pollinator attractants via color, shape and pattern (Abrol 2012). The male component of the flower is called the stamen which bears the filament and anther. Anthers commonly bear two pollen sacs at the upper end (Glimn-Lacy and Kaufman 2006). Within the anthers, pollen is produced through the process of microsporogenesis. Among woody angiosperms, pollen is generated through meiosis and further maturation occurs in the anthers (Ramirez and Davenport 2010). Once pollen becomes mature the anthers dehisce or split open releasing pollen grains (Abrol 2012). The female part of the flower is called the carpel or pistil and is composed of the upper end, stigma, mid part, style and lower part ovary (Fig. 1.3). Pollen contacts the stigmatic surface during pollination, and then germinates through the style reaching the ovary, which contains the ovules (Glimn-Lacy and Kaufman 2006). The ovule contains the megaspores; one of these develops into an embryo sac containing an egg (Glimn-Lacy and Kaufman 2006).

Droughts and Pollination

Dry forests are environments characterized by drought conditions that extend for long periods with very few rainy periods. Under these circumstances trees have become adapted to survive under drought conditions (Fig. 4.1). Worldwide 42% of all intertropical vegetation and 49% of the vegetation of Mesoamerica (southern Mexico and Central America) and the Caribbean comprises tropical dry forest (Murphy 1995). Dry forest tree species distribution has been affected by climate change, e.g. southern Ecuador (Aguirre et al. 2017). To date, dry forests and forests worldwide face climate change impacts. These could cause several consequences within forests by varying the frequency, intensity, length, and timing of fire, drought, insect and pathogen outbreaks, invasive species, hurricanes, landslides, etc. (Dale et al. 2001).

Plant-Insect Phenology and Pollination

Climate change has been known to impact plant pollination by changing flowering phenology and by distressing the activity of pollinators, e.g. flight (Abrol 2012). Similarly, phenological decoupling of plant–pollinator interactions (Settele et al. 2016) have been reported. Specifically, plants and insects have different responses to changing temperature, creating temporal (phenological) and spatial (distributional) disparities that cause problems at the population level (Reddy et al. 2013). Mismatches could impact plants by impairing decreased insect visitation that means less pollen deposition, whereas pollinators could face reduced food availability (Reddy et al. 2013). However, in some circumstances, pollinator–plant synchrony does not cause mismatches, due to generalist pollinator species keeping pace with changes in forage-plant flowering by switching between host plants (Fig. 5.1) (Settele et al. 2016). Animal biology and ecology associated with pollination i.e. population, reproductive aspects, and activity - flight, etc., are essential for understanding the impacts manifested by climate change. Relatively very little research has been conducted on the physiology of many crucial pollinators influenced by warming temperatures (Scaven and Rafferty 2013). This is evident in many tropical regions worldwide, where, animal pollinators comprise much more species and interactions, when compared to temperate conditions (Figs. 1.2 and 5.2).

Precipitation, Flooding and Pollination

Climate change has altered the rainfall patterns worldwide. In the last century, the amount of annual precipitation and occurrence of extreme precipitation events have increased worldwide (Rosenzweig et al. 1996). Under climate change conditions, unseasonal rains often occur during dry periods, or as extended rainy seasons causing flooding events. Magrin et al. (2014) reported an increase in climate change driven extreme events i.e. flooding, droughts, heavy rains, landslides, heat waves in Central and South America (Fig. 3.1). The rainfall pattern drives climate regulation within a biome (Scarano and Ceotto 2015). It is also a key to controlling watershed levels and soil stability on mountain slopes (Scarano and Ceotto 2015).

Ecophysiological Adaptations and Climate Change

Tropical plants have developed a number of ecophysiological adaptations for thriving at high elevations. These include restriction of root growth, shoot growth decline, high leaf pubescence, high leaf thickness and purple color anthocyanin rich leaves (Fischer 2000). Additionally, fruit trees such as Lulo (Solanum quitoense) tend to branch excessively when grown above their elevation range between 1,600 and 2,450 m in the Colombian Andes (Erazo 1991; Fischer 2000; Fischer et al. 2012). This species synthesizes more purple-colored anthocyanins in leaves, shoots and flowers when grown above 2,400 m (Erazo 1991). Most ecophysiological adaptations developed by fruit trees that live at high elevations in the tropics have been developed over the course of evolution.