The consequences of increased atmospheric carbon dioxide for trees

So I thought I’d start off 2016 with a detailed look at how trees can be impacted by increased atmospheric carbon dioxide. This post is taken from an assignment I did recently, so is referenced quite significantly (I have provided the reference list, complete with links, at the bottom of the post). Of course, what I have written is not a complete guide to how elevated carbon dioxide impacts trees – it’s only a mere brick in an entire structure that makes a house, that is yet to be completed even in spite of all the research that has been undertaken over the years (will this ‘house’ ever even be built completely?). Us humans are – after all – looking at only a tiny segment of our world’s existence, and as trees have existed for millions of years – at times in conditions where the atmosphere’s composition has been different – we don’t really have an ‘absolute’ reference point that will stand unequivocally in the face of scrutiny. There’s also the risk of projections not actually manifesting into actuality, because of the infinite number of potentially ’causers’ and ‘effectors’ across the world’s ecosystems (within individual ecosystems, and across a cohort of ecosystems) Further, a lot of research is limited to what researchers can get funding for, which is actually a rather disconcerting aspect when assessing the works from any field of scientific research!

Anyway, on with this morning’s post…

Increased atmospheric carbon dioxide is not necessarily bad for trees. In fact, as trees require carbon dioxide for photosynthesis, there are many benefits of higher levels of carbon dioxide within the air. For instance, increased photosynthetic rates are brought about by the increase in exposure to atmospheric carbon dioxide (Rogers et al., 1983, Tissue et al., 1997, Medlyn et al., 1999), which may therefore increase productivity of forest ecosystems for many years – particularly in summer, when temperatures are higher (Tissue et al., 1997), and most notably on leaves exposed to the sun (a 98% increase in photosynthetic capacity compared to 41% for shade leaves) (Herrick & Thomas, 1999). Assuming other factors, such as water availability and nutrient availability, are not lacking, increased carbon dioxide can in fact be beneficial for forests (Broadmeadow et al., 2005, Kimmins, 1997).

Such elevated carbon dioxide levels also aid with growth – up to 30% in young trees (Medlyn et al., 2001) – primarily down to the increase in photosynthesis, which has an added effect of increasing leaf area also due to better growth. Such a leaf area increase can then further aid with photosynthetic rates; observed long-term increases in net photosynthesis are however typically lower than the heightened short-term response (Liberloo et al., 2006; Hyvönen et al., 2007). In addition, increased leaf mass improves the ecology of the soil as nutrient cycling from the increased leaf litter brings about an increase in nutrient availability (Johnson et al., 2001); again however, how sustainable this is in the longer term is questionable (Hyvönen et al., 2007; Kirby & Watkins, 2015).

Looking out across an agricultural field, lined with trees. How will these trees, and also the crops, be impacted by elevated CO2 levels?

Continuing, the efficiency of water usage by plants with elevated atmospheric carbon dioxide conditions has the potential to increase, with studies showing that plants exposed to higher concentrations of carbon dioxide not wilting as they would under normal drought conditions (Rogers et al., 1983; Guehl et al., 1994). However, much like the increase in the ability to photosynthesise, it is unclear whether such a continued efficiency is sustainable long-term (Eamus, 1991). Further to this, the increase in leaf area as a result of increased carbon dioxide may mean that water usage is not more efficient when assessing water efficiency to overall leaf area as a ratio (Picon et al., 1996).

Increases in nitrogen uptake are also present within elevated atmospheric carbon dioxide conditions – via combinations of increased fine root production, increased rates of soil organic matter decomposition, and increased allocation of carbon to mycorrhizal fungi. Regardless of the specific mechanism, research indicates that the larger quantities of carbon entering the forest soil system under elevated conditions results in greater nitrogen uptake, even in nitrogen-limited ecosystems (Finzi et al., 2007). Such indications may have beneficial implications for forests of the future, whereby, even in nitrogen depleted areas, growth is stimulated – of course, as increased carbon dioxide causes greater uptakes of soil nitrogen, if nitrogen is significantly depleted or entirely lacking in a soil ecosystem, increased carbon dioxide may actually have a negligible or negative impact upon forest health (Norby & Iversen, 2006).

How will this young sapling, if it reaches maturity, fare in conditions where ambient CO2 is in greater abundance?

Despite the benefits, elevated carbon dioxide levels also come with many adverse impacts. Whilst water-use efficiency is a common consequence of increased carbon dioxide within the atmosphere, this does mean there is decreased evapotranspiration. Such reduction in moisture being lost from trees may then have the implication of altered rainfall patterns and increased temperature – more infrequent rainfall and decreased cloud cover, respectively (Field et al., 1995) – given the reduction in evapotranspiration, which can then impact upon the trees themselves, inducing water stress down the line. Not only this, but increased carbon dioxide (and thus increased temperature) brings about, in itself, differing rainfall patterns. Whilst the UK might not initially suffer readily, Eastern Europe and the Mediterranean will experience severe drought and increased occurrence of wild fires (Kirby & Watkins, 2015), both of which may spell disaster for the constituent tree populations.

As the climate warms (in part, due to increased atmospheric carbon dioxide), the emergence of new continental pests and diseases within the UK will also increase. Already, we have seen a significant increase in the abundance and diversity of exotic pests and diseases within the UK – most within the last 5-10 years (Percival & O’Callaghan, 2015) – though as the climate warms and trading patterns change in response to different demands from different regions of the world due to the change in climate, transport may act as an increasing vector for new pest emergence (Brasier, 2008; Kirby & Watkins, 2015). Endemic native species might be particularly vulnerable to such exotic pests and pathogens with which they have not co-evolved (Anderson et al., 2004). Research even suggests that pests and diseases “can be considered biotic forcing agents capable of causing consequences of similar magnitude to climate forcing factors”, which could be disastrous when compiled with the effects of a changing climate (Flower & Gonzalez-Meller, 2015). Further, as the effects of climate change on tree–pathogen interactions cannot be accurately predicted given their complexity (Pautasso et al., 2012; Sturrock et al., 2011), it is difficult to predict, with accuracy, the impact future pests will have on forest health – though it is likely not to be promising.

Warming in general may also have different impacts on different communities within a forest ecosystem. Higher canopy species are likely to be impacted before the under-canopy species, given their micro-climate will not change at the same rate as the more exposed upper canopy micro-climate would. Such impacts of warming and changing weather patterns in response to the warmer, more volatile weather system, may therefore have lag periods for particular forest communities (Kirby & Watkins, 2015). Coppice woodlands may in fact be particularly vulnerable to warming, as their tendency to not have, when so heavily managed, reproduced via seed (as the trees were harvested before seed-bearing age), means they may be considered ‘stuck in the past’ from a genetic perspective; any regeneration would have largely been via layering or suckering, in place of sexual reproduction through seed germination (Buckley & Mills, 2015).

The phenological processes these trees go through each year will change in response to, in part, altered CO2 levels in the atmosphere.

Increased atmospheric carbon dioxide has also been found to alter, though varying between species, phenology – bud break, frost hardiness, flowering, fruiting, and seed production (Asshoff et al., 2006; Medlyn et al., 2001). For instance, the bud break during spring of Pinus sylvestris (Scots pine) is significantly hastened under elevated carbon dioxide levels, as is bud setting during autumn (Jach & Ceulemans, 1999), whilst the frost hardiness of some species is improved under elevated carbon dioxide conditions and in others reduced. For those that suffer reduced frost hardiness, survivability over winter may suffer in response (Bigras & Bertrand, 2006; Lutze et al., 1998; Repo et al., 1996).

Curiously, UK woodlands under private ownership may be at most risk of the threats of a warming climate and the evolving relationship between trees and both pests and diseases. A recent survey of private woodland owners highlighted that they are “not generally convinced of a need to adapt”, as they “feel the future is uncertain, more usually in relation to tree disease than to climate change itself” (Lawrence & Marzano, 2014). Therefore, perhaps the impending and growing threat is not simply due to climatic changes, but the lack of willingness to respond pro-actively.

Sources:

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Asshoff, R., Zotz, G., & Koerner, C. (2006) Growth and phenology of mature temperate forest trees in elevated CO2. Global Change Biology. 12 (5). p848-861.

Bigras, F. & Bertrand, A. (2006) Responses of Picea mariana to elevated CO2 concentration during growth, cold hardening and dehardening: phenology, cold tolerance, photosynthesis and growth. Tree Physiology. 26 (7). p875-888.

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Buckley, P. & Mills, J. (2015) Coppice Silviculture: From the Mesolithic to the 21st Century. In Kirby, K. & Watkins, C. (eds.) Europe’s Changing Woods and Forests: From Wildwood to Managed Landscapes. UK: CABI.

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