Future expectations of forest soils: increasing productivity within environmental limits using new knowledge

ABSTRACT

Global demands for forest products are continually increasing. This is leading to greater demands on forest soils as rotation length decreases, harvesting and climate change related disturbance increase, and the use of inputs such as fertilisers, herbicides, pesticides and other chemicals increase. More often the question of how can we increase productivity within environmental limits is being asked by communities, consumers and nations. This paper explores current research that is seeking new knowledge on tree responses to biostimulants, balanced nutrition additions, soil–microbe–plant relationships and our ability to influence the outcomes of these interactions in ways that benefit forest productivity and resilience all with a focus on producing more for less, within environmental limits and without compromising wood properties. Greater awareness of the wider benefits of forest may broaden the current expectations on forest soils, leading to new research directions.

KEYWORDS:

• Forest soils
• productivity gap
• mycorrhizae
• plant growth promotion
• radiata pine

Introduction: the importance of forest soils to New Zealand

In New Zealand, plantation forests occur on 9 of the 15 soils orders recognised in the New Zealand soil classification system (Hewitt 1998). These plantations are spread throughout New Zealand on Allophanic, Brown, Oxidic, Pallic, Podzol, Pumice, Raw, Recent and Ultic soil orders (Watt et al. 2005). Although the forests and their associated soils are widespread, it is easy to take the soils beneath New Zealand’s planted forest estate for granted. Yet the fact that these soils supported annual export returns from forest products of $5.47 billion (NZFOA 2017) suggests an important role for these soils in the wider context. The sizeable returns make forestry the third biggest exporter earner for the New Zealand economy. With approximately 1.72 million hectares of planted forest, this represents average (2011–2015) export earnings per hectare of soil of $2819 compared to $6328 and $1470 for soils that support the dairy and red meat, and animal fibre sectors, respectively. The planted forest estate consists of 90% Pinus radiata (D. Don) (radiata pine) and 6% Pseudotsuga menziesii (Mirb.) Franco (Douglas-fir) with the rest of the area consisting of eucalyptus, cypresses and other exotic hardwood and softwood species. The recognition of sustainable forest management principles is high with 67% or more than 1.228 million hectares of the planted forest estate and their soils certified under the Forest Stewardship Council principals for sustainable forest management. The forestry sector also contributes nearly 2% of GDP, employing approximately 25,000 people directly and indirectly involving considerably more people in supporting trades and services, mostly in rural areas (NZFOA 2016). It is now easy to see the importance of forest soils to New Zealand.

By international standards, the rotation length of planted forests in New Zealand is considered to be short and the forests are recognised as being very productive. They produce 1.1% of the world’s timber supply from less than 0.05% of the world’s forest cover. The short rotations currently mean that some 30% of the forests are now in their second rotation and approximately 20% of the forests are in their third rotation (MAF 2008a) with some forests established in the 1920s now entering their fourth rotation, exceeding some early expectations of the soils' ability to supply nutrients (Will 1968) even after nearly 100 years of forest production. Inputs such as nutrients have been minimal and remain so although the need for greater inputs is clearly recognised (Smaill and Clinton 2016) and research is underway to ensure that this occurs in a sustainable fashion.

What pressures are there on forests and forest soils?

There is increasing demand for timber and non-timber products and services (food, fibre, carbon sequestration, clean water, recreation, biodiversity and erosion mitigation) from both planted and natural forests (Payn et al. 2015). The World Business Council for Sustainable Development (WBCSD 2015) looks to forests as being key to global sustainability now and into the future. The WBCSD has suggested that the world even needs more forests to meet current and future expectations for wood products and for environmental services and benefits from forests. The WBCSD recently identified that up to 300% more fibre is required to meet expected global shortage, which would see the annual demand for wood triple by 2050 to more than 10 billion m³ as a result of population growth, rise of the middle class, changing demands, for example, bioenergy and biofuels and the rise of the bioeconomy. In addition to these increased demands, forests currently directly affect the livelihoods of 20% of world population, provide approximately 75% of clean freshwater, support 9% of world’s primary energy demands while removing and storing vast quantities of CO₂. Forests also support habitat for many important species with approximately 80% of terrestrial biodiversity found in forests (WBCSD 2015). This global demand for more forests and their products is also seen in New Zealand. The New Zealand Wood Council outlined in its 2012 strategy (Woodco 2012), the aspiration to grow forest-based export receipts to $12 billion per annum by 2022. This strategy is also supported by the New Zealand Forest Owners Association (NZFOA 2012) who aspires to doubling forest productivity in terms of volume of wood produced to meet the projected increased demand for forest products. It is important to define what is meant by productivity, recognising it can be measured and reported in a variety of ways (Moore and Clinton 2015). Throughout the text for the purposes of this discussion increased productivity is used in the context of greater volume of wood production per area of land as opposed to more efficient use of inputs to generate greater profitability (see Moore and Clinton 2015 for a more detailed discussion of these issues). These two forest industry strategies were well aligned at the time with the previous Government’s Business Growth Agenda (MBIE 2012) that had the aim of doubling national export earnings by 2025.

In addition to these increasing demands on forests, other pressures (land-use change, changes to New Zealand’s Emissions Trading Scheme (NZETS), intensification) also affect the future of forests and, when combined, these pressures bring a focus to the soils that support the forests reliant on them. As an example, land-use change and intensification in other sectors is putting pressure on existing forested land in New Zealand, challenging forestry as an economically competitive land use. In addition, the changes to the NZETS in relation to deforestation in January 2008 by the New Zealand Government, coinciding with low carbon prices, led to large areas of planted forests being cleared for conversion to dairy farms. If prices for logs remain the same, then the productivity of remaining forests will need to increase if export earnings are to increase from the same land base. To double the productivity of the current forest estate will require far greater intensive forest management practices. A first step to increase the productivity would be to fully utilise the site (Powers 1999). Once the site is fully utilised, treatments aimed at improving the site could be used such as significantly increasing the use of inputs such as fertiliser, herbicides, pesticides and chemicals and also far greater use of natural resources such as soil and water combined through increased plant uptake and utilisation due to improved genetics and intensive management practices such as soil cultivation.

Future expectations of forest soils

To meet the many previously identified demands on forest soils, forest managers need to evaluate their current management practices and strategies for the efficient use of resources and provision of timber and non-timber benefits over time. They also need to consider new approaches to managing natural resources such as forest soils and the use of a range of inputs. Continued sustainable management of forests is essential if the world's forests are to continue to meet society's expectations of forests, including bioenergy and increased contribution to human health and well-being.

Increasing productivity

There is an increasing recognition of the productivity gap (Mueller et al. 2012) in the agricultural sector and the need to close this to increase the supply of products. The productivity gap refers to the difference between observed yields and those attainable (absolute biophysical potential yields based on modelling physiological use of resources and inputs) in a given region (Mueller et al. 2012). In the light of the future demands on forests, there is also the need to identify the extent of the productivity gap for forests (Moore and Clinton 2015). Forest productivity is a function of the climate, site quality and genetics and management (Dyck et al. 1994). If one of these factors is limiting, for example by soil nutrient deficiency or the stand is poorly stocked, then current productivity will be less than potential productivity (Powers 1999). Ameliorating the nutrient deficiency or increasing stocking will improve productivity to a new potential productivity limited by genetic potential or local climate. Due to the long-term nature of the forest cycle, there is the potential to manipulate both the productivity of existing forests and also future forests that will be planted following the harvest of current forests. Several approaches have recently been used to estimate the size of the productivity gap for New Zealand planted forests (e.g. Moore and Clinton 2015).

Full site occupancy is important for maximising productivity. The recent work of Watt et al. (2017) has shown that, on average, current New Zealand radiata pine forests are understocked by 100 or more stems per hectare (approximately 20% of current stocking). Increase stocking by 100 additional trees per hectare could mean 20% more demand on soil resources and greater nutrient removals at harvest. Increasing the stocking does not change the soils' ability to supply nutrients unless there is an associated biological phenomenon we have not yet observed. Further work is needed to test the assumption that a 20% increase in stocking will result in 20% more demand on soil resources.

There is clearly a productivity gap that could be closed and in doing so achieve the goals of the industry strategy outlined earlier. This would generate greater revenues for industry and increase exports. So how could we close this gap? An obvious approach is through genetic improvement. Genetic gain of up to 25% in terms of stem volume through tree improvement programmes for radiata pine has also been recently quantified (Kimberley et al. 2015). Again, unless there has been a matched increase in resource use efficiency for nutrients per unit of wood grown or increased nutrient input, then the gain in volume will also result in more demand on soil resources and greater nutrient removals at harvest. Changing the genotype does not change the soils’ ability to supply nutrients unless there is an associated biological phenomenon we have not yet observed.

The manipulation of forest soils offers many possible options for increasing productivity and closing the productivity gap. Simply put, producing larger logs, and more of them with better wood characteristics, will increase both the value and productivity of existing forests. One way to do this is to increase the sophistication of fertiliser use in forestry (Smethurst 2010) and to bring it into line with state-of-the-art fertiliser technology while adopting a systems biology approach to match plant nutrition with forest soil resources. If we recognise that as well as being nutrients, phosphorus (P), boron (B), and particularly, nitrogen (N) can act as signals, regulating plant gene expression, physiology, growth and development (Smaill et al. 2011; Gutierrez 2012), we could transform fertiliser use in forestry. As an example, B has been identified as an important factor in quorum sensing (Chen et al. 2002), the process by which microbes communicate. So applying B to a B-deficient soil should not only address plant needs in terms of overcoming B deficiency and improving growth, but also address microbial demands for improving communication. As a result of increased microbial communication, there may be more microbial activity that ultimately benefits plant growth. Not only may plant growth be improved, but the quality of the resulting wood may also be improved as B acts to increase pectin cross-linking in cell walls. In turn, this improves cell wall strength (O’Neill et al. 2004) and improves plant health due to improved plant immune system responses (Ruuhola et al. 2011). The results of these types of studies will be important as they will demonstrate the need to look at a whole of soil–plant response and not just focus on a measure of plant growth when it comes to adding value to forests.

So how will increases in productivity be achieved and isn’t forest management already intensive?

Overall productivity as well as profitability of planted forests could be increased by planting more land or better land, but there has been very little new planting in recent years for a number of reasons, including: the need for upfront capital to establish trees, the (perceived) long lag between establishment and achieving harvesting returns, uncertainty over future prices, lack of awareness of the returns that can be achieved from forestry, and concerns over future carbon liabilities and being locked into forestry as a land use. The potential for planting more land to achieve a number of objectives and where this land is (Watt et al. 2011) and what is the potential productivity and how profitable this land might be (Barry et al. 2014) have been explored, but the results of these studies have yet to be put into practice due to the lack of planting and in fact deforestation of exotic planted forests that has been occurring, with further areas of deforestation predicted (MPI 2016).

Integral to planting more forests is the question of improved silvicultural regimes and species choices to produce fit-for-purpose products. I have previously discussed some of the potential unintended consequences of new silvicultural regimes and of tree improvement programmes in terms of more demands on soils. It is beyond the scope of this current discussion to go into the consequences of species choice on demands on soil resources and their use, but there are many possible implications as a result of changing species. Possible effects could occur as a result of differences in rooting depth, organic matter inputs, nutrient demand and cycling, N fixation and many more.

I often say that there is more than one genome in the forest. This concept is increasingly being recognised along with the idea that it may be possible to manage and improve the many genomes found throughout a forest, which may offer benefits in addition to those traditionally derived from tree breeding strategies (Wakelin 2018). The various and sometimes multiple roles that these other important symbionts play in the life of trees is not always well understood due to the complexity of interactions and the actual roles themselves. For example, a number of studies have shown how mycorrhizae can enhance productivity in trees (e.g. Stenström and Ek 1990; Onwuchekwa et al. 2014) through many processes (e.g. nutrient and water uptake), although the evidence of enhancing water uptake is not clear (Ortega et al. 2004; Lehto and Zwiazek 2011). Understanding how genotype, aquaporin and mycorrhizal activity all interact to actively or passively influence hydraulic conductivity for conifers in situ would be a big step forward in the science of tree water relations and the role mycorrhizae play. Given the focus on the current economic traits in tree breeding programmes, the value of including resource efficiency traits in tree breeding programmes needs to be highlighted possibly in the new context of climate change and impacts of intensification that put more pressure of forest soils. A potential driver for this change in or addition to the focus of breeders is the speed at which climate change and intensification are occurring compared to the speed on current tree breeding programmes. In the future, it may not just be a genomic approach based solely on tree breeding from the point of view of the tree but from the point of view of the many genomes in the forest. This approach could also take advantage of potential biotechnology approaches to increase the productivity of forests (Fenning et al. 2008; Dubouzet et al. 2013). Biotechnology should not only be considered in the light of just the tree either but include ideas such as engineering the rhizosphere (Zhang et al. 2015) by manipulating bacterial and fungal communities to increase nutrient and carbon cycling and production of plant growth promoting substances.

Is fertiliser the only answer?

Smaill and Clinton (2016) reported that the current fertiliser use in forests is minimal with estimates of less than 700 T of N fertiliser applied nationally, which means in reality that most stands are receiving no fertiliser beyond or even at the time of establishment. Inputs are generally confined to the establishment phase with traditional use of herbicides or cultivation where appropriate. This suggests that there is a large opportunity to increase productivity through initiating the greater use of inputs that supplement site resources or increase the availability, uptake and efficient use of existing resources. To date, only a limited number of interventions have been used to increase forest productivity although there are number of potential approaches available from other sectors that could be tried.

If fertiliser is to be used widely, then the predictability and magnitude of tree fertiliser response need to be greatly improved to increase the efficiency of fertiliser use and minimise unintended consequences. This requires response models and systems models that consider a whole of plant response to applied nutrients and competition for resources that limit tree growth. Increased fertiliser costs, coupled with more stringent requirements around nutrient inputs, mean that greater efficiency of and value from fertiliser application will become increasingly important.

There are also some other challenges to increasing forest productivity in New Zealand. The current age class distribution shows that most forests in New Zealand are older than 16 years (NZFOA 2016) and are generally considered post-silviculture, that is to say, they require no further treatment in terms of thinning or pruning or further inputs such as herbicides and fertilisers. This point of view is reinforced by the limited number of interventions and lack of recent experience in trying new approaches. The other issue is that forest managers do not want to compromise wood properties through the use of fertilisers (e.g. Beets et al. 2001). As previously stated, increased tree growth will consume more resources, and remains unclear if forest soils can sustain this over multiple rotations both nationally (Garrett et al. 2015a) and internationally (Achat et al. 2015). Shorter rotations will also increase the frequency of nutrient removal and soil disturbance. Any intensification has to be sustainable and within limits and will possibly require interventions other than more inputs that can reduce the effects of more disturbance due to harvesting for example. New harvesting systems using robotics and automation are currently being investigated (e.g. Raymond 2012; Parker et al. 2016) although their environmental effects have not been evaluated yet. More demand for soil resources such as nutrients and physical disturbance during harvesting may lead to more soil degradation (Delong et al. 2015) through loss of soil organic matter and nutrients along with erosion as a result of disturbance, which may be further magnified by impacts of climate change.

Current research to address future demands

Davis et al. (2010) provide a detailed review of the large volume of research carried out in New Zealand on forest nutrition research. In 2013 a new research programme entitled ‘Growing Confidence in Forestry’s Future’ starting with funding support from both the New Zealand Government and the forestry sector (http://gcff.nz). This programme focuses on improving the productivity of current and future forests using practices that are within environmental and social limits. Understanding and manipulating soil resources is a key means through which productivity gains will be achieved. This research is looking at novel ways to manage and manipulate soil resources. For example, we are extending our work with Biuret with Douglas-fir (e.g. Xue et al. 2004), a known biostimulant to radiata pine. New research to date has confirmed the positive effects of Biuret on conifer growth under controlled conditions and we have now proceeded to field trialling. We are also reviewing the impacts of B on tree growth beyond just ameliorating obvious deficiency to consider the benefits on sites where there is no obvious B deficiency (Olykan et al. 2008; Khan et al. 2012). With respect to mid-rotation stands, we are also investigating the use of site-specific balanced fertiliser regimes to overcome site-specific soil nutrient limitations and using a forestry-specific nutrient balance model (NuBalm, Smaill et al. 2011) to predict nutrient requirements throughout the rotation. We are also furthering our understanding of the influence of N supply on growth (Bown et al. 2010) and extending our knowledge to how different N forms may influence the composition of the root microbiome of radiata pine (Gallart et al. 2018). What is becoming more obvious with time is how specific genotypes are interacting with soil properties to determine growth and nutrition outcomes (e.g. Hawkins et al. 2010; Xue et al. 2013; Yongjun et al. 2015), suggesting that by selecting genotypes of radiata for specific sites we may be able to manage the future of soils in this way. Indeed the potential for understanding plant genotype × environmental (G × E) interactions is seen as a key area for opportunity to not only close the current productive gap, but actually, increase the fundamental potential for forest productivity in New Zealand. A number of trials have been set up to explore this, and these are expanding to consider the wider forest ‘biome’ from the soil up.

Also, the extent to which microbial activity can be manipulated is a key question (Smaill et al. 2010), particularly in order to increase the availability and effectiveness of soil resources and the certainty with which this can be achieved. What processes ought we focus on – increased weathering, e.g. Smaill et al. (2014a) and Smaill et al. (2014b), increased N and P availability, enhanced plant growth promotion, or enhanced plant immune system response (e.g. Ping and Boland 2004; Berendsen et al. 2012)? Interactions between plants and soil microbes are clearly something to focus on when it comes to managing soil resources and moderating the demands on forest soils in the future. This is particularly important in forests where large amounts of organic matter in and on the forest floor can provide suitable habitat and an energy/resource base to support a wide range of soil microbes to carry out various functions. Early work showed the production of volatiles in the forest floor material from radiata pine forests (Lill and McWha 1976), and recent work (e.g. Ping and Boland 2004; Insam and Seewald 2010; Maffei 2010) suggests that this could be a fruitful area for further research so as to better understand forest soil–plant interactions.

What we do not know very well is to what extent fertilisers (e.g. Gallart et al. 2018) influence soil microbial communities, and whether this both positively and/or negatively influences soil nutrient supply or microbe–plant interactions? There has been concern that fertilisers reduce soil microbial activity in terms of gene expression and can even reduce the mycorrhizal effectiveness or change the nature of species involved in the symbiotic relationship. Looking ahead, are there other new compounds that will have positive effects on microbial communities, stimulating greater soil microbial activity and enhancing plant growth?

New advances in molecular biology and plant and microbial sciences are creating an opportunity to further explore these interactions in a more rigorous fashion in order to inform forest management. The effect of soil properties and processes on cambial activity in plants is one area where this convergence of scientific disciplines may provide important insights and potentially ensure forests continue to provide a diversity of values.

Intensification pressures on soils

Chemical applications (e.g. herbicides, fungicides, pesticides, fertilisers, etc.) are fundamental to intensification, but there are increasing requirements to understand the environmental impacts of their use. Forestry is no exception with several recent studies examining questions around environmental fate and impacts of various chemical inputs to forests (Davis et al. 2012; Garrett et al. 2015b; Neary and Baillie 2016).

Chemical inputs can occur at various stages of the crop life cycle, pre- and post-planting and pre-harvest. Not only will the frequency increase, but the quantity and types of products applied may increase with climate change. Ultimately, it may not be the impacts of one particular application of one type of product on the environment that will be the issue, but attention may start to focus on the cumulative effects of multiple applications of a wider range of products applied over a longer timeframe. A key concern is the future ability of soils to buffer or filter the greater use of chemicals, particularly if their use alters the biological capacity to deal with them over time. Do we know enough about the short-term and long-term effects on soil biological activity to be confident in our predictions about long-term use? We do not have good intelligence about how markets/consumers will respond in the long term.

Environmental limits

The ability of forest soils to buffer the impacts of intensification of forest management will become increasingly important. As already stated, one of the goals of the NZFOA science and innovation plan (NZFOA 2012) is to improve productivity (double it) and consistency of wood in a sustainable way. To do this, new innovative approaches will be required such as novel biological agents to promote plant growth and manipulation of sites, including nutrient amendments, to increase site carrying capacity. As well as increasing productivity, these approaches will need to maximise wood quality and operate within environmental limits and maintain existing environmental values, e.g. Baillie and Neary (2015). Limits on catchment nutrient loadings will restrict current and future fertiliser use, particularly N and P (Davis et al. 2012). At the same time, international certification processes such as Forest Stewardship Council certification are challenging the use of a range of chemicals, including some Cu-based compounds used in forest management (Baillie et al. 2017). More research is required to avoid unnecessary restriction on forest productivity, but also to ensure environmental quality is not compromised.

New knowledge and understanding

When thinking about future forests and new plantings, we can anticipate the impacts of new genetics; new species choices; local, national and international policies; and climate change realities that will have many and varied influences on the supply of timber and non-timber products and services from forests. The ability of forest soils to accommodate all of these potential changes will be the key to any successful strategy.

Current efforts are focused on improving the genetics of existing species for superior tree growth and wood characteristics with a much lesser focus on other important plant traits such as roots, which are the main interface between plants and soil resources such as nutrients and water. Do we know enough about the interactions between tree roots and forest soils, in particular, mechanisms that enable plants to access important soil resources while avoiding environmental issues such as leaching of N? A lot of effort is spent on selecting plant traits (e.g. White et al. 2013) while leaving nature to determine the make-up of the soil microbial community. There is little effort on selecting and breeding soil microbes to improve the use of soil resources or enhance plant growth. Is this something that needs addressing to get more out of forest soils with fewer inputs?

I have raised the issue that we need to understand the role of soil microbes in signalling soil conditions to plants and to their tissues where the signal is translated into a growth response (Ping and Boland 2004). Ultimately we want to understand the role of soil microbes in communicating soil conditions to plants. Then we can influence plant growth in order to improve wood quality by manipulating the activity of belowground microbial communities using appropriate amendments that could include biostimulants, rhizobia and or mycorrhizal fungi.

Forest therapy – a walk a day keeps the doctor away

Finally, we also need to know more about the role of forest soils in the health of forests and how this links to the health of humans and our well-being. New understanding resulting from research into these links will undoubtedly lead to greater expectations of forest soils and changes in their management so as to further benefit human kind. There is increasing evidence that soils play an important role in human well-being beyond just the production of food and fibre. For example, Wall et al. (2015) outlined a range of pathways where land and soil management interact with the loss of soil biodiversity and function, leading to impacts on human health through increased risk of human diseases caused by human pests and pathogens, less nutritious food and by lack of clean water and air.

With respect to forests and relationship to human health, recent studies have shown linkages to human health (e.g. Donovan et al. 2013; Hansen et al. 2017). The concept of forest ‘bathing’ or Shinrin-yoku is a traditional Japanese practice of immersing oneself in a forest and taking in the forest atmosphere (Hansen et al. 2017). The benefits of forest bathing were recently reviewed (Cho et al. 2017) and although the focus has been on compounds produced by trees, in general, the role of volatiles from forest soils is not well understood nor is the potential to manage these emissions to further benefit human health. This is clearly an area where more research will bring new value to forest soils and require further consideration of the constraints and requirements of future forest management to provide the full range of benefits and services from forest soils.

Conclusions

Pressures on forest soils will continue to increase as expectations of the forests they support continue to grow. Shorter rotations and greater use of a range of inputs are becoming the new norm, and impacts of management and climate change are creating more frequent and enduring disturbances that directly impact on soils. Expectations of forest soils are changing and as a consequence new research will need to be initiated to address how these expectations can be realised in the context of forest management activities. What is clear is that we will not have all the answers overnight. Planted forests have a determinant life, which extends well beyond the life of any current funding mechanism. This creates some risk if we are to capture value from current research designed to look at long-term rotation length outcomes in terms of productivity of forests and quality of goods and services produced. To encourage more long-term research it is important to show the benefits from previous research examining long-term questions such as impacts of intensive harvesting on long-term site productivity (e.g. Smith et al. 1994, 2000). This nationally important trial series has recently been the focus of intensive sampling of soils and forest biomass pools with results reported to forest industry prior to scientific publication. Although these trials have addressed the original question, the challenge remains to find a mechanism to support research underpinning the ability of forest soils to meet future expectations and contribute to such issues as meeting global demand for forest and wood products and forest services and benefits that improve human well-being.

Acknowledgements

Opinions expressed in this paper are those of the author and do not necessarily present those of the New Zealand Forest Owners Association of Forest Growers Research.

Disclosure statement

No potential conflict of interest was reported by the author.

Additional information

Funding

This research was supported by the ‘Growing Confidence in Forestry’s Future’ programme, which is jointly funded by the New Zealand Ministry of Business, Innovation and Employment (contract No C04X1306) and the Forest Growers Levy Trust.