Ecoleap: From ecophysiology to forest productivity

The ECOLEAP project has two main objectives: The first is to improve current knowledge of the links between environmental factors (temperature, fertility, etc.), physiological processes (photosynthesis, respiration, etc.) and forest productivity. The second involves using this knowledge to develop tools for predicting forest ecosystem productivity on a regional scale but with a spatial resolution fine enough to be of use to forest managers. These tools will enhance the monitoring of forest productivity at the level of entire regions, the integration of new technologies that harness digital remote sensing and geographic information systems and the evaluation of the impact of external events such as climate change and insect outbreaks.

UNDERSTANDING FOREST PRODUCTIVITY

Stand-level forest productivity is usually calculated from the site index, which is derived from the height and age of dominant trees. The site index incorporates the effect of the three components of forest productivity: vegetation characteristics, climatic environment and site properties. This method of calculation, albeit useful, is not really suitable for quantifying the effect of a change in growing conditions resulting from an insect outbreak, a modification of forest composition or climatic variation. The ECOLEAP researchers are working on developing more functional approaches, based on growth mechanisms, that can be used to determine the separate contributions that vegetation, climate and site make to forest productivity.

Measure of the amount of carbon captures by the roots.

Forest productivity is currently evaluated with reference to the net annual growth of merchantable stems of commercial species. To understand how the forest produces this volume of wood and how a change in the climatic environment can affect productivity, it is necessary to examine the processes of photosynthesis and tree respiration, as this is where the interactions between growth and the physical environment are centred . A major component of ECOLEAP therefore involves conducting field studies and modelling at the tree and plot levels. The goals of this research include enlarging understanding about how photosynthesis and respiration vary as a function of the species, temperature and soil dryness, how carbon captured by the foliage is allocated to the different parts of the tree and how this allocation affects the tree’s ability to assimilate resources from its environment.

Research is also undertaken to estimate the pools of carbon and nutrients that are present in the soil, along with the relationship between litter quality and nitrogen mineralization. All of these elements play a role in tree growth.

Carbon allocation is of particular interest, since the trunk has a lower priority in this regard than the rest of the tree. Fine roots, for example, absorb nearly half of the carbon fixed by the tree, whereas the trunk often receives only about a quarter. The allocation data acquired from several study sites are incorporated into predictive models of forest productivity.

	Net primary productivity Net primary productivity (NPP) is equal to photosynthesis minus respiration. This somewhat theoretical measure serves as a basis for understanding and quantifying forest productivity. NPP can obviously be modelled by taking account of photosynthesis by the foliage and respiration by the different tree constituents. Field verification of NPP requires making measurements of the above and below ground parts of trees. The above ground NPP of the trees in a stand is estimated from tree diameter growth values by applying allometric relationships that allow this increase to be correlated with the annual carbon allocation to the stem, branches and foliage. Allometric equations are established for this purpose through destructive sampling. It is also necessary to take account of the tree’s continual loss of above ground parts, including needles, twigs and branches. These above ground losses, which are not considered during destructive sampling, must be quantified and added to the estimate derived from the allometric equations so that the amount of above ground material that the tree has produced can be determined. Measure of photosynthesis on fir. Below ground NPP is largely invested in the continuous production of fine roots, which generally have a life span of only a few months. Fine root production can use up as much as 30% to 50% of the tree’s carbohydrate production. Measuring this productivity involves assessing the fine roots in the soil by taking cores and quantifying the root renewal rate using a minirhizotron. The minirhizotron is a device made up of transparent tubes that are inserted into the soil and a camera that takes photos of root growth, mortality and decomposition along the tube walls. Carbon released by the tree through root exudations and that expended by the tree to sustain mycorrhizae is not measured with this approach, but may represent up to 25% of the below ground allocation of carbon. The researchers quantify this allocation through measurements of soil respiration.

FROM LEAF TO LANDSCAPE

Example of forest productivity model.

To gain insight into the factors governing the productivity of forest ecosystems, studies need to be undertaken at the plot, tree, branch and leaf levels. In practice, however, the applications resulting from this work will apply to stands, landscapes and even regions. An important part of the ECOLEAP project centres on the development of spatial extrapolation tools. Basic research is being conducted at a series of sites scattered across the following bioclimatic domains mostly in Quebec and New Brunswick: sugar maple–basswood, sugar maple–yellow birch, balsam fir–white birch and black spruce–moss. The wide range of climates and species covered by these sites permits spatial extrapolation of the information obtained, that is, it can be used to identify broad temperature and precipitation gradients. Basic forest productivity models developed at the site level are thus applied to generate simpler functions that can estimate NPP on a regional scale for sites and stands for which little information is available. These regional models are used, on a stand-by-stand basis, to assess the effects of environmental conditions on forest productivity as a function of the species involved and certain physical characteristics of the site.

Three models have already been developed through this research work: FineLEAP, TreeLEAP and StandLEAP. FineLEAP is able to integrate the complex interactions that occur between foliage and crown light exposure and thus predict how photosynthesis will vary as a function of environmental conditions (light, temperature, moisture, etc.). Modelling of NPP (NPP = photosynthesis – respiration) at this scale provides a basis for calibrating simpler models that can compute NPP for stands or large regions. TreeLEAP is a stand-level model that can be used to take account of competition and mortality in calculating productivity following a silvicultural intervention such as thinning. StandLEAP is a tool that can use the values obtained from the other models to map and simulate the NPP of extensive regions.

Spatial data such as satellite images and digital terrain models play a key role in the ECOLEAP project. They provide inputs to models that can be harnessed to estimate forest productivity over vast expanses of territory. They can also be used to map certain forest properties and track changes in them over time.

PROMISING APPLICATIONS

Regional models, although simplified, need to be supplied with spatial information covering the entire field of application.

Measure of light transmission through forest canopy.

This need for spatial information has given rise to techniques whose potential for application exceeds the scope of the ECOLEAP project. One of these tools is a procedure for mapping above-ground biomass by drawing on inventory data and ecoforest maps. Two others versions of this procedure that are under development will use airborne or satellite remote sensing images to enhance the regional-scale estimates or to cover vast expanses of land.

Another application that is under development relates to detecting changes. Annual monitoring of forest productivity requires the ability to quickly map disturbances over extensive areas. ECOLEAP researchers are testing a technique based on differential analysis of satellite images taken over consecutive years. This technique will make it possible to generate digital maps illustrating the changes that have occurred in the forest landscape within a given area during the period between the image acquisition dates.

THE EFFECT OF DISTURBANCES

Computing commercial productivity from net primary productivity is not an easy task, because it requires the ability to estimate tree mortality in the stand. Whereas carbon fixation and the gross increase in timber volume can be estimated for a given stand, mortality cannot be generalized to the stand level easily. Studies are under way to improve the ability to predict balsam fir mortality following spruce budworm defoliation and self-thinning associated with competition for light. This research should aid in enhancing our ability to predict the impacts that insect pests have on forest productivity.

MULTIFACETED RESEARCH

Study of photosynthesis on trees.

The growth of a tree depends on a variety of factors, all of which are dealt with in the ECOLEAP project. Every aspect that is studied is essential to our understanding of productivity. Researchers must be familiar with the exchanges that occur at the level of leaves, branches, roots and trees as well as the plot and stand levels. All of these elements play a role in forest productivity and knowledge acquired about them goes towards the development of models that can be applied on a regional scale.

The work being done to enhance our understanding forest productivity is multifaceted and new aspects are continually coming to the fore. New needs and questions emerge as research proceeds, leading to knowledge acquisition. In the field, ingenuity and technology go hand in hand. Scaffolding is used to reach tree crowns; a track system may be improvised (using iron tracks and clothesline) to move the light measuring apparatus; cameras are adapted for use in monitoring root development; special devices are used to measure soil respiration; and probes are inserted into tree trunks to measure the rise of sap and transpiration. All of this equipment is part of the researchers’ day-to-day tools.

TEAMWORK AND PARTNERING

The ECOLEAP project involves teamwork and partnering. In moving from the conceptual phase to the operational phase, the researchers stay in close contact with their research partners (provincial, national and international) and with forest managers. These contacts and exchanges help to ensure the scientific quality and relevance of the work, and pave the way for the gradual integration of ECOLEAP-derived concepts and tools into forest management operations.

	Ecophysiology Ecophysiology is the study of the interaction between ecological factors and the physiological processes of living beings. Measure tree transpiration. Ecophysiologists who study forests look at how trees respond to their environment. The ecophysiology of photosynthesis is an area of study in which theoretical leaf-level notions are applied to enhance understanding of the mechanics of sunlight capture and use by tree crowns and to quantify climate change impacts on photosynthesis, including changes in the length of the growing season. Studies of this sort enable researchers to devise growth models that link environmental factors and forest productivity.

A NATIONAL NETWORK OF RESEARCH SITES

A national network of research sites has been established for ECOLEAP. Data series are available for most of these sites, allowing the attainment of the project objectives. The project leaders have also created a Pilot Region Network in order to give forest modelling experts and forest managers access to an information base containing a wealth of field observations and reference data on forest, climate, soil and topography topics. The network consists of six regions covering an area of up to 4,000 km² in Newfoundland, Quebec, Saskatchewan and Alberta.

The pilot regions will play an important role in the development, evaluation and validation of new forest modelling methods. They provide sufficient precision for updating models, a solid foundation for generalizing principles and the capacity to assess errors. They are also necessary for developing operational procedures to bridge the gap between fine and coarse scale data sets. The role of pilot regions is therefore twofold: they are used to demonstrate the validity of newly devised methods and they provide a practical link between the local scale (tree and stand) and the regional scale.

	Biophysical site index Developing practical tools is a key goal of ECOLEAP. Collaborative work with the Quebec Department of Natural Resources has enabled project researchers to use the databases of the provincial permanent and temporary sample plot network in developing a biophysical site index. This mappable index is estimated from a soil texture index, a stand structure variable and climatic indices. The biophysical site index has been tested on seven commercial species in Quebec: yellow birch, white birch, trembling aspen, sugar maple, jack pine, balsam fir and black spruce. It can be used in many cases to estimate volume growth with a level of precision similar to that obtained with the conventional site index, which is based on measurement of the height and age of dominant trees. Modifications that are currently being made to simplify the use of this new biophysical index.

	The ECOLEAP team Approximately 12 people at the LFC are working on the ECOLEAP project. The lead researchers are Pierre Y. Bernier, F Eng PhD, Huor Ung, F Eng PhD, Gilles Robitaille, PhD, David Paré, F Eng PhD, Frédéric Raulier, PhD and Robert Boutin, F Eng. They are assisted by a team of professionals and technicians in fields such as geomatics, remote sensing and statistics. Other researchers, from universities and provincial, national and international research institutes, are involved in this undertaking as well.