Forest Structure Dynamics Research Articles

Using the tree growth model MOSES to assess the dynamics of Dinaric old-growth mixed beech–fir forest ecosystems

The limiting factor in studying the dynamics of old-growth forests is the lack of long-term data. A model for general Dinaric old-growth beech–fir forest ecosystem dynamic has not yet been fully developed. Only general schemes, primarily developed for natural forests in Central Europe, have been used in the research of old-growth forest dynamics. One example of a model for simulating growth of uneven-aged mixed-species stands is called MOSES (modeling stand response). The results given by MOSES indicate general trends of the structure dynamics (stand density, volume, diameter of breast height (dbh) distribution, mortality and other) over time. Comparisons of the predicted and measured dbh distribution show very good prediction capability for all species on the plot, when the period of simulation is up to 50 years. Regarding our results, we can conclude that MOSES is a useful tool for analyzing the complexity of old-growth forest structure dynamics and the resulting predictions could easily be improved by including local data for model calibration to address species-specific regional effects.

Carbon monoxide emissions in summer 2010 in the central part of the Russian Plain and estimation of their uncertainties with the use of different land-cover maps

This study is devoted to estimation of carbon monoxide (CO) emissions during the wildfires of the anomalously hot 2010 summer in the central part of the Russian Plain. CO emissions from the forest wildfires have been estimated with use of the Active Fires (AF) (MODIS MCD14ML) and Burned Areas (BA) (MODIS MCD45) methods for AVHRR/UDM, Global Land Cover 2000 (GLC 2000), GlobCover, and MCD12Q1 vegetation maps. A comparison of the vegetation maps and investigation of forest structure dynamics for the period from 2005 to 2009 have been carried out. It is shown that the major uncertainties during the estimation of CO in decreasing order are the following: distinctions in emission-calculation methods, differences in the vegetation maps used, differences in satellite data from Terra and Aqua, and the insufficient registration of forest structure dynamics. For additional comparison of estimations obtained by an independent method with the use of orbital (MOPITT, AIRS, and IASI) and ground-based (Moscow and Zvenigorod) spectroscopic measurements of CO content were presented.

Forest dynamics following eastern hemlock mortality in the southern Appalachians

Understanding changes in community composition caused by invasive species is critical for predicting effects on ecosystem function, particularly when the invasive threatens a foundation species. Here we focus on dynamics of forest structure, composition and microclimate, and how these interact in southern Appalachian riparian forests following invasion by hemlock woolly adelgid, HWA, Adelges tsugae. We measured and quantified changes in microclimate; canopy mortality; canopy and shrub growth; understory species composition; and the cover and diversity in riparian forests dominated by eastern hemlock Tsuga canadensis over a period of seven years. Treatments manipulated hemlock mortality either through invasion (HWA infested stands) or girdling (GDL) hemlock trees.Mortality was rapid, with 50% hemlock tree mortality occurring after six years of invasion, in contrast to more than 50% mortality in two years following girdling. Although 50% of hemlock trees were still alive five years after infestation, leaf area lost was similar to that of girdled trees. As such, overall responses over time (changes in light transmittance, growth, soil moisture) were identical to girdled stands with 100% mortality. Our results showed different growth responses of the canopy species, shrubs and ground layer, with the latter being substantially influenced by presence of the evergreen shrub, rhododendron Rhododendron maximum. Although ground layer richness in the infested and girdled stands increased by threefold, they did not approach levels recorded in hardwood forests without rhododendron. Increased growth of co‐occurring canopy trees occurred in the first few years following hemlock decline, with similar responses in both treatments. In contrast, growth of rhododendron continued to increase over time. By the end of the study it had a 2.6‐fold higher growth rate than expected, likely taking advantage of increased light available during leaf‐off periods of the deciduous species. Increased growth and dominance of rhododendron may be a major determinant of future responses in southern Appalachian ecosystems; however, our results suggest hemlock will be replaced by a mix of Acer, Betula, Fagus and Quercus canopy genera where establishment is not limited by rhododendron.

20th century tree‐line advance and vegetation changes along an altitudinal transect in the Putorana Mountains, northern Siberia

Kirdyanov, A. V., Hagedorn, F., Knorre, A. A., Fedotova, E. V., Vaganov, E. A., Naurzbaev, M. M., Moiseev, P. A. & Rigling, A. 2012 (January): 20th century tree‐line advance and vegetation changes along an altitudinal transect in the Putorana Mountains, northern Siberia. Boreas, Vol. 41, pp. 56–67. 10.1111/j.1502‐3885.2011.00214.x. ISSN 0300‐9483.Ongoing climatic changes potentially affect tree‐line ecosystems, but in many regions the observed changes are superimposed by human activities. We assessed how the forest‐tundra ecotone has changed during the last century in the Putorana Mountains, northern Siberia, an extremely remote and untouched area in Eurasia. A space‐for‐time approach was used to determine the spatio‐temporal dynamics of forest structure and biomass along an altitudinal transect. From the closed larch forest to the upper tree line, the mean age of Larix gmelinii (Rupr.) decreased considerably from 220 to 50 years ago. At the current upper species line, there is a strong and successful germination of larch, with 1500 saplings per hectare, indicating an ongoing filling‐in, a densification of a formerly open forest and an upslope shift of the tree‐line position (approximately 30 to 50 m in altitude during the last century). The forest expansion coincided with large increases in winter precipitation during the 20th century. In contrast, tree growth rates were significantly positively related to summer temperatures, neither of which increased markedly. The total aboveground biomass decreased from approximately 40 t ha−1 in the closed larch forest to 5 t ha−1 at the tree line. Our study demonstrates that ongoing climatic changes lead to an upslope expansion of forests in the remote Putorana Mountains, which alters the stand structure and productivity of the forest‐tundra ecotone. These vegetation changes are very probably of minor importance for aboveground carbon sequestration, but soil carbon data are needed to estimate the impact of the forest expansion on the total ecosystem carbon storage.

Effect of climatic and palaeoenvironmental changes on the occurrence of Holocene bats in the Swiss Alps

A large-scale palaeozoological study compared 45 14C-dated bat remains from the southern and northern Swiss Alps with palaeovegetational and palaeoclimatic data. Four thermophilous (warm-demanding) and four psychrophilous (cold-tolerant) bat species, mainly forest dwellers, were selected for the study. Myotis blythii is the oldest bat species recorded in the Alps, i.e. on the southern side, going back to the early Holocene at 10 500 cal. BP. Our study showed that thermophilous species (e.g. Myotis bechsteinii and Rhinolophus hipposideros) were most abundant during the Holocene climatic optimum in Central Europe (10 000—4000 cal. BP), when warm-demanding mixed forests were dominant. Psychrophilous species such as Myotis brandtii also occurred during the climatic optimum, but most of the samples fall into the onset of the late Holocene (Sub-Atlantic period), when summer temperatures were already declining. These species declined in the southern Alps after 4000 cal. BP, when fire was intensively used by humans to convert portions of the forest into open land. This fire practice modified forest species composition and structure, with effects on forest-dwelling bat communities. We conclude that during the early and mid Holocene bat community compositions mainly depended on climate and related vegetation and forest structure dynamics. With increasing land use during the mid and late Holocene, anthropogenic changes of forest composition and creation of open habitats increasingly co-determined bat-population dynamics in the Alps. These Swiss findings are in agreement with previous results from eastern Central Europe.

Species resistance and community response to wind disturbance regimes in northern temperate forests

Summary Severe winds are the predominant cause of natural disturbance in temperate forests of north‐eastern and north‐central North America. Conceptual models of the effects of wind disturbance have traditionally focused on the impacts of catastrophic disturbances and have painted a simple picture of how disturbance acts to maintain tree species diversity. These models ignore variation among species characteristics that could have important consequences for both resistance to and recovery from disturbance. We integrated an empirically parameterized, mechanistic model of windstorm mortality (WINDSTORM) and a seed‐mass‐based dispersal and recruitment model into a spatially explicit, individual tree model of forest dynamics (SORTIE) in order to create simulated long‐term ‘experiments’ designed to explore the sensitivity of forest composition and structure to species‐specific resistance to and recovery from disturbance. We found that species‐specific variation in resistance to wind mortality interacted strongly with: (i) shade tolerance characteristics, (ii) the medium‐term history of disturbances, (iii) the long‐term average severity of the disturbance regime and (iv) seedbed substrate dynamics to influence tree population dynamics and successional trajectories. We also examined how local and long‐distance dispersal affect response to wind disturbance. Ignoring differences among local dispersal characteristics overestimates the importance of dispersal‐limited species. Our results show that long‐distance immigration maintains species coexistence only if the immigration rate is very high relative to local dispersal. Despite this, stand‐scale models that ignore long‐distance dispersal can underestimate population dynamics of dispersal‐limited species. Our results indicate that landscape‐scale heterogeneity in structure and species composition in these forests is facilitated by synergisms between the stochastic nature of wind disturbance and complex interactions between species traits that govern: (i) resistance to disturbance, (ii) local dispersal and seedling establishment and (iii) competitive (i.e. growth/survival) traits that do not adhere to strict tradeoffs. We conclude that wind disturbance has much more complex and variable effects on long‐term dynamics of forest structure and composition than suggested by previous models. In particular, non‐catastrophic disturbances can generate important variation in forest dynamics that can either lead to dominance by a single or small number of species, or facilitate species coexistence.

A REVIEW OF FOREST GAP MODELS

Forest gap models, initially conceived in 1969 as a special case of individual-tree based models, have become widely popular among forest ecologists for addressing a large number of applied research questions, including the impacts of global change on long-term dynamics of forest structure, biomass, and composition. However, they have been strongly criticized for a number of weaknesses inherent in the original model structure. In this paper, I review the fundamental assump- tions underlying forest gap models, the structure of the parent model JABOWA, and examine these criticisms in the context of the many alternative formulations that have been developed over the past 30 years. Four assumptions originally underlie gap models: (1) The forest is abstracted as a composite of many small patches of land, where each can have a different age and successional stage; (2) patches are horizontally homogeneous, i.e., tree position within a patch is not considered; (3) the leaves of each tree are located in an indefinitely thin layer (disk) at the top of the stem; and (4) successional processes are described on each patch separately, i.e., there are no interactions between patches. These simplifications made it possible to consider mixed-species, mixed-age forests, which had been difficult previously mainly because of computing limitations. The structure of JABOWA is analysed in terms of the functional relationships used for formulat- ing the processes of tree establishment, growth, and mortality. It is concluded that JABOWA contains a number of unrealistic assumptions that have not been questioned strongly to date. At the same time, some aspects of JABOWA that were criticized strongly in the past years are internally consistent given the objectives of this specific model. A wide variety of formulations for growth processes, establishment, and mortality factors have been developed in gap models over the past 30 years, and modern gap models include more robust parameterizations of environmental influences on tree growth and population dynamics as compared to JABOWA. Approaches taken in more recent models that led to the relaxation of one or several of the four basic assumptions are discussed. It is found that the original assumptions often have been replaced by alternatives; however, no systematic analysis of the behavioral effects of these conceptual changes has been attempted to date. The feasibility of including more physiological detail (instead of using relatively simple parame- terizations) in forest gap models is discussed, and it is concluded that we often lack the data base to implement such approaches for more than a few commercially important tree species. Hence, it is important to find a compromise between using simplistic parameterizations and expanding gap models with physiology-based functions and parameters that are difficult to estimate. While the modeling of tree growth has received a lot of attention over the past years, much less effort has been spent on improving the formulations of tree establishment and mortality, although these processes are likely to be just as sensitive to global change as tree growth itself. Finally, model validation issues are discussed, and it is found that there is no single data source that can reliably be used for evaluating the behavior of forest gap models; instead, I propose a combination of sensitivity analyses, qualitative examinations of process formulations, and quantitative tests of gap models or selected submodels against various kinds of empirical data to evaluate the usefulness of these models for assessing their utility for predicting the impacts of global change on long-term forest dynamics.

Pattern and Process and the Dynamics of Forest Structure: A Simulation Approach

(...) Relationships of leaf area to sapwood area, stem respiration to sapwood volume and vigour to growth efficiency provide equations for tree growth, leaf-area dynamics and canopy structure. The model simulates mixed-age, mixed-species populations of trees on a 1000-m 2 patch. The test site, Fiby urskog (central Sweden), has been unmanaged since a catastrophic storm in 1795. (...)

Forest Structure Dynamics Following Wildfire and Prescribed Burning in the New Jersey Pine Barrens

Forest Structure Dynamics Following Wildfire and Prescribed Burning in the New Jersey Pine Barrens