Left: In canopies of un-thinned or just moderately thinned mixed species stands (95% and 75% line, respectively) the sum of the crown cross section area mostly exceeds pure stands (100% reference ...line) due to multiple crown overlaps. Right: Mixing species with varying morphological traits can cause a selection effect (a–c) but beyond this may trigger a morphological plasticity which can lead to denser canopy space filling, completer light capture, and productivity gains of mixed versus pure stands (d–f). Display omitted
•Canopy space filling can increase with species number.•Complementary crown shapes may increase canopy space filling.•Mixing can modify allometry and inner space filling of tree crowns.•Denser canopy space filling can increase light interception.•Denser canopies may increase stand productivity and resilience.
Mixed-species forest stands are well explored in their favourable ecological, economical, and socio-economical functions and services compared with pure stands, but still poorly understood in their structure and functioning. Canopy structure and tree morphology affect the environmental conditions within the stand, the tree growth, and by this most forest functions and services. Here, I review how canopy structure and crown morphology in mixed stands can differ from pure stands and how this depends on the selection of tree species and interactions between them. The focus is on the macrostructure of canopy and crowns derived from the trees’ positions, their convex crown hulls, and their space filling with branches.
In mixed canopies the sum of the crown projection area, but not the ground coverage by crowns, mostly exceeds pure stands due to multiple crown overlaps. The interspecific differences in crown shape and allometric scaling cause a ‘selection effect’ when complementary species are combined. In interspecific environment furthermore ‘true mixing effects’ like intraspecific shifts in size, shape, and inner space filling of crowns may occur. The much denser and more plastic canopy space filling in mixed stands may increase light interception, stand density, productivity, and growth resilience to disturbances. I discuss the relevance of interspecific interactions for forest management, model building, and theory development and draw perspectives of further research into stand canopy and crown structure.
•Species interactions in mixed-species forests are dynamic, spatially and temporally.•Complementarity increased with decreasing soil N when interactions increased N.•Complementarity increased with ...decreasing water availability when interactions reduced competition for water.•In some stands complementarity increased with increasing site quality.•Few studies have examine the processes driving these dynamics.
Mixed-species forests and plantations sometimes have greater levels of ecosystem functions and services, including productivity, than monocultures. However, this is not always the case and there are many examples where mixtures are not more productive. Whether or not mixtures are more productive depends on the net effects of different types of interactions, and these are dynamic, changing through space and time. Many studies have examined how species interactions influence the growth of mixtures, but few have examined how spatial and temporal differences in resource availability or climatic conditions can influence these interactions. This review examines these spatial and temporal dynamics. The processes driving the dynamics are discussed using the production ecology equation, where plant growth is a function of resource availability, multiplied by the fraction of resources that are captured by the trees, multiplied by the efficiency with which the resources are used. Relative complementary effects depended on the types of species interactions and how resource availability changed. Complementary effects increased as soil nitrogen or water availability decreased when mixtures contained nitrogen fixing species, or when interactions were assumed to reduce competition for water. In contrast, some studies found that complementary effects increased with increasing site qualities, however in those studies there were no measurements of soil resource availability or any complementarity mechanisms. In those studies it was assumed that as growing conditions improved, competition for light increased and complementary effects resulted from interactions that improved light absorption or light-use efficiency. Multiple types of interactions can occur simultaneously in mixtures (e.g. nitrogen fixation, increased light absorption, and increased water-use efficiency) and so different resource availability-complementarity patterns will probably occur for a given pair of species, depending on the resource being examined. Less than half of the studies actually measured variables of the production ecology equation to indicate the processes driving the patterns. Several questions are listed that cannot yet be answered with confidence. Finally, stand structural characteristics, such as density, have also been shown to strongly increase or decrease complementarity effects and these need to be taken into account when interpreting results, but the mechanisms driving these density patterns were rarely quantified.
•We study tree-ring responses to climate of Abies alba in western European mountains.•We compare responses of pure stands to those of three different mixtures.•Climatic drivers are temperature at ...high elevation and drought at low altitude.•Mixture reduces A. alba sensitivity to summer drought.•Mixture effects depend on local climatic conditions.
In most dendroecological studies, climate–tree growth relationships are established for trees growing on pure stands. However, response to climate may be affected by inter-species interactions and local constraints, which beg the question of the effect of mixture on tree growth response under various ecological conditions. To assess these effects, climate–tree growth relationships of pure Abies alba stands were compared to those of three different mixtures: A. alba with Fagus sylvatica, with Picea abies and with both species. 151 stands (456 A. alba trees) were sampled in the Vosges mountains in north-eastern France under three contrasted climates, from low altitude and dry conditions (mean precipitation in July <85mm and altitude <600m) to high altitude and humid conditions (P July >115mm and alt. >900m). We sampled adult trees and homogeneous stand conditions to clearly assess differences in sensitivity to climate. Climate–tree growth relationships were evaluated from 12 A. alba chronologies (four mixtures×three climatic conditions) through pointer years and response function analyses. Late previous summer conditions and current summer soil water deficit and temperature played a major role on A. alba growth. Results showed greater sensitivity to temperature at high elevation, and to summer drought at low altitude and under dry conditions. Mixture allowed maintaining a higher level of A. alba growth during extreme climatic events and reduced A. alba response to summer drought especially under the driest contexts. Different facilitation processes may explain mixture effects such as changes in rooting depth, water input by stemflow and rainfall interception. This differentiated functioning of mixed forests highlights their importance for adapting forest management to climate change.
► We quantified stand-level pattern in terms of individual trees, clumps, and openings. ► We used AET and Deficit to develop climate analog reference conditions. ► We developed a method to translate ...reference patterns into prescription guidelines. ► Our ICO method created patterns in agreement with reference conditions. ► Reference spatial patterns offer a proven resilience strategy.
Stand-level spatial pattern influences key aspects of resilience and ecosystem function such as disturbance behavior, regeneration, snow retention, and habitat quality in frequent-fire pine and mixed-conifer forests. Reference sites, from both pre-settlement era reconstructions and contemporary forests with active fire regimes, indicate that frequent-fire forests are complex mosaics of individual trees, tree clumps, and openings. There is a broad scientific consensus that restoration treatments should seek to restore this mosaic pattern in order to restore resilience and maintain ecosystem function. Yet, methods to explicitly incorporate spatial reference information into restoration treatments are not widely used. In addition, targets from reference conditions must be critically evaluated in light of climate change. We used a spatial clump identification algorithm to quantify reference patterns based on a specified inter-tree distance that defines when trees form clumps. We used climatic water balance parameters, downscaled climate projections, and plant associations to assess our historical reference sites in the context of projected future climate and identify climate analog reference conditions. Spatial reference information was incorporated into a novel approach to prescription development, tree marking, and monitoring based on viewing stand structure and pattern in terms of individuals, clumps, and openings (ICO) in a mixed-conifer forest restoration case study. We compared the results from the ICO approach with simulations of traditional basal area and spacing-based thinning prescriptions in terms of agreement with reference conditions and functional aspects of resilience. The ICO method resulted in a distribution of tree clumps and openings within the range of reference patterns, while the basal area and spacing approaches resulted in uniform patterns inconsistent with known reference conditions. Susceptibility to insect mortality was lower in basal area and spacing prescriptions, but openings and corresponding opportunities for regeneration and in situ climate adaptation were fewer. Operationally, the method struck a balance between providing clear targets for spatial pattern directly linked to reference conditions, sufficient flexibility to achieve other restoration objectives, and implementation efficiency. The need to track pattern targets during implementation and provide immediate feedback to marking crews was a key lesson. The ICO method, especially when used in combination with climate analog reference targets, offers a practical approach to restoring spatial patterns that are likely to enhance resilience and climate adaptation.
► Most eucalypt plantations are managed in regions with high environmental stresses. ► Clonal plantations with interspecific hybrids strongly contributed to improve site-genotype adaptations. ► ...Continuous gains in productivity of eucalypt plantations have been obtained in Brazil. ► There are a number of risks associated with intensive, high yielding plantations. ► Integration of breeding and silviculture are imperative to sustain productivity.
Organized forestry in Brazil began in the late 1960s, stimulated by a government policy which subsidized afforestation programs from 1967 to 1989 to develop an internationally-competitive wood-based industry, managed by the private sector. Currently, planted forests in Brazil total about 6.9million ha, from which 4.9million ha is planted with eucalypt (around 25% of world plantation), 1.6million ha with pine, and 0.42Mha with other species. Roundwood consumption of forest plantations totaled 170.1millionm3 in 2011, eucalypt plantation accounted for 80.6% of this total.
Most eucalypt plantations are managed in short rotations (6–8years) and are established in regions with water, nutritional and frost stresses of low to high degrees. The mean annual increment is 40m3ha−1year−1 roundwood, ranging from 25 to 60m3ha−1year−1 depending on the level of environmental stress. Improving natural resources use efficiency by breeding and matching genotypes to sites and using appropriate site management practices is a key challenge to sustain or increase productivity.
The wide range of eucalypt species and hybrids with different climatic and edaphic suitability associated with the easy propagation by seeds and cloning allow the adaptation of plantations to various tropical and subtropical regions in Brazil. The possibility of using eucalypt wood in a range of purposes has led large and small enterprises to establish eucalypt forests for multiple uses. The desirable characteristics in association with the accumulated knowledge on eucalypt silviculture encourage the use of this genus in most plantations. The most important factors in the selective process for a genotype are wood characteristics, productivity level, susceptibility to pests and diseases, drought tolerance, especially in tropical regions (frost free), and frost tolerance in subtropical regions (mostly without water deficit). In regions with pronounced seasonality and moderate to long drought periods, the planting of hybrid genotypes predominates, propagated by cloning. Under subtropical conditions, the planting of single species predominates, propagated by seed. Clonal plantations with interspecific hybrids have been fundamental for eucalypt adaptation in regions under water and nutritional stresses. Given the rapid advances in eucalypt breeding, regarding adaptation to water stress and resistance to diseases and pests, and the adoption of clonal propagation techniques, genotypes are rapidly becoming obsolete and are replaced by more productive ones after harvesting. Thus, the replanting of crops has become a common procedure after the second half of the 1990s in Brazil.
This paper describes the basic requirements for integrating genetic and silvicultural strategies to minimize abiotic and biotic constraints in eucalypt plantations.
The use of plastic mulch in agriculture has increased dramatically in the last 10 years throughout the world. This increase is due to benefits such as increase in soil temperature, reduced weed ...pressure, moisture conservation, reduction of certain insect pests, higher crop yields, and more efficient use of soil nutrients. However, disposing of used plastic films, which cause pollution, has led to development of photodegradable and biodegradable mulches. Here we review the use of plastic mulches in agriculture, with special reference to biodegradable mulches. Major topics discussed are (1) history of plastic mulch and impact on crop yield and pest management, (2) limitations of polyethylene mulches and potential alternatives, (3) biodegradable and photodegradable plastic mulches, (4) field performance of biodegradable mulches, and (5) use of biodegradable plastic mulches in organic production. We found that (1) despite multiple benefits, removal and disposal of conventional polyethylene mulches remains a major agronomic, economic, and environmental constraint; (2) early use of photodegradable plastic mulch during the 1970s and 1980s, wrongly named biodegradable mulch films, discouraged adoption of new biodegradable mulch films because they were too expensive and their breakdown was unpredictable; (3) biodegradable plastic films are converted through microbial activity in the soil to carbon dioxide, water, and natural substances; (4) polymers such as poly(lactic acid), poly(butylene adipate-coterephthalate), poly(ε-caprolactone), and starch-based polymer blends or copolymers can degrade when exposed to bioactive environments such as soil and compost; (5) with truly biodegradable materials obtained from petroleum and natural resources, opportunity for using biodegradable polymers as agricultural mulch films has become more viable; and (6) the source of polymer and additives may limit use of some biodegradable mulches in organic production. More knowledge is needed on the effect of biodegradable mulches on crop growth, microclimate modifications, soil biota, soil fertility, and yields.
•Pre-fire field data was used to assess Rim Fire severity in frequent-fire forests.•Regardless of forest conditions, plume-dominated fire burned at higher severity.•Under less extreme weather, fire ...severity was mainly low or moderate.•Shrubs, time since last fire, elevation and burning index were related to severity.
The 2013 Rim Fire, originating on Forest Service land, burned into old-growth forests within Yosemite National Park with relatively restored frequent-fire regimes (⩾2 predominantly low and moderate severity burns within the last 35years). Forest structure and fuels data were collected in the field 3–4years before the fire, providing a rare chance to use pre-existing plot data to analyze fire effects. We used regression tree and random forests analysis to examine the influence of forest structure, fuel, fire history, topographic and weather conditions on observed fire severity in the Rim Fire, as estimated from an initial fire severity assessment based on the relative differenced normalized burn ratio (RdNBR). Plots that burned on days with strong plume activity experienced moderate- to high-severity fire effects regardless of forest conditions, fire history or topography. Fire severity was also highly negatively associated with elevation, with lower severity observed in plots over 1700m. Burning index (a composite index of fire weather), time since the last fire, and shrub cover had strong positive associations with fire severity. Plots that had experienced fire within the last 14years burned mainly at low severity in the Rim Fire, while plots that exceeded that time since last fire tended to burn at moderate or high severity. This effect of time since last fire was even more pronounced on days when the burning index was high. Our results suggest that wildfire burning under extreme weather conditions, as is often the case with fires that escape initial attack, can produce large areas of high-severity fire even in fuels-reduced forests with restored fire regimes.
Silviculture for old-growth attributes Bauhus, Jürgen; Puettmann, Klaus; Messier, Christian
Forest ecology and management,
07/2009, Volume:
258, Issue:
4
Journal Article, Conference Proceeding
Peer reviewed
Open access
Silviculture to maintain old-growth forest attributes appears to be an oxymoron since the late developmental phases of forest dynamics, described by the term old-growth, represent forests that have ...not experienced human intervention or timber removal for a long time. In the past, silvicultural systems applied to old-growth aimed to convert it into simplified, more productive regrowth forests substantially different in structure and composition. Now it is recognised that the maintenance of biodiversity associated with structural and functional complexity of late forest development successional stages cannot rely solely on old-growth forests in reserves. Therefore, in managed forests, silvicultural systems able to develop or maintain old-growth forest attributes are being sought. The degree to which old-growth attributes are maintained or developed is called “old-growthness”. In this paper, we discuss silvicultural approaches that promote or maintain structural attributes of old-growth forests at the forest stand level in (a) current old-growth forests managed for timber production to retain structural elements, (b) current old-growth forests requiring regular, minor disturbances to maintain their structure, and (c) regrowth and secondary forests to restore old-growth structural attributes. While the functions of different elements of forest structure, such as coarse woody debris, large veteran trees, etc., have been described in principle, our knowledge about the quantity and distribution, in time and space, of these elements required to meet certain management objectives is rather limited for most ecosystems. The risks and operational constraints associated with managing for structural attributes create further complexity, which cannot be addressed adequately through the use of traditional silvicultural approaches. Silvicultural systems used in the retention and restoration of old-growthness can, and need, to employ a variety of approaches for managing spatial and temporal structural complexity. We present examples of silvicultural options that have been applied in creative experiments and forestry practice over the last two decades. However, these largely comprise only short-term responses, which are often accompanied by increased risks and disturbance. Much research and monitoring is required still to develop and optimise new silvicultural systems for old-growthness for a wide variety of forest ecosystem types.
•Productivity of Picea abies and Abies alba were compared in mixed and monospecific neighbourhoods.•Inter-specific competition was often less intense than intra-specific competition.•For A. alba, ...complementarity increased with increasing maximum temperatures.•For P. abies, complementarity increased with decreasing maximum temperature, increasing minimum temperature and site quality.•The magnitude of all complementarity effects was amplified with increasing stand density.
Interactions between plant species can be dynamic, changing spatially and temporally with variability in climatic, soil and stand conditions. We examined how inter- and intra-specific interactions between Abies alba Mill. and Picea abies (L.) Karst. varied with climate, site quality and stand density in the Black Forest of south-western Germany, using spatially explicit neighbourhood indices. The mixing response, a measure of complementarity, was quantified as the increase in growth of individual trees in a mixed-species neighbourhood compared to a mono-specific neighbourhood. Both species benefited from growing in mixed-species neighbourhoods, but this complementarity effect (−60% to >200%) depended on climatic conditions, site quality and stand density. Complementarity increased for A. alba with increasing mean maximum temperatures in August, those for P. abies increased with mean minimum temperatures in May and site quality, and in each case the magnitude of the effect was amplified with increasing stand density. Complementarity is often considered to become more important in less productive ecosystems, but this study showed that for the given pair of species, complementarity effects can increase as growing conditions improve. A simple model is proposed that describes how relationships between productivity and complementarity change depending on the resources limiting productivity.
•We assess changes in forest spatial structure following fire exclusion and logging.•The historical forest had greater structural heterogeneity.•The contemporary forest lacked gaps, and was dominated ...by large, dense tree groups.•This homogenization of structure is likely associated with a loss of resilience.•Spatial measures are presented that may be applied to forest restoration treatments.
Fire suppression and past logging have dramatically altered forest conditions in many areas, but changes to within-stand tree spatial patterns over time are not as well understood. The few studies available suggest that variability in tree spatial patterns is an important structural feature of forests with intact frequent fire regimes that should be incorporated in restoration prescriptions. We used a rare dataset consisting of mapped locations for all trees ⩾10cm in three 4-ha plots in 1929 before logging and in 2007/2008, 78 and 79years after logging, to assess changes in three spatial components of forest structure: individual trees, tree clusters and gaps. Comparing 1929 old growth to modern conditions, area in gaps decreased from 20% to zero, the percentage of stems that were single trees from 6% to 2% and in small or medium clumps (2–9 trees) from 28% to 9%, while trees in large clumps (⩾10trees) increased from 66% to 89%. Concurrent with these changes, canopy cover increased from 45% to 62%, and the average number of trees in a clump increased from 11 to 26, resulting in much more homogenous conditions across the stand. These changes also altered tree size and species associations within different structural groups (i.e., single tree, small, medium and large clumps). In an effort to account for the alteration of the fire regime that had already taken place in 1929 (the last fire recorded in tree rings was in 1889) we also analyzed spatial patterns of 1929 conditions removing all trees <25cm. In this analysis, 35% of the plot area was in gaps and canopy cover averaged 36%. Tree clusters had an average of 5.2 trees per clump, with 13% of trees being singles, 30% in small clumps, 24% in medium clumps and 33% in large clumps. Our results provide metrics that quantify spatial patterns and composition of individual trees, tree clumps, and gaps under the historical fire regime that may be useful to forest managers. Our study demonstrates that the contemporary forest is more homogeneous than it was historically, and variability that likely provided diverse microclimate and habitat conditions and fostered resilience to a variety of stressors and disturbances such as fire, insects and drought has been lost. Future management may benefit from restoring these structural components.
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