Klaus v.Gadow

a Faculty of Forestry and Forest Ecology, Georg-August-University G?ttingen, Büsgenweg 5, D-37077, G?ttingen, Germany

b Department of Forestry and Wood Science, Faculty of AgriSciences, Stellenbosch University, Matieland, South Africa

ABSTRACT The objective of this study is to introduce a coherent theoretical perspective for sustaining forested landscapes at different scales: meta(forest region),gamma (forest property)and alpha(forest stand),with some details on practical implementation.The main part introduces an integrated set of approaches under three headings:a)gamma design, b) alpha jardinage, and c) continuous observation.The study is based on evidence gathered in the broadleaf and conifer mixed forests of north-eastern China,in European temperate forests,in various forest regions in India,in the afro-montane forests of Southern Africa,and in the pine-oak forests of the Mexican Sierra Madre Occidental.

1.Introduction

The post-World War II economic boom was a period of rapid economic development.Many countries experienced unusually high growth rates and full employment.Timber was in high demand,and the focus of forest management was on wood production.More recently, the emphasis has shifted from sustainable production towards sustaining forest resilience.Many of today's forest ecosystems are the direct result of the production-oriented policies of the 1950s and 1960s.Trees take a long time to reach maturity,and this is the reason why the time-lag between a policy change and the desired effect on forest structure and species composition is so extensive.

It is widely accepted that any deviation from Holocene standards will present human societies with great challenges (Albert et al., 2016).This realization gives rise to speculation about quick fixes and simple solutions.Efforts aimed at forest restoration include invitations to plant a million trees (https://planet-tree.de/?gad=1&gclid=Cj0KCQjw4s-kBhDq ARIsAN-ipH20fcOxI-_2LZlf5AkpN-eD-im9d6kNCM3-9JZ_XCTQ1Q4VQy 6w80waAqpoEALw_wcB), or even a trillion trees (https://www.trilli ontreecampaign.org/).More demanding, and ultimately much more rewarding is any active engagement in solving the complex problem of sustaining existing natural or planted forest ecosystems.In a comprehensive review, Ellis (2021) examines evidence of human land use from archaeology, paleoecology and environmental history that reveals major transformations of our planet.Because of human activities, the current biosphere differs significantly from that of the Holocene.The atmospheric carbon dioxide concentration,the rate of terrestrial biodiversity loss,and the changes in land use are threatening the ecological resilience and productivity of forest ecosystems(Steffen et al.,2015).

1.1.Biomes and anthromes

The anthropogenic transformations of the biosphere may be interpreted as disturbance or as a process of cultural evolution.Such interpretations are expressed by specific paradigms.1Weaver and Clements(1938) pictured the natural world as an orderly system of well-defined successions towards a “climax”, a state of long-term stability.The Biomes paradigm implies that the biosphere is shaped by the biophysical environment.Human influence is characterized by impact or footprint.Sustaining the biosphere means minimizing human impact.The ideal world is one without humans.

The Anthromes paradigm offers the view that the biosphere is shaped by Humans.Ecosystems are primarily a function of population density and affluence.People's priorities differ depending on where they live.Environmental issues and sustainable development have a high priority in affluent societies.Subsistence issues and economic development are important in regions with a low per capita income (Eisenmenger et al.,2020).Humans manipulate and modify the environment, and these activities determine the particular mix of services and benefits that ecosystems provide.The two paradigms offer contrasting yet complementary points of view.Together they present a fairly realistic perspective of the threats, and the opportunities, for sustaining forest ecosystems.

1.2.New forest strategies

A traditional view of a managed forest is that its development follows a perpetual series of growth cycles that are defined by a rotation and a yield table.A cycle begins with the planting of trees and ends with the harvest of the“final crop”.Productivity is measured in terms of the mean annual increment at rotation age(MAIR).The present value of a specific silvicultural program is equal to the sum of the annual net incomes,discounted to the age of establishment.2Refer to Clutter et al.(1983), Chang (1998) and Chang and Gadow (2010)for economic criteria used to evaluate alternative forest development trajectories in plantations and natural forests.During the past 300 years, the theory and practice of sustaining forest ecosystems were based on yield tables (Paulsen, 1795).The result of this history is a world covered by extensive areas of “yield table forests”, - ecosystems that have been managed following the long-term trajectories of a yield table.Standards are agreements on technical specifications that are used as rules or definitions to facilitate manufacturing, trade, and communication.Optimizing standard silviculture(planting,pruning,thinning,rotations)was an important research topic in many forested regions,especially between the 1960s and the 1980s(Brodie et al.,1978).Standardized silviculture,however, creates uniform habitats and may lead to reduced ecosystem stability and resilience.Adapting to changes of the climate, and public policy, requires a re-appraisal of long-term silvicultural trajectories and yield table standards.

Since its inception in 2015, the 2030 United Nations Agenda for Sustainable Development developed a blueprint for a sustainable world.Timber production should be complemented by other objectives.Especially important is the objective of sustaining ecological resilience.On July 4, 2022, a dialogue began in the German Federal Ministry of Food and Agriculture(BMEL)to prepare a new“forest strategy”for the Federal Government (Waldstrategie, https://www.bundestag.de/dokumente/t extarchiv/2022/kw27-pa-landwirtschaft-901206).The new strategy,intended to be in place by summer 2024,is meant to provide the basis for a new version of the Federal Forest Law.In a public debate on the subject“Adapting forests and forestry to climate change”, it was agreed that there had to be a process of “forest transformation”.The specific proposals varied, but there was general agreement that self-regenerating multi-species forests are preferred over planted monocultures.Such changes in environmental policy represent a considerable challenge for science.

Accordingly,the objective of this study is to present a perspective for sustaining forest ecosystems that is (1) universal (applicable to any forested landscape); (2) practical (useful for guiding local forest conservation and management) and(3)plausible(based on evidence).

2.Scale effects of forest design

Resilience is the capacity of an ecosystem to successfully adapt and recover from some adverse condition.Evidence shows that the resilience and productivity of a forested landscape increases with increasing diversity and structural complexity (Lian et al., 2022), and with greater spatial asynchrony of individual communities within a forest region(Qiao et al., 2021, 2022).Following Whittaker's (1972) hierarchy, this section introduces the multiple path (MP) approach, a hierarchical concept for sustaining the productivity and resilience of a forested landscape(gamma scale),the jardinage approach for sustaining individual forest communities(alpha scale),and some details of new observational infrastructures for gathering necessary evidence about forest structure and dynamics.

2.1.Gamma scale design

The fixed treatment schedules and long-term yield table trajectories of traditional forest management are unsuitable for meeting the challenge of change.Messier et al.(2019) consider forests as complex adaptive systems that require careful analysis and continuous re-appraisal within realistic time windows.A practical theory and effective tools are essential for designing forested landscapes in response to human demand and changing environmental conditions (Tress and Tress,2001).

In the context of this study, a forested landscape is meant to be a wooded land area in private, community or public ownership (gamma scale).The landscape typically consists of a mix of local communities(alpha scale) that are commonly known as “stands”.The current condition of the landscape is the result of the public policies and ownership preferences of the past.Any future design of the landscape as a whole presents an opportunity to adapt to new challenges and assumed risks.The most important design phase involves the framing of a number of possible development trajectories, or “paths” for each stand within the landscape.Each development path in each stand defines a specific sequence of activities.The objective of the multiple path(MP)theory is to develop multiple trajectories of all the alpha units in the landscape.The optimum combination of paths within the landscape depends on current preferences and practical constraints.A concise description of the MP theory was given by Pukkala and Kangas(1993):

where U is the utility of a given path combination;n refers to the number of products/services considered in the design;wiis the relative weight of service i（0 ≤wi≤1;∑mj=1wi=1）;qiis the realized amount of service i for a given path combination;ui(qi)is the partial utility function for service i（0 ≤ui（qi）≤1）.Alternative formulations and refinements of the basic concept include period weighting and various formulations as a mathematical programming model.The MP model represents a spatio-temporal design that provides a desirable mix of products and services for the landscape(or ownership)as a whole.Fig.1 presents a simplified diagram to illustrate the essential elements of the MP theory: the hierarchy(gamma landscape; alpha stands), and the flexibility, i.e.alternative development paths for each individual stand.

An extensive forested landscape of strategic importance is the(meta)region Madrense in Mexico,an important ecotone of the Mexican Sierra Madre Occidental that includes at least 24 species of Pinus,54 species of Quercus and 7 species of Arbutus(González-Elizondo et al.,2012,Fig.2).The natural Pine-Oak forests of the Madrense region exhibit a range of complex structures and changing spatial patterns.Most of the forest land is owned by local communities known as egidos or comunidades.Surprisingly, after more than 100 years of selective harvesting by the local communities, the natural tree species composition was found to be similar to that existing in undisturbed forests(Corral-Rivas et al.,2016).Fig.2 also presents a map of individual stands (alpha scale) within the Egido Tambores(gamma scale)of the Municipio San Dimas in the State of Durango/Mexico.The stands are identified by different colouring.

Another meta scale landscape of strategic national importance is the north-eastern forest region of China.The region includes several mountain ranges with cold temperate coniferous forests, temperate mixed coniferous and broad-leaved forests, and warm temperate deciduous broad-leaved forests (latitude 39°42′48′′-53°19′21′′N, longitude 119°48′1′′-134°01′0′′E).New research based on an extensive network of permanent observational field plots, reveals the important contribution of “asynchronous” response to change.Species asynchrony refers to the discordant responses of species with distinct functional traits to environmental fluctuations at the alpha scale.Species asynchrony rather than richness determines ecosystem stability (Yue et al., 2022).Spatial asynchrony refers to the variation in the aggregate responses to disturbance among the alpha communities at the gamma scale.Communities with different species compositions respond more effectively (“in asynchrony”) to the common environment than communities with similar species composition(Wang and Loreau,2016;Qiao et al.,2022,2023;He et al., 2022;Lin et al., 2023).Refer to Wang et al.(2019)for a detailed description of the statistical methods.Such evidence confirms the importance of hierarchical MP designs.

Fig.1.Simplified diagram to illustrate the essential elements of the MP theory.(a) Map of a hypothetical forest landscape/property (Whittaker's gamma scale) that includes three alpha communities:stands A,B and C.(b)Two possible development trajectories(“paths”)for each stand are shown in different colors.(c)Two paths in three stands add up to 23 possible developments of the landscape as a whole.One of the 8 combinations will be optimal in terms of the risks and preferences, the resilience and productivity, and any particular restrictions specified by the land owner (graphic adapted from Hinrichs (2006)).

Fig.2.(a)Forested landscape of the Mexican Sierra Madre Occidental(meta scale)that provides essential goods and services of strategic national importance.(b)Map showing individual stands(alpha scale)within the ownership of the Egido Tambores(gamma scale)of the Municipio San Dimas in the State of Durango/Mexico(Both maps were kindly provided by Prof.Javier Corral-Rivas of the Universidad Juárez del Estado de Durango).

2.1.1.Other natural landscapes

The MP theory is applicable to a wide range of landscapes including natural forests, novel ecosystems, coastal dune landscapes, mountain catchment areas, and plantation forests.The following three examples illustrate the generality of the theory.

Coastal dune landscapes are important because of their buffering role against shifting dunes that threaten adjoining agricultural land.Coastal dune landscapes are often affected by recreation, urban development and exploitation of sand and mineral resources.A multiple path design of a dune landscape may typically involve four distinct phases:(1)stratification and spatial subdivision of dune types(e.g.tidal beach areas,hummock dunes, protected dune valleys); (2) definition of alternative path trajectories(e.g.branch packing,seeding hummock dunes,planting of saltbushes and other suitable species, fencing of dune valleys); (3)establishing an optimum design by finding the most appropriate path combination given a set of objectives and local constraints.

Mountains.Early criticisms of the effects of forest plantations on water supply led to the establishment of large-scale and long-term paired catchment experiments in South Africa to observe post-afforestation trends in evapotranspiration (Bosch and Gadow, 1990).Uncontrolled fires occur quite frequently.A scientific design can be helpful to reduce fuel loads and to increase the variety of vegetation types and the mix of successional stages of the natural flora.When considering a spatially explicit design of a mountain ecosystem, two types of information are useful, (1) a spatial organisation of alpha communities, and (2) a technical classification of the vegetation(e.g.,in the Suurberg Mountains of South Africa: Tristachya-Themeda veld; Festuca-Myrsine veld; Bobartia--Trachypogon veld; Erica-Restio veld invaded by Pinus radiata).Common objectives may include: removal of alien vegetation; increasing water yields by creating a spatial mix of successional stages;reducing fuel loads and preventing uncontrolled fires.

Planted forests.The Indira Gandhi Canal in India provides water for agriculture and human consumption in remote areas of western Rajasthan.Drought resistant tree species, including Acacia senegal, Acacia nilotica,Prosopis cineraria,Tecomella undulata and Salvadora persica,were planted on either side of the canal to protect the water course against shifting sand dunes.The main canal as well as subordinate canals provide an essential water supply to small villages.The tree plantings along the water course were subdivided into blocks(alpha scale)covering an area of three ha(300 m along the canal×100 m extending into the desert).In a pilot study involving selected blocks planted with Tecomella undulata of varying density and age, Tewari and Gadow (2010) developed an MP design with the aim to improve the quality of harvested wood(density,energy content),and to ensure an even flow of wood yields for the local communities.Constraints included a minimum age of 21 years for harvested trees and a limit on the annual harvest area.

The multiple path approch is not a new concept.It is a robust theoretical framework for designing forested landscapes that has been implemented in different parts of the world.A range of application examples is presented in Gadow and Pukkala(2007).

2.2.Alpha scale jardinage

Two and a half centuries ago,the French term jardinage3An appropriate English equivalent may be“gardening”or“forest gardening".appeared in the literature to refer to a method of harvest control that differed fundamentally from the yield table systems developed for planted forests(Schütz et al., 2012).Dralet (1820) in the French Pyrenees found that clearfelling was detrimental to soil conservation,and that it was essential to maintain a continuous forest cover.Gurnaud (1882) introduced a method of control to ensure sustainable harvesting in the communal forests of Couvet in Switzerland.The approach, involving the periodic removal of trees in specific diameter classes, was refined by Biolley(1901),and became later known as the Plenterwald system.

2.2.1.Yield table forests and jardinage forests

The objective of yield table forest management is long-term sustainable wood production.The objective of jardinage forest management is short-term re-adjustment towards a desirable residual state.De Liocourt(1898) found that it was possible to derive a frequency distribution of tree diameters (dbh's) that closely resembled those found in selectively managed forests near Gérardmer in France using the mathematics of number sequences.Many uneven-aged forests exhibit a negative exponential dbh-class distribution,also known in the literature as an inverse J-shaped distribution.In a balanced inverse J-shaped distribution, a constant quotient q exists between successive dbh classes(Virgilietti and Buongiorno, 1997; Cancino and Gadow, 2001; Picard and Gasparotto,2016).This quotient defines the curve's shape and permits the calculation of an ideal number of trees for each dbh class in a forest with a given basal area.A harvest is defined by the difference between the“real”and the“ideal” number of trees in the different tree diameter classes.

A yield table estimates the removals and defines the long-term development of a forest.Jardinage systems define the “residual”.The objective of the jardinage approach is to define, and periodically reestablish, a desirable post-harvest residual community through shortterm structural adjustments.Numerous examples have been published to illustrate the method.Guldin(1991),for example,calculated the ideal dbh distribution for a forest using a target basal area(B=180 ft2·acre-1),a maximum residual dbh (36 inches), a q parameter (1.21) and a dbh-class width of 2 inches.Natural forests display a great variety of diameter distributions (Korpel, 1995; Westphal et al., 2006).In a much-cited critical analysis, Mitscherlich (1952) found a wide range of balanced uneven-aged forest structures and concluded that the concept of an ideal diameter distribution, as applied in the Liocourt guide curve model, has no biological foundation.Can the jardinage approach be extended to make it more ecologically relevant?

2.2.2.Extensions of the jardinage approach

An advantage of the jardinage approach is the reduced planning horizon that allows short-and medium-term adjustment following changes in environmental conditions or owner policy.The original idea of the Plenterwald residual normality is a practical concept, but the focus is on tree size which limits the potential.Greater resilience and improved production can be achieved by gradually steering the system towards desirable species mixtures, by including specific habitat requirements(Messier et al., 2019), and by maintaining minimum residual densities(Corral et al.,2018).

An interesting variant of the original jardinage approach was developed in the Southern Cape forest region of South Africa.The Southern Cape Afrotemperate Forests are part of the Afromontane group of forests(White, 1978; Mucina and Geldenhuys, 2006).The region is of considerable ecological interest because of the unique composition of tree species (Hao et al., 2021).Main canopy species are Pterocelastrus tricuspidatus, Podocarpus falcatus, Pharus latifolius, Psydrax obovata, Olea capensis subsp.macrocarpa, Apodytes dimidiata, Curtisia dentata and Ocotea bullata(Seydack et al.,2011;Seifert et al.,2014).A specific jardinage system was developed during the 1960s and 1970s to sustain the diversity and productivity of these unique forests(von Breitenbach,1974).A minimum residual density was prescribed to sustain production.Four tree cohorts were defined to sustain diversity.The tree classes were assigned Roman numerals: I = mature trees of commercially important species that have reached a minimum (species-specific) threshold diameter; II = immature trees of commercially important species; III =other non-commercial tree species; IV = undesirable invasive species(Fig.3).

Obviously,a specific classification of tree cohorts needs to be developed for local conditions and specific objectives.Lujan-Soto et al.(2015)reviewed different approaches of simplifying species-rich forest communities.A height-growth ordination that had been used in previous studies, provided the motivation for a new method of vertical stratification, based on the separation between canopy and permanent subcanopy species.The canopy species group was further subdivided into mature and immature individuals using the relationship between diameters and heights within a bivariate mixed normal distribution(Zucchini et al., 2001).The shrub layer was neglected in the temperate pine-oak forests of Durango/Mexico, but that layer was found to be functionally quite important in the Southern Afromontane forests and the multi-species temperate forests of north-eastern China.The particular trait mix of the residual tree species defines the community resilience.Community production is primarily affected by the residual biomass density(Corral-Rivas et al.,2018).Conservation objectives may include certain habitat requirements, e.g., deadwood or certain “habitat trees”.Examples of extensions of the original jardinage approach were presented for temperate forests in China and Mexico (Gadow et al., 2013) and Europe(Gadow,2020).

3.Forest observational infrastructures

Forest observational infrastructures include forest inventories, field experiments,and special sites for continuous observation(Gadow et al.,2021).The objective of a forest inventory is to assess the state of the forest resource,usually at regular intervals,and to prepare a report with details about the state of the resource for a given area and point in time.Examples are local or national forest inventories(McRoberts et al.,2007;Tomppo et al., 2010; Zeng et al., 2015; Alekseev et al., 2019).Field experiments are established to test a response to active manipulation.Treatments may include different planting densities or fertilizer applications (Burkhart and Tomé, 2012), analyses of competition, structure and species mixtures (Kramer, 1988, p.97; Lee, 1993; Vanclay, 1994;Crawley,2005;Pretzsch,2009),effects of afforestation on water yield in mountain catchments(Bosch and Gadow, 1990).

Continuous observation provides information that is useful for understanding ecosystem structure and functioning.Observations are collected without manipulating the system.To evaluate the response to specific disturbances such as a forest fire or a harvest event,assessments should immediately follow a disturbance.The selection of sites for observational field plots is not random or systematic as in forest inventories, but based on particular data requirements and objectives.Permanent observational field plots are usually larger than sample plots,and measurements are usually more detailed.India has a long tradition in establishing and maintaining forest observational field plots.Numerous plots were established during the 1930s in Bihar, Karnataka, Kerala,Tamil Nadu, Uttar Pradesh, Andhra Pradesh, and during the period 1950-1990 in Assam, Uttar Pradesh, Karnataka, Kerala, Orissa and Maharashtra(Tewari et al.,2014).Three designs were recommended for the Forest Survey of India(Gadow,2017):

· “Small”monitoring plots:Circular plots of 0.1 ha to capture spatiotemporal variations of vegetation,altitude,soil conditions and human disturbance.Using this design, an extensive network of more than 400 circular plots of 0.1 ha each is maintained in north-eastern China(Qiao et al.,2022;He et al.,2022).

· “Medium-sized”monitoring plots:Square plots of 1 ha allocated to different forest types, with due consideration of particular local requirements to capture detailed forest structure and to record the changes through time, including species area relations and species abundance distributions.This design has been widely used, for example in dry tropical forests (Remadevi et al., 2023) and for comparing natural forests in Iran, Ukraine, Germany, Italy, Mexico,Peru,China,India,and South Africa(Hao et al.,2021).

· “Big”monitoring plots:Square plots of 4 ha each,allocated to major forest types in collaboration with specialised university departments to study scale dependent phenomena and dynamics.Very large observational infrastructures have been established in China to study maximum density patterns and scale effects in temperate natural forests(Zhang et al.,2015).

During field work it was found that one “small” plot can be established and assessed within one day by an experienced field crew.The initial establishment of a“medium-sized”and a“big”plot required about a week and a month of field work,respectively.New remote technology may eventually replace the laborious field work.Especially important are intelligent systems for reliable identification of tree species.

The ultimate aim of all field studies is to provide external validity,the ability to generalize from a limited set of observations.Traditional forest plots are often too small to capture scale effects on community structure(Zhang et al., 2010) or species-habitat associations and demographic variation (Zhang et al., 2012).Forest observational sites have been designed to study seedling survival (Wang et al., 2014), reproductive allocation of dioecious tree and shrub species (Yan et al., 2015), neighbouring competition and habitat effects of a dioecious tree species(Zhang et al., 2009), or effects of density and structure on forest production(álvarez-González et al.,2014;Corral-Rivas et al.,2018).

The uneven-aged communal forests of the Mexican Sierra Madre Occidental have been subject to Continuous Cover Forestry (CCF) with selective harvesting for more than a century.A monitoring network called Monafor (“Monitoreo Nacional Forestal”, covering an area between 16.02°-30.33°N and 87.77°-108.68°W) was developed with support by the National Forestry Commission Conafor to ensure that tree harvesting is sustainable.In addition, the Monafor system provides the infrastructure for data storage and to generate information for the development of forest models.The database is available online at http://forestales.ujed.mx/monafor.Examples of forest models are local and generalized height-diameter relations in mixed uneven-aged forests(Crecente-Campo et al.,2014),studies for developing tree species cohorts(Lujan-Soto et al.,2015)and new biomass models(López-Serrano et al.,2015).

Fig.3.(a) Schematic illustration of the currently existing basal area distribution of four tree cohorts: I = mature trees of commercially important species that have reached a minimum (species-specific) threshold diameter; II = immature trees of commercially important species; III = other non-commercial tree species; IV =undesirable invasive species; (b) Thick lines show a desirable residual basal area distribution for each group; (c) The removable surplus is shown in red color.

4.Discussion

There is a fairly clear distinction between opinion, hypothesis and theory.An opinion is a statement of personal belief that has not been,or cannot be,tested.A hypothesis is a testable assumption that is often based on some preliminary evidence.A scientific theory is an idea that explains observed phenomena and that is based on one or several hypotheses that could not be rejected.The attributes of a useful theory are (1) universality(applicable to any arbitrary situation);(2)practicability(useful for guiding practical applications);and(3)plausibility(based on evidence).It is through the testing, refinement, and consolidation of theories that scientific disciplines develop.This requires a shift of perspective from‘theory’as an accepted statement to theorizing as an iterative process of advancing knowledge.In Goethe's Faust Mephisto says to the pupil4Goethe,J.W.v.(1962).Goethes Faust(W.Kaufman,Trans.).Anchor Books.:“All theory is gray,my friend.But forever green is the golden tree of life”.The contrast of gray and green creates an impression of the remoteness and redundancy of theory.Yet theory can be useful in identifying commonalities and essential characteristics among many specific instances and thus become instrumental in guiding action based on improved understanding.

4.1.To act or not to act

Since the 1960s,environmentalists have been campaigning to create protected areas that are entirely free from human intervention.Some of the arguments were based on the Western scientific mindset that saw“nature” as being largely devoid of human influence (Wapner, 2014;Purdy, 2015).It is widely recognized today that humans have always been an integral part of ecosystem dynamics.Active management can be effective in safeguarding the continued viability of the world's forest ecosystems,for a number of reasons:

Hazard potential:Today,much of the northern Mediterranean arc is a flammable forested landscape due to the lack of professional land management.According to WWF(2019),there is only one effective way to counter the growing risk of forest fires, and that is to focus on prevention through active management and risk planning.

Reduced biodiversity: The protection of Bia?owie˙za National Park resulted in a severe reduction of biodiversity because only very few tree species were capable of withstanding the heavy browsing pressure of the large herbivores that are protected(Kuijper et al.,2010).

Productivity and resilience:Old trees provide essential habitat for a variety of organisms thus fulfilling unique functions.However,as forests age,the trees develop a syndrome of changes(reduced leaf area,reduced photosynthetic activity,possible changes in allocation)that acts to lower primary production (Ryan et al., 2008).Active management can be effective in enhancing forest productivity, and at the same time preserving old trees, deadwood, and biological diversity for improved resilience.

Ecological lag time:The time-lag between a major ecological impact(fire, drought, bark beetles) and the recovery of an ecosystem to its adaptive capacity and functioning, may be considerably reduced by active management(Watts et al.,2020;Puettmann and Bauhus,2023).

Illegal logging: A joint study by the United Nations Environment Programme and Interpol found that illegal logging accounts for up to 30% of the global logging trade and contributes to more than 50% of deforestations in Central Africa, the Amazon Basin,South East Asia and Europe (Tacconi, 2008; Kleinschmit et al., 2016).Areas that are not protected by an infrastructure of active land management appear to be an especially attractive target for organized crime.

Protected areas have always been part of cultural landscapes, for example to study compositional changes of managed and unmanaged forests over long periods of time (Ducey et al., 2023).“Naturalness”,however, is a concept that is widely employed in the management of protected areas,yet it is ambiguous,subjective,and contentious.It is not possible to measure and objectively evaluate the degree of what is“natural”, because of shifting baselines and differing values (refer to Sprugel,1991,for a detailed and exhaustive analysis).Protection is just another management option that may, or may not, enhance forest resilience and functioning.

5.Hypotheses for future studies

A positive mindset can be helpful in the face of a threat.Approaching a problem with a solution-focused mind helps to overcome adversity and to pursue a worthwhile goal.Four hypotheses are proposed for future studies that aim to sustain the resilience and productivity of a forest ecosystem:

Hypothesis I.Active management can significantly reduce the time-lag between impact and restored functioning of forest ecosystems.

Hypothesis II.An effective forest management infrastructure is a precondition for preventing unsustainable forest use and illegal logging.

Hypothesis III.Jardinage forests, supported by an effective infrastructure of forest observation have a greater capacity to adapt to change than yield table forests.

Hypothesis IV.The multiple path theory is effective in safeguarding the resilience and productivity of a forested landscape irrespective of region or management history.

The scientific approach involves the collection of evidence through appropriate research infrastructures,and to use the evidence to test these and other relevant hypotheses.The scientific method is helpful in reaching a compromise in any discussion about the future use of forest ecosystems.

6.Conclusions

This study introduces a perspective for sustaining forest resilience and productivity in times of change, and at multiple scales.Evidence from different forest regions shows that forested landscapes with asynchronous stands are more stable than homogenous landscapes,and that local stands that include asynchronous species are more resilient than biologically homogenous communities.Jardinage systems are more effective than yield table systems for adapting forests to change because the time-lag between a major disturbance and recovery may be considerably shortened by reducing the planning horizon and focusing on the residual,instead of the timber harvest.Examples of new forest observational studies are presented that are more effective than traditional growth and yield plots in gathering the evidence that is essential for dealing with the challenge of change.Based on observations in different regions of the world, a combination of multiple path design, jardinage management and continuous observation, is a recipe for sustaining forest ecosystems in a variety of biophysical and political environments.

Availability of data and materials

Not applicable.

Declaration of interest

The author declare that he has no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.