Recent Afforestation in the Iowa River and Vorskla River Basins: a Comparative Trends Analysis Yury G

Agronomy Publications Agronomy

2016 Recent Afforestation in the Iowa River and Vorskla River Basins: A Comparative Trends Analysis Yury G. Chendev Belgorod State University

Jason A. Hubbart West Virginia University

Edgar A. Terekhin Belgorod State University

Anthony R. Lupo Belgorod State University

Tom J. Sauer United States Department of Agriculture

SeFoe nelloxtw pa thige fors aaddndition addal aitutionhorsal works at: http://lib.dr.iastate.edu/agron_pubs Part of the Agronomy and Crop Sciences Commons, Climate Commons, Environmental Monitoring Commons, and the Forest Management Commons The ompc lete bibliographic information for this item can be found at http://lib.dr.iastate.edu/ agron_pubs/131. For information on how to cite this item, please visit http://lib.dr.iastate.edu/ howtocite.html.

This Article is brought to you for free and open access by the Agronomy at Iowa State University Digital Repository. It has been accepted for inclusion in Agronomy Publications by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. Authors Yury G. Chendev, Jason A. Hubbart, Edgar A. Terekhin, Anthony R. Lupo, Tom J. Sauer, and C. Lee Burras

This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/agron_pubs/131 Article Recent Afforestation in the Iowa River and Vorskla River Basins: A Comparative Trends Analysis

Yury G. Chendev 1, Jason A. Hubbart 2,*, Edgar A. Terekhin 1, Anthony R. Lupo 1,3, Tom J. Sauer 4 and C. Lee Burras 5 1 Department of Natural Resource Management and Land Cadastre, Belgorod State University, Pobeda Street 85, Belgorod 308015, Russia; [email protected] (Y.G.C.); [email protected] (E.A.T.); [email protected] (A.R.L.) 2 Institute of Water Security and Science, Schools of Agriculture and Food, and Natural Resources, Davis College, West Virginia University, 3109 Agricultural Sciences Building, Morgantown, WV 26506, USA 3 Department of Forestry and Department of Soil, Environmental, and Atmospheric Sciences, University of Missouri, 302 Anheuser-Busch Natural Resources Building, Columbia, MO 65211-7250, USA 4 USDA National Laboratory for Agriculture and the Environment, 1015 N. University Boulevard, Ames, IA 50011-3611, USA; [email protected] 5 Department of Agronomy, Iowa State University, 1126 Agronomy Hall, Ames, IA 50011-3611, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-304-293-2472

Academic Editors: M. Altaf Arain and Timothy A. Martin Received: 6 September 2016; Accepted: 5 November 2016; Published: 15 November 2016

Abstract: Afforestation trends were compared between two continentally-distinct, yet similar ecoregions to characterize similarities or differences in forest advancement due to natural and anthropogenic forcings. Temporal changes in forest cover were analyzed using high resolution aerial and satellite photographs for Southeast Iowa, USA, and satellite photographs for the western Belgorod Oblast, Russia. An increase in forested area was shown to occur over a 44-year period from 1970–2014 in Iowa where afforestation was reﬂected by the aggregation of smaller forest units. In the Belgorod region the opposite occurred in that there was an increase in the number of smaller forested units. The rate of forest expansion into open grassland areas, previously used as haying lands and pastures, was 14 m decade−1 and 8 m decade−1 in Iowa and the Belgorod Oblast, respectively. Based on current trends, predicted times for complete forest coverage in the study areas was estimated to be 80 years in Iowa and 300 years in the Belgorod Oblast. In both the Iowa and Belgorod Oblast, there was an increase in annual precipitation at the end of the 20th and the beginning of the 21st centuries, thus providing a contributing mechanism to forest advancement in the study regions and implications for future management practices.

Keywords: afforestation; climate change; Iowa River Basin; Vorskla River Basin; remote sensing

1. Introduction Investigations of the forested extent of the biosphere and anthropogenic impacts on forest ecosystems are greatly needed given the implications for forest ecosystem and natural resource management, conservation and policy [1]. Widespread deforestation is often implicated as a contributing factor of global climate change [2,3]. However, recent studies of afforestation indicate both anthropogenic and natural process drivers represent a combined focus that integrates contemporary issues of climate change [4–8]. Previous studies showed that natural vegetation communities have reacted strongly to changes in climate during the last 10 thousand years (KY) (the Holocene). For example, dry climatic conditions during the Middle Holocene (8–4 KY before present (BP)) in the USA, the Eurasian East European

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Plain and Western Siberia were reflected by the development of widespread grasslands. Later, during the Late Holocene (the most recent 4 KY BP), forest advancement into grasslands occurred in many locations due to climatic cooling and an increase in precipitation [9–13]. More recently, the climate of the Midwestern States of the USA (e.g., Iowa, Missouri, Kansas) has become warmer and moister since the early 1980s [14,15]. These conditions would seem to favor increased tree growth in the Midwest, as studies have shown that tree growth is positively correlated with precipitation in the mid-latitudes [16,17], but not always in the tropics [18]. It was shown that in the Northern Hemisphere during the atmosphere circulation epoch of 1890–1920s, the annual amount of precipitation exceeded the long-term climatic norm [19]. Thus, from the 1920s to the mid-1950s, a new atmospheric regime became established during which global temperatures increased and precipitation decreased, which caused more frequent droughts in the steppe and dry-steppe regions of East Europe. From the mid-1950s to the decade encompassing the turn of the century (the mid-1990s to the mid-2000s), the climate became wetter over the central USA and southwest Russia. With an increase in precipitation over southwestern Russia, many ecosystem processes significantly changed, for example the ground water level increased, and the composition of natural vegetation communities changed sharply. Hygrophilous forms of plants appeared, and the activity of wind-erosion processes decreased [19]. Conceivably, these ecosystem-level changes may alter expected climate change trajectories. It is therefore important to investigate the extent of these changes, particularly in forested ecosystems, as forests can disproportionately affect global climates relative to many other vegetated ecosystems. An accepted method for studying ecosystem dynamics is via comparative analysis of archived remote sensing data. Remote sensing technology has been shown to be useful in examining the impact of natural and anthropogenic changes on land cover spatially and temporally [20–28]. Aerial photography has a longer history of use for monitoring the environment and is often used in concert with satellite information [20,29,30]. The usefulness of satellite information in this regard was recognized shortly after the launch of the first satellites [31]. While aerial and satellite information is available at various resolutions, medium resolution imagery (e.g., 30 m) may underestimate the extent of forest cover as demonstrated in an urban environment of upstate New York, USA [26]. A study of a rainforest biome in Africa [32], however, indicated such a resolution is adequate for examining changes in forest cover. During the last decade, a number of authors have utilized very fine resolution imagery, whether it was acquired from aerial photographs or satellites [25,30], including the use of visual analyses [32,33]. Many of these studies included an analysis of regional and local-scale changes for the purpose of monitoring the local environment [30,33–35]. Ultimately, the use of layered maps and high resolution imagery has become a common way to estimate changes in forest cover area regionally and globally [25,30,36]. As local regional and/or global studies dedicated to the investigation of recent afforestation are not plentiful, addressing this lack of information by investigating differences in forest advancement due to natural and anthropogenic forcings using a time series aerial photographic, a GIS approach, was the primary objective of this study. Sub-objectives included (a) estimating the rate of afforestation during the later 20th century and into the 21st century in Iowa, USA, and the Belgorod Oblast in Russia and (b) identifying physical and geographic drivers of afforestation in those regions. Conceivably, afforestation in a warmer and wetter climate may help mitigate future climate warming, and such information could inform regional land use and policy decisions.

2. Materials and Methods

2.1. Study Regions Study regions included Johnson County, Iowa, USA, and the Borisovka District in the Belgorod Oblast of southwest Russia (Figure1). These regions are situated in different continents, but have similar ecoregion characteristics given that they are mid-latitude forest regions with similar long cool seasons and warm humid summers, are both located in transitional zones between forests and Forests 2016, 7, 278 3 of 21

Forests 2016, 7, 278 3 of 22 grasslands in moderate climates and were opportunistic research activities for the authors. The Iowa, USA,USA, study study area area was was located located east east of of CoralvilleCoralville Lake reservoir reservoir in in Newport Newport and and Graham Graham Townships. Townships. PriorPrior to to 1840, 1840, the the area area was was a a typical typical oak-hickory oak‐hickory broad broad-leaf‐leaf forest forest land land that that stretched stretched for for hundreds hundreds of of milesmiles along along the the main main rivers rivers of of the the state state (Figure(Figure2 2).). InIn 1870, the western western half half of of the the study study area area (within (within NewportNewport Township) Township) was was almost almost completely completely covered with with timber timber [37], [37 ],and and the the eastern eastern region region was was a a combinationcombination of of prairie, prairie, timber timber and and agricultural agricultural lands [37,38]. [37,38]. The The western western and and the the eastern eastern regions regions werewere described described in in 1870 1870 as as follows follows [ 37[37]] (pp. (pp. 15–16):15–16): “Newport “Newport Township Township is is located located on on the the east east bank bank of of thethe river, river, just just above above Iowa Iowa City City... … ItIt isis nearly all covered covered with with timber, timber, and and although although it is it not is not quite quite so so goodgood as as an an agricultural agricultural Township, Township, its its timber timber makes makes it it of of full full as as great great aa valuevalue toto thethe County at large large as mostas most any Township any Township in the in County”.the County”. The The authors authors continue; continue; “Graham “Graham Township Township is is oneone of the finest ﬁnest in thein County, the County, being being mostly mostly all prairie, all prairie, with with a few a few ﬁne fine places places which which give give considerable considerable timber timber for for fuel fuel and buildingand building purposes”. purposes”.

FigureFigure 1. 1.Study Study areas areas in in the the USA USA andand Russia: ( (11)) Iowa Iowa River River Basin; Basin; (2 ()2 Vorskla) Vorskla River River Basin. Basin.

Forests 2016, 7, 278 4 of 21 Forests 2016, 7, 278 4 of 22

FigureFigure 2. Pre-settlement2. Pre‐settlement forest forest cover cover (shown (shown inin black) in the the State State of of Iowa Iowa [39]. [39 ].The The study study area area is is markedmarked by by the the red red box. box.

In Russia, the study area was located in Belgorod Oblast in the southwest region of European Russia.In Russia, The area the studyis situated area north was located of the Vorskla in Belgorod River, Oblast a branch in of the the southwest Dnieper River, region and of Europeanmainly Russia.corresponds The area to the is situated border of north the Belgorod of the VorsklaOblast District, River, the a branch Borisovka of the District. Dnieper As in River, the Iowa and study, mainly correspondsthe study area to the in the border Belgorod of the Oblast Belgorod was almost Oblast completely District, the covered Borisovka by broadleaf District. forest As before in the the Iowa study,arrival the of study intensive area inhuman the Belgorod activity Oblast(before was1600). almost The area completely is thus covered an approximate by broadleaf Russian forest beforeenvironmental the arrival equivalent of intensive of humanthe natural activity forests (before in Iowa 1600). (Figure The 3). area The isnative thus forests an approximate of the study Russian area environmentalhere were represented equivalent by of predominantly the natural forests by English in Iowa oak (Figure (Quercus3). The robur native L., 1753) forests with of thea mixture study areaof hereash were (Fraxinus represented excelsior by L., predominantly 1753), maple by(Acer English platanoides oak (,Quercus Acer tataricum robur L., L., 1753) 1753) with and a linden mixture (Tilia of ash (Fraxinuscordata excelsiorMiller, 1768)L., 1753), [40]. mapleThe areal (Acer extent platanoides of study, Acer regions tataricum withinL., the 1753) Iowa and and linden Belgorod (Tilia cordatawas Miller,approximately 1768) [40]. 166 The km areal2 and extent 420 km of2 study, respectively. regions within the Iowa and Belgorod was approximately 166 km2 and 420 km2, respectively.

Forests 2016, 7, 278 5 of 21 Forests 2016, 7, 278 5 of 22

FigureFigure 3. Pre-settlement3. Pre‐settlement forest forest cover cover (shown (shown inin green)green) in the Belgorod Belgorod Oblast Oblast [41]. [41 ].The The study study area area is is markedmarked by by the the red red rectangle. rectangle.

2.2.2.2. Orthophotographs Orthophotographs For the Iowa study area, orthophotographs from aerial surveys, as well as satellites were For the Iowa study area, orthophotographs from aerial surveys, as well as satellites were obtained obtained from the Iowa Geographic Map Server [42]. The collection on the server includes black‐ from the Iowa Geographic Map Server [42]. The collection on the server includes black-white white aerial photos (from the 1930s to the 1990s) and colored satellite images (since approximately aerial photos (from the 1930s to the 1990s) and colored satellite images (since approximately 2000). 2000). The highest resolution for all photos is one square meter per pixel. For the current The highest resolution for all photos is one square meter per pixel. For the current investigation, investigation, a resolution of two square meters per pixel was used. Using the aerial photos from the a resolutionU.S. Department of two square of Agriculture meters per from pixel wasMay used. 1967–October Using the 1974 aerial (Figure photos from4), it thewas U.S. shown Department that of Agricultureafforestation from began May in the 1967–October study area in the 1974 1970s (Figure [42].4 Thus,), it was for the shown purposes that afforestation of comparison began to current in the studyforest area cover, in the we 1970s used [42 data]. Thus, starting for the at purposes1970. For ofcontext, comparison Figure to 4 current shows forestthe time cover, series we usedof dataorthophotographs starting at 1970. that For context,was used Figure to make4 shows this determination. the time series of orthophotographs that was used to make thisIn determination.order to characterize a trend of afforestation in the Iowa study area, orthophotographs for threeIn order years to(1970, characterize 1992 and 2014) a trend were of studied. afforestation Lacking in other the Iowa information, study area, changes orthophotographs were assumed to for threebe yearsapproximately (1970, 1992 linear and between 2014) were these studied. periods. Lacking For the other Belgorod information, Oblast study changes area, were data assumed from to besatellite approximately images were linear available between for only these two periods. periods For in 1970 the Belgorod [43] and 2014 Oblast [44] study due to area, the datalimited from satelliteavailability images of weresatellite available imagery for during only the two 1980s periods and 1990s in 1970 for [public43] and use. 2014 The [resolution44] due to of the satellite limited availabilityimages for of Belgorod satellite imageryOblast area during for both the 1980sstudy anddates 1990s was 1.8 for m public per pixel. use. TheNo horizontal resolution shift of satellite was imagesobserved. for Belgorod Thus, for Oblast both the area Iowa for and both Belgorod study datesstudy wasareas, 1.8 the m same per pixel.time period No horizontal was used shiftfor the was observed.investigation Thus, of for change both the in forest Iowa andcover Belgorod (1970–2014). study In areas,both areas, the same the analyzed time period orthophotographs was used for the were measured manually (i.e., digital interrogation was not possible given available information for investigation of change in forest cover (1970–2014). In both areas, the analyzed orthophotographs each site) for the propagation of woodland. The authors recognize that manual interpretations can were measured manually (i.e., digital interrogation was not possible given available information for introduce subjectivity; however, great care was taken to use the same methods, by the same each site) for the propagation of woodland. The authors recognize that manual interpretations can individual between all sites, thereby maximizing consistency and quality control. Distances were introducecalculated subjectivity; using pixel however, counts while great accounting care was taken for the to sloping use the terrain same methods,using topographic by the same maps. individual These betweenresults all served sites, therebyas the basis maximizing for forested consistency area change and qualityestimations control. and Distances maps created were by calculated overlaying using pixelgeographic counts while information accounting systems for the (GIS) sloping layers terrain over orthophotographs. using topographic In maps. addition, These maps results of drainage served as the basis for forested area change estimations and maps created by overlaying geographic information Forests 2016, 7, 278 6 of 21 Forests 2016, 7, 278 6 of 22 systemsnetworks (GIS) were layers created over for orthophotographs. each study area and In used addition, for the maps identification of drainage of forested networks watershed were created and forravine each study types areaand andthe calculation used for the of identiﬁcationlinear rates of offorest forested advancement watershed or retreat and ravine relative types to those and the calculationfeatures. ofSoil linear maps rates at the of 1:15,800 forest advancement scale (cm to m) or retreatwere constructed relative to using those aerial features. photographs Soil maps [38]. at the 1:15,800Topographic scale (cm maps to m) from were [42] constructed were used usingto create aerial a drainage photographs network [38 map]. Topographic for the Iowa maps study from area. [42 ] wereThe used map to of create the drainage a drainage network network for mapthe Belgorod for the Iowa study study area area.was created The map using of the topographic drainage networkmaps forat the a scale Belgorod of 1:10,000 study (cm area to was m). createdThis region’s using topography topographic was maps the opposite at a scale of of the 1:10,000 dense drainage (cm to m). Thisnetwork region’s in topography the Iowa study was area the as opposite evidenced of the by densethe absence drainage of vast network and relatively in the Iowa flat (less study than area 2 as evidenceddegrees byof change) the absence watershed of vast regions. and relatively ﬂat (less than 2 degrees of change) watershed regions.

FigureFigure 4. Series4. Series of of aerial aerial (before (before 2000) 2000) and and satellite satellite (after (after the the 2000s) 2000s) orthophotographs orthophotographs in a fragment of theof study the study area eastarea ofeast Corallville of Corallville Lake Lake Iowa, Iowa, USA. USA.

TheThe developed developed forest forest area area change change maps maps were were used to predict predict the the time time needed needed for for complete complete forest forest coveragecoverage of of the the study study areas areas (i.e., (i.e., the the amount amount ofof change, per per unit unit time time between between orthophotographs). orthophotographs). TheseThese maps maps served served as as the the primary primary map’s map’s layerslayers for the forest area area photographs photographs and and were were also also used used to calculateto calculate linear linear forest forest advancement.advancement. It It is isacknowledged acknowledged that that the time the timefor complete for complete coverage coverage of both of bothareas areas by by forest forest is ishypothetical hypothetical since since land land use use and and climate climate changes changes may may influence inﬂuence those those rates. rates. Nonetheless, the estimation based on current rates of advancement is of use to land managers and Nonetheless, the estimation based on current rates of advancement is of use to land managers and policy makers wishing to make informed future planning decisions. policy makers wishing to make informed future planning decisions. 2.3. Data Analysis 2.3. Data Analysis For the Iowa study area, aerial photographs and satellite imagery (1970, 1992 and 2014) were For the Iowa study area, aerial photographs and satellite imagery (1970, 1992 and 2014) were projected into the North American Datum (NAD) 1983 coordinate system (Projection UTM, Zone projected into the North American Datum (NAD) 1983 coordinate system (Projection UTM, Zone 15N). 15N). Satellite images for the Belgorod study area were similarly geographically referenced, but into Satellitethe WGS images 84 reference for the system Belgorod (Projection study area UTM, were Zone similarly 36N). Geo geographically‐referencing and referenced, geometric correction but into the WGSwere 84 referenceperformed system using (Projection the software UTM, package Zone 36N).ERDAS Geo-referencing IMAGINE, wherein and geometric geospatial correction data are were performedcombined using and quickly the software organized package for geocorrection, ERDAS IMAGINE, analysis, wherein visualization geospatial and map data output. are combined The and quickly organized for geocorrection, analysis, visualization and map output. The study areas Forests 2016, 7, 278 7 of 21 of Iowa and Belgorod Oblast were delineated in the software package ArcGIS 10.1, resulting in an integrated archive of aerial photographs, satellite images and vectored layers of data (for each study period and location) for wooded lands at the 1: 9000 scale, which enabled the assessment of linear change at the one meter level. This allowed for an unbiased and accurate quantitative assessment of forested area and the rates of deforestation or afforestation [42–44]. Precipitation data (mm) for Iowa were compiled from daily rainfall observations at several locations, archived and subsequently made available via the National Climate Data Center in Asheville, North Carolina, USA, or available from the Midwest Regional Climate Center at the University of Illinois [45]. Precipitation (mm) and temperature (◦C) data used for the Belgorod region were obtained from the All Russia Research Institute of Hydrometeorological Information-World Data Centre [46]. Data supplied descriptive statistics and were used to support observed results and future projections. The rate of forests edge advancement (m/year) was estimated in places where open areas remained in the form of corridors or glades with meadow-steppe vegetation between sprawling forests. These are areas where forests have not yet fully occupied open areas during the study period (1970–2014), but may in the future. The analysis of a subset of sites within slopes of ravines and river valleys for northern and southern exposures was also performed. Selection criteria for those sites included (a) conﬁrmed slope exposure as per the digital model of relief and aerial and/or satellite images analysis and (b) the area of the forested slope could not be dissected by roads or railways, settlements or agricultural land, especially croplands, and must have grassland connectivity. This latter condition was complicated given that forests in both Iowa and the Belgorod Oblast study areas were, in many places, situated among agricultural lands. Nevertheless, it was possible to choose a representative number of forest test plots for which the measurements were conducted, except for the Belgorod Oblast, where all forested areas bordered cultivated land. Forest advancement estimates were performed in GIS by measuring the distance between forest borders in the Iowa study area during the periods 1970–1992 and 1992–2014 and in Belgorod Oblast study area for the period 1970–2014. The distance between adjacent linear measurements of forest edge increments ranged from 15–30 m. Examples of measuring distance between forests boundaries for different years of surveys for ravine slopes of southern and northern exposures, as well as in the watershed for the Iowa study area are shown in Figure5. Forests 2016 7 Forests 2016, ,, 278 7, 278 8 of8 22 of 21

Figure 5. Measurement of forests edges’ frontal advancement (e.g., Iowa study area, 1992–2014). Figure 5. Measurement of forests edges’ frontal advancement (e.g., Iowa study area, 1992–2014). Legend: (1) borders of forest in the orthophotograph from 1992; (2) borders of forest in the Legend: (1) borders of forest in the orthophotograph from 1992; (2) borders of forest in the orthophotograph of 2014; (3) increments of the forest edge on northern slopes; (4) increments of orthophotograph of 2014; (3) increments of the forest edge on northern slopes; (4) increments of the the forest edge on southern slopes; (5) increments of the forest edge on the watershed. forest edge on southern slopes; (5) increments of the forest edge on the watershed.

Forests 2016, 7, 278 9 of 21 Forests 2016, 7, 278 9 of 22

3. Results 3. Results Analysis indicated relatively heterogeneous forest stand and cover dynamics from 1970–2014 (FiguresAnalysis 6 and indicated 7). There relatively was an overall heterogeneous increase in forest forest stand area within and cover the Iowa dynamics and Belgorod from 1970–2014 Oblast (Figuresstudy 6areas and7 ).(Figures There was6 and an 7). overall Forest increase advancement in forest in areathe withinIowa study the Iowa area andduring Belgorod the periods Oblast study1970–1992 areas (Figures and 1992–20146 and7). occurred Forest advancement at a uniform inrate the (Table Iowa 1). study During area the during earlier the period periods of 1970–1992analysis and(1970–1992), 1992–2014 occurredthe total forested at a uniform area increased rate (Table (on1). average) During thefrom earlier approximately period of 14.8%–19.8% analysis (1970–1992), of the thetotal total area, forested and for area 1992–2014, increased the (on increase average) was from from approximately 19.8%–24.8%. Results 14.8%–19.8% also indicated of the total a decrease area, and forin 1992–2014, the number the of increasedistinct forest was fromblocks, 19.8%–24.8%. as smaller blocks Results combined also indicated to form larger a decrease forest blocks in the in number the of distinctIowa region forest (Table blocks, 1). In as the smaller Belgorod blocks Oblast combined study area to formduring larger the same forest period, blocks the in same the Iowatendency region (Tablefor forest1). In growth the Belgorod was observed Oblast (Table study 2). area However, during unlike the same the Iowa period, site, the there same was tendency an increase for in forestthe growthnumber was of observed smaller area (Table forest2). However,blocks in the unlike Belgorod the Iowa Oblast. site, Overall, there was the anincrease increase from in 1970–2014 the number in of smallerforest area cover forest in the blocks Belgorod in the Oblast Belgorod study Oblast. area was Overall, 36.6%, the while increase in Iowa, from this 1970–2014 increase in was forest 67.5% cover in the(Table Belgorod 1). In the Oblast Belgorod study Oblast, area wasin spite 36.6%, of the while large in increase Iowa, thisin square increase area, was forest 67.5% growth (Table appeared1). In the Belgorodto be primarily Oblast, in in spiteravines, of the which large were increase previous in square grasslands area, (pastures forest growth or hayfields) appeared (Figures to be primarily 6 and 7). in This accounted for the doubling of the number of forest blocks in Belgorod, each with a smaller ravines, which were previous grasslands (pastures or hayﬁelds) (Figures6 and7). This accounted for footprint (Table 1). the doubling of the number of forest blocks in Belgorod, each with a smaller footprint (Table1).

FigureFigure 6. 6.Dynamics Dynamics of of forest forest cover cover for for the the period 1970–2014 (Iowa (Iowa study study area). area). Shaded Shaded area: area: (1) (1)forests forests in 1970–2014;in 1970–2014; (2) (2) forest forest loss; loss; (3) (3) forest forest expansion. expansion.

TableTable 1. Changes1. Changes in in forest forest characteristics characteristics duringduring the period between between 1970 1970 and and 2014 2014 in inthe the Iowa Iowa and and BelgorodBelgorod Oblast Oblast study study areas areas (based (based on on aerialaerial andand satellitesatellite orthophotographs). Data Data presented presented as as Iowa/BelgorodIowa/Belgorod Oblast. Oblast. Year Forest Cover Number of Forest Mean Block Total Forested Forest Cover Number of Mean Block Total Forested Year (km2) Blocks Area (ha) Area (%) (km2) Forest Blocks Area (ha) Area (%) 1970 24.6/91.0 1109/667 2.2/13.7 14.8/21.0 19921970 32.9 24.6/91.0 1109/667702 2.2/13.74.7 14.8/21.019.8 1992 32.9 702 4.7 19.8 2014 41.1/121.0 663/1439 6.2/8.4 24.8/28.1 2014 41.1/121.0 663/1439 6.2/8.4 24.8/28.1

Forests 2016, 7, 278 10 of 21 Forests 2016, 7, 278 10 of 22

FigureFigure 7. Dynamics7. Dynamics of forestof forest cover cover for for the the period period 1970–2014 1970–2014 (Belgorod (Belgorod Oblast Oblast study study area). area). Shaded Shaded area: (1) forestsarea: (1) in forests 1970–2014; in 1970–2014; (2) forest (2) loss; forest (3) loss; forest (3) forest expansion. expansion.

Figure8 shows land cover in 1970 versus 2014 within the Iowa and Belgorod Oblast study areas. To obtain a subset of information in addition to the earlier articulated methods, land types were divided into three physiographic groups depending on landscape position (Figure8) as follows: (a) included areas situated on undulating surfaces in the watersheds or on watershed slopes. The typical tendency for this land type was the formation of denser forests replacing sparse forest or individual groups of trees. Group (b) contained plots within the gentler slopes of relatively vast river valleys and big ravines. Forests 2016, 7, 278 11 of 21

In this land type, areas of forest advanced in two ways: (1) gradual encroachment upward along slopes (when forest was situated earlier in the ﬂoodplain or ravine bottoms); or (2) downward along slopes (when forests contacted underlying along-slope grasslands). In Group (b), haying lands and pastures (grasslands), which in 1970 were transitional zones between forest and arable land areas, were, in the majority of cases, completely afforested by 2014. In these areas, simpliﬁed spatial combinations of land management occurred (e.g., previously forest, pasture, arable land or forest, haying, arable land became forest, arable land). However, in some cases, forested area was replaced by hay or pasture lands or the appearance of glades or other open areas (Figure8b). Group (c) consisted of watershed slopes, combined with relatively small ravines. In these areas, haying lands and pastures within ravines were replaced by forests or dense forests advanced into sparse park-type forests (Figure8c).

Table 2. Results of the statistical analysis for forest advancement (m) in different geomorphological positions of the Iowa study area during the periods 1970–1992 and 1992–2014 for Iowa.

Mean Rate of Position in Relief N Min Max Max-min X ± δX δ Advancement, (m/Year) 1970–1992 Flat watersheds and gentle watershed slopes 36 12 64 52 27.3 ± 1.9 11.2 1.37 Slopes of ravines (northern exposure) 65 10 112 102 32.6 ± 2.9 23.9 1.48 Slopes of ravines (southern exposure) 96 10 132 122 41.2 ± 3.0 29.3 1.87 1992–2014 Flat watersheds and gentle watershed slopes 44 10 54 44 25.1 ± 1.8 11.9 1.14 Slopes of ravines (northern exposure) 120 10 92 83 31.1 ± 1.7 19.1 1.41 Slopes of ravines (southern exposure) 193 10 94 84 31.1 ± 1.4 20.0 1.41

Results confirmed that field cultivation at the forest edge restrained forest advancement. This was clearly demonstrated by forest fragments visible in the 1970 and 2014 surveys of aerial photographs (1970) and satellite images (2014) presented in Figure9. In particular, the south central area in Figure9a and central area in Figure9b were arable lands for the entire period 1970–2014. Conversely, in the absence of cultivation, some tree species spread into formerly cultivated areas, presumably at least in part due to wind-induced seed distribution [47,48]. Thus, in areas without plowing and other anthropogenic activities, afforestation was apparent. For example, hay and pasturelands present in 1970 were replaced by forests by 2014 (Figure9). Afforestation in the Belgorod Oblast was shown to begin within two years of abandonment. This trend was visible by manual inspection near the edge of the forest in contact with the abandoned field. Next to the Acer negundo (box elder) plantation of the windbreak and among the grasses and weeds in the abandoned field, many one- and two-year-old trees were observed at a distances of up to 20–25 m from the forest edges. Forest edge advancement within the Iowa study region was investigated from different relief positions (northern and southern slopes of ravines, watersheds and watershed slopes) during the periods 1970–1992 and 1992–2014. There was a tendency toward a decreased rate of afforestation during the second part of the study period (Table2), and the forest advancement rate between watersheds and ravine slopes was not statistically significant (CI = 0.05). For example, rates of forest advancement on ravine slopes during both study periods were higher than lesser sloped areas (Table2). The mean rate of the forest edge advancement in the Iowa study area for the period 1970–2014 was 1.3 m year−1 in watersheds; 1.4 m year−1 in ravines with a northern exposure; and 1.6 m year−1 in ravines with a southern exposure (Table2). The mean rate of forest advancement in the Iowa study area was 1.4 m year−1. The Belgorod Oblast study area was characterized by slower rates of forest edge advancement or about 50% less than that found in Iowa (Table3). The mean rates of forest advancements for ravine slopes of northern and southern exposures were approximately equal, about 0.8 m year−1 (Table3). Forests 2016, 7, 278 12 of 21

Forests 2016, 7, 278 12 of 22

Figure 8. Physiographic types of afforestation process: (a) on watershed summits and slopes; (b) on Figure 8. Physiographicslopes of river types valleys, of large afforestation ravines; (c) in ravine process: systems. (a )Left: on examples watershed of afforestation summits according and slopes; (b) on slopes of river valleys,to fragments large of the ravines; maps of forestland (c) in ravine cover dynamics systems. in Iowa Left: and examplesBelgorod Oblast. of afforestationMap indices: according to fragments ofsymbols the maps of forests of dynamics forestland are the cover same as dynamics in Figure 7. in Iowa and Belgorod Oblast. Map indices: symbols of forestsResults dynamics confirmed are that the field same cultivation as in Figureat the forest7. edge restrained forest advancement. This Forests 2016, 7, 278 13 of 22 was clearly demonstrated by forest fragments visible in the 1970 and 2014 surveys of aerial photographs (1970) and satellite images (2014) presented in Figure 9. In particular, the south central area in Figure 9a and central area in Figure 9b were arable lands for the entire period 1970–2014. Conversely, in the absence of cultivation, some tree species spread into formerly cultivated areas, presumably at least in part due to wind‐induced seed distribution [47,48]. Thus, in areas without plowing and other anthropogenic activities, afforestation was apparent. For example, hay and pasturelands present in 1970 were replaced by forests by 2014 (Figure 9). Afforestation in the Belgorod Oblast was shown to begin within two years of abandonment. This trend was visible by manual inspection near the edge of the forest in contact with the abandoned field. Next to the Acer negundo (box elder) plantation of the windbreak and among the grasses and weeds in the abandoned field, many one‐ and two‐year‐old trees were observed at a distances of up to 20–25 m from the forest edges.

Figure 9. OrthophotographsFigure 9. Orthophotographs fragments fragments of surveys of surveys from from different different periods, 1970 1970 (left) (left) and 2014 and (right) 2014 (right) [42–44]; [42–44]; (a) Iowa study area; (b) Belgorod Oblast study area. The appearance of forests took place (a) Iowa studymostly area; in ( ravinesb) Belgorod and slopes. Oblast Outlines study of plowed area. lands The remain appearance largely unchanged. of forests took place mostly in ravines and slopes. Outlines of plowed lands remain largely unchanged. Forest edge advancement within the Iowa study region was investigated from different relief positions (northern and southern slopes of ravines, watersheds and watershed slopes) during the periods 1970–1992 and 1992–2014. There was a tendency toward a decreased rate of afforestation during the second part of the study period (Table 2), and the forest advancement rate between watersheds and ravine slopes was not statistically significant (CI = 0.05). For example, rates of forest advancement on ravine slopes during both study periods were higher than lesser sloped areas (Table 2). The mean rate of the forest edge advancement in the Iowa study area for the period 1970–2014 was 1.3 m year−1 in watersheds; 1.4 m year−1 in ravines with a northern exposure; and 1.6 m year−1 in ravines with a southern exposure (Table 2). The mean rate of forest advancement in the Iowa study area was 1.4 m year−1. The Belgorod Oblast study area was characterized by slower rates of forest edge advancement or about 50% less than that found in Iowa (Table 3). The mean rates of forest advancements for ravine slopes of northern and southern exposures were approximately equal, about 0.8 m year−1 (Table 3).

Forests 2016, 7, 278 13 of 21

Table 3. As in Table2, except for the Belgorod Oblast study area during the period between 1970 and 2014.

Mean Rate of Position in Relief n Min Max Max-Min X ± δ δ X Advancement, (m/Year) Slopes of ravines 175 10 90 80 33.1 ± 1.4 18.2 0.75 (northern exposure) Slopes of ravines 137 10 132 122 35.9 ± 2.3 27.4 0.82 (southern exposure)

4. Discussion An analysis of aerial photographs, coupled with available information in the literature provided the basis for some explanations pertaining to the relative influence of anthropogenic and natural factors for the increase in forest land cover within the study areas. In some locations, forest-plantations were established, which on the photographs were identified clearly by visible rows of planted trees. The reason for the appearance of forest-plantations and/or abandoned agricultural lands (mainly on slopes) may at least in part be attributed to depletion of soil fertility as a result of erosion on slopes [49]. In some areas, there were changes in the number of forested pixels apparently due to residential construction and/or the planting of trees near new development. This process resulted in a net loss of forested area (see red areas in Figures6 and7). In Iowa, the development of a recreation facility on former farm lands in the area of study also resulted in afforestation. An increase in forest land cover near the Coralville Lake shore, after the creation of a reservoir in 1958, was a direct effect of changes in land management (Figure4). Land was retired from agricultural use and converted into recreational zones, namely the Coralville Lake Wildlife Area [50]. Aerial photography from 1970 showed many open areas that were covered with young trees. At that time, forests covered about 40% of the area within two kilometers of the lake shore just 12 years after the reservoir was created. Forty-four years later (2014), forests covered 80% of that same part of the lake’s shoreline (Figure6). In the absence of forest plantations in the photography from 1970 and 2014, it is possible to conclude that the change in forest cover was largely due to natural expansion by forest vegetation near the shoreline of the Coralville Lake reservoir (as measured by changes in the number of forested pixels). In Iowa, the process of natural forest expansion into the prairie is confirmed by examining the properties of soils that have clear signs of meadow-steppe soil formation, which at present are situated in forested lands [38]. In a number of locations in Iowa, there are current areas of prairie that were formerly forested during the period of intensive settlement during the second-half of the 19th century [37,51]. It is of interest to note that the forest advancement shown in the current work is also favored by the climatic trends that have existed for about 800–900 years associated with the completion of a climatic episode that was warmer and more arid at the end of the first and the beginning of the second millenniums A.D. [9,52,53]. There is also significant discussion in the literature regarding pre-settlement conditions in Iowa, USA. The pre-settlement conditions of the forest and prairie were regulated largely by periodic fires [54–56]. The authors of one of the oldest publications on the geography of Iowa reported that if not for the occurrence of frequent prairie fires into the 19th century, it is possible that the entire area within a few decades would have been covered by forest [51]. In the Belgorod Oblast, the same patterns noted in Iowa, USA, of natural afforestation during the Late Holocene were observed. In a number of places currently forested, grassland soils or chernozems (Russian equivalent of Mollisols in USA soil taxonomy) were found buried under prehistoric mounds from the bronze epoch [12]. Applying the paleosoils for the reconstruction of natural environments within Belgorod Oblast and combining this with modern trends for forest area growth and replacement of these by human activity (grasslands), it is inferred that the characteristics of the environmental succession were very close to those of Iowa for the last 700–800 years. Natural afforestation was especially rapid during the so-called Little Ice Age Period (15th–19th centuries) [57]. Forests 2016, 7, 278 14 of 21 Forests 2016, 7, 278 15 of 22

DuringDuring recent recent decades decades in Iowa Iowa and and the the Belgorod Belgorod Oblast, Oblast, quantifiable quantifiable changes changes in climate in (e.g., climate (e.g.,increased increased precipitation) precipitation) have have been beenobserved. observed. The comparison The comparison of annual of and annual summer and season summer (June– season (June–August)August) only only precipitation precipitation in Iowa in Iowa during during two periods two periods (1931–1960 (1931–1960 and 1981–2010) and 1981–2010) indicates indicates that by that bythe second second-half‐half of of the the 20th 20th century, century, the the climate climate of of Iowa Iowa was was more more humid, humid, particularly particularly in the in the summer summer season.season. This This observation observation is is consistent consistent withwith many published investigations investigations [15,58–60]. [15,58–60 In]. Inparticular, particular, withinwithin the the study study area, area, the the annual annual precipitation precipitation increased by by approximately approximately 100 100 mm mm (four (four inches); inches); moremore than than half half of of this this amount amount during during the the summersummer season (Figure (Figure 10). 10). Similar Similar climate climate change change was was confirmedconfirmed for for the the Belgorod Belgorod Oblast Oblast (Figure (Figure 11 11).). The comparison comparison of of climate climate maps, maps, which which reflect reflect average average temperaturetemperature and and precipitation precipitation during during 1971–2000, 1971–2000, with with maps maps of the of same the same variables variables during during the previous the thirty-yearprevious period thirty‐year (1951–1980), period (1951–1980), showed that, showed for the that, last for quarter the last of thequarter 20th of century, the 20th there century, were there distinct were distinct increases in the annual amount of precipitation. This was reflected by the shift to the increases in the annual amount of precipitation. This was reflected by the shift to the north of the north of the January isotherms and to the south of the July isotherms (Figure 11). January isotherms and to the south of the July isotherms (Figure 11).

FigureFigure 10. 10.Average Average values values of of annual annual and and summer precipitation precipitation (in (in inches) inches) in in Iowa Iowa during during two two periods: periods: (A,(CA),C 1931–1960;) 1931–1960; (B, D()B 1981–2010,D) 1981–2010. [45]. The[46](Courtesy study area of is shownthe Midwest by the redRegional rectangle. Climate Center, http://mrcc.isws.illinois.edu). The study area is shown by the red rectangle.

Forests 2016, 7, 278 16 of 22 ForestsForests2016 2016, 7,, 278 7, 278 16 of15 22 of 21

FigureFigure 11. 11.Climatic Climatic mean mean precipitation precipitation (left) (left) and and temperatures temperatures (right) (right) for for the the Belgorod Belgorod Oblast Oblast during during the periodsFigurethe periods 1951–1980 11. Climatic 1951–1980 and mean 1971–2000 and precipitation 1971–2000 (adapted (left)(adapted from and [ 61temperaturesfrom,62 ]).[61,62]). The study (right)The areastudy for isthe area shown Belgorod is shown by the Oblast redby theduring rectangle. red therectangle. periods 1951–1980 and 1971–2000 (adapted from [61,62]). The study area is shown by the red rectangle. In addition, and in response to climatic changes, natural factors favoring the modern increase In addition, and in response to climatic changes, natural factors favoring the modern increase in inforest forestIn cover addition, cover for for Iowa and Iowa in(but response (but also the also to Belgorodclimatic the Belgorod changes, Oblast) Oblast)naturalinclude factorsevolving include favoring hydrologic evolving the modern hydrologicerosion increase networks erosion in networksforest[8,63,64] cover [ 8with,63 for,64 coupled ]Iowa with (but coupledshallow also the groundwater shallow Belgorod groundwater Oblast) supplies include that supplies areevolving known that hydrologic are to support known erosion todense support networks riparian dense riparian[8,63,64]forests forests [61]. with Similarly, [coupled61]. Similarly, anshallow additional an groundwater additional explanation supplies explanation for forest that cover forare forestknown increase cover to issupport the increase strongly dense is thedissected riparian strongly dissectedforestsrelief within [61]. relief Similarly,dense within drainage densean additional networks, drainage explanation the networks, density for of theforest which density cover in some increase of whichplaces is reaches inthe some strongly 5–6 places km/km dissected reaches2 in 5–6reliefIowa km/km (Figurewithin2 in dense12) Iowa and drainage (Figure 2–3 km/km 12 networks,) and2 in 2–3Belgorod the km/km density Oblast2 in of Belgorod(Figure which in13). Oblastsome In effect, places (Figure the reaches slope 13). In sections,5–6 effect, km/km thewhich2 slopein sections,Iowaobtain (Figure which additional obtain12) and moisture additional 2–3 km/km with moisture2 surfacein Belgorod withand Oblastsubsurface surface (Figure and soil subsurface 13). drainage, In effect, soil create the drainage, slope land sections,forms create landthat which are forms thatobtainfavorable are favorable additional for forest for moisture expansion. forest expansion. with surface and subsurface soil drainage, create land forms that are favorable for forest expansion.

Figure 12. Drainage network (contours of ravines and river ﬂoodplains) in the Iowa study area. Forests 2016, 7, 278 17 of 22 Forests 2016, 7, 278 16 of 21 Figure 12. Drainage network (contours of ravines and river floodplains) in the Iowa study area.

FigureFigure 13. 13.Drainage Drainage network network in in the the BelgorodBelgorod Oblast study area area (southern (southern fragment) fragment) (a) ( aand) and flat ﬂat watershedswatersheds distribution distribution (b (b).). MapMap indexes:indexes: ( 1)) bottoms of of ravines; ravines; (2 ()2 )rivers; rivers; (3 ()3 flat) ﬂat watersheds watersheds (steepness(steepness of surfaceof surface less less than than 2 ◦2°).).

The comparative analysis of plant cover distributions in the orthophotographs from 1970 and The comparative analysis of plant cover distributions in the orthophotographs from 1970 and 2014 2014 (Figures 6 and 7) made it possible to identify the three modes of forest cover increase presented (Figuresearlier6 and(Figure7) made 14). it Anthropogenic possible to identify forest the regeneration three modes was of forest characterized cover increase by the presented simultaneous earlier (Figureappearance 14). Anthropogenic of woody vegetation forest regeneration (mainly windbreaks) was characterized (Figure 14a). by Natural the simultaneous forest regeneration appearance was of woodydetermined vegetation by identifying (mainly windbreaks) particular forest (Figure segments 14a). Naturalor patches forest in the regeneration 1970 photos wasand then determined locating by identifyingthem within particular forested forest stands segments in the 2014 or photos. patches In in places the 1970where photos there were and thensparse locating forests or them separate within forestedtrees, standsforest advancement in the 2014 photos. could proceed In places in whereall directions there were from sparseeach group forests of ortrees separate or isolated trees, trees forest advancement(Figure 14b). could It was proceed thus assumed in all directions based on fromFigures each 5 and group 9a that of trees this oris the isolated manner trees by (Figurewhich the 14 b). It waslargest thus blocks assumed of forests based in on the Figures Iowa study5 and 9areaa that were this formed. is the manner In the case by whichof the theforest largest expanding blocks of forestsoutward in from the Iowa the boundaries study area of were dense formed. forest present In the at case the ofstart the of forest the study expanding period, frontal outward advances from the boundariesof forest ofboundaries dense forest took present place (Figure at the start 14c). of the study period, frontal advances of forest boundaries took placeIt is (Figure of use 14 forc). land management purposes to estimate the time to complete forest cover representedIt is of use forby this land 44 management‐year period analysis. purposes It to is estimate acknowledged the time that to completesuch calculations forest cover are somewhat represented by thisspeculative 44-year since period future analysis. changes It is in acknowledged climate or anthropogenic that such calculations activity may are accelerate somewhat or decelerate speculative sincethese future rates. changes Regardless, in climate based on or current anthropogenic results, considering activity may that accelerate(a) the average or decelerate linear rate of these forests rates. growth during the period between 1970 and 2014 (14 m per decade) and assuming forests existed Regardless, based on current results, considering that (a) the average linear rate of forests growth originally at the lowest elevations only (ravine bottoms and flooded parts of river valleys), as well as during the period between 1970 and 2014 (14 m per decade) and assuming forests existed originally at that (b) the average distance between the bottoms of the drainage network depressions and the the lowest elevations only (ravine bottoms and ﬂooded parts of river valleys), as well as that (b) the central parts of the divided uplands (110 m), it was estimated that the time for the total areal forest average distance between the bottoms of the drainage network depressions and the central parts of the divided uplands (110 m), it was estimated that the time for the total areal forest coverage in the western part of the study Iowa study area would be approximately 80 years. This estimate would be Forests 2016, 7, 278 17 of 21 Forests 2016, 7, 278 18 of 22

coverage in the western part of the study Iowa study area would be approximately 80 years. This considerably less if it included forest cover in the bottoms of the erosion network, as well as on slopes estimate would be considerably less if it included forest cover in the bottoms of the erosion network, and summits between ravines, as well as projections of future climate, which is expected to be warmer as well as on slopes and summits between ravines, as well as projections of future climate, which is andexpected wetter duringto be warmer the growing and wetter season during [3,60 the]. Forgrowing the Belgorod season [3,60]. study For area, the Belgorod the same study calculations area, the and assumptionssame calculations indicated and that assumptions the average indicated distance that between the average the bottoms distance of thebetween drainage the bottoms network of equaled the 233drainage m (Table network4), and equaled the estimated 233 m (Table time for 4), theand totalthe estimated areal forest time coverage for the total would areal be forest approximately coverage 300would years. be approximately 300 years.

FigureFigure 14. 14.Processes Processes of increasesof increases in forestin forest cover cover within within study study areas areas in Iowa in Iowa and Belgorodand Belgorod Oblast Oblast during theduring period the from period 1970–2014: from 1970–2014: (a) artiﬁcial (a) artificial forest-plantations; forest‐plantations; (b) growth (b) growth of forest of forest from manyfrom many centers; (c)centers; frontal growth(c) frontal from growth boundaries from boundaries of forests. of forests.

Table 4. Results of the statistical analysis of the distance between the bottom of ravines or border Table 4. Results of the statistical analysis of the distance between the bottom of ravines or border parts parts of river floodplains and divided watersheds. of river ﬂoodplains and divided watersheds.

Index n Min Max Max-Min X ± δ δ Index n Min Max Max‐Min X ± δX δ X Iowa Study Area Distance Between Ravine Bottoms or Iowa Study Area BordersDistance of Between Rivers Flooded Ravine PlainsBottoms and or 245 50 298 248 110 ± 3 39 BordersCentral of Rivers Parts of Flooded Watersheds Plains and 245 50 298 248 110 ± 3 39 Central Parts of Watersheds Belgorod Oblast Study Area Distance Between Ravine Bottoms or Belgorod Oblast Study Area BordersDistance of Between Rivers Flooded Ravine PlainsBottoms and or 310 30 1022 992 233 ± 8 149 BordersCentral of Rivers Parts of Flooded Watersheds Plains and 310 30 1022 992 233 ± 8 149 Central Parts of Watersheds Distance Between Ravine Bottoms or BordersDistance of Between Rivers Flooded Ravine PlainsBottoms and or 121 30 296 266 89 ± 7 72 BordersEdges of of Nearest Rivers Flooded Flat Watersheds Plains and 121 30 296 266 89 ± 7 72 Edges of Nearest Flat Watersheds

TheThe main main characteristic characteristic of of the the natural natural environment environment for the for forest the‐ forest-steppesteppe zone of European zone of European Russia Russiais the is presence the presence of forests of forests and grasslands and grasslands in relatively in relatively flat watersheds flat watersheds [65]. Therefore, [65]. there Therefore, is interest there is interestby land bymanagers land managers and policy and makers policy in makers estimations in estimations of the time of needed the time for neededthe formation for the of formation forest‐ of forest-steppesteppe from a fromformer a formersteppe environment. steppe environment. In the Belgorod In the study Belgorod area, study the mean area, distance the mean from distance the fromravine the ravinebottoms bottoms or river orflood river plain flood borders plain to borders the nearest to the flat nearest watersheds flat watersheds was about 90 was m (Table about 4). 90 m (TableThus,4). considering Thus, considering the calculated the calculated mean rate mean of forest rate advancements of forest advancements to grasslands to of grasslands 0.8 m year of−1, 0.8the m yearestimated−1, the estimated time of forest time‐steppe of forest-steppe advancement advancement from a previous from steppe a previous environment steppe environment may be as little may as be as little110–115 as 110–115 years. years. ClimaticClimatic conditions conditions favorable favorable forfor thethe formationformation of of a a “climax “climax prairie” prairie” in in the the Midwest, Midwest, USA, USA, includingincluding Iowa, Iowa, were were optimal optimal during during the the so-called so‐called “Thermal Maximum Maximum of of Holocene” Holocene” or orthe the interval interval betweenbetween 6.0 6.0 and and 4.0 4.0 KY KY BP BP [9 [9,11,50].,11,50]. For For thethe Central Chernozem Chernozem Region Region of of Russia, Russia, including including Belgorod Belgorod Oblast, this period is associated with the interval from 4.3–4.0 KY BP [12,66]. This was a period of Oblast, this period is associated with the interval from 4.3–4.0 KY BP [12,66]. This was a period of time time when forests within the study areas were located mainly in the bottoms of the drainage network. when forests within the study areas were located mainly in the bottoms of the drainage network. Later, more humid climatic conditions prompted the successional conversion of prairie-grassland into forest Forests 2016, 7, 278 18 of 21 in many places. Conceivably, in places with dense drainage networks (such as the study areas here), the formation of large forested areas as a result of forest expansion from ravines and valley confluences could have occurred during a relatively short period time during the transition from Middle to Late Holocene. The calculations made here substantiate this observation. In the future, the study regions and adjacent regions in much of Iowa and the Belgorod Oblast are expected to be subjected to warmer and wetter climatic conditions. The results of this study may suggest that improved conditions for forest growth in warmer climates could be advantageous in efforts to mitigate climate change [2,3,67]. This is due to the fact that forest ecosystems (biomass and soils) are highly productive in terms of carbon sequestration processes [3,68,69]. In addition, planting trees has long been used as a strategy to mitigate urban heat island effects [70] by multiple mechanisms, including decreased albedo [71].

5. Conclusions From 1970–2014, a large increase in forest cover occurred in Johnson County, Iowa, to the east of Coralville Lake reservoir in Iowa, USA, and in Borisovka District of Belgorod Oblast, to the north of Vorskla River, Russia. Forest cover increased in the Belgorod Oblast by 36.6% during the study period, while in Iowa, the increase in forest cover was 67.5%. Several factors supporting these ﬁndings include (but are not limited to): the development of a recreational area adjacent to river areas (in Iowa), changes to conservation practices, a change in priorities of farmer landownership, the warming and moistening climate and the dense drainage network. Three mechanisms of forest advancement were investigated including: (a) the simultaneous appearance of forest as a result of the artiﬁcial planting of trees; (b) the growth of forest from groups or isolated trees; and (c) forest edge advancement. The average linear rate of forest growth, which in the context of mainly natural factors occupying meadow and steppe areas (pastures and haying lands), was approximately 14 m (Iowa study area) and 8 m (Belgorod study area) per decade, respectively. Based on the 44-year study period, it was projected that the entire study area could be forested in about 80 years in Iowa and in approximately 300 years in Belgorod Oblast. The time period may be reduced given projections for a warmer and wetter future climate (assumes no further anthropogenic impacts). Rapid forest advancement similar to that shown in the current work may also have occurred during previous epochs when greater precipitation may have been observed, such as the Middle to Late Holocene or after 1000 A.D. It could thus be inferred from this work that the forests are “returning” to their historic, pre-human settlement, positions on the landscape in Iowa and Belgorod Oblast. Therefore, afforestation rates in association with a warmer and wetter climate may encourage afforestation, which may be advantageous for climate change mitigation.

Acknowledgments: This study was supported by the Russian Science Foundation, Project No. 14-17-00171 “Regional responses of environmental components on climate change varying periodicity: South forest-steppe of the Central Russian Upland”, and the Council for International Education/Fulbright Program, as well as the anonymous reviewers who helped make this paper a stronger contribution. Author Contributions: Yury G. Chendev, Tom J. Sauer, Edgar A. Terekhin and C. Lee Burras conceived of and designed the experiments. Yury G. Chendev and Anthony R. Lupo performed the experiments. Yury G. Chendev and Anthony R. Lupo analyzed the data. Tom J. Sauer, C. Lee Burras, Anthony R. Lupo, Jason A. Hubbart, Edgar A. Terekhin and C. Lee Burras contributed reagents/materials/analysis tools. Yury G. Chendev, Edgar A. Terekhin, Anthony R. Lupo and Jason A. Hubbart wrote the paper. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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