Brazilian montane rainforest expansion induced by Heinrich Stadial 1 event

The origin of modern disjunct plant distributions in the Brazilian Highlands with strong floristic affinities to distant montane rainforests of isolated mountaintops in the northeast and northern Amazonia and the Guyana Shield remains unknown. We tested the hypothesis that these unexplained biogeographical patterns reflect former ecosystem rearrangements sustained by widespread plant migrations possibly due to climatic patterns that are very dissimilar from present-day conditions. To address this issue, we mapped the presence of the montane arboreal taxa Araucaria, Podocarpus, Drimys, Hedyosmum, Ilex, Myrsine, Symplocos, and Weinmannia, and cool-adapted plants in the families Myrtaceae, Ericaceae, and Arecaceae (palms) in 29 palynological records during Heinrich Stadial 1 Event, encompassing a latitudinal range of 30°S to 0°S. In addition, Principal Component Analysis and Species Distribution Modelling were used to represent past and modern habitat suitability for Podocarpus and Araucaria. The data reveals two long-distance patterns of plant migration connecting south/southeast to northeastern Brazil and Amazonia with a third short route extending from one of them. Their paleofloristic compositions suggest a climatic scenario of abundant rainfall and relative lower continental surface temperatures, possibly intensified by the effects of polar air incursions forming cold fronts into the Brazilian Highlands. Although these taxa are sensitive to changes in temperature, the combined pollen and speleothems proxy data indicate that this montane rainforest expansion during Heinrich Stadial 1 Event was triggered mainly by a less seasonal rainfall regime from the subtropics to the equatorial region.

The origin of disjunct vegetation types in mountain landscapes of southeastern and central Brazil that display a strong affinity to wet montane floras of northern South America remains unknown. Earlier hypotheses 1,2 suggested cold and wet migration corridors possibly in the Eocene or Miocene, later affected by the Quaternary climatic change, might have allowed ancient contact between plant populations now isolated on distant mountaintops.
In this study we investigate the impact on tropical montane vegetation of an enhanced South American Summer Monsoon (SASM) regime between 18.1 and 14.7 kcal yr BP in synchrony with glacial episodic iceberg discharge in the North Atlantic, known as Heinrich Stadial 1 (HS1), as indicated by speleothem isotope records [3][4][5][6][7] . Oxygen isotopes in the Botuverá cave speleothems have indicated that wet phases prevailed during the last glacial cycle in southern Brazil 8 . Additional support for this scenario comes from calcite deposits at lake margins within caves and expansion of wet forests in the HS1 of northern Bahia, currently covered by semi-arid vegetation (caatinga), suggested by abundant plant megafossils in calcareous tufas, belonging to arboreal and herbaceous taxa presently found in the Atlantic and Amazon rainforests 9 .
We hypothesize that the intensified precipitation within the area climatologically affected by the South Atlantic Convergence Zone (SACZ) and the Intertropical Convergence Zone (ITCZ) promoted conditions suitable for the establishment of north-south migration corridors for the expansion of montane forest. These possible former connections between southeast and central Brazil, from 31° to 22°S lat. and southeastern Amazonia could, therefore, explain much of the modern occurrence of disjunct humid and cold-adapted taxa in elevated areas of cerrado and semi-arid caatinga reaching 4°S, with counterparts in the tepuis of the Guyana Shield, including those in Roraima (northern Brazilian Amazonia) and in Venezuela.

Methods
We infer vegetational and correlated climatic changes during the HS1 event by analyzing selected arboreal pollen taxa, in most cases with abundance higher than 5%, in Brazilian Late Quaternary pollen records, with morphological features that allow identification to genus level and in some cases only to family 10  Forst., Hedyosmum Sw., Ilex L., Myrsine L., Symplocos Jacq., and Weinmannia L., as well as the families Myrtaceae Juss., Ericaceae Juss. and Arecaceae Bercht. & J. Presl (sin. Palmae Juss., palms), chosen based on their frequent presence in glacial pollen signals of tropical America [11][12][13][14] . It is noteworthy that in southern and southeastern Brazil, Podocarpus is represented by two species in the highlands, i.e. P. lambertii Klotzsch ex Endl. and P. sellowii Klotzsch ex Endl. Pollen rain analyses in this area 15 indicate that its pollen counts can be as low as 0.8% to indicate significant presence in native coastal rainforests.
These taxa are stenopalynous; although some of them may contain thousands of members, their pollen is represented only by one morphological type, and thus does not permit separation of species i.e., Myrtaceae Juss. Although palm pollen can be assigned to different genera, it frequently appears in pollen records simply as Arecaceae or Palmae.
Montane species of the families Myrtaceae, Ericaceae and Arecaceae were verified in herbarium collections of The Field Museum of Natural History and the Department of Botany of the Institute of Biological Sciences of the University of Sao Paulo and in the Flora do Brasil 2020 16 . Myrtaceae is represented by a total of 17 genera and 53 species. Ericaceae, an almost exclusive montane family of shrubs and trees comprised of 12 genera, 99 species and 27 varieties, commonly found in high altitude montane ecosystems of Brazil where they are subjected to sub-zero temperatures at certain periods of the year 2 . The palm family Arecaceae is represented on montane with subtropical humid climate by 5 genera and 19 species. A list of montane species for these three families are given in Supplementary 45 with 01-minute spatial resolution, draped by a custom hypsometric color scale. For the continental area, shaded relief illumination is from 060°N, 30° above horizon, with 40 times vertical exaggeration. In the oceanic area, illumination is from 060°N, 20° above horizon, with 5 times vertical exaggeration. Raster shading and color scale creation 46  Northward migration during HS1 conditions. The fossil pollen data reveal constant presence of humid forest elements in montane corridors following three major routes during the HS1 event: a northward expansion linking southern-southeastern Brazil to southern Amazonia, via Serra do Mar/Mantiqueira, central Brazil, Serra Geral, Serra de Carajás, hereafter Southern-southeastern Brazil to southern Amazonia (route SSA) and a northward expansion linking southern-southeastern Brazil to northeastern Brazil, via Serra do Mar/Mantiqueira, Serra do Espinhaço, Chapada Diamantina, Serra de Ipiabapa, hereafter Southern-southeastern Brazil to northeastern Brazil (route SSN) and a more restricted distribution connecting southern/southeastern to central Brazil, extending from 30°S to 18°S, hereafter Southern-Southeastern Brazil (route SSB; see Fig. 2, lower right). The SSA and SSN pattern are evident for Podocarpus, Ilex, Myrsine, Hedyosmum, Myrtaceae and Arecaceae, describing an arch-like direction. Some of these taxa extended from the southern/southeastern coast at latitudes as low as 30°S, to the central elevated regions of the Brazilian subtropics, thus reaching southeastern Amazonia at 5°S (Podocarpus, Hedyosmum and Arecaceae) and 4°S within the modern Cerrado/Amazon rainforest ecotone of northern Maranhão State (Myrtaceae, Myrsine) (routes SSA and SSN, respectively; Fig. 2). It is noteworthy that although most species participate in these different pathways, their environmental characteristics are fundamentally different. SSA is associated with the humid and warm Amazonian lowlands, while SSN is associated with www.nature.com/scientificreports www.nature.com/scientificreports/ xeric environments such as the semi-arid caatinga of the northeast, and SSB is restricted to humid and cool habitats of the southern high-elevation mountains.
We hypothesize that modern plant distributions on isolated mountaintops of northeastern Brazil indicate that various rainforest arboreal taxa 2,47 possibly reached these regions through a secondary fork-like branch emerging at 22°S and extending until 18°S into the Brazilian highlands, very likely to have been maintained by orographic rains and/or adiabatic cooling. In addition, from 18°S northwards, the SSN route could have reached latitudes within the present-day domain of semi-arid caatinga. Although this additional route during HS1 cannot be confirmed by palynological analyses due to the lack of study sites in that region, macrobotanical evidence in calcareous tufas deposited at ca. 17 kcal yr BP 48 indicate the presence of Atlantic/Amazonian rainforest elements such as Aparisthmium Endl., Annonaceae Juss., Chrysobalanaceae R. Br., Drymonia coccinea (Aubl.) Wiehler, Luehea grandiflora Mart., Prunus sellowii Koehne, Sloanea L., Tetrastylidium Endl. at 10°S lat. during Heinrich Stadial events in Northeastern Brazil.
Route SSB, utilized by Araucaria, Drimys, Symplocos, Weinmannia and Ericaceae, appeared to have had a more restricted distribution in Southeastern Brazil limited within a latitudinal band between 30°S and 18°S. This pathway appears to have two different latitudinal components: a coastal distribution at low elevation from 30°S to 22°S (Araucaria, Drimys, Symplocos, Weinmannia, and Ericaceae) and a high montane component from 22°S to 18°S (Araucaria and Drimys).
All migration pathways suggest that during HS1 a long chain of mountains, starting at the Serra do Mar, followed by the Serra da Mantiqueira and deriving into the Serra do Espinhaço and the Brazilian Central Plateau, functioned as an efficient corridor for the migration of montane elements as proposed by earlier biogeographers 49 , www.nature.com/scientificreports www.nature.com/scientificreports/ possibly under an enhanced humid phase in the SACZ and ITCZ climate domains [5][6][7] . Also, the occurrence of cold-adapted trees such as Araucaria beyond their present-day northern limit suggests the possibility that intensified polar air incursions into the interior of South America might have dropped the temperatures during the HS1 period as has been inferred for the last glacial period 37,50 . During that time, the tilt of the Earth´s axis, the eccentricity of the orbit and the longitude of the perihelion may have affected the solar radiation at the top of the atmosphere. Therefore, it is possible that under a scenario of less intensified solar radiation during HS1 polar circulation became somewhat stronger while the Hadley circulation was weakened and this mechanism is likely to have generated a larger displacement of polar air towards northern South America 51 .
Under these conditions of enhanced humidity and lowered temperatures, the maintenance of both routes appears to be controlled by different pollen and seed dispersal abilities. Most taxa that migrated further along the SSA route have wind-dispersed pollen and are dioecious, two evolutionary traits directly linked to high dispersal potential 52 , in addition to seed dispersal by birds ( Table 1), all of which may have granted them increased ecological amplitude and greater colonization ability. Moreover, these taxa can occur in both late and early successional stages, which can be interpreted as highly adapted to these vegetational changes.
Taxa displaying the SSN route, in general, are characterized by slightly less efficient dispersal of pollen in terms of distance by entomophily as well as zoochory, other than bird-dispersed, seeds. We hypothesize that these these main routes were actually transects of high montane microrefugia alignments during the humid and cold phase of HS1 and probably in earlier glacial phases. Therefore, the small nuclei of montane vegetation acted as sources of immigrants to other microrefugia along theses routes, where all dispersal syndromes were highly efficient, thus permitting geneflow between these populations. Support for this hypothesis comes from a modernal relictual Podocarpus microrefuge in semi-arid vegetation at Morro do Chapeu, Bahia at 11°S 53,54 . There, rocky otucrops reduce mean annual temperature by 5 °C and augment humidity by 12%. Under this scenario, final coalescence of these microrefugia may have formed rather contiguous humid forested corridors. Testing these hypotheses requires more pollen records especially at lower latitudes of southern Amazonia and northeastern Brazil. This paleoclimatic scenario is supported by a marine pollen record off the coast of northeastern Brazil 55 , containing continental sediments generated in the region of modern semi-arid vegetation. This record shows an HS1 pollen zone, characteristic of humid climate, with significant percentages of SSA route taxa such as Hedyosmum, Ilex, and Myrtaceae, together with Symplocos and Cyathea Sm. It is clear that entomophilous pollination and animal-dispersed seeds did not hinder the very long dispersal ability of Drimys possibly due to the beneficial effects of nearby microrefugia and therefore shorter dispersal distances needed for population expansion. The data suggest that the SSN route could have extended well into northeastern Brazil, where Drimys, Hedyosmum, The main feature of route SSB is the northward expansion of Araucaria, a major element in ombrophilous rainforest of high elevations of southern/southeastern Brazil in consortium with Podocarpus. It can be considered a good proxy for relatively low temperature and high humidity 12,13,27,56,57 . Our hypothesis suggests that its migration pattern reaching 18°S was likely to be controlled by low temperature regimes and low precipitation variability throughout the year during HS1. Although low modern temperature regimes at the Serra do Espinhaço (site 17) are conducive to sustaining gymnosperm forests as they did during HS1, the site no longer has adequate humidity due to irregular distribution of precipitation during the long dry season.
Hereafter represented by Podocarpus, demonstrate wider latitudinal expansion abilities, ranging from 30°S to 0°S, than those displayed by SSB, hereafter represented by Araucaria (Supplementary Figs. 1-11), where taxa were restricted to latitudes between 30°S and 18°S.
Climatic changes and montane vegetation expansion during HS1. Pollen histories of arboreal taxa common in humid and cold forests of glacial age in Brazil 12,13,23,37,40 support the hypothesis of intensification of transient climatic systems under glacial regime during HS1. We suggest that continental surface temperatures lowered by the effects of polar air incursions in South America had a significant forcing effect on modern plant biogeographical patterns by fostering long-distance migration of currently montane elements. One of the best-known lines of indirect evidence for this hypothesis is given by the vegetation of the Pico das Almas (13°34′S), in the Chapada Diamantina mountain range, northeastern Brazil. This vegetation is floristically more related to that of the Andean paramo and subparamo forests 2 with genera like Podocarpus, Drimys, Symplocos, Weinmannia, and Hedyosmum (SSA pattern), and to the flora of the tepuis of Venezuela. The second line of indirect evidence is the pollen record of the Icatu site at 10°S, currently under semi-arid vegetation in northeastern Brazil, which shows during the Pleistocene/Holocene transition 58 at c. 11 kcal yr BP abundant cold and humid-adapted taxa such as Podocarpus, Ilex, Myrtaceae and Hedyosmum coexisting with Humiria Aubl., a taxon of shrubs to large trees of the Guyanas and Guyana-influenced Amazon and currently present at Serra do Espinhaço range above 1000 m altitude 59 .
Although alternative and viable hypotheses for such biogeographical patterns may suggest that connections could have been established during the cold phases of the Oligocene 1,2 , following the tropical decline of the Eocene, Late Pleistocene pollen data points to a powerful reorganization of ecosystems in South America during the terminal phases of the last glacial cycle. To test this Late Quaternary expansion of cold and humid forests, i.e. augmentation of the potential distribution of the fundamental niche of our selected montane taxa into northern Brazil, we generated a Species Distribution Model (SDM) for Podocarpus and Araucaria for the HS1 phase in the Brazilian Highlands, by correlating observations of taxa occurrences in palynological records, representatives of the SSA, SSN and SSB routes, with monthly mean of precipitation and surface temperature from climatic layers from the CCSM3 Trace21k dataset 44 (Fig. 3).
The resulting maps highlight areas of habitat suitability over large sections of the SSA and SSB routes especially those between 30°S and 14°S, a region where the South American Monsoonal System was intensified during HS1 6,7,60 . The disjunct high suitability for Podocarpus for the Carajás lakes region (site 25) and very low suitability for Lagoa do Caçó 41 (site 27) might be due to the absence of continental pollen records in these equatorial and subequatorial regions in Brazil during HS1. On the other hand, a marine pollen record off the coast of Ceará 61 , showing expansion of cold-adapted montane arboreal elements during HS1, provide strong evidence for high humidity levels brought about by the southward displacement of the ITCZ 6 . This scenario is confirmed www.nature.com/scientificreports www.nature.com/scientificreports/ by CCSM3 Trace21k dataset 44 simulation data analyses for current semi-arid Apodi region (<250 m elevation, mean annual temperature 28.5 °C), nearby Caçó lake, indicating average annual accumulation of 2650 mm associated with high precipitation levels of monthly mean Convective Precipitation Rate (PRECC) and mean annual surface temperatures (TS) of around 22.6 °C, of c. during HS1 and (Fig. 4). Therefore, we estimate a temperature depression of c. 5 °C, which is supported by similar cooling between ca. 14-17 Krs BP, revealed by noble gases paleotemperature record in groundwater in northeastern Brazil 62 . In addition to conducive temperature, precipitation values are also well above the ecological requirements for sustaining a cold and humid forest vegetation with Podocarpus at Caco Lake, currently under mean annual precipitation of 1400-1500 mm and mean annual temperature of 25 °C 38 .
The frequency of Podocarpus in pollen records analyzed depict a strong latitudinal control. In the southernmost range of its distribution (30°S-22°S) it could be found in elevations ranging from sea level to 1157 m, followed by an intermediate range from north of 22°S to 10°S where it was restricted to elevations higher than 610 m (Fig. 5a). In its northernmost limit, lying between 6°S and 3°S, Podocarpus eventually reached the Amazonian  www.nature.com/scientificreports www.nature.com/scientificreports/ lowlands, thus descending from 540 m to 80 m elevation (Fig. 5a, lower right), next to Caçó Lake. Araucaria, on the other hand, a good representative of the SSB route, shows a dissimilar distribution pattern in the HS1 pollen records ranging from 30°S at sea level to 750 m at 26°S with its northern limit during the glacial times at 18°S in 1246 m elevation, with maximum elevation reached at 1820 m at 22°S. The separating line between sites north and south of 18°S (Fig. 5b) suggests the northernmost limit for incursion of polar air masses during HS1, a fact that possibly imprinted a biogeographical boundary still observed in the modern southeastern Atlantic rainforest with a higher plant species turnover 63 north of 18°S. Molecular genetic data from three common species of tree frogs widely distributed along the Atlantic rainforest 64 support forest stability in the central core area of this vegetation type in the late Pleistocene. Combining these with our results, it becomes clear that stability of forest physiognomic persisted during the Late Pleistocene concurrent with migration of cold-adapted plant species northwards propelled by H1 events, which in turn might have contributed to the establishment of biogeographical compartmentalization of the montane coastal vegetation specially north of 18°S.

Modern parameters and the climate of HS1 for the brazilian highlands. Podocarpus L' Her. ex
Pers. and Araucaria (Bertol.) Kuntze stand out for beeing more ecologically informative in terms of precipitation and temperature requirements in southern and southeastern Brazil. Species Distribution Model (SDM) maps for A. angustifolia, P. lambertii and P. sellowii, shown in Fig. 6, confirm a geographical restriction of Araucaria in southern/southeastern Brazil as opposed to a large distribution of Podocarpus, ranging rom 30°S to ca 5°S. Data cleaning methods was used to check the quality of data of modern distribution 42 , i.e. validation of the taxonomic identification in relation to the available literature with its corresponded latitude and longitude coordinates was performed. Georeferencing errors were evaluated and discarded from the database and a filter was applied to select only occurrences within the Brazilian territory.
At the northernmost edge of this distribution range, Podocarpus is found in relictual populations in orographically controlled humid "islands" within semi-arid climates 53,54 , isolated in the Pleistocene, as indicated by genetic data 65 . Anotherline of evidence for expanded montane forest in this region is the discovery 9,48 of fossilized leaves of rainforest taxa found in the late glacial calcareous tufas in a modern Caatinga region adds more support for this climatic mechanism during Heinrich Stadial events throughout the last glacial period. However, the mode that humid adapted plants migrated to present-day semi-arid regions in South America is still unknown.
The current distribution of Araucaria angustifolia reaches its northern limit at 21°S in any elevation, at altitudes of about 400 to 1750 m above sea level (asl) under less than three months dry season, whereas from 24° to 31°S this taxon occurs as low as at sea level up to ca. 1200 m, under year-round high humidity levels. www.nature.com/scientificreports www.nature.com/scientificreports/ In order to investigate the factors controlling its modern distribution as well as Podocarpus lambertii occurrences, we used a 15-year long data set of rainfall data CPC Morphing technique (CMORPH) 66 , corrected by the Brazilian meteorological station network data 67,68 , between 2000 and 2015. Afterwards, R-scripts were written to generate descriptive statistics for the occurrence of each taxon. Box plots and histograms were generated based on values of 1° × 1° latitude grid of total daily accumulated precipitation data. These analyses yielded mean values of precipitation for 6 months periods, for the seasons, months and annual precipitation for that historic series. PCA biplots shown on Fig. 7 indicate that annual and summer variables are the main loading values for the PC1 (84.11%) and PC2 (15.15%) components, respectively. The simultaneous occurrence of Araucaria angustifolia and Podocarpus lambertii is discernibed as a main central cluster (Fig. 7a) for 1750 mm annual precipitation. Outside this range, only Podocarpus can survive as it tolerates higher precipitation variability (Fig. 7b). The latitudinal control for Araucaria is significantly stronger when compared to Podocarpus that can thrive in microrefugia in latitudes outside the Araucaria-Podocarpus associations typical of south/southeastern Brazil (Fig. 7d,e). The PCA diagram of the combined distribution, with overlapping clusters, of both taxa shows that the modern occurrences of Araucaria angustifolia and Podocarpus lambertii in Brazil are controlled mainly by mean total annual precipitation of 1680 ± 180 mm and 1520 ± 220 mm respectively based on a 15-year climatic series (2000-2015). These values range well within the classical precipitation values used in the literature for both taxa for the Brazilian highlands.
PCA biplots for Araucaria (Fig. 8a) reveals annual and summer variables are the main loading values for the PC1 (71.47%) and PC2 (27.07%) component. PCA loadings indicate that the distribution of Araucaria angustifolia is primarily influenced by anual acumulated precipitation and well distributed precipitation long the year, both in dry and wet seasons, respectively. Figures 8b,c indicates low precipitation variability as a significant parameter for explaining Araucaria distribution (Fig. 8d) under modern conditions. Therefore, it's clear that this taxon does not tolerate dry conditions even during the dry months, showing an optimum value of precipitation around 500 mm for the period of March to August.
We suggest that individualistic species reshuffling in the three migration routes during HS1 was constrained by their physiological responses within these temperature and precipitation ranges. Morevorer, the climatic confinment in terms of humidity stability possibly explains why Araucaria did not migrate further than lat 18°S in route SSB during HS1.

conclusions
Our analyses provide strong evidence for the establishment of migration corridors of montane forest taxa, facialtitated by the presence of microrefugia, connecting different ecosystems in a continental scale, during humid and cold conditions related to Heinrich Stadial 1, from 18.1 to 14.7 kcal yr BP. Analysis of published Brazilian fossil pollen records corroborate with a climatological scenario of high humidity sustained by the South American Monsoonal System under lowered temperatures resulting in more regular polar air mass incursions, reaching 18°S latitude, thus forming frequent cold fronts advancing northwards, into Amazonia and northeastern Brazil. Consequently, the two main migration routes, here coined SSA and SSN, respectively, were established. On those montane pathways, different plant taxa were able to disperse taking full advantage of certain reproductive traits, especially anemophilous pollen and bird dispersed seeds. It's possible, that effective migration along these routes were facilitated by the presence of previous montane forests microrefuges, which might have expanded downslope towards the lowlands during HS1, thus forming coalesced vegetated corridors. The proximity bwtween these established microrefugia permitted a series of local population expansions along the three montane routes during these cold and humid events.
It becomes clear that mean annual temperature depression played a major role during the wet and cold HS1 phase in Brazil of at least 5 °C, as indicated by temperature estimation based on noble gas paleotemperature records not for Last Glacial Maximum but also for HS1 62 . In addition, the presence of modern humid microrefuge containing montate taxa such as Poducarpus 53 within hyper-xerophylous caatinga is indicating a previous wetter climate conditions that took place during the establishment of SSN route.
The Brazilian highlands latitudinal range of 30°S to 18°S, conducive to expansion of montane vegetation with Araucaria during HS1, has probable ecological limits controlled by temperature. However, in its northern boundary of the range, intense monsoon rainfall caused shorter seasonality in comparison to its current limit at 21°S while mean annual temperatures are today approximately similar. Currently, at 18°S latitude, long dry seasons of 5 to 6 months inhibit the growth of Araucaria angustifolia. In comparison, its fossil pollen occurrence during HS1 implies in a well-distributed precipitation throughout the year. By integrating the pollen data with speleothem isotope records 3,5,7 , we can assume that South American Monsoon regime during HS1 was longer with higher annual precipitation rates.
Unlike Araucaria and other taxa that followed the SSB migration route, Podocarpus and others were able to migrate further north and beyond the highland domain, thus reaching distant lowlands regions due to their ability to disperse pollen and seeds more efficiently, and to germinate and grow in understory and dark forests. Their enlarged biogeographical range during HS1 primarily reflects a close approximation of their fundamental niches in opposition to their realized niche after the onset of Holocene warm climates. This significant habitat reduction is likely to have established modern disjunct distribution patterns between southern/southeastern and northeastern Brazil.
We, therefore, conclude that, although we do not devalue a possible scenario of early migration corridors during the Eocene/Miocene 1,2 of Brazil, a significant imprint of the HS1 in delineating modern disjunct distributions there is unquestionable.
One of the major implications of this study deals with possible impact of rising temperature predicted for future climatic scenarios on a substantial loss of tropical montane biodiversity. Our investigation suggests that the change in rainfall distribution may potentialy enhance this process.