The research was conducted near Agüero in the province of Huesca, northeastern Spain, on a south-facing slope at 750 metres elevation under a continental Mediterranean climate. This is a world of two favourable seasons and two hostile ones: spring and autumn bring the moisture that fuels growth, while winter frost and summer drought impose hard limits on what a tree can do and when. The study plot, an 800 by 100 metre stretch of open scrub and scattered trees on a Calcisol formed over Miocene clays, held more than 60 adult trees of each oak species, evenly distributed and showing no sign of habitat segregation. That coexistence is precisely what makes the site scientifically valuable, because it allows the two species to be compared under identical conditions.

The team’s phenological work rested on a long-term research programme running from 1997 to 2010. For the new analysis, 15 individuals of each species were followed from 2006 to 2009, with monthly sampling that intensified to every two or three weeks in spring. Observers estimated the percentage of each crown displaying 14 carefully defined phenophases, from winter dormancy and bud burst through shoot growth, bud formation, flowering, fruit development and leaf senescence. Branch samples were preserved in a pheno-morphological herbarium, creating a verifiable historical record. The result was a detailed seasonal calendar for each species, showing when every visible stage of the annual cycle begins, peaks and ends.

The organogenetic component demanded even greater patience. Because leaf units are packed densely inside tiny buds, complete dissection of even a single bud is extraordinarily laborious. The researchers therefore limited this part of the study to a single year, 2007, and two replicate trees per species, selecting fruit-bearing individuals so that reproductive structures would be represented. Across the year they fully dissected 123 buds of Q. faginea and 111 of Q. ilex down to the apical meristem, measuring every component under a stereomicroscope at 10 to 100 times magnification: scales, cataphylls, leaf primordia, stipules, axillary buds, hypsophylls and the primordia of male and female inflorescences.

What emerged from the dissections was a picture of remarkable preformation. Organogenesis in both species began in early spring, around the time of bud swelling in the deciduous oak and bud burst in the evergreen, and finished by mid-September, with bud development complete by November. By late summer, each winter bud contained the entire embryonic shoot for the following year, including axillary buds and fully preformed male inflorescences. Female inflorescence meristems, by contrast, were initiated surprisingly early but remained minimally developed, persisting as simple axillary meristems through winter and resuming growth only when the bud swelled in late winter, with flower initiation following in the bract axils and continuing until anthesis.

The two species ran strikingly similar programmes on slightly different clocks. Q. faginea burst its buds between mid-March and early April, roughly a month before Q. ilex, whose cycle began between mid-April and early May. In both species, most phenophases peaked in spring, and phenology and organogenesis proved tightly synchronised, advancing in parallel through the year. The deciduous oak produced larger buds with more nodes, while the evergreen packed in more leaf primordia despite smaller buds. A key anatomical difference emerged in the position of female flowers: Q. ilex placed them in the axils of mid to distal leaves, most often between the fourth and seventh leaves below the hypsophyll, whereas Q. faginea placed them more distally, typically in the three outermost leaves. That positional difference, the authors found, explains why the evergreen initiated its female inflorescences almost 15 days earlier than the deciduous species, despite starting organogenesis later.

This timing matters because the Mediterranean summer forces hard choices. Plant growth proceeds in two phases: slow, temperature-sensitive cell division, followed by potentially rapid cell expansion that can swell cell volumes ten- to twenty-fold. When shoot extension, fruit development and bud organogenesis all demand resources at once, something must give. The study’s first hypothesis, that organogenesis and shoot extension compete when they overlap, was supported. The evergreen oak, with its later bud burst and slower leaf maturation extending into September, finds its window for bud development compressed precisely when shoot growth is most vigorous. The deciduous oak, by finishing shoot extension early, enjoys a longer, less contested window. This, the authors argue, explains the alternating years of high and low shoot production observed in Q. ilex but not in Q. faginea: big shoot years leave less room for bud development, producing smaller buds and reduced growth the following year.

The second hypothesis fared less well, and the surprise it produced may be the study’s most consequential finding. The team had expected female inflorescence initiation to coincide with peak bud development and the onset of maximum fruit growth, providing a direct mechanism for the post-mast crash in flowering. Instead, initiation in both species preceded peak fruit development, and the evergreen started earliest of all. The authors propose a refined mechanism: in mast years, fruit development begins earlier than usual, with peduncles of pollinated flowers accelerating from May onwards, so that advanced fruit growth overlaps with the initiation of inflorescence meristems and the formation of new buds. In ordinary years this overlap never occurs, which is why normal fruit crops leave bud development largely unscathed, while mast years exact a heavy toll, yielding smaller buds and reduced flowering the following season. Supporting evidence comes from earlier work showing significantly reduced bud size in Q. ilex after an exceptionally abundant mast event, and from fruit-removal experiments in which mid-June defruiting shifted allocation toward leaves and female inflorescences.

The study also illuminates how these oaks survive the dry season at all. Summer-active meristems are shielded by bud scales, by scaly cupules around immature acorns, or by bark, while fine roots tap subsoil moisture. Unprotected organs, such as the lammas shoots that occasionally appear on 5 to 10 percent of branches, and occasionally far more in wet years, remain the exception. Buds, the authors note, behave as lower-priority sinks than shoots and developing fruits, succeeding only when water is ample or competition is low. Their completion also depends on early secondary growth that widens xylem vessels and sustains hydraulic conductivity before summer deficits bite.

For a biodiversity hotspot facing intensifying drought, the implications are sober. The deciduous oak’s early start extends its growing season and carbon assimilation, but a late frost or an excessively long, dry summer can wreck bud development and cascade into reduced shoot production, flowering and fruiting the next year. The evergreen, holding three cohorts of leaves and able to lengthen leaf longevity under drought, enjoys more flexibility. Ultimately, the authors conclude, the capacity of these oaks to persist and respond to climate change will hinge on their ability to synchronise phenological patterns with seasonal conditions and to maintain adequate summer growth, a balance now visible, node by node, inside the buds themselves.

Subject of Research: Bud organogenesis and phenology of coexisting evergreen and deciduous Mediterranean oak species

Article Title: An integrated study of bud development and phenology enhances our understanding of coexistence and reproduction in two Mediterranean Quercus species

Article References: Montserrat-Martí, G., Cera, A., & Hodgson, J. G. (2026). An integrated study of bud development and phenology enhances our understanding of coexistence and reproduction in two Mediterranean Quercus species. Web Ecology, 26(2), 203-221. https://doi.org/10.5194/we-26-203-2026

Image Credits: AI Generated

DOI: 10.5194/we-26-203-2026

Keywords: Quercus ilex, Quercus faginea, phenology, organogenesis, bud development, masting, Mediterranean climate, oak reproduction, resource competition, drought, shoot growth, Web Ecology

The research team turned to MaxEnt, short for maximum entropy modelling, one of the most widely used tools in modern biogeography. The technique works by comparing the environmental conditions at locations where a species has been recorded with the conditions available across the whole study area, then estimating the probability of occurrence at every grid cell. Rather than requiring absence data, which is notoriously unreliable, MaxEnt builds a picture of a species’ ecological niche from presence records alone. For a country as topographically and climatically varied as Algeria, which spans Mediterranean coastline, high plateaus and the northern fringe of the Sahara, this approach is particularly powerful because it can capture the full breadth of conditions a species tolerates.

To drive the models, the researchers assembled occurrence records for both oak species, drawing on field knowledge and global biodiversity databases, and combined them with a suite of environmental predictor variables covering temperature, precipitation and terrain. Crucially, they projected the models forward using future climate scenarios for three time horizons, allowing them to track not just whether habitat will be lost or gained, but where the geography of suitability will shift. The performance of the models was assessed using the Area Under the Curve, or AUC, a standard statistical measure of discrimination. Values above roughly 0.8 are generally considered good, and above 0.9 excellent. Both oak models exceeded 0.96, a strikingly high score that indicates the models separated suitable from unsuitable terrain with near-perfect reliability.

The single most influential variable shaping the distribution of both species was the Martonne aridity index, a classic climatic measure that combines annual precipitation with mean temperature to express the degree of water availability across a landscape. Its dominance in the models makes ecological sense. Mediterranean oaks live at the dry edge of what trees can tolerate, and their growth, survival and regeneration are tightly governed by the balance between water arriving as rain and water lost to the atmosphere. In Algeria, where warming is outpacing the global average and droughts are becoming more frequent and severe, this finding underscores that aridity, not temperature alone, is the master variable controlling the fate of these forests.

When the researchers projected the models into the future, the two species told different stories. For cork oak, the annual rate of highly suitable habitat is expected to decline under every future scenario, with projected losses ranging from 0.040 percent to 0.191 percent per year. The numbers may look small, but compounded over decades they translate into a meaningful contraction of the species’ core range in Algeria. Cork oak is already under pressure across the western Mediterranean from land abandonment, overgrazing, fire and unexplained decline phenomena, so any additional climatic squeeze on its best habitat is cause for concern for foresters and conservationists alike.

Holm oak, by contrast, showed far greater stability. Its highly suitable habitat is projected to change only slightly, with shifts ranging from a loss of 0.021 percent to a gain of 0.164 percent per year depending on the scenario. This hardy evergreen is famously drought-tolerant, capable of surviving on thin soils and withstanding long summer dry seasons, and the model results reflect that toughness. In practical terms, the study suggests that holm oak could even hold its ground or modestly expand in some parts of Algeria as conditions change, making it a candidate species for stabilising forests in an increasingly arid landscape.

Perhaps the most vivid finding concerns elevation. The average elevation of suitable habitat is projected to shift under each climate scenario, ranging from 866 to 1,025 metres for cork oak and from 978 to 1,172.5 metres for holm oak. In other words, as the lowlands warm and dry, the climatic comfort zone for both species climbs the mountainsides, and holm oak consistently occupies higher ground than cork oak. This pattern echoes a well-documented global phenomenon in which mountain plant communities shift upslope in response to warming. The catch, as ecologists have repeatedly warned, is that mountains are cones: the higher a species climbs, the less land remains available, and slow-moving trees may lag far behind the pace of climate change.

The models also revealed a striking directional signature. Under most future scenarios, the highly suitable habitat for cork oak shifts towards the northeast of the country, while for holm oak it shifts towards the northwest. These compass bearings matter enormously for conservation planning. Northeastern Algeria, with the humid coastal ranges of Kabylia and El Kala, retains some of the wettest conditions in the country, and the projected northeastward drift of cork oak suitability suggests these areas will become increasingly critical refuges. The northwestward shift for holm oak points to a different set of priority landscapes, and the divergence between the two species means that a single, one-size-fits-all protected area network will not serve both trees equally well.

The practical value of this work lies in its maps. By identifying where habitat is highly suitable now and where it is likely to remain suitable through the end of the century, the study gives Algerian foresters a decision-support tool of real consequence. Rather than reacting to forest decline after it happens, managers can use these projections to prioritise which forest stands to protect, where to focus regeneration efforts, and which zones should be preserved as climate refugia. The authors emphasise that the maps are intended precisely for this purpose: helping foresters select the best conservation strategy for cork oak and holm oak and choose appropriate forest zones to be preserved before the climate window closes.

The study also sits within a growing body of Algerian research on climate-driven range shifts, including recent work on Atlas cedar, Aleppo pine, Phoenician juniper and the endemic oak Quercus afares, all pointing in a consistent direction: the country’s forest flora is being reshuffled by warming and drying. What makes the new findings compelling is the contrast between two closely related, sympatric oaks responding so differently to the same climatic pressures. As the Mediterranean basin warms faster than the global average, the fate of these forests will depend on whether conservation planning can anticipate the movement that the models now make visible. For Algeria’s cork and holm oaks, the future is not yet written, but the map of where it will unfold has finally been drawn.

Subject of Research: Species distribution modelling of cork oak and holm oak habitats in Algeria under climate change

Article Title: Modelling the current and future potential distributions of Quercus suber and Q. ilex (Fagaceae) in Algeria

Article References: Rais, H., Laala, A., Meghzili, I., Mennour, H., & Boucenna, H. (2026). Modelling the current and future potential distributions of Quercus suber and Q. ilex (Fagaceae) in Algeria. Plant Biosystems, 160(5), Article 239. https://doi.org/10.1007/s44473-026-00241-2

Image Credits: AI Generated

DOI: 10.1007/s44473-026-00241-2

Keywords: Quercus suber, Quercus ilex, Algeria, MaxEnt, species distribution modelling, climate change, aridity index, habitat suitability, Mediterranean forests, cork oak, holm oak, forest conservation