Marin Hills

California buckeye has begun its midsummer dormancy across the inland hills, dropping leaves while still bearing open flowers and developing fruits. Observations from early July across a broad belt from the Presidio through Orinda show the same pattern: this native deciduous tree is shedding in response to heat and water stress, not the cold dormancy that characterizes temperate deciduous species elsewhere. It is a survival strategy finely tuned to the Mediterranean climate.This synchronized phenology — leaf fall, flowering, and fruiting happening together — is no accident. The Ohlone people harvested the forming nuts in this exact window, leaching out the bitter toxins and rendering them edible. That knowledge, built on generations of observation of this annual rhythm, reflects an ecological understanding that is only now being formally documented in scientific phenology records. The timing was never arbitrary; it was precisely calibrated to the tree's own biology.

Bay water temperatures hit 20°C this week, a threshold where petroleum compounds become significantly more toxic and dissolved oxygen drops sharply. The timing is sobering: the refinery upstream fell into permit violation just as seasonal warming arrived, doubling the chemical stress on water already listed by the EPA as impaired.The bay has shown real recovery over the past decade — native fish and invertebrates have returned, water clarity has improved. But that recovery and active industrial discharge are now occupying the same system. Warmer water means faster chemical reactions and less oxygen for organisms that evolved in cooler, cleaner conditions. The margin between restoration and fresh damage has never been thinner.

Tiburon mariposa lily (Calochortus tiburonensis) is fading from the serpentine hills here — a bloom window that closes by late June each year. This lily exists nowhere else on Earth, and its toxic, nutrient-poor soils have been its shield: where those minerals discourage invaders, the plant persists. Science only named it in 1971; for millennia before that, it simply bloomed and endured.About 200 observations documented the bloom across May and into June this year. The Marin dwarf-flax, another federally threatened serpentine endemic, blooms alongside it — two rare plants sustained by the same hostile substrate that has excluded the competition and development that would have erased them long ago.

The rocks along the Tiburon shoreline are wet from the last tide and warming fast in the summer sun. Where the water pulled back, the intertidal zone is exposed: barnacle-crusted granite, mats of green algae, and the dark crevices where striped shore crabs spend the hours when the sea is away from them.Striped shore crabs are flat, quick, and about the size of a closed fist. Their carapace is mottled olive and brown with thin pale lines running across it, which makes them nearly invisible against wet rock until they move. And they do move, constantly, during low tide. They forage along the intertidal margin, turning over small stones, scraping algae, picking apart anything soft enough to eat: dead fish, invertebrates, plant matter, whatever the tide left behind. They are not specialists. They eat what is available, and in summer, with water temperatures rising in the bay and biological productivity near its peak, there is a great deal available. The crabs are most active during the hours after a low tide retreats, working the exposed zone before the water returns. This is not a choice so much as a constraint: they breathe through gills and must stay wet, so they work close to the water's edge and retreat into crevices or beneath rocks when conditions dry out too much or when something larger arrives.That something larger is often a black-crowned night heron, which forages along these same rocks in the low-light hours of early morning and evening. The herons stand still for long intervals, then strike downward with the bill in a single fast movement. Crabs are a regular part of what they take. Brown pelicans work the open water just offshore, diving for fish, but they occasionally land on exposed rocks and will take a crab given the chance. The competition for intertidal prey during summer is real and layered: multiple species reading the same tidal schedule, converging on the same narrow strip of rock at the same time. The crabs are not passive in this. They are fast enough to escape under cover if they have a moment's warning, and their flattened bodies let them press into gaps that a bird's bill cannot follow.The red-tailed hawk is less directly tied to the intertidal zone, but it is present in the hills and open slopes above Tiburon and hunts the transition zones between habitat types. A hawk working the edge where rocky shore meets scrub is watching for movement. A crab caught in the open on a flat rock, away from cover, is exposed in the same way a vole is exposed crossing a gap in the grass. Red-tailed hawks are generalist predators; they take what presents itself. Whether one takes a shore crab opportunistically or simply passes overhead is a matter of timing and distance, but the pressure from above is part of why these crabs stay close to cover even when foraging.The broader intertidal community here includes species the crabs interact with more continuously. Barnacles and mussels are prey for the crabs when small enough to manage. The algae the crabs graze supports its own chain of smaller invertebrates. Striped shore crabs also turn up as prey for larger fish during high tide, when the water covers the rocks and the crabs move into the shallows. Their role shifts depending on the hour and the water level: forager during low tide, prey during high.Summer is when the crabs are breeding. Females carry egg masses beneath their abdomens, held against the body by the curled tail flap. You can see this if you find a female under a rock: a dense, spongy cluster of orange eggs pressed against her underside. She will stay in cover more consistently while brooding, limiting her foraging time. The larvae, when they hatch, are planktonic and will spend weeks drifting before settling back into the intertidal zone.Look down at the rocks nearest the water's edge. If the tide has pulled back in the last hour, watch for movement near the line where wet rock meets dry. The crabs are there, working the margin, pulling back when a shadow crosses the stone.

The summer air carries the scent of dry grass and salt from the bay. Here on the slopes above Tiburon, the longest days of the year have arrived, and with them, something rare unfolds in the coastal scrub. Step outside if you can, or find a window that opens to summer light. The heat builds slowly in the morning, and by noon, the hillsides shimmer.In this narrow band of serpentine soil, the Tiburon mariposa lily opens its three-petaled flowers to the June sun. Each bloom spans three inches across, white petals marked with purple chevrons at the base, surrounding a cluster of yellow anthers. The lily grows from a bulb buried six inches deep in the rocky soil, sending up a single stem that can reach two feet tall. This threatened wildflower exists nowhere else on earth except these serpentine slopes in Marin County. The plant spends eleven months of the year as an underground bulb, emerging only when the soil warms and the rains have stopped.The mariposa lily shares this harsh ground with other rarities. Long-rayed brodiaea, also threatened, clusters its purple flowers on nearby slopes. Marin dwarf-flax, another endemic, opens tiny white flowers among the rocks. These native plants have adapted to serpentine soil, which contains high levels of magnesium and heavy metals that most plants cannot tolerate. The invasive pale flax and common cat's-ear struggle here, giving the natives an advantage in their own territory. The rocky, nutrient-poor soil that challenges most plants becomes a refuge for species that evolved with it. Each lily plant may live for decades, blooming only when conditions align perfectly. The bulbs can remain dormant through drought years, waiting for the right combination of winter rain and summer heat.Bees and butterflies find the mariposa lilies during their brief flowering window. The flowers produce nectar deep in their cups, accessible to long-tongued pollinators. Each successful pollination produces a three-chambered seed capsule that splits open in late summer, releasing seeds that may take seven years to produce their first flower. Most seeds never germinate. Those that do must navigate the narrow window between too much moisture and too little, between the spring rains and the summer drought. The population here numbers in the hundreds, each plant a survivor of this precise timing.Close your eyes and feel the summer heat building on your skin. Somewhere on these slopes, white petals catch the light, opening wider as the temperature climbs. The air shimmers above the serpentine, and the scent of warming earth rises from the ground.

The hillsides around Tiburon hold their color differently in late spring. Where winter rains once darkened the soil, patches of ruby chalice clarkia now bloom in clusters of deep pink, their four-petaled flowers catching the morning light. The Douglas-fir trees show fresh green needle tips, and the poison oak leaves have settled into their glossy three-leaflet patterns. This is the season when the landscape and its smallest aerial residents move in careful synchrony.Anna's Hummingbirds are deep into their nesting season now, the males' territorial calls sharp and frequent from the coyotebrush and willow branches. A female hovers at the clarkia blooms, her needle-thin bill probing each flower for nectar. She moves methodically from bloom to bloom, her wings beating so rapidly they blur into transparency. The timing is precise. The clarkia flowers produce their richest nectar in the morning hours, and the hummingbirds have learned this schedule. As she feeds, pollen grains stick to the feathers around her bill and throat, pale yellow dust that she will carry to the next patch of flowers.The relationship runs deeper than simple feeding. Ruby chalice clarkia evolved alongside hummingbirds, its deep pink petals and tubular flower shape perfectly matched to the birds' feeding behavior. The flower's stamens extend just far enough that a hummingbird must push past them to reach the nectar, ensuring pollen transfer. Other pollinators visit these flowers, bees and butterflies among them, but the hummingbird's hovering flight and long bill make it the most effective pollinator for this particular plant. The female Anna's Hummingbird needs this concentrated energy source now more than ever. She is building a nest, a tiny cup of plant fibers and spider silk bound together and camouflaged with lichen, hidden in the fork of a willow branch. Each day she adds more material, and each day she must fuel both her own metabolism and the development of her eggs.The clarkia blooms will continue for several more weeks, their peak abundance timed to support not just the hummingbirds' nesting season but the broader community of pollinators that emerge in late spring. Native bees work the flowers in the afternoon heat, and Gray Buckeye butterflies visit when the light begins to soften toward evening. The pitted onion and Ithuriel's Spear add their own nectar sources to this seasonal feast, their purple and blue flowers creating a mosaic across the grasslands. Listen for the hummingbird's call, a sharp chip that cuts through the morning air, often repeated in rapid succession when she encounters another bird near her feeding territory. The sound carries farther than you might expect from such a small bird, a declaration that these particular flowers, this particular patch of hillside, are spoken for.