The below are some rambling thoughts about the "Valley oak", which grows in California and is possibly the biggest species of oak that exists, and speculation into the reasons why this oak species is well adapted to the specific conditions that exist in the climate where it is naturally found. Quercus lobata is in the white oak family (which includes English oak). I can provide an insight into why this species is so big. As you go north in California, 30 miles inland from the coast, the oaks predominate right before the redwoods begin. As we all know, the redwoods are a contender for the tallest trees species in the world. So it would not be so surprising that the oak species just south of it would be a contender for the biggest oak species. The oaks do not naturally grow right near the coast, but they also do not grow too far from coast either (with the exception of spots near the foothills of the Sierra Nevada range or near river bed flood plains). You can see a detailed map of their true distribution here. They grow in the area that is only just a little bit drier and hotter than where the redwoods can naturally grow. In fact they almost may be a subspecies with Quercus garryana, which grows all the way up to the Puget Sound in Washington state. In the California climate, Quercus lobata typically leafs out in early Spring and then commonly drops many of its leaves in the Summer, when moisture runs low. Then they begin dropping the remaining leaves in mid October, but are still able to take advantage of some of the increased moisture in October before the temperatures begin going too low. (Needle shaped leaves can still function at a lower temperature than broadleaf shapes because of evaporative cooling effects) So why would this type of tree grow in this area? Because 30 miles away from the coast the daytime temperatures in early Spring will be warmer than near the cold waters of the coast, and so broadleaf shaped leaves can have the advantage, so long as there is adequate moisture and rainfall (which there is not if you go too far from the coast). March to early May is the time of year that makes deciduous oak trees favorable to grow in this area. Unlike further north in the Pacific Northwest, this area gets more rainfall in March than in November, so needle shaped leaves to be able to carry out photosynthesis right before the winter do not make as much sense. These oak trees will still have a small number of their leaves in November and can still take advantage of moisture during warmer days. I'll go further and speculate on how oak trees (Quercus garryana) and Douglas fir both coexist and each fill their own niche in the Puget Sound region of Washington state. The oak trees will typically be found in the drier more open areas, with grasses, where fire occasionally goes through. This might at first seem a paradoxical, because in a dry summer climate we might not expect the broadleaf species to thrive in drier areas than the needle shaped leaf species, which can conserve water better. However, I think it may have more to do with temperatures during the early Spring. Because in places with a high density of Douglas fir trees, I think that drops the daytime temperature due to all the shading, as well as all the evaporation of water from the high density of trees. That means the leaves need more air flow through their leaves to counter the temperature difference, since the leaves will be even colder than the surrounding air. During the summer, on the other hand, the trees might need to be better at conserving water because of the high density of trees and the competition for water (and then it can be the reverse, the leaves exposed to direct sunlight will be a warmer temperature than the air, so more air flow will help reduce water loss). Fire tolerance will have something to do with it as well, since oak trees are more tolerant to fire whereas Douglas Fir has more flammable resins. (But those resins help the tree resist rot and insects during the wetter part of the year, especially when there is more shade and the moisture does not evaporate as fast) So to summarize, the oaks (in this region) thrive in places with warm early Spring temperatures, a time of year when they are able to get a combination of moisture and warmth.
Since a thread is nothing without pictures, here's a picture someone posted of a valley oak, at an unknown location
The "ramblings" can be backed up by generalising statistics. My colleagues in Stockholm and Chicago have compiled distribution data of all oaks for investigating and mapping general leaf habit trends in course of the evolution of the genus, to see how much of it is adaptive or legacy (I was involved to some degree in the early phase), to determine their general niches and biome coverage. White oaks are a lineage that can everything, so it's worth to go a level down/deeper, systematically. Quercus lobata is a Dumosae, which represent the first diverging lineage of white oaks, and Q. lobata is sister to the remainder within them. The Dumosae go back a long way (~35 myrs old lookalikes can be found in northeastern Russia, since the ~15 myrs ago we have them in [N.] California) and are possible one of the first lineages that evolved leaf-shedding, which, in oaks, often comes with lobedness. The don't have a lot of species but all their species lineage are relatively old as well (> 10 myrs according to some preliminary tests). A feature of the Dumosae is their leaf-habit variability, sometimes even within species, you have lobed-deciduous to unlobed-evergreen or something in between or a combination of it. And while being variable in their appearances, most of them show (today) a (strong) preference for summer-dry, mild climates. Quercus lobata is near exclusively Csa+Csb, i.e. it cannot stand frequent frost or permanent snow cover in winter like the redwoods or the Douglas fir. Biome-wise it prefers what Olson et al. classified as "Mediterranean Forest, Woodland & Scrub", dry, more or less open summer-hot and dry forests but extends into "Temperate Grasslands, Savannas & Shrublands" (mild winters, periodic rainfalls, more extensive droughts, few trees, like on the photo). In short, exactly the transition you have at the (North) Californian coast and pre-Rockies as soon as you leave the seaside as you pointed out. In contrast, Q. garryana is mostly part of "Temperate Conifer Forests" (the winter-cooler or montane equivalent of the former; also the redwood biome) and the local climate setting of the Puget Sound is just to the spot compared to its general niche. What distinguishes the two settings, except for the more snow-/frost-prone winters, is also the length of the growing season. The bulk rule is, to support large trees you need at least 9 months above 10 °C. Which could be another reason the Valley Oaks can afford to become so large but don't go as far north as their relatives; they start early in the spring enjoying a perfect settings for a deciduous large tree and having ~10 months of growing season outsize any competitor after a few years. Notably, all the oldest fossils of the Dumosae from the Russian Far East showing leaves indistinguishable or very similar to Q. lobata grew under mild but humid conditions, the typical niche of deciduous and lobed oaks also in eastern N. America (or Eurasia) and are found in association with e.g. early beeches that do not tolerate any summer draught, not even the light ones. From an historical-evolutionary point of view, the Valley Oak succeeded by having found a way to adapt to the emerging and increasingly summer-dry climates that replaced the original more perhumid climates in northern California in since 20 myrs ago despite being deciduous and lobed. Maybe because, it went gigantic. Anyhow, providing a monumental example of the capacity of oaks to thrive, where most (angiosperm) trees with seemingly the same toolkits would fail.
The shape of the leaf makes sense. The main growing season of Q. lobata is very late winter to spring to the earliest part of summer. In this climate, with cool winds coming in from the cold Pacific ocean, the temperatures are mild, not too hot. The tree does not need to have leaves adapted to arid conditions. While it's true the late summers get very hot and dry, I suspect the tree stops doing much photosynthesis during that time, and probably the leaf pores are more closed to conserve water. Having a leaf shape designed for more air flow is not really going to benefit the tree, because during its peak growth time the tree wants more heat so it can begin the growing season earlier, and the wind flow in March is too cool because of the marine influence. This is the reason these oaks are not found closer to the coast. In early June, the tree has to conserve water, so I think probably the tree can photosynthesize and open its pores only in the morning, when it is not too hot. It's not like the air is dry in its climate range either, even if the rainfall is low. Other desert oak species might need to have more scraggly leaves because they need to take advantage of the water during a more limited part of the year. In these areas the time when the wet season lines up with sun and warm enough daytime temperatures will also be hot and desiccating on the leaves. In drier more continental climates, the temperature swing between night and day is greater, and when cold air is warmed, its capacity to be able to take up moisture is suddenly increased and it will be more drying. These trees have to do as much photosynthesizing as they can, which includes the hot part of the day, before the short wet season ends. Even though that wet season is cooler than the dry season. During part of the cooler wet season, temperatures may be too cold to do photosynthesizing, while for most of the rest of the daytime during the wet season, when temperatures are warm enough, the temperatures will be hot with drying air. It may seem paradoxical, but in that type of climate the trees do not have the luxury of only photosynthesizing during the part of the day with moderate temperatures, since that window of time does not last very long in the day, before the wet season comes to an end. Temperature change as the sun appears in the sky is also more rapid in dry climates. For these tree species, more air flow over the leaves is beneficial, even if that air has a drying effect, because the air will still be cooler than the temperature of the leaf (or ground surface) exposed to the sun. Loss of water will be almost entirely proportional to how much the leaf pores are open (gas exchange), which is tied to photosynthesis, rather amount of air flow passing over the leaf. Reducing temperature difference between leaf and air helps reduce evaporation when the pores are open and gas exchange is taking place. The time overlap of availability ground moisture and optimal temperatures (warm but neither too cold nor very hot) is much greater for Q. lobata in its growing range than it is for other oak species in other areas. The fact that both Q. lobata and Q. garryana have very lobed shape leaves is indicative of a dry summer climate, however, since it does help increase the air flow. But conditions are not so hot and dry that the trees need to have smaller scraggly leaves.
I'm sure you may be right, but I'd be curious why growing season would have an effect on tree size. I understand the trees would grow faster, of course, in the longer growing season area, but wouldn't we expect them to eventually catch up in size? Snow accumulation can be a significant issue for deciduous branched trees, and snow accumulation and ice (in some years) can break branches. But the Pacific Northwest does have Big Leaf maples and apparently they are able to handle the weight of snow. Why would it be more difficult for the larger sized oak tree Q. lobata than Q. garryana? Or maybe something to do with danger of water in the vascular part of the trunk freezing and causing air bubbles? But Douglas Fir can grow very tall. Perhaps Q. lobata just is unable to compete with Douglas Fir in the more northern range where there is enough moisture? And perhaps the areas where Q. garryana grows just do not have quite enough moisture for the larger sized Q. lobata to grow? I'm not really sure which of these two oak species need more moisture, or whether they may be the same. Further north may get more rain, but its also true the percentage of rain tends to drop off earlier in the year, and Q. garryana tends to grow in the areas more dry than where forests of Douglas Fir are found. (The oaks like more heat, and will not grow well in an area shaded by a dense dark forest in the PNW climate, since the moisture comes during the cool part of the year. Similarly you won't find oaks growing very close to the water of the Puget Sound, for the same reason)
That's where legacy and competition comes in. The maple has no closer related species competing with it, but Q. garryana and Q. lobata are (relatively distant, but still) siblings. One can observe this in quite a lot of species, especially in oaks, when they are cousins they can be equally dominant in a forest, when they are siblings (evolutionary speaking) they either start to mingle, in oaks there are the notorious 'syngameons' of eastern N. America, or the case of Acer rubrum-saccharinum, or only one species survives: a single beech species in N. America, where you have 3 to 4 in China under the same setting (and a max. of 2 in W. Eurasia and Japan, but mostly it's one, too). My guess would be, it's a combination of all the things you mentioned, and that it's just too much to bear for Q. lobata to thrive as far north in the Puget Sound area (the frosts in relatively moist situations is a good explanation, being larger, the vessel may be, too and more susceptible to frost collapse) but Q. garryana can still cope or prevailed when their lineages were still sympatric and competing with each other. By all we know from the phylogenetic history, Q. lobata is the older form, Q. garryana is part of a later diverged and further evolved branch of the Dumosae. Q. lobata could be sort of ancient relict, the ancient prototype, and as such it would be trapped in its area, but the lineage that evolved Q. garryana was the one more tolerant to restricting factors and expanded at some point. The general climate is a Csb climate in both places but the morning fogs are really hard to quantify in mm, but we have observed settings in the Csb climates of N. America and Europe that functionally approach perhumid climate settings because of frequent moring fogs or are more similar to them because of increased rainfall in spring, with other vegetation elements that one wouldn't expect in summer-dry climates. In Washington, we have the transition from Csb to Cfb (and extremely moist in the Olympic N.P. because of the luv setting, whereas Puget Sound area is lee-ward and much drier). A transitional setting may favour one species (lineage) over the other. This is particularly mosaic-like and diffuse in Mexico. In the climate envelope data for the oaks, there are species of red and white oaks, which classify as winter-dry and summer-dry when one maps their distribution, which is unusal for oaks (or Fagaceae in general). So I looked into the station data, and it turned out that they are in fact neither-nor but show a strong preference for spring- and autumn-wet situations, with relatively dry winters (less/no frost damage) and summers with drought; and some also show conspicuous topography-correlated distributions (luv- or lee-side of moutain ranges) or altitudinal constraints (fog zone?). The big-leaved maples and Q. garryana may benefit from winters relatively dry in the Puget Sound because of the local microclimate; but already too moist for Q. lobata. And then there's the introgression thing always to keep in mind: Q. garryana, its precursors, may have picked up some genes along the way, that increased its fitness for the Puget Sound situation, while Q. lobata picked up some making it more summer draught tolerant. And then there's something, we know oaks did in the past: one species migrating into the area of another may overtake it completely, phenotypically it's gone, but in the genomes you find the lost parentage. Traditionally, there had been little research in that respect in the U.S. – it's more laborious to study hybrid complexes than trivial data situations – but it starts as it's now much more fashionable and much quicker, with the genetic data pouring down en masse than to "next generation sequencing" approaches. And the Pacific West is the more interesting area, fewer species, but older, early diverged lineages and higher probability to find ancient mixes, with all the non-oak relatives still around, too (Notholithocarpus and Chrysolepis). And the golden-cup oaks, not too forget, and Q. sadleriana. The West is truly much more interesting and puzzling than the East, if I'd have a position in the States, I would never have bothered about anything else than the trees of the Pacific West from Yukon down to Sinaloa.
