I have been watching specials by Discovery Channel on dinosaurs including the

extremely huge plant eaters. My guess is that they were considerably taller

and larger than a lot of other "objects" around them and because of geographic

area needed to feed a breeding population, limited in numbers.

As to the first count that the largest sauropods were taller than most things around them: Sauropods are big animals, no doubt there, however much of there mass isn't necessarily straight up, but rather stretched out like a wiener dog (for lack of a better analog). There are exceptions to this of course, but in general the largest beasts were no taller than 6+ meters at the shoulder (usually the highest spot, exception being brachiosaurids). Limited in numbers is a relative term, we cannot very accurately determine what mean populations sizes would have been. We can tell readily which animals are most often preserved but that may well be preservational bias. Though interestingly enough, in the Jurassic (all coniferous forests, no deciduous trees yet) the planet was full of really big critters, not necessarily in terms of shear numbers in a population, but in diversity. With that being said there would be finite resources for any growing population, though they were most likely (due to trackway evidence) migratory, and traveling in herds, and based on teeth and cranial morphology selectively eating. The most useful analog would be an elephant herd.

But how would they have handled lightning? Wouldn't the largest creatures

being so huge been much more likely to have taken hits?

We now enter the realm of even more speculation, but a perfect question none the less. My assertion would be that these animals hold an roughly equal chance of being struck by lightning as the other animals within its ecosystem. By the "footprint", or surface area of their body seen from the air, they take up more space individually than any other animal. On an individual basis one could surmise that they would have a higher probability of taking a hit, though I believe it would still be a very low probability. Certainly not enough to make any noticeable difference in a population. At least it is obvious that large sauropods lived through the Jurassic and much of the Cretaceous. I have not heard, nor seen documentation of lightning or even proposed lightning for burned bones. Even the bones that are burnt will likely have an indeterminable origin of the burn. I would speculate that lightning has had little effect on these animals' populations over all. I would further speculate that a few did die, annually if not semi-annually, due to lightning strikes. Recently, here in Wyoming, we had close to 60 elk near Jackson Hole, die on a hillside due to lightning strikes. Perhaps, the sauropods wore "insulated" padding or booties on there feet effectively isolating them from the grounding effect. Can you imagine an Apatosaur running around in goulashes?

How did they control heat with the massive size during heat waves or with loss of heat during rain

or cold periods?

Great question! And one with an answer! The hotter or colder it gets its is good to be big (assuming food and water is available…those little necessities of living). Again elephants will make a great example. When a heat wave hits populations of elephants it is the largest of the population that survive. It all has to do with volume. There is a ratio between surface area and volume. Take a sphere for example with radius of one. The surface area is 4(Pi)r^2 whereas the volume is 4/3(Pi)r^3. So the surface area (SA) for a sphere with radius of one, would be ~12.566, and the volume (V) would be ~4.186. Ok two more examples, take radius of four, the SA would be ~201.056, and V = 268.07, and lastly a radius of 20: SA = ~5026.4, and the V = ~33,509.33 as you can see the larger the radius, and therefore the SA, the V starts to go VERY high. So small mammals like mice have a huge SA compared to their V. Where as an elephant (or sauropod) has a relatively low SA compared to its Huge V. Now what allows an animal to heat up? The sun, and metabolism. The sun heats via a surface area that collects or absorbs the radiation of the sun in the form of visible light, UV, etc. Metabolism, the ongoing process of all cells also produces heat, from chemical breakdown (the REALLY short answer). So if an animal has a huge surface area it will cool down, and heat up extremely rapidly, compared to its larger counterparts. When it heats up on the African savanna the largest elephants are the slowest to have their body temp rise, and thus remain cooler. When it gets cold, say like the Ice Ages, the elephants though somewhat hairier, are/were quite adept at surviving these frigid temperatures. Because of the metabolism heating them from the inside because the elephant has more volume for creating the heat, than surface area to radiate it, it remains warmer than its smaller counterparts. Now that may be a little confusing, but I hope you can see the relationship between SA and V. This relationship was what allowed the sauropods to be so big in warm, and cold climates.

How would the ground handle the weight distribution when

saturated?

That really depends on HOW saturated. It will support more mass is it is slightly saturated then if it is completely dry, this is due to the natural hydrostatic pressure of water in what are otherwise collapsible pore space. There is however a threshold that once crossed will actually suspend the particles of sediment and become more squishy than when it is dry, or even partially wet. An experiment take a box of sand and a five gallon bucket filled with water. Place the bucket in the sand while the sand is dry. It will displace an amount of sand. If you smooth the sand back out, wet it down, and set the bucket on again you will notice it does not displace as much. Now supersaturate the sand and set the bucket on it, you will notice it sinks more than the other two times. This approximates what happens when large dinosaurs walk on wet ground. Now, like many extant animals, extinct animals' feet exhibit characteristics conducive to spreading weight over various types of land environments. The last two sauropod quarries that I worked on were buried with sediment that was supersaturated. One was a debris flow (mudslide) where the sediment was saturated beyond the threshold of stability and caused a slow moving mud to creep down the shallow slope. The other was a flooding event called a crevasse splay. A huge amount of mud expanded out onto the flood plain trapping an apatosaur. It died, and preserved in basically an upright position, the legs were in the standing position, and the body decayed and was buried by another event.

Were trees literally that much larger to provide them with

protection at their heights?

The trees were not larger to protect the sauropods but because of them. Plants and herbivores are often in what is called an evolutionary arms race. The taller the trees got the better protected the reproductive portion was. As the trees were selected for height (those that were shorter had reproductive parts eaten) the tallest sauropods had an advantage of the shorter ones. The tallest had a food source that had very limited competition, and thus had more resources and more offspring. The exaggerated features such as extreme height in trees, and long (upright) necks in some sauropods such as brachiosaurs are a direct result of this arms race. So there were always trees taller than the tallest sauropods. There is an extremely large cost (in terms of energy, thus resources) to having a super large size, and hugely long necks. They would not have these unless they allowed the animal a better use of resources. Therefore the trees were already large, but only because the evolution of sauropods selected them in that direction.

I guess it just seems to me that the larger the land animal, the more weather

would be a hinderance to longevity...

All in all, not really. Weather today is much the same as it was in the past. It is definitely a factor in an animal's life, but the populations that live in any given ecosystem were well adapted to those conditions. Those that weren't went extinct, or natural selection selected for the survivors. Hope this answers your question(s).

Cheers

JohnC