This is a Pacific or Western plum (Prunus subcordata, Bentham) and it is a native species found growing wild in a relatively limited region east of the Coast Range from southern Oregon to central California. It occurs in greatest abundance in Lake and Klamath counties in Oregon and Modoc and Sierra counties in California. While the greatest concentration of the native thickets in California seems to be in the general vicinity of Mount Shasta, the plum is found in more or less abundance east and south of the Nevada line, especially in the foothills of the Sierra Nevadas. It has been reported as far south as Yosemite Valley. The plum usually forms thickets of small to large shrubby trees along streams in canyons, on hillsides, or in the open area of pine forest. In other cases the thickets are found on ridges which are thought to be the shore lines of prehistoric lakes. There is a general similarity in the sites on which the plum thrives and spread throughout the region, so much so that one can almost predict the location of the thickets. The largest trees are found in these thickets growing on the deeper, sandy-loam soils in the canyons where water is ample, and the richer bottomlands adjoining the old lake beds. The hardiness of the species and its ability to adapt itself to adverse conditions is typified by the scrubby little thickets found growing out of rock slides at high elevations. An occasional thicket is found, however, where drought or exposure has all but killed out the stand. The region in which the plum is native has an elevation of 1,000 to 5,000 feet, although it has been observed growing in favorable locations in the mountains of the region at elevations as high as 6,000 feet. At these higher elevations the plums are less apt to bear crops and are late in ripening.

Thanks everyone for your replies. Unfortunately I don't have a photo of the fruit. The last few years it did develop a fungus (which I think is called plum pocket). You can see a withered plum in the second picture. Before that though, the fruit was always good and sweet. theaceofspades: you may be right about it being planted from a pit. That sounds like something that my grandfather, whose property it was, would have done. destin_gardener: It didn't have any thorns. From my online reading, I think that yes, it is likely a Canada or American plum. It was really a hearty tree for being so far north, so I think a native tree is probably a good guess. Thanks again. And if anyone has an idea, please chime in!

There are several possible culprits that are well known as leaf chewing pests. It will be up to you to find them. Try looking carefully after dark (with a flashlight) or just after a rainfall. Grasshoppers, caterpillars, some beetle species, earwigs, pillbugs, slugs and snails are all "critters of interest ". Slugs and snails usually leave a tell-tale slime trail...evidence of their recent visits. If we know the kinds of plants that are being attacked, it can sometimes lead to an ID.

Thanks everybody. I did some searching for common repelants for lettuce feasting insects and came up with garlic and pepper as commonly being effective. I may also look into using BT, though I'd prefer to stick with stuff I have around my house while I can. I do think it's some sort of insect, I still havent been able to observe anything though. I feel like if it were some sort of rodent there would be some kind of trace left behind, a whole or disturbance in my netting, footprints, trampled seedlings, something, none of which I've observed. It's not the whole plant that's being eaten, just the two leaves of the seedling with the stalk left there still standing, which quickly dies with no photosynthetic surface. I'm going to keep applying a garlic and pepper spray every 3-5 days and see how that works. Since I applied it Saturday I havent noticed a significant decline in the # of seedlings, so perhaps it's having some effect.