Saguaro populations are showing signs of rebounding or stabilizing at Saguaro National Park after decades of decline, some research is finding.
But other research from a much smaller area of the park shows a steady decline in saguaros since the 1980s.
The longer-term outlook for the Sonoran Desert's signature plant is uncertain because of evidence that it's getting harder to find the youngest saguaros in recent years. That's possibly due to drought or climate change, researchers say. But it may be a long time before researchers know why younger saguaro populations are dropping - or even if they are.
At a weekend symposium on Saguaro Park and climate change, researchers laid out results from three sets of data plots in the national park's east and west units. The numbers were sometimes contradictory, but at least some data showed total saguaro population increases in recent decades. The symposium was at the Arizona-Sonora Desert Museum.
A key problem in monitoring and making long-term forecasts for saguaros is that saguaro populations are nurtured and recruited in episodes - periods lasting many years - and it's hard to know how this cycle will play out in the future, said researcher Tom Orum, who has been monitoring six plots at Saguaro Park-East.
"We're entering into a period where there is little recruitment, and we don't know how significant the next episode of recruitment will be or when it will happen," he said.
• Official censuses from the park's east and west units show that total saguaro counts rose 75 percent from 1990 to 2010. The censuses have been taken every decade since 1990 at 35 plots on flatlands, steep slopes and foothills. But researchers found only 8 percent of the number of germinating baby saguaros in 2000 as they did in 1995.
• Data taken by two retired University of Arizona researchers showed total saguaros at the six plots near the end of East Broadway dropped from 1,400 in 1941 to about 900 in 1995 and 660 today. The number of baby saguaros apparently has dropped sharply as well.
In 1995, they contained 38 saguaros under 4 inches tall and 119 plants 4 to 8 inches tall. By 2010, they found one saguaro under 4 inches and seven between 4 and 8 inches. Orum and Nancy Ferguson are the most recent in a series of reseachers to study these plots since 1941.
• Retired ecologist Ray Turner, who has studied saguaros at the park since 1961, said his data show a clear increase in saguaros at Saguaro-West since 1973. Total saguaros rose at Saguaro-East from the mid-70s until the mid-90s before dropping into the early part of this century.
Orum doesn't know for sure why his Saguaro-East counts showed a decline when the park service's showed increases but said it could be due to differences in terrain studied.
But it's clear that at Saguaro East, "we've lost all the big ones" in recent decades, said Turner, best known for writing books that compare pictures of the same spot from different eras to show environmental change. "Virtually all the ones with arms are gone."
Such comparative photography has illustrated the park's earlier saguaro declines. At the Saguaro-East visitor's center, pictures of the same spot in 1935, 1960, 1995 and 2004 show a dramatic drop-off in large cacti.
Yet during much of that time, many baby saguaros were born that didn't show in the pictures. Those babies were part of the more recent increases, researchers said.
The increases have had several causes, said Turner, Orum and Don Swann, a Saguaro Park biologist:
• Cattle that for decades trampled saguaro seedlings were removed in 1958.
• Palo verdes, which protect seedlings, had time to recover from heavy timber-cutting in the early 20th century for use as firewood and in lime kilns. The recovery ended a series of catastrophic freezes.
• From 1960 into the early 1990s, the Tucson area was in a generally wet cycle, which helped the cacti.
But since the early to mid-90s, the area has been in a dry cycle. It's possible the lower numbers of baby saguaros are due to drought, the researchers said. If so - and if global warming is heralding a new era of drought in the Southwest as many researchers have predicted - then this could be the start of another saguaro decline, researchers said.
But it's too early to know if drought is the culprit with baby saguaros, Swann and Orum said. It's still hard today to even see saguaros germinated as recently as 2002, Orum said.
Swann added he can't even say how many more years of data are needed to determine if climate change is affecting the saguaros. For more than a decade, climate experts have warned global warming could reduce saguaro populations or drive them to higher elevations.
"Saguaros grow so slowly, you don't fully understand what is going on with population until decades after the changes," Swann said.
Contact reporter Tony Davis at 806-7746 or firstname.lastname@example.org