'OHI'A DIEBACK AND FOREST LIFE CYCLES

Hawaiian rain forests are a mosaic of diflereni sta es in the The dieback stage is now a weak $nk in the life

forests of Hawai'i.

Grant Gerrish

I n rccent vears, the dying off of many 'ohi'a (Merrocidmspolyntnrpha) trees in Hawaiian forest\ led to intensive research that ha. grml) added to our i.nderst:~ndin,: of thebe forests. 1 \ s i l l use the recearch th :~ t war inrti- gated in response to thir phenomenon, known as 'ohi'o dieback or 'ohi'a decline, as a starting point for a discussion o i the dvnani~c c l edopn l~m of forest communities~ -

F h t , let me describe the different types of fnrcrt that occur in Hawai'i. Plantarion forests. which have been planted hy man for economic purposes, are usuall) made up of tree species not native to Hnwlii'i (alien y~ecies), such as eucalvptus (Eucalyptus spp.) or tropical ash (Frarinrcs ulrd,,;). Ilowever, foresters sometimes plant the native koa (Acacia koa) for ils economic val~le. In either case. ~lantation forests are not herestine from the standooint of consen,ation bkluse the" do not contain 311 the p h s and anikclk of naclral forest ecoysterns, and eventually all the trees will he cut clown. In add~tion to wlantation forests that are "un~lanted or "unmanaeed." forests made uo of 'alien tree species grou ing w~ld, ruch as guwa (~.$i&rm \pp.) and k i i ~ e ' ~ or mesquitr. (Prosopirpollidn), are not of inrcrest here, except in the nera- tive sense that alien trees mav dismwt native forests.

Forcsh oi native trees inilude <onx that occur in :$reas uith moderate or low rainfall, for example the mrrntnne (Supltora chq~oplrylla) forest on Maun3 Kea and the aulu (Supindusoahucnc~c) forest near Mokule'ia, O'ahu. Ihese native forest>, cdlled se;rtonal or &y forest,, are imercsring and important to biological conservation hecmse only sm:~lI :Irene of them remain. Most of the native llawai~an forests. however. are rnin forests. Understanding Hawaiian rain forests is extremely important to consewation because the majority of Hawaiian plant and animal s ecies live there, and because they are the only tropical rain forests in the bnited States (not including Puerto Rico and the trust territories). Native rain forests are the subject of the rest of this pa er

Rain forests mav be detnneb as forests that receive a minimum of 3.9 in. (100 mm) of rain every month (see also ('uddih this volume). Becausc tho llawaiian Mands are within the Tropics, these [crests sre tropical rain forests. Rain forests occur on all the maior Hawaiian Island< Examoles are the forests on the summits of the ~o'olau and Wai'anac Mountains (if Oahu, the forests around Koke'e State Park on Kaua'i, the forests of Kipahulu

Valley on Maui, and the forests of Hamakua and those around Hawaii Volcanoes National Park on the Big Island (Hawai'i).

In all these forests, the most im ortant, or dominant, trees are the 'ohi'a (in the family Myrtaceae) and koa (family Leguminosae--also called Fabaceae). The most extensive rain forests are on the windward slopes of Mauna Kea and Mauna Loa on the Big Island.

'OHI'A DIEBACK

In the late 1960s and the 1970s people noticed that many 'ohi'a trees in Big Island forests were dead or dying. In 1972, more than half of the rain forest area on the windward side of the Big Island was in a state of severe decline. Dieback continues today at a slower pace. The standing trunks of many of these dead trees can still be seen along the Saddle Road or near Hawaii Volcanoes National Park. Because so many trees died in a relatively short time, it a peared that some disease was responsible for dieback or decline of the Poreit. Research was begun to find the cause of the dieback.

Surprisingly, no disease or other single cause of the dieback was ever found. However, many other factors were discovered. Four important find- ings are listed:

1. Some pathogens (disease-causing organisms) and insects were found associated with dying trees. One is an endemic Hawaiian beetle (PIagifI~mysus bilineaius) whose larvae tunnel through the living bark of 'ohi'a trees; however, it was found that these hark beetles onlv attack

not in dvine trees in other licatibns. Further. $was s6metimes found

ihese Dathoeens can attack trees that are alreadv weakened and hasten the de'aths a trees.

2. Climatic records showed that no droughts or floods had occurred which mieht have caused the dieback.

3. R ~ ' \ : . I I : I I ~ , ~ > atim)wrcd that only the tree\ in the forest canopy wcrc d!ing. S c e J l i ~ p :lnd ra l~ l inp were not : idwr~cl \ ; i ife~wd. I-urtherrnme. lulh~n ing dieha;& in most :trza\, good rqenersricrn o l ohi';l seedlings ;tiid \;lpliii:\ :~pp(.:m ant1 a new iurest ssno[)v rcwlr5.

4. A wsrch c~i I~istorlcal rccmli re\r.aled that diehack had hccn i ~ b i e n u l hciorc. In 'ohi'a forc~tc on m e r a l l l a \ ~ i ~ ~ i . ~ n island\. T ~ L . he\[ h c u - mented previous dieback occurred near Nahiku, Maui, on the Hana coast in the early part of this century. The pathologist who researched the die- back found no disease pathogen but concluded that the trees were dying because of natural changes taking place in the soil. Most of the Maui area was replanted with alien tree species. In parts of the area, 'ohi'a forests have come back.

We now believe that dieback is a normal process in the dynamics of some ecosystems. Tree death seems to be caused by many factors that work throughout the life of a tree. Essentially, the trees die of old age and

accumulated environmental stress. This process can he hest understood when dieback is put into its place in the life cycle of the forest.

A RAIN FOREST LIFE CYCLE

llawaiian rain forests begin on new lava flows from active volcanoes. To- day this means on the slopes of Mauna Loa and Kilauea. Plants be in to colonize law flowc within a year or two aiter they cool. This is the %egin- ning of the process ecdoyist\ c~11 succession, referring to the changing se- ouence of plant communities that deve lo~ on a t~iece of new land. Uanv text- hooks desiribe primruy (initial) successidn as ari excmtiatingly slow process that begins with a few lichens and proceeds to mosses, ferns, and finally seed plants. In rainy areas of Hawai'i, such as the district of Puna on the island of Hawai'i. the seeds of 'ohi'a and a few other oioneer ~ l a n t s quickly find thcir \ray to new lava nows andgerminate in'cracks a'nd holes \r here mi~isl~tre and bits of soil first accumulale. Lichens, moqses. and small ferns are also sienificant ~ i o n e e r forms.

Initially, pl&t growth is slow on a lava flow, hut the first plants speed the proccrs by adding organic matter from thcir old leaves, branches, and roots. Soil develops, more plant? invade, and coloni7ation of the flow accelerates. In these future rain forests, small 'ohi'a trees contmue lo dominate the vegetation. Evcntually there are enough of them that their hranches form a complete canopv over the around and create shade. Thi5 chanees erowine conditions to <he extent thzt oioneer ~lants . includine .ohi.g cak no G g e r germmate in the young ft;rest. lihwev&. other rFecies of plants, includ~na tree ferns and nranv sr)ccics of native fernr and shruhs, noiv find the shadc. moist forest floor iuiiahle for erowth.

As the forest Lontinucs to develop, the upper &nopy i~ made up of 'ohl'a trees that are approx~mately the same size and age. The forest grows towards maturltv with few vnung 'ohi'a treer helow the cartouv. We know some of the stresseithat beginto wzaken trees 3s they age. In this example of an early succession;~l 'ohra forest, the soil will still he quite shallow and the .wow- ing trees will compete strongly for the limited supply of nutrients arid water These environmental stresses will combine with internal stresses associated with aging to reduce the vigor of the now-mature trees. This is the point where forest diehack appears in the cvcle.

Since the canopy tiees in the forest are all approxinurely the slime age or of the wme generation, they tend to lose \.igor at thc same ttme. A strong shock, such as a droueht or a wind storm that rips awav maw leaves, mxv tr~ggcr the diehack oiall or most trees of the stand. The dyi'ng trees are' readily attacked by psthngens, and the iorert canopy becomes harren. At this point, the forert life mcle heeins again. With the tree crowns dead. light again strikes the ground and-plantctl~at require fu l l sunlight can hecchc e5tahlished. Amung theqe ;!re netr 'ohi'a seedlings that wll one day iorm another forest canopy.

'l'he new forest will not he the same as the one that first invaded the fresh lava tlow. t'ollow~ng d~ellack, cond~trons are generally more favorable than those on a raw law flow: the soil is more dcvelo~ed. manv ~ l a n t s~ec ies are already established, and the dead 'ohi'a trees theinselves irk a souke of nutrients.

Manv 'ohi'a trees that become established in the second forest are of a ~~~ ~~~-~~~

different-variety than in the first, a variety that is adapted to the ncw con- ditions. It is a general rule that the number of s~ecies of native ~ l a n ~ r and animals increases through the course of succession; thus, the second forest will he biologically richer. However, the dynamics of succession will remain the .same. N o more young 'ohi'a trees will become established once a closed canopy of trees is formed; The forest will continue to develop until once again the mature 'ohi'a trees lose vigor and enter a dieback phase.

This cycle may continue through millions of ears of succession and evolution in the Hawaiian forests, but each new ? orest may be different from the preceding one. A comparison of the rain forests in the Ko'olau Mountains of O'ahu with the forests of Mauna Loa, which I have been des- cribing, illustrates some of the changes which have occurred over a very long forest succession. On O'ahu, the 'ohi'a are generally smaller and of a different variety or form than the 'ohi'a of early succession. The forests of O'ahu contain many more species, including numerous trees associated with the 'ohi'a. The land itself has chaneed. The broad. voune slooes of Mauna

~~ ~~

L03 prd\i& large segments or u n i 6 r r n hahitat wh& thc'for;sts may be syn- chronizcil i n t11c ,:me I ) U I ( J ~ I ~ I C lile CVCIC. On the older Islands, streams

together.

IMPLICATIONS

Witnessing the impressive dieback of large forests gives us perspective. Trees and forests, being slow-growing, may seem permanent to people. Die- back and forest regeneration show that these ecosystems are dynamic and ever-chaneine. This knowledee has imolications for the conservation of the island biora. ;s ecially in ref&ence to ihe size of natural areac that r n u v be preserved. Rawailan rain forests arc a motaic ofdiffcrcnt <t:i.ges in ihe forest life cvcle. At anv one time forests can be found that are mature and healthy, dying, or regenerating. A functioning forest system will not be preserved by creating a reserve around a segment of forest that is healthy today, for that forest will certainly lose vigor and undergo dieback at some Lime in the future. As 1 pointed our, there are plants and animals that live !n each stage of forest succession; therefore, it is essential that a reserve include an area large enough to contain all the rain forest life cycle stages, and that there be reserves on all islands representing all the successional stages (see Holt, this volume).

In the past, forests dieback were described as "decadent," and sometimes people doomed and should he replaced with something more an interpretation that does not con- P sider that the forest l i e cycle has a regeneratiye stage. It is unfortunately true that the dieback stage is now a weak link I? the ltfe cycle m sonle parts of Hawai'i. At this stage it may he easler for allen plants, such as flre tree (Myrica f q a ) or banana poka (Pmsiflora mollirsitna), to invade the sunny openings in the forest. Natural forest areas that include 'ohi'a forests may thus require special management of alien spectes, especially during the dieback phase.

Important References

Cuddihy, L.W. [this volume] Vegetation zones of the Hawaiian Islands. Hadges. C.S., KT. Adee, J.D. Stein, H.B. W d , and R.D. Doty. 1986. Decline ofOhia

(Metrosiderm polymorpha) in Hawaii: A Review. U S . Forest S C M C ~ Pacific Southwest Forest and Range Experiment Station General Technical Report PSW-86, Berkeley, California

Holt, A. [this volume) Proledion of natural habitats. Muelle3-Dombois, D. 1985. 'Ohi'a dicback and protection management of the Hawaiian raia

forcst. Pp. 403-421 IN C.P. Stone and J.M. Scott (ed8.l. Hawoi'i's Tenesrriol . ' Ecosynemr: Prcsewolion ondMuwgemmr. Univ. Ilawaii P rcs for Univ. tlauaig Coopcralne National Park Resources Studics Uait, Honolulu.

hfuellcr-r>ombois, D. 1985. 'Ohr'l ditluck in Haui i : 1M ~nrhcsir and cvalukm. Pacific Science 39(2):150-170.

Mucller-Dornbois, D. 1987. Natural dicback in forests. Bioscience 37(8):575-583. Mueller-Dombois, D., K.W. Bridges, and H.L. Carson (eds.). 1981. Island Ecosyslernr

Biolw'cal Orgarrizarion in SelecredHawoiiau Communities. Hutchinson Ross Publ. Co., Stroudsburg, Pennsylvania.

Mueller-Dombois. D., J.E. Canfield, RA. Holt, and G.P. Buelow. 1983. Tree-goup dealh in Nonh American and Hawaiim forests: a oathalo~ical oroblem or a new nroblem irw - . ~ ~~ r ~ ~ - ~ ~ ~ ~-~ vegetation ecology? Pl#yo~oe?~ologia ll(lj:ll7-137.

Pacific Science Congress. 1983. Canopy dieback and dynamic processes in Pacific k,rcsts. Pacific Science 37(4): entire issue

Papp, R.P . J.T. KIiejun& R S Smith, and R.R. Scharpf. 1 9 4 Association of Nogrrhn~yn~s brlmt.un<s (Coluqwra: Ccramhycidac) m u Phylophora brlinrom with the decline of ohia-lchua forc.1~ on the i s h d of Hawaii. tiwar Somw 125:157-190.

US. Department of Aariculture. 1981. A Biolo&al Evohtolio,t ofOhio Decline ott rite ~, ~ . - ~ ~ ~. . . . ~. Isl&d of f f a w o i i . - ~ S . Forest Service, pacific Southwest Region, San Francisco, and Hawaii Slalc Depl. Land and Natural Rcsourceg Honolulu.